San Diego Hydrology Model 3.0 Interim Version User Manual

San Diego Hydrology Model 3.0 Interim Version User Manual

San Diego Hydrology Model

3.0 Interim Version

User Manual

Clear Creek Solutions, Inc. www.clearcreeksolutions.com

May 2016

To download the San Diego Hydrology Model 3.0 and the electronic version of this user’s manual, please go to www.clearcreeksolutions.com/downloads

If you have questions about SDHM 3.0 or its use, please contact:

Clear Creek Solutions, Inc.

360-943-0304 (8 AM – 5 PM Pacific time) ii

End User License Agreement

End User Software License Agreement (Agreement). By clicking on the “Accept”

Button when installing the San Diego Hydrology Model 3.0 (SDHM 3.0) software or by using the San Diego Hydrology Model 3.0 software following installation, you, your employer, client and associates (collectively, “End User”) are consenting to be bound by the following terms and conditions. If you or User do not desire to be bound by the following conditions, click the “Decline” Button, and do not continue the installation process or use of the San Diego Hydrology Model 3.0 software.

The San Diego Hydrology Model 3.0 software is being provided to End User pursuant to a sublicense of a governmental licensee of Clear Creek Solutions, Inc. Pursuant to the terms and conditions of this Agreement, End User is permitted to use the San Diego Hydrology Model 3.0 software solely for purposes authorized by participating municipal, county or special district member agencies of signatory programs which are organized on a county-wide basis for implementation of stormwater discharge permits issued by the California Regional Water Quality

Control Board, under the National Pollutant Discharge Elimination System. The End User is not permitted to use the San Diego Hydrology Model 3.0 software for any other purpose than as described above.

End User shall not copy, distribute, alter, or modify the San Diego Hydrology Model 3.0 software.

SDHM 3.0 incorporates data on soils, climate and geographical features to support its intended uses of identifying site-appropriate modeling parameters, incorporating user-defined inputs into long-term hydrologic simulation models of areas within the County of San Diego, and assisting design of facilities for flow duration control as described in the accompanying documentation.

These data may not be adequate for other purposes such as those requiring precise location, measurement or description of geographical features, or engineering analyses other than those described in the documentation.

This program and accompanying documentation are provided 'as-is' without warranty of any kind.

The entire risk regarding the performance and results of this program is assumed by End User.

Clear Creek Solutions Inc. and the governmental licensee or sublicensees disclaim all warranties, either expressed or implied, including but not limited to implied warranties of program and accompanying documentation. In no event shall Clear Creek Solutions Inc, or authorized representatives be liable for any damages whatsoever (including without limitation to damages for loss of business profits, loss of business information, business interruption, and the like) arising out of the use of, or inability to use this program even if Clear Creek Solutions Inc., has been advised of the possibility of such damages. Software Copyright © by Clear Creek

Solutions, Inc. 2005-2016; All Rights Reserved. iii

FOREWORD

The San Diego Hydrology Model 3.0 (SDHM 3.0) is a tool for analyzing the hydromodification effects of land development projects and sizing solutions to mitigate the increased runoff from these projects. This section of the User Manual provides background information on the definition and effects of hydromodification and relevant findings from technical analyses conducted in response to regulatory requirements. It also summarizes the current Hydromodification Management Standard and general design approach for hydromodification control facilities, which led to the development of the

SDHM 3.0.

Effects of Hydromodification

Urbanization of a watershed modifies natural watershed and stream processes by altering the terrain, modifying the vegetation and soil characteristics, introducing pavement and buildings, installing drainage and flood control infrastructure, and altering the condition of stream channels through straightening, deepening, and armoring. These changes affect hydrologic characteristics in the watershed (rainfall interception, infiltration, runoff and stream flows), and affect the supply and transport of sediment in the stream system. The change in runoff characteristics from a watershed caused by changes in land use conditions is called hydrograph modification, or simply hydromodification.

As the total area of impervious surfaces increases in previously undeveloped areas, infiltration of rainfall decreases, causing more water to run off the surface as overland flow at a faster rate. Storms that previously didn’t produce runoff under rural conditions can produce erosive flows. The increase in the volume of runoff and the length of time that erosive flows occur ultimately intensify sediment transport, causing changes in sediment transport characteristics and the hydraulic geometry (width, depth, slope) of channels. The larger runoff durations and volumes and the intensified erosion of streams can impair the beneficial uses of the stream channels.

Regulatory Context

The California Regional Water Quality Control Board (Water Board) requires stormwater programs to address the increases in runoff rate and volume from new and redevelopment projects where those increases could cause increased erosion of receiving streams. Phase

1 municipal stormwater permits in San Diego County contain requirements to develop and implement hydromodification management plans (HMPs) and to implement associated management measures.

Development of the San Diego Hydrology Model

The concept of designing a flow duration control facility is relatively new and, as described above, requires the use of a continuous simulation hydrologic model. To facilitate this design approach, Clear Creek Solutions (CCS) has created a user-friendly, automated modeling and flow duration control facility sizing software tool adapted from its Western Washington Hydrology Model (WWHM). The WWHM was developed in

2001 for the Washington State Department of Ecology to support Ecology’s Stormwater iv

Management Manual for Western Washington

1

and assist project proponents in complying with the Western Washington hydromodification control requirements. The

San Diego Hydrology Model 3.0 (SDHM 3.0) is adapted from WWHM Version 4, but has been modified to represent San Diego County hydrology and enhanced to be able to size other types of control measures and low impact development (LID) techniques for flow reduction as well.

SDHM 3.0 is a useful tool in the design process, but must be used in conjunction with local design guidance to ensure compliance for specific projects. The reader should refer to Appendix C and local stormwater program guidance for additional information and suggestions for using SDHM 3.0.

Acknowledgements

The following individuals are acknowledged for their contributions to the development of

SDHM 3.0 and User Manual:

 Doug Beyerlein, Joe Brascher, and Gary Maxfield of Clear Creek Solutions, Inc., for development of SDHM 3.0 and preparation of the SDHM 3.0 User Manual.

 Darren Bertrand and Brian Haines of Environmental Science Associates (ESA) for overall SDHM 3.0 project management, as part of Task Order No. 10.

 HMP Monitoring Subworkgroup members Stuart Kuhn (County of San Diego),

Eric Mosolgo (City of San Diego), and Rene Vidales (County of San Diego) for general guidance and direction in the construction and implementation of SDHM

3.0.

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Washington State Department of Ecology. 2001. Stormwater Management Manual for

Western Washington. Volume III: Hydrologic Analysis and Flow Control Design/BMPs.

Publication No. 99-13. Olympia, WA. v

This page has been intentionally left blank. vi

TABLE OF CONTENTS

End User License Agreement .......................................................................................... iii

FOREWORD.................................................................................................................... iv

INTRODUCTION TO SDHM 3.0 ................................................................................... 1

SDHM 3.0 OVERVIEW ................................................................................................... 3

QUICK START ................................................................................................................. 5

MAIN SCREENS ............................................................................................................ 43

MAP INFORMATION SCREEN..................................................................................... 44

GENERAL PROJECT INFORMATION SCREEN ......................................................... 45

SCHEMATIC EDITOR .................................................................................................... 46

STANDARD ELEMENTS ............................................................................................. 47

BASIN ELEMENT ........................................................................................................... 48

TRAPEZOIDAL POND ELEMENT ............................................................................... 52

VAULT ELEMENT ......................................................................................................... 57

TANK ELEMENT ............................................................................................................ 59

IRREGULAR POND ELEMENT .................................................................................... 61

GRAVEL TRENCH BED ELEMENT ............................................................................. 64

SAND FILTER ELEMENT ............................................................................................. 66

CHANNEL ELEMENT .................................................................................................... 68

FLOW SPLITTER ELEMENT ........................................................................................ 70

TIME SERIES ELEMENT ............................................................................................... 72

SSD TABLE ELEMENT .................................................................................................. 73

LID ELEMENTS ............................................................................................................ 77

BIORETENTION/RAIN GARDEN ELEMENT ............................................................. 78

INFILTRATION PLANTER ELEMENT ........................................................................ 85

FLOW-THROUGH PLANTER ELEMENT .................................................................... 88

POROUS PAVEMENT .................................................................................................... 91

DISPERSION ................................................................................................................... 94

LATERAL BASIN ELEMENT (Pervious) ...................................................................... 97

LATERAL I BASIN ELEMENT (Impervious) ............................................................... 98

DRY WELL ELEMENT .................................................................................................. 99

INFILTRATION TRENCH ELEMENT ........................................................................ 101

INFILTRATION BASIN ELEMENT ............................................................................ 103

GREEN ROOF ELEMENT ............................................................................................ 105

RAINWATER HARVESTING ...................................................................................... 108

ADDITIONAL INFORMATION ................................................................................ 109

OUTLET STRUCTURE CONFIGURATIONS............................................................. 110 vii

INFILTRATION ............................................................................................................. 116

AUTO POND, AUTO VAULT, AUTO TANK ............................................................. 117

STAGE-STORAGE-DISCHARGE TABLE .................................................................. 119

POINT OF COMPLIANCE ............................................................................................ 120

CONNECTING ELEMENTS ......................................................................................... 123

ANALYSIS SCREEN ................................................................................................... 127

FLOW DURATION ....................................................................................................... 129

FLOW FREQUENCY .................................................................................................... 130

DRAWDOWN ................................................................................................................ 131

HYDROGRAPHS........................................................................................................... 132

REPORTS SCREEN .................................................................................................... 135

TOOLS SCREEN ......................................................................................................... 141

LID ANALYSIS SCREEN ........................................................................................... 145

OPTIONS....................................................................................................................... 151

DURATION CRITERIA ................................................................................................ 152

SCALING FACTORS .................................................................................................... 154

APPENDIX A: DEFAULT SDHM 3.0 HSPF PERVIOUS PARAMETER VALUES

......................................................................................................................................... 155

APPENDIX B: DEFAULT SDHM 3.0 HSPF IMPERVIOUS PARAMETER

VALUES ........................................................................................................................ 171

APPENDIX C: ADDITIONAL GUIDANCE FOR USING SDHM 3.0 ................... 173

APPENDIX D: BIORETENTION MODELING METHODOLOGY..................... 179

APPENDIX E: SDHM 3.0 COMPLEX MODEL EXAMPLE ................................. 185

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INTRODUCTION TO SDHM 3.0

SDHM 3.0 is the San Diego Hydrology Model. SDHM 3.0 is based on the WWHM

(Western Washington Hydrology Model) stormwater modeling platform. WWHM was originally developed for the Washington State Department of Ecology. More information about WWHM can be found at www.clearcreeksolutions.com. More information can be found about the Washington State Department of Ecology’s stormwater management program and manual at www.ecy.wa.gov/programs/wq/stormwater/manual.html.

Clear Creek Solutions is responsible for SDHM 3.0 and the SDHM 3.0 User Manual.

This user manual is organized so as to provide the user an example of a standard application using SDHM 3.0 (described in Quick Start) followed by descriptions of the different components and options available in SDHM 3.0. The LID Elements section presents some ideas of how to incorporate LID (Low Impact Development) facilities and practices into the SDHM 3.0 analysis. Appendices A and B provide a full list of the

HSPF parameter values used in SDHM 3.0. Appendix C contains additional guidance and recommendations by the stormwater programs that have sponsored the SDHM 3.0 development.

Throughout the user manual notes using this font (sans-serif italic) alert the user to actions or design decisions for which guidance must be consulted that is external to the SDHM 3.0 software, either provided in Appendix C of this user manual or by the local municipal permitting agency.

Purpose

The purpose of SDHM 3.0 is to size hydromodification management or flow control facilities to mitigate the effects of increased runoff (peak discharge, duration, and volume) from proposed land use changes that impact natural streams, wetlands, and other water courses.

SDHM 3.0 provides:

 A uniform methodology for San Diego County

 A more accurate methodology than single-event design storms

 An easy-to-use software package

SDHM 3.0 is based on:

 Continuous simulation hydrology (HSPF)

 Actual long-term recorded precipitation data

 Potential evapotranspiration data

 Existing vegetation (for Predeveloped conditions)

 Regional HSPF parameters

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Changes from SDHM2015 (and SDHM2011) to SDHM 3.0 Interim Version

 Revised, updated rainfall data (1984-2014).

 Revised, updated potential evapotranspiration (PET) data.

 SDHM 3.0 automatically selects rainfall record and multiplication factor based project site location on map.

 Rainfall and runoff data changed from hourly to 15-minute time step.

 SDHM 3.0 flow durations percent limit is 110% for 100% of the flow duration levels.

 New SDHM 3.0 land covers: natural vegetation (NatVeg), dirt, rock.

 New SDHM 3.0 urban landscape without irrigation (UrbNoIrr).

 New SDHM 3.0 HSPF parameter values.

 Flow frequency method changed from Cunnane to Weibull.

The changes that are crossed out are changes that will be in the final version of

SDHM 3.0, but are not included in the interim version. The final version will be released following additional review and analysis of the revised, updated rainfall data.

Note: Because of the above listed changes, projects that have been created in

SDHM2015 (and earlier SDHM2011) will not work in SDHM 3.0. To convert these older projects to SDHM 3.0 the user needs to manually re-input the project information into a new SDHM 3.0 project file. The project results will change and the hydromod mitigation facility may need to be re-sized.

SDHM 3.0 Computer Requirements

 Windows 2000/XP/Vista/7/8/10 with 300 MB uncompressed hard drive space

 Internet access (only required for downloading SDHM 3.0, not required for executing SDHM 3.0)

 Pentium 3 or faster processor (desirable)

 Color monitor (desirable)

Before Starting SDHM 3.0

 Knowledge of the site location and/or street address.

 Knowledge of the actual distribution of existing site soil by category (A, B, C, or

D).

 Knowledge of the planned distribution of the proposed development (buildings, streets, sidewalks, parking, lawn areas) overlying the soil categories.

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SDHM 3.0 OVERVIEW

The SDHM 3.0 software architecture and methodology is the same as that developed for

BAHM (Bay Area Hydrology Model) and WWHM and uses HSPF as its computational engine

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. Like BAHM and WWHM, SDHM 3.0 is a tool that generates flow duration curves for the pre- and development condition and then sizes a flow duration control facility and outlet structure to match the Predevelopment curve. The software package consists of a user-friendly graphical interface with screens for input of Predeveloped and development (Mitigated) conditions; an engine that automatically loads appropriate parameters and meteorological data and runs continuous simulations of site runoff to generate flow duration curves; a module for sizing or checking the control measure to achieve the hydromodification control standard; and a reporting module.

The HSPF hydrology parameter values used in SDHM 3.0 are based on regional values.

SDHM 3.0 uses the San Diego County long-term hourly precipitation data records selected by the County of San Diego.

SDHM 3.0 computes stormwater runoff for a site selected by the user. SDHM 3.0 runs

HSPF in the background to generate a hourly runoff time series from the available rain gage data over a number of years. Stormwater runoff is computed for both Predeveloped and development (Mitigated) land use conditions. Then, another part of the SDHM 3.0 routes the development stormwater runoff through a stormwater control facility of the user’s choice.

SDHM uses the Predevelopment peak flood values from a partial duration series of individual peak events to compute the Predevelopment 2-year through 10-year flood frequency values

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or user-input values. The development (Mitigated) runoff 2-year through 10-year flood frequency values are computed at the outlet of the proposed stormwater facility. The model routes the development (Mitigated) runoff through the stormwater facility. As with the Predevelopment peak flow values, partial duration development (Mitigated) flow values are selected by the model to compute the developed

2-year through 10-year flood frequency.

The Predevelopment 2-year peak flow is multiplied by a percentage (10, 30, or 50 percent) to set the lower limit of the erosive flows, in accordance with the current HMP performance criteria

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. The Predevelopment 10-year peak flow is the upper limit. A comparison of the Predevelopment and development (Mitigated) flow duration curves is conducted for 100 flow levels between the lower limit and the upper limit. The model counts the number of hourly simulation intervals that Predevelopment flows exceed each

2

3

SDHM 3.0 is based on WWHM Version 4.

The actual flood frequency calculations are made using the Weibull flood frequency

4 equation.

In SDHM 3.0, this low flow limit is a user-defined variable, to allow flexibility based on whether the site is located in an area with a lower, medium, or high susceptibility to erosive flows.

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San Diego Hydrology Model 3.0 Interim Version User Manual – May 2016 of the flow levels during the entire simulation period. The model does the same analysis for the development Mitigated flows.

Low impact development (LID) practices have been recognized as opportunities to reduce and/or eliminate stormwater runoff at the source before it becomes a problem.

They include compost-amended soils, bioretention, porous pavement, green roofs, rain gardens, and vegetated swales. All of these approaches reduce stormwater runoff.

SDHM 3.0 can be used to determine the magnitude of the reduction from each of these practices and the amount of stormwater detention storage still required to meet HMP requirements.

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QUICK START

Quick Start very briefly describes the steps to quickly size a stormwater detention pond/basin using SDHM 3.0. New users should read the descriptions of the SDHM 3.0 screens, elements, and analysis tools before going through the steps described below.

1. Select appropriate lower erosive flow threshold.

The lower threshold for the flow duration analysis is based on critical shear stress in the downstream channel. Check with the reviewing agency to determine the appropriate lower threshold for the project site.

The default lower threshold value is 10 percent of the 2-year flow. If appropriate, this lower threshold can be changed by the user to a higher percent.

Go to View, Options to change the lower threshold value.

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The lower and upper thresholds can also be set using specific user-defined flow values, if allowed by the reviewing agency.

These user-defned flow values are calculated by the user outside of SDHM 3.0 and then input by the user as the new flow duration criteria.

The user has the option of changing the lower and upper thresholds based on the following USGS regional regression equations:

Q2 = 3.60*(A^0.672)*(P^0.753)

Q10 = 6.56*(A^0.783)*(P^1.07)

Where A = drainage area (sq. miles)

P = mean annual precipitation (inches)

The lower threshold equals 0.10Q2.

Mean annual precipitation values for the standard 14 San Diego County rain gages are shown below.

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Bonita

Descanso

Encinitas

Fallbrook

Fashion Valley

Granite Hills

9.1 inches

20.5

9.3

13.9

10.4

12.8

Kearny Mesa

Lake Henshaw

Lake Wolford

Morena Lake

Oceanside

Poway

10.8

22.6

16.8

16.6

11.3

11.6

Ramona

Santa Ysabel

13.0

21.1

Note that when changing the default threshold value(s) this must be done for each point of compliance if there is more than one point of compliance.

For this example we will leave the lower threshold set at the default value of 10% of the

2-year flow value based on the USGS equation using the Fashion Valley rainfall.

A = 11 ac = 0.0171875 square miles

P = 10.4 inches

Q2 = 1.368 cfs

Q10 = 3.337 cfs

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Lower threshold = 0.10Q2 = 0.1368 cfs

Upper threshold = Q10 = 3.337 cfs

Click the Update button when finished.

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2. Select the project site location.

Locate the project site on the map. Use the map controls to magnify a portion of the map, if needed. Select the project site by left clicking on the map location. A red dot will be placed on the map identifying the project site.

Select the appropriate rainfall station from the list on the right and SDHM 3.0 will automatically load the appropriate precipitation data. (The selection of the appropriate

rainfall station will be automated in the final version of SDHM 3.0.)

For this example we will use the Fashion Valley rain gage. The rain gage data will be multiplied by the precipitation factor (in this example: 1.000). (The adjustment of the

precipitation factor to the project site will be automated in the final version of

SDHM 3.0.).

The site name, address, and city information is optional. It is not used by SDHM 3.0, but will be included in the project report summary.

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3. Use the tool bar (immediately above the map) to move to the

Scenario Editor. Click on the General Project Information button.

The General Project Information button will bring up the Schematic

Editor.

The schematic editor screen contains two scenarios:

Predeveloped and Mitigated.

Set up first the Predeveloped scenario and then the Mitigated scenario.

Check the Predeveloped scenario box.

Left click on the Basin element under the Basic Elements heading. The Basin element represents the project drainage area. It is the upper left element.

Select any grid cell (preferably near the top of the grid) and left click on that grid. The basin will appear in that grid cell.

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To the right of the grid is the land use information associated with the basin element.

Select the appropriate soil, land cover, and land slope for the Predeveloped scenario.

Soils are based on NRCS general categories A, B, C, and D.

Land cover is based on the native or existing vegetation or lack of for the

Predevelopment area and the planned vegetation for the planned development (Mitigated scenario). Non-urban land cover has been divided into natural vegetation (NatVeg), dirt, and rock. Dirt and rock includes landscapes that are vegetated, but the majority of the surface cover is non-vegetation.

The Mitigated scenario developed landscape will consist of urban vegetation (lawns, flowers, planted shrubs and trees). It may be irrigated (Urban) or non-irrigated

(UrbNoIrr).

Land slope is divided into flat (0-5%), moderate (5-15%), and steep (>15%) land slopes.

HSPF parameter values in SDHM 3.0 have been adjusted for the different soil, land cover, and land slope categories.

For this example we will assume that the Predevelopment land use is 10 acres of D soil with native vegetation (NatVeg) on a moderate slope (5-15%) and 1 acre of C soil with rock on a flat (0-5%) slope.

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The total drainage area is 11 acres for this land use basin (default name “Basin 1”).

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The exit from this land use basin will be selected as our point of compliance for the

Predeveloped scenario. Right click on the basin element and highlight Connect to Point of Compliance (the point of compliance is defined as the location at which the runoff from both the Predeveloped scenario and the Mitigated scenario are compared).

The Point of Compliance screen will be shown for Predeveloped Basin 1. The POC

(Point of Compliance) outlet has been checked for both surface runoff and interflow

(shallow subsurface flow). These are the two flow components of stormwater runoff. Do not check the groundwater box unless there is observed and documented base flow on the project site.

Click the Connect button in the low right corner to connect this point of compliance to the Predeveloped land use basin.

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After the point of compliance has been added to the land use basin the basin element will change. A small box with a bar chart graphic and a number will be shown in the lower right corner of the basin element. This small POC box identifies the exit from this land use basin as a point of compliance. The number is the POC number (e.g., POC 1).

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Click on the Run Scenario button to run the Predeveloped scenario and generate the

Predeveloped runoff.

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After the Run Scenario button has been clicked SDHM creates an HSPF UCI (User

Control Input) file, runs HSPF for the entire simulation period, generates the resulting runoff, and stores the runoff time series data in the database (HSPF WDM file).

When HSPF is running the information of the simulation start and end date and current simulation date status are posted at the top of the screen. This information disappears when the HSPF computations are finished.

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4. Set up the Mitigated scenario.

First, check the Mitigated scenario box and place a land use basin element on the grid.

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For the Mitigated land use we have:

6.5 acres of D soil, urban no irrigation vegetation, moderate slope

1.0 acre of D soil, urban (with irrigation) vegetation, moderate slope

1.0 acre of impervious, flat slope

2.5 acres of impervious, moderate slope

The total Mitigated area is 11 acres.

The impervious land categories include roads, roofs, sidewalks, parking, and driveways.

All are modeled the same, except that steeper slopes have less surface retention storage prior to the start of surface runoff and therefore generate runoff more quickly.

If including permeable or porous pavement do not include this area in the land use basin element. This area should be placed in the separate porous pavement element.

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We will add a vault at the downstream end of the Mitigated land use basin.

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The vault element is placed below the land use basin element on the grid. Right click on the land use basin and select Connect To Element. A green line will appear with one end connected to the land use basin.

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With the mouse pointer pull the other end of the line down to the vault and click on the vault element box. This will bring up the From Basin to

Conveyance screen. As with the

Predeveloped scenario we want to only connect the surface flow and the interflow (shallow subsurface runoff) from the basin to the pond. Click OK.

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A line will connect the land use basin to the vault.

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Right click on the vault element to connect the vault’s outlet to the point of compliance.

Highlight Connect to Point Of Compliance and click.

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The Point of Compliance screen will be shown for the vault. The vault has one outlet (by default). The outflow from the vault will be compared with the Predeveloped runoff.

The point of compliance is designated as POC 1 (SDHM 3.0 allows for multiple points of compliance). Click on the Connect button.

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The point of compliance is shown on the vault element as a small box with the letter “A” and number 1 in the bar chart symbol in the lower right corner.

The letter “A” stands for Analysis and designates that this is an analysis location where flow and stage will be computed and the output flow and stage time series will be made available to the user. The number 1 denotes that this is POC 1.

You can have an analysis location without having a point of compliance at the same location, but you cannot have a point of compliance that is also not an analysis location.

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5. Sizing the vault.

A stormwater vault can be sized either manually or automatically (using Auto Vault).

For this example Auto Vault will be used. (Go to page 53 to find more information about how to manually size a vault or other HMP facility.)

Click on the Auto Vault button and the Auto Vault screen will appear. The user can set the vault depth (default: 4 feet), vault length to width ratio (default: 1 to 1), vault side slopes (limited to: 0 to 1), and the outlet structure configuration (default: 1 orifice and riser with rectangular notch weir).

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To optimize the vault design and create the smallest vault possible, move the Automatic

Vault Adjuster pointer from the left to the right.

Two outlet structure options are available in Auto Vault:

1. 1 orifice and a rectangular notch weir on the riser

2. 3 orifices and a flat weir on the riser

For this project example we will select the three-orifice outlet structure option.

The vault does not yet have any dimensions. Click the Create Vault button to create initial vault dimensions, which will be the starting point for Auto Vault’s automated optimization process to calculate the vault size and outlet structure dimensions.

Running Auto Vault automates the following SDHM 3.0 processes:

1. The hourly Predevelopment runoff is computed for the 30-50 years of record.

2. The Predevelopment runoff flood frequency is calculated based on the partial duration peak flows.

3. The range of flows is selected for the flow duration based on the lower and upper flow thresholds.

4. This flow range is divided into 100 increments.

5. The number of hourly Predevelopment flow values that exceed each flow increment level (Predevelopment flow duration) are counted to create the flow duration curves and accompanying tabular results.

Next, SDHM 3.0 computes the development runoff (in the Mitigated scenario) and routes the runoff through the vault. But before the runoff can be routed through the vault the vault must be given dimensions and an outlet configuration. Auto Vault uses a set of rules based on the Predeveloped and Mitigated scenario land uses to give the vault an initial set of dimensions and an initial outlet bottom orifice diameter and riser height and diameter. This information allows SDHM 3.0 to compute a stage-storage-discharge table for the vault.

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With this initial vault stage-storage-discharge table SDHM 3.0:

1. Routes the hourly development runoff through the vault for the 30 years of record to create to the Mitigated flow time series.

2. Counts the number of hourly Mitigated flow values that exceed each flow increment level (this is the Mitigated flow duration).

3. Computes the ratio of Mitigated flow values to Predeveloped flow values for each flow increment level (comparing the Predeveloped and Mitigated flow duration results).

If any of the 100 individual ratio values is greater than allowed by the flow duration criteria then the vault fails to provide an appropriate amount of mitigation and needs to be resized.

Flow duration results are shown in the plots above. The vertical axis shows the range of flows from 10% of the 2-year flow (0.14 cfs) to the 10-year flow (3.34 cfs). The horizontal axis is the percent of time that flows exceed a flow value. Plotting positions on the horizontal axis typical range from 0.001% to 1%, as explained below.

For the entire 30-year simulation period all of the hourly time steps are checked to see if the flow for that time step is greater than the minimum flow duration criteria value (0.14 cfs, in this example). For a 30-year simulation period there are approximately 250 thousand hourly values to check. Many of them are zero flows. The 10% of the

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Predevelopment 2-year flow value is exceeded less than 0.1% of the total simulation period.

This check is done for both the Predevelopment flows (shown in blue on the screen) and the Mitigated flows (shown in red).

If all of the Mitigated flow duration values (in red) are to the left of the Predevelopment flow duration values (in blue) then the pond mitigates the additional erosive flows produced by the development.

If the Mitigated flow duration values (in red) are far to the left of the Predevelopment flow duration values (in blue) then the pond can be made smaller and still meet the flow duration criteria.

Auto Vault goes through an iteration process by which it changes the vault dimensions and outlet configuration, then instructs SDHM to again compute the resulting Mitigated runoff, compare flow durations, and decide if it has made the results better or worse.

This iteration process continues until Auto Vault finally concludes that an optimum solution has been found and the Mitigated flow duration values (in red) are as close as possible to the Predevelopment flow duration values (in blue).

The user may continue to manually optimize the vault by manually changing vault dimensions and/or the outlet structure configuration. (Manual optimization is explained

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San Diego Hydrology Model 3.0 Interim Version User Manual – May 2016 in more detail on page 53.) After making these changes the user should click on the

Optimize Vault button to check the results and see if Auto Vault can make further improvements.

The final vault dimensions (bottom length, bottom width, and effective depth) and outlet structure information (riser height, riser diameter, riser weir type, and orifice diameters and heights) are shown on the vault screen to the right of the Schematic grid.

NOTE: If Auto Vault selects a bottom orifice diameter smaller than the smallest diameter allowed by the local municipal permitting agency then the user should consult with the local municipal permitting agency to determine an appropriate solution. Additional mitigating BMPs may be required to meet local hydromodification control requirements. Please see Appendix C or consult with local municipal permitting agency for more details. For manual sizing information see page 53.

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5. Review analysis.

The Analysis tool bar button (third from the left) brings up the Analysis screen where the user can look at the results. Each time series dataset is listed in the Analyze Datasets box in the lower left corner. To review the flow duration analysis at the point of compliance select the POC 1 tab at the bottom and make sure that both the 501 POC 1 Predeveloped flow and 801 POC 1 Mitigated flow are highlighted.

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The flow duration plot for both Predeveloped and Mitigated flows will be shown along with the specific flow values and number of times Predeveloped and Mitigated flows exceeded those flow values. The Pass/Fail on the right indicates whether or not at that flow level the flow control standard criteria were met and the pond passes at that flow level (in this example from 10% of the 2-year flow to the 10-year). If not, a Fail is shown; one Fail fails the mitigation facility design.

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Vault drawdown/retention time is computed on the Analysis screen.

NOTE: This information is not required for basic sizing of the flow duration facility, but can assist the user in determining the overall suitability of the mitigated design in meeting additional, related requirements for treating stormwater runoff and minimizing risk of vector (mosquito) breeding problems.

See page 131 for more descriptions of this SDHM feature, and Appendix C for discussion and references for these requirements.

Click on the Stage tab at the bottom to get the Mitigated vault stage time series.

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Click on the tab labeled Drawdown. This is where the vault drawdown/retention time results will be shown.

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Select the vault you want to analyze for drawdown/retention time (in this example there is only one vault: Vault 1) by clicking on the dataset and highlighting it.

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Click on the Analyze Stage button and the computed vault stages (vault water depths) are summarized and reported in terms of drain/retention time (in days).

For this example, the 10-year stage computed for the 30-year simulation period is 3.20 feet. This 10-year stage has a drawdown time of less than one day.

Stormwater storage facilities may have drain times in excess of the allowed maximum time. This can occur when a stormwater facility has a small bottom orifice. If this is not acceptable then the user needs to change the facility outlet configuration, manually run the Mitigated scenario, and repeat the analyze stage computations. A situation may occur where it is not possible to have both an acceptable drawdown/ retention time and meet the flow duration criteria.

NOTE: See Appendix C or the local municipal permitting agency for an overview of other requirements that may apply regarding drawdown time, and suggestions for addressing situations where it is not possible to meet all drawdown/retention time guidelines and also meet the flow duration criteria. The user manual assumes that the flow duration criteria take precedence unless the user is instructed otherwise by the local municipal permitting agency.

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6. Produce report.

Click on the Reports tool bar button (fourth from the left) to generate a project report with all of the project information and results. Select either a Text Report (Word format) or a PDF Report.

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Scroll down the Report screen to see all of the results.

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7. Save project.

To save the project click on File in the upper left corner and select Save As.

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Select a file name and save the SDHM 3.0 project file. The user can exit SDHM 3.0 and later reload the project file with all of its information by going to File, Open.

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8. Exit SDHM.

To exit SDHM 3.0 click on File in the upper left corner and select Exit. Or click on the X in the red box in the upper right hand corner of the screen.

NOTE: A more complex SDHM 3.0 example project is included in Appendix E.

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MAIN SCREENS

SDHM 3.0 has six main screens. These main screens can be accessed through the buttons shown on the tool bar above or via the View menu.

The six main screens are:

 Map Information

 General Project Information

 Analysis

 Reports

 Tools

 LID (Low Impact Development) Analysis

Each is discussed in more detail in the following sections.

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MAP INFORMATION SCREEN

The Map Screen contains county information. The precipitation gage and precip factor are shown to the right of the map. For this interim version of SDHM 3.0 the user selects the rainfall record from the list of precipitation stations shown in the lower right side of the screen. The precip factor will be 1.00. This method of selecting the rain gage and precip factor will change in the final version of SDHM 3.0.

The user locates the project site on the map screen by using the mouse and left clicking at the project site location. Right clicking on the map re-centers the view. The + and – buttons zoom in and out, respectively. The cross hair button zooms out to the full county view. The arrow keys scroll the map view.

The user can provide site information (optional). The site name and address will help to identify the project on the Report screen and in the printed report provided to the local municipal permitting agency.

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GENERAL PROJECT INFORMATION SCREEN

The project screen contains all of the information about the project site for the two land use scenarios: Predeveloped land use conditions and the Mitigated (developed) land use conditions. To change from one scenario to another check the box in front of the scenario name in the upper left corner of the screen.

Predevelopment is defined as the natural conditions prior to the existing and proposed land use development. Runoff from the Predevelopment scenario is used as the target for the Mitigated scenario compliance. The model will accept any land use for this scenario.

Mitigated is defined as the developed land use with mitigation measures (as selected by the user). Mitigated is used for sizing stormwater control and water quality facilities.

The runoff from the Mitigated scenario is compared with the Predeveloped scenario runoff to determine compliance with flow duration criteria.

Below the scenario boxes are the Elements. Each element represents a specific feature

(basin, pond, etc.) and is described in more detail in the following section.

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SCHEMATIC EDITOR

The project screen also contains the Schematic Editor. The Schematic Editor is the grid to the right of the elements. This grid is where each element is placed and linked together. The grid, using the scroll bars on the left and bottom, expands as large as needed to contain all of the elements for the project.

It is recommended that all movement on the grid be from the top of the grid down, although it is no longer required.

The space to the right of the grid will contain the appropriate element information.

To select and place an element on the grid, first left click on the specific element in the

Elements menu and then drag the element to the selected grid square. The selected element will appear in the grid square.

The entire grid can be moved up, down, left, or right using the Move Elements arrow buttons.

The grid coordinates from one project can be saved (Save x,y) and used for new projects

(Load x,y).

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STANDARD ELEMENTS

The following pages contain information about these standard elements:

 Basin

 Trapezoidal Pond

 Vault

 Tank

 Irregular Pond

 Gravel Trench Bed

 Sand Filter

 Channel

 Flow Splitter

 Time Series

 SSD Table

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BASIN ELEMENT

The Basin element represents a drainage area that can have any combination of soils, land cover, and land slopes. A land use basin produces three types of runoff: (1) surface runoff, (2) interflow, and (3) groundwater. Surface runoff is defined as the overland flow that quickly reaches a conveyance system. Surface runoff mainly comes from impervious surfaces. Interflow is shallow, subsurface flow produced by pervious land categories and varies based on soil characteristics and how these characteristics are altered by land development practices. Groundwater is the subsurface flow that typically does not enter a stormwater conveyance system, but provides base flow directly to streams and rivers.

The user can specify where each of these three types of runoff should be directed. The default setting is for the surface runoff and interflow to go to the stormwater facility; groundwater should not be connected unless there is observed base flow occurring in the drainage basin.

Table 1 shows the different pervious land types represented in the Basin element.

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Table 1. SDHM 3.0 Pervious Land Types

PERLND No.  Soil Type 

1  A 

2  A 

Land Cover 

Natural Vegetation 

Land Slope 

Flat (0‐5%) 

Natural Vegetation  Moderate (5‐15%) 

31 

32 

33 

34 

27 

28 

29 

30 

23 

24 

25 

26 

19 

20 

21 

22 

35 

36 

37 

38 

39 

40 

15 

16 

17 

18 

11 

12 

13 

14 

10 

Natural Vegetation 

Dirt 

Dirt 

Dirt 

Rock 

Rock 

Rock 

Natural Vegetation 

Steep (>15%) 

Flat (0‐5%) 

Moderate (5‐15%) 

Steep (>15%) 

Flat (0‐5%) 

Moderate (5‐15%) 

Steep (>15%) 

Flat (0‐5%) 

Natural Vegetation  Moderate (5‐15%) 

Natural Vegetation  Steep (>15%) 

Dirt 

Dirt 

Flat (0‐5%) 

Moderate (5‐15%) 

Dirt 

Rock 

Rock 

Rock 

Steep (>15%) 

Flat (0‐5%) 

Moderate (5‐15%) 

Steep (>15%) 

Natural Vegetation  Flat (0‐5%) 

Natural Vegetation  Moderate (5‐15%) 

Natural Vegetation 

Dirt 

Steep (>15%) 

Flat (0‐5%) 

Dirt 

Dirt 

Rock 

Rock 

Moderate (5‐15%) 

Steep (>15%) 

Flat (0‐5%) 

Moderate (5‐15%) 

Rock 

Natural Vegetation 

Steep (>15%) 

Flat (0‐5%) 

Natural Vegetation  Moderate (5‐15%) 

Natural Vegetation  Steep (>15%) 

Dirt 

Dirt 

Dirt 

Rock 

Flat (0‐5%) 

Moderate (5‐15%) 

Steep (>15%) 

Flat (0‐5%) 

Rock 

Rock 

Urban 

Urban 

Urban 

Urban 

Moderate (5‐15%) 

Steep (>15%) 

Flat (0‐5%) 

Moderate (5‐15%) 

Steep (>15%) 

Flat (0‐5%) 

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53 

54 

55 

56 

49 

50 

51 

52 

45 

46 

47 

48 

41 

42 

43 

44 

Urban 

Urban 

Urban 

Urban 

Urban 

Urban 

Urban 

Urban 

Moderate (5‐15%) 

Steep (>15%) 

Flat (0‐5%) 

Moderate (5‐15%) 

Steep (>15%) 

Flat (0‐5%) 

Moderate (5‐15%) 

Steep (>15%) 

Urban, No Irrigation  Flat (0‐5%) 

Urban, No Irrigation  Moderate (5‐15%) 

Urban, No Irrigation 

Urban, No Irrigation 

Steep (>15%) 

Flat (0‐5%) 

Urban, No Irrigation  Moderate (5‐15%) 

Urban, No Irrigation  Steep (>15%) 

Urban, No Irrigation  Flat (0‐5%) 

Urban, No Irrigation  Moderate (5‐15%) 

57 

58 

59 

Urban, No Irrigation 

Urban, No Irrigation 

Steep (>15%) 

Flat (0‐5%) 

Urban, No Irrigation  Moderate (5‐15%) 

60  D  Urban, No Irrigation  Steep (>15%) 

The user does not need to know or keep track of the HSPF PERLND number. That number is used only for internal tracking purposes. The HSPF parameter values for each

PERLND are listed in Appendix A.

The user inputs the number of acres of appropriate basin land use information. Pervious land use information is in the form of soil, land cover, and land slope. For example, “A,

Natural Vegetation (NatVeg), Flat” means NRSC soil type A, native grass/shrub cover, and flat (0-5%) land slope.

There are four basic soil types: A (well infiltrating soils), B (moderate infiltrating soils), and C (poor infiltrating soils), and D (really poor infiltrating soils).

There are five basic land cover categories: natural vegetation, dirt, rock, urban irrigated landscaped vegetation, and urban non-irrigated landscaped vegetation..

Natural land cover has been divided into natural vegetation (NatVeg), dirt, and rock. Dirt and rock includes landscapes that are vegetated, but the majority of the surface cover is non-vegetation.

Urban vegetation consists of lawns, flowers, planted shrubs and trees. Urban vegetation may be irrigated (Urban) or non-irrigated (UrbNoIrr).

Land slope is divided into flat (0-5%), moderate (5-15%), and steep (>15%) land slopes.

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HSPF parameter values in SDHM 3.0 have been adjusted for the different soil, land cover, and land slope categories. SDHM 3.0 HSPF soil parameter values take into account the hydrologic effects of land development activities that result from soil compaction when “Urban” or “UrbNoIrr” is specified.

Impervious areas are divided into three different slopes (see Table 2). Impervious areas include roads, roofs, driveways, sidewalks, and parking. The slope categories are flat, moderate, and steep.

Table 2. SDHM Impervious Land Types

IMPLND No. Surface

1 Impervious

2 Impervious

Slope

Flat (0-5%)

Moderate (5-15%)

3 Impervious Steep (>15%)

The user does not need to know or keep track of the HSPF IMPLND number. That number is used only for internal tracking purposes.

The HSPF parameter values for each IMPLND are listed in Appendix B.

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TRAPEZOIDAL POND ELEMENT

In SDHM 3.0 there is an individual pond element for each type of pond and stormwater control facility.

The pond element shown above is for a trapezoidal pond. This is the most common type of stormwater pond.

A trapezoidal pond has dimensions

(bottom length and width, depth, and side slopes) and an outlet structure consisting of a riser and one or more orifices to control the release of stormwater from the pond. A trapezoidal pond includes the option to infiltrate runoff, if the soils are appropriate and there is sufficient depth to the underlying groundwater table.

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The user has the option to specify that different outlets be directed to different downstream destinations, although usually all of the outlets go to a single downstream location.

Auto Pond will automatically size a trapezoidal pond to meet the required flow duration criteria. Auto Pond is available only in the Mitigated scenario.

Quick Pond can be used to instantly add pond dimensions and an outlet configuration without checking the pond for compliancy with flow duration criteria. Quick Pond is sometimes used to quickly create a scenario and check the model linkages prior to sizing the pond. Multiple clicks on the Quick Pond button incrementally increase the pond size.

The user can change the default name “Trapezoidal Pond 1” to another more appropriate name, if desired.

Precipitation and evaporation must be applied to the pond unless the pond is covered.

The pond bottom elevation can be set to an elevation other than zero if the user wants to use actual elevations. All pond stage values are relative to the bottom elevation.

Negative bottom elevations are not allowed.

The pond effective depth is the pond height (including freeboard) above the pond bottom.

It is not the actual elevation of the top of the pond.

Pond side slopes are in terms of horizontal distance over vertical. A standard 3:1 (H/V) side slope would be given a value of 3. A vertical side slope has a value of 0.

The pond bottom is assumed to be flat.

The pond outlet structure consists of a riser and zero to three orifices. The riser has a height (typically one foot less than the effective depth) and a diameter. The riser can have either a flat top or a weir notch cut into the side of the top of the riser. The notch can be either rectangular, V-shaped, or a Sutro weir. More information on the riser weir shapes and orifices is provided later in this manual.

After the pond is given dimensions and outlet information the user can view the resulting stage-storage-discharge table by clicking on the “Open Table” arrow in the lower right corner of the pond information screen. This table defines the pond’s hydraulic characteristics.

The user can use either Auto Pond to size a pond or can manually size a pond. Follow the following steps for manual sizing a pond using an outlet configuration with one orifice and a riser with rectangular notch (this is usually the most efficient design):

1. Input a bottom orifice diameter that allows a discharge equal to the lower threshold (e.g., 10% of 2-year) Predevelopment flow for a stage equal to 2/3rds the height of the riser. This discharge can be checked by reviewing the pond’s stage-storage-discharge table (Pond Table).

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2. Input a riser rectangular notch height equal to 1/3 of the height of the riser.

Initially set the riser notch width to 0.1 feet.

3. Run Predevelopment and Mitigated scenarios.

4. Go to Analysis screen and check flow duration results.

5. If pond passes flow duration criteria then decrease pond dimensions.

6. If pond fails flow duration criteria then change (in order of priority) bottom orifice diameter, riser notch width, pond dimensions.

7. Iterate until there is a good match between Predevelopment and Mitigated flow duration curves or fatigue sets in.

Pond input information:

Bottom Length (ft): Pond bottom length.

Bottom Width (ft): Pond bottom width.

Effective Depth (ft): Pond height from pond bottom to top of riser plus at least 0.5 feet extra.

Left Side Slope (H/V): ratio of horizontal distance to vertical; 0 (zero) for vertical pond sides.

Bottom Side Slope (H/V): ratio of horizontal distance to vertical; 0 (zero) for vertical pond sides.

Right Side Slope (H/V): ratio of horizontal distance to vertical; 0 (zero) for vertical pond sides.

Top Side Slope (H/V): ratio of horizontal distance to vertical; 0 (zero) for vertical pond sides.

Riser Height (ft): Height of overflow pipe above pond bottom.

Riser Diameter (in): Pond overflow pipe diameter.

Riser Type (options): Flat or Notched

Notch Type: Rectangular, V-Notch, or Sutro.

For a rectangular notch:

Notch Height (feet): distance from the top of the weir to the bottom of the notch.

Notch Width (feet): width of notch; cannot be larger than the riser circumference.

For more information on riser notch options and orifices see discussion in OUTLET

STRUCTURE CONFIGURATIONS section.

Infiltration: Yes (infiltration into the underlying native soil)

Measured Infiltration Rate (in/hr): Native soil infiltration rate.

Infiltration Reduction Factor: 1/Native soil infiltration rate safety factor (see page 116)..

Use Wetted Surface Area (sidewalls): Yes, if infiltration through the pond side slopes is allowed.

If infiltration is used then the user should consult the Infiltration discussion on page 116.

NOTE: See Appendix C or consult with the local municipal permitting agency for additional considerations regarding infiltration and determination of the appropriate infiltration reduction factor.

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A pond receives precipitation on and evaporation from the pond surface. The

Precipitation Applied to Facility and Evaporation Applied to Facility boxes should be checked.

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NOTE: The detention pond section diagram shows the general configuration used in designing a pond and its outlet structure. This diagram is from the

Washington State Department of Ecology’s 2014

Stormwater Management

Manual for Western Washington. Consult with your local municipal permitting

agency on specific design requirements for your project site.

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VAULT ELEMENT

The storage vault has all of the same characteristics of the trapezoidal pond, except that the user does not specify the side slopes (by definition they are zero) and the vault is assumed to have a lid (no precipitation or evaporation).

Auto Vault and Quick Vault work the same way as Auto Pond and Quick Pond. Go to page 53 to find information on how to manually size a vault or other HMP facility.

Auto Vault is available only in the Mitigated scenario.

Vault input information:

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Bottom Length (ft): Vault bottom length.

Bottom Width (ft): Vault bottom width.

Effective Depth (ft): Vault height from vault bottom to top of riser plus at least 0.5 feet extra.

Riser Height (ft): Height of overflow pipe above vault bottom.

Riser Diameter (in): Vault overflow pipe diameter.

Riser Type (options): Flat or Notched

Notch Type: Rectangular, V-Notch, or

Sutro.

For a rectangular notch:

Notch Height (feet): distance from the top of the weir to the bottom of the notch.

Notch Width (feet): width of notch; cannot be larger than the riser circumference.

For more information on riser notch options and orifices see discussion in OUTLET

STRUCTURE CONFIGURATIONS section.

Infiltration: Yes (infiltration into the underlying native soil)

Measured Infiltration Rate (in/hr): Native soil infiltration rate.

Infiltration Reduction Factor: 1/Native soil infiltration rate safety factor (see page 116).

Use Wetted Surface Area (sidewalls): Yes, if infiltration through the vault sides is allowed.

If infiltration is used then the user should consult the Infiltration discussion on page 116.

NOTE: See Appendix C or consult with the local municipal permitting agency for additional considerations regarding infiltration and determination of the appropriate infiltration reduction factor.

A vault is usually covered and does not receive precipitation on and evaporation from the vault surface. The Precipitation Applied to Facility and Evaporation Applied to Facility boxes should not be checked unless the vault top is open to the atmosphere.

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TANK ELEMENT

A storage tank is a cylinder placed on its side. The user specifies the tank’s diameter and length.

Auto Tank and Quick Tank work the same way as Auto Pond and Quick Pond.

Auto Tank is only available in the Mitigated scenario.

The Quick Tank option creates a tank, but does not check for compliance with the flow duration criteria.

Tank input information:

Tank Type: Circular or Arched

For Circular:

Diameter (ft): Tank diameter.

Length (ft): Tank length.

For Arched:

Height (ft): Tank height.

Width (ft): Tank width (at widest point).

Length (ft): Tank length.

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Riser Height (ft): Height of overflow pipe above tank bottom; must be less than tank diameter or height.

Riser Diameter (in): Tank overflow pipe diameter.

Riser Type (options): Flat or Notched

Notch Type: Rectangular, V-Notch, or Sutro.

For a rectangular notch:

Notch Height (feet): distance from the top of the weir to the bottom of the notch.

Notch Width (feet): width of notch; cannot be larger than the riser circumference.

For more information on riser notch options and orifices see discussion in OUTLET STRUCTURE

CONFIGURATIONS section.

Infiltration: Yes (infiltration into the underlying native soil)

Measured Infiltration Rate (in/hr): Native soil infiltration rate.

Infiltration Reduction Factor: 1/Native soil infiltration rate safety factor (see page 116).

Use Wetted Surface Area (sidewalls): Yes, if infiltration through the tank sides is allowed.

If infiltration is used then the user should consult the Infiltration discussion on page 116.

NOTE: See Appendix C or consult with the local municipal permitting agency for additional considerations regarding infiltration and determination of the appropriate infiltration reduction factor.

A tank is covered and does not receive precipitation on and evaporation from the tank surface. The Precipitation Applied to Facility and Evaporation Applied to Facility boxes should not be checked.

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IRREGULAR POND ELEMENT

An irregular pond is any pond with a shape that differs from the rectangular top of a trapezoidal pond. An irregular pond has all of the same characteristics of a trapezoidal pond, but its shape must be defined by the user.

The Auto Pond option is not available for an irregular-shaped pond. Go to page 53 to find information on how to manually size an irregular pond or other HMP facility.

To create the shape of an irregular pond the user clicks on the “Open PondPad” button.

This allows the user to access the PondPad interface (see below).

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PondPad Interface

The PondPad interface is a grid on which the user can specify the outline of the top of the pond and the pond’s side slopes.

The user selects the line button (second from the top on the upper left corner of the

PondPad screen). Once the line button is turned on the user moves the mouse over the grid to locate the pond’s corner points. The user does this in a clockwise direction to outline the pond’s top perimeter. The user can select individual points by clicking on the point button immediately below the line button. Once selected, any individual point can be moved or repositioned.

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The default side slope value is 3 (3:1). The side slopes can be individually changed by right clicking on the specific side (which changes the line color from black to red) and then entering the individual side slope value in the slope text box.

The grid scale can be changed by entering a new value in the grid scale box. The default value is 200 feet.

PondPad Controls and Numbers

Clear: The Clear button clears all of the lines on the grid.

Line:

Point:

The Line button allows the user to draw new lines with the mouse.

The Point button allows the user to move individual points to alter the pond shape and size.

Sq Ft: Converts the computed pond area from square feet to acres and back.

Grid Scale: Changes the length of a grid line. Default grid scale is 200 feet.

Grid X: Horizontal location of the mouse pointer on the grid

(0 is the upper left corner).

Vertical location of the mouse pointer on the grid

(0 is the upper left corner)

Grid Y:

Area:

Slope:

Top area of the pond (either in square feet or acres).

Side slope of the selected line (side of the pond).

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GRAVEL TRENCH BED ELEMENT

The gravel trench bed is used to spread and infiltrate runoff, but also can have one or more surface outlets represented by an outlet structure with a riser and multiple orifices.

The user specifies the trench length, bottom width, total depth, bottom slope, and left and right side slopes.

The material layers represent the gravel/rock layers and their design characteristics

(thickness and porosity).

Quick Trench will instantly create a gravel trench bed with default values without checking it for compliancy with flow duration criteria.

The gravel trench bed input information:

Trench Length (ft): Trench bed length.

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Trench Bottom Width (ft): Trench bed bottom width.

Effective Total Depth (ft): Height from bottom of trench bed to top of riser plus at least

0.5 feet extra.

Bottom Slope of Trench (ft/ft): Must be non-zero.

Left Side Slope (ft/ft): H/V ratio of horizontal distance to vertical; 0 (zero) for vertical trench bed sides.

Right Side Slope (ft/ft): H/V ratio of horizontal distance to vertical; 0 (zero) for vertical trench bed sides.

Infiltration Rate (in/hr): Trench bed gravel or other media infiltration rate.

Layer 1 Thickness (ft): Trench top media layer depth.

Layer 1 Porosity: Trench top media porosity.

Layer 2 Thickness (ft): Trench middle media layer depth (Layer 2 is optional).

Layer 2 Porosity: Trench middle media porosity.

Layer 3 Thickness (ft): Trench bottom media layer depth (Layer 3 is optional).

Layer 3 Porosity: Trench bottom media porosity.

Riser Height (ft): Height of trench overflow pipe above trench surface.

Riser Diameter (in): Trench overflow pipe diameter.

Riser Type (options): Flat or Notched

Notch Type: Rectangular, V-Notch, or Sutro.

For a rectangular notch:

Notch Height (feet): distance from the top of the weir to the bottom of the notch.

Notch Width (feet): width of notch; cannot be larger than the riser circumference.

For more information on riser notch options and orifices see discussion in OUTLET

STRUCTURE CONFIGURATIONS section.

Native Infiltration: Yes (infiltration into the underlying native soil)

Measured Infiltration Rate (in/hr): Native soil infiltration rate.

Infiltration Reduction Factor: 1/Native soil infiltration rate safety factor (see page 116).

Use Wetted Surface Area (sidewalls): Yes, if infiltration through the trench side slopes is allowed.

If infiltration is used then the user should consult the Infiltration discussion on page 116.

NOTE: See Appendix C or consult with the local municipal permitting agency for additional considerations regarding infiltration and determination of the appropriate infiltration reduction factor.

Gravel trench bed receives precipitation on and evaporation from the trench surface. The

Precipitation Applied to Facility and Evaporation Applied to Facility boxes should be checked.

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SAND FILTER ELEMENT

The sand filter is a water quality facility. It does not infiltrate runoff, but is used to filter runoff through a medium and send it downstream. It can also have one or more surface outlets represented by an outlet structure with a riser and multiple orifices.

The user must specify the facility dimensions (bottom length and width, effective depth, and side slopes. The hydraulic conductivity of the sand filter and the filter material depth are also needed to size the sand filter (default values are 1.0 inch per hour and 1.5 feet, respectively).

NOTE: When using the sand filter element check with Appendix C or the local municipal permitting agency to determine the required treatment standard (percent of the total runoff volume treated by the sand filter).

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The filter discharge is calculated using the equation Q = K*I*A, where Q is the discharge in cubic feet per second (cfs). K equals the hydraulic conductivity (inches per hour). For sand filters K = 1.0 in/hr. Sand is the default medium. If another filtration material is used then the design engineer should enter the appropriate K value supported by documentation and approval by the reviewing authority.

Design of a sand filter requires input of facility dimensions and outlet structure characteristics, running the sand filter scenario, and then checking the volume calculations to see if the Percent Filtered equals or exceeds the treatment standard percentage. If the value is less than the treatment standard percentage then the user should increase the size of the sand filter dimensions and/or change the outlet structure.

The sand filter input information:

Bottom Length (ft): Sand filter bottom length.

Bottom Width (ft): Sand filter bottom width.

Effective Depth (ft): Height from bottom of sand filter to top of riser plus at least 0.5 feet extra.

Left Side Slope (ft/ft): H/V ratio of horizontal distance to vertical; 0 (zero) for vertical sand filter sides.

Bottom Side Slope (ft/ft): H/V ratio of horizontal distance to vertical; 0 (zero) for vertical sand filter sides.

Right Side Slope (ft/ft): H/V ratio of horizontal distance to vertical; 0 (zero) for vertical sand filter sides.

Top Side Slope (ft/ft): H/V ratio of horizontal distance to vertical; 0 (zero) for vertical sand filter sides.

Riser Height (ft): Height of sand filter overflow pipe above sand filter surface.

Riser Diameter (in): Sand filter overflow pipe diameter.

Riser Type (options): Flat or Notched

Notch Type: Rectangular, V-Notch, or Sutro.

For a rectangular notch:

Notch Height (feet): distance from the top of the weir to the bottom of the notch.

Notch Width (feet): width of notch; cannot be larger than the riser circumference.

For more information on riser notch options and orifices see discussion in OUTLET

STRUCTURE CONFIGURATIONS section.

Infiltration: Yes (infiltration through the filter material)

Hydraulic Conductivity (in/hr): Filtration rate through the sand filter.

Filter material depth (ft): Depth of sand filter material (for runoff filtration).

Sand filter receives precipitation on and evaporation from the sand filter surface. The

Precipitation Applied to Facility and Evaporation Applied to Facility boxes should be checked.

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CHANNEL ELEMENT

The Channel element allows the user to route runoff from a land use basin or stormwater facility through an open channel to a downstream destination.

The channel cross section is represented by a trapezoid and is used with Manning’s equation to calculate discharge from the channel. If a trapezoid does not accurately represent the cross section then the user should represent the channel with an independently calculated SSD Table element or use the Use X-Sections option.

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The user inputs channel bottom width, channel length, channel bottom slope, channel left and right side slopes, maximum channel depth, and the channel’s roughness coefficient (Manning’s n value). The user can select channel type and associated Manning’s n from a table list directly above the Channel

Dimension information or directly input the channel’s

Manning’s n value.

The channel is used to represent a natural or artificial open channel through which water is routed. It can be used to connect a basin to a pond or a pond to a pond or multiple channels can linked together.

Channel input information:

Channel Bottom Width (ft): Open channel bottom width.

Channel Length (ft): Open channel length.

Manning’s n coefficient: Open channel roughness coefficient (user menu selected or input).

Slope of Channel (ft/ft): Open channel bottom slope.

Left Side Slope of Channel (ft/ft): H/V ratio of horizontal distance to vertical; 0 (zero) for vertical channel sides.

Right Side Slope of Channel (ft/ft): H/V ratio of horizontal distance to vertical; 0 (zero) for vertical channel sides.

Maximum Channel Depth (ft): Height from bottom of channel to top of channel bank.

Infiltration: Yes (infiltration into the underlying native soil)

Measured Infiltration Rate (in/hr): Native soil infiltration rate.

Infiltration Reduction Factor: 1/Native soil infiltration rate safety factor (see page 116).

Use Wetted Surface Area (sidewalls): Yes, if infiltration through the channel side slopes is allowed.

If infiltration is used then the user should consult the Infiltration discussion on page 116.

NOTE: See Appendix C or consult with the local municipal permitting agency for additional considerations regarding infiltration and determination of the appropriate infiltration reduction factor.

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FLOW SPLITTER ELEMENT

The flow splitter divides the runoff and sends it to two difference destinations. The splitter has a primary exit (exit 1) and a secondary exit (exit 2). The user defines how the flow is split between these two exits.

The user can define a flow control structure with a riser and one to three orifices for each exit. The flow control structure works the same way as the pond outlet structure, with the user setting the riser height and diameter, the riser weir type (flat, rectangular notch, Vnotch, or Sutro), and the orifice diameter and height.

For more information on riser notch options and orifices see discussion in OUTLET

STRUCTURE CONFIGURATIONS section.

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The second option is that the flow split can be based on a flow threshold. The user sets the flow threshold value (cfs) for exit 1 at which flows in excess of the threshold go to exit 2. For example, if the flow threshold is set to 5 cfs then all flows less than or equal to 5 cfs go to exit 1. Exit 2 gets only the excess flow above the 5 cfs threshold (total flow minus exit 1 flow).

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TIME SERIES ELEMENT

SDHM 3.0 uses time series of precipitation, evaporation, and runoff stored in its database

(HSPF WDM file). The user has the option to create or use a time series file external from SDHM in SDHM 3.0. This may be a time series of flow values created by another

HSPF model. An example is offsite runoff entering a project site. If this offsite runoff is in an existing WDM file and is the same period as SDHM 3.0 data and the same simulation time step (hourly) then it can be linked to SDHM 3.0 model using the Time

Series element.

To link the external time series to SDHM 3.0 the user clicks on the Choose WDM button and identifies the external WDM file. The external WDM’s individual time series files are shown in the Time Series Out box. The selected input dataset is the time series that will be used by SDHM 3.0.

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SSD TABLE ELEMENT

The SSD Table is a stage-storage-discharge table externally produced by the user and is identical in format to the stage-storage-discharge tables generated internally by SDHM

3.0 for ponds, vaults, tanks, and channels.

The easiest way to create a SSD Table outside of SDHM 3.0 is to use a spreadsheet with a separate column for stage, surface area, storage, and discharge (in that order). Save the spreadsheet file as a space or comma-delimited file. A text file can also be created, if more convenient.

The SSD Table must use the following units:

Stage: feet

Surface Area: acres

Storage: acre-feet

Discharge: cubic feet per second (cfs)

A fifth column can be used to create a second discharge (cfs). This second discharge can be infiltration or a second surface discharge.

Certain rules apply to the SSD Table whether it is created inside or outside of SDHM 3.0.

These rules are:

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1. Stage (feet) must start at zero and increase with each row. The incremental increase does not have to be consistent.

2. Storage (acre-feet) must start at zero and increase with each row. Storage values should be physically based on the corresponding depth and surface area, but

SDHM does not check externally generated storage values.

3. Discharge (cfs) must start at zero. Discharge does not have to increase with each row. It can stay constant or even decrease. Discharge cannot be negative.

Discharge should be based on the outlet structure’s physical dimensions and characteristics, but SDHM 3.0 does not check externally generated discharge values.

4. Surface area (acres) is only used if precipitation to and evaporation from the facility are applied.

To input an externally generated SSD Table, first create and save the table outside of

SDHM 3.0. Use the Browse button to locate and load the file into SDHM 3.0.

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To input columns of values beyond (to the right of) the Storage column click on the “Not

Used” title and select the appropriate option. Use “Manual” when the discharge has been included in the external spreadsheet.

Use “Outlet Structure” to input riser and orifice dimensions.

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The fifth column can be used for a second surface outlet (manual or outlet structure), infiltration, or aquifer recharge. Aquifer recharge differs from infiltration in how the model separately accounts for it.

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LID ELEMENTS

The following pages contain information about these LID elements:

 Bioretention/Rain Garden

 In-Ground Planter

 Flow-Through Planter

 Porous Pavement

 Dispersion

 Lateral Basin (Pervious)

 Lateral I Basin (Impervious)

 Dry Well

 Infiltration Trench

 Infiltration Basin

 Green Roof

 Rainwater Harvesting

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BIORETENTION/RAIN GARDEN ELEMENT

The bioretention swale element is also known as a landscape swale or rain garden. The

SDHM 3.0 bioretention swale element is a special conveyance feature with unique characteristics. The element uses the HSPF hydraulic algorithms to route runoff, but the

HSPF routing is modified to represent the two different flow paths that runoff can take.

The routing is dependent on the inflow to the swale and the swale soil capacity to absorb additional runoff. HSPF Special Actions is used to check the swale soil capacity to determine the appropriate routing option. More technical details on how the bioretention element is modeled in SDHM 3.0 are included in Appendix D.

A bioretention swale is a swale in which the native soils have been excavated and replaced with amended soil. At the downstream end of the swale a weir controls the surface discharge from the swale and detains runoff, encouraging it to infiltrate into the amended soil. Infiltration from the amended soil to the native soil is also possible, depending on the properties of the native soil. Swales can include an underdrain pipe.

The amended soil placed in the swale is assumed to have storage capacity equal to its porosity and volume. Runoff infiltrates from the surface of the swale to the amended soil at an infiltration rate set by the user. The infiltration rate cannot exceed the available storage capacity of the amended soil. The available storage capacity is determined each time step by HSPF Special Actions. Once the amended soil is saturated then water has the opportunity to infiltrate into the underlying native soil at the native soil’s infiltration rate. The native soil infiltration is input by the user and is assumed to be constant throughout the year.

Inflow to the swale can exceed the amended soil infiltration rate. When this occurs the extra water ponds on the surface of the swale. The extra water can then infiltrate into the soil during the next time step or can flow out of the swale through its surface outlet if the ponding exceeds the surface outlet’s storage.

Runoff in both the surface storage and amended soil storage is available for evapotranspiration. Surface storage evapotranspiration is set to the potential evapotranspiration; the amended soil evapotranspiration pan evaporation factor is set to 0.50 to reflect reduced evapotranspiration from the amended soil.

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The user is required to enter the following information about the bioretention swale:

The bioretention swale dimensions are specified in terms of swale length, bottom width, freeboard, over-road flooding, effective total depth, bottom slope, and left and right side slopes.

Swale Length (ft): length dimension of swale surface bottom.

Swale Bottom Width (ft): width dimension of swale surface bottom.

Freeboard (ft): depth of surface ponding before weir/street overflow occurs.

Over-road Flooding (ft): maximum depth of flow over weir/street.

Effective Total Depth (ft): the total depth of the amended soil layer(s) plus freeboard plus over-road flooding plus vertical orifice elevation plus vertical orifice diameter; effective total depth is computed by SDHM 3.0.

Bottom Slope of Swale (ft/ft): the slope of the swale length; must be greater than zero.

Left Side Slope (ft/ft): H/V ratio of horizontal distance to vertical; 0 (zero) for vertical swale sides.

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Right Side Slope (ft/ft): H/V ratio of horizontal distance to vertical; 0 (zero) for vertical swale sides.

In the amended soil water movement through the soil column is dependent on soil layer characteristics and saturation rates for different discharge conditions. Details of how soil water movement is modeled in SDHM 3.0 are presented in Appendix D.

The amended soil user inputs:

Layer Thickness (feet): depth of amended soil.

Type of amended soil: 24 different soil types are included; the user can also create their own soil type using the Edit Soil Type button.

Note that there can be a maximum of three different amended soil layers.

Infiltration to the native soil can be turned on by setting Native Infiltration to YES. The parameters for native soil infiltration are:

Measured Infiltration Rate (inches per hour): infiltration rate of the native soil.

Infiltration Reduction Factor: between 0 and 1 (1/Native soil infiltration rate safety factor

(see page 116).

Use Wetted Surface Area (sidewalls): YES or NO; YES allows infiltration to the native soil through the sidewalls of the swale; otherwise all infiltration is through the bottom only.

If infiltration is used then the user should consult the Infiltration discussion on page 116.

NOTE: See Appendix C or consult with the local municipal permitting agency for additional considerations regarding infiltration and determination of the appropriate infiltration reduction factor.

The user has two swale surface outlet configuration choices: (1) riser outlet structure or

(2) vertical orifice + overflow.

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Riser outlet structure option:

The input information required for the riser outlet structure is:

Riser Height above Swale Surface (feet): depth of surface ponding before the riser is overtopped.

Riser Diameter (inches): diameter of the stand pipe.

Riser Type: Flat or Notched.

Notch Type: Rectangular, V-Notch, or Sutro.

For a rectangular notch:

Notch Height (feet): distance from the top of the weir to the bottom of the notch.

Notch Width (feet): width of notch; cannot be larger than the riser circumference.

For more information on riser notch options and orifices see discussion in OUTLET

STRUCTURE CONFIGURATIONS section.

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The input information required for the vertical orifice plus overflow is:

Vertical Orifice Diameter (inches): diameter of vertical opening below the weir.

Vertical Orifice Elevation (inches): vertical distance from the top of the amended soil surface to the bottom of the vertical orifice.

Width of Over-road Flow (feet): weir/street length.

Diagram of bioretention swale with vertical orifice plus overflow:

Width of Over-road Flow

Over-road Flooding

Native Soil

Underdrain

Freeboard

Vertical Orifice Diameter

Vertical Orifice Elevation

Amended Soil

Layer 1

Layer 2

Layer 3

Native Soil

Effective

Total

Depth

Native Soil

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To use the underdrain click the Underdrain Used box and input an underdrain pipe diameter (feet), underdrain outlet orifice diameter (inches), and underdrain offset. The offset (inches) is the height of the bottom of the underdrain pipe above the bottom of the lowest amended soil layer.

The amended soil layer fills with stormwater from the top on down to where it can drain to the native soil (if Native Infiltration is set to YES) and/or the underdrain pipe (if

Underdrain Used box is checked).

Water enters the underdrain when the amended soil becomes saturated down to the top of the underdrain. The underdrain pipe fills and conveys water proportionally to the depth of amended soil saturation. When the amended soil is fully saturated the underdrain pipe is at full capacity. Discharge from the underdrain pipe is controlled by the underdrain orifice diameter.

If native infiltration is turned on then native infiltration will start when/if:

1. Water starts to fill the underdrain (if an underdrain is used).

2. Water enters the amended soil (if Use Wetted Surface Area (sidewalls) is set to YES).

3. Water saturates the amended soil layer(s) to 2/3rds of the total amended soil depth (if there is no underdrain and Wetted Surface Area is set to NO).

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There is a simple swale option. It is computationally much faster than the standard bioretention swale.

The standard bioretention swale routine uses HSPF Special Actions to check the available amended soil storage and compares it with the inflow rate. Because of the check done by

HSPF Special Actions simulations using bioretention swales take much longer than simulations not using bioretention swales. Simulations that normally take only seconds may take multiple minutes when one or more bioretention swales are added, depending on the computational speed of the computer used.

One solution to this problem is to use the simple swale option (check the Use Simple

Swale box). The simple swale does not include HSPF Special Actions. It is less accurate than the standard swale. Tests have shown that the simple swale option may produce slightly larger bioretention facilities. If appropriate, model the bioretention swale both ways and see how close the simple swale answer is to the standard swale method. The standard swale method will always be more accurate than the simple swale.

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INFILTRATION PLANTER ELEMENT

NOTE: To access the Infiltration Planter element click on the LID Toolbox bar.

An infiltration planter allows stormwater to enter the planter above ground and then infiltrate through the soil and gravel storage layers before exiting through a discharge pipe or into the native soil beneath the planter.

The infiltration planter dimensions and parameters are:

Planter Length (ft): Length of planter box.

Planter Bottom Width (ft): Width of planter box.

Infiltration Planter

Freeboard: Additional storage height above top of riser.

Effective Total Depth (ft): Planter height from bottom of planter to top of riser plus freeboard.

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Soil Layer 1 Type: Select from Soil Type pulldown menu.

Soil Layer 1 (ft): Planter soil layer depth.

Soil Layer 2 Type: Select from Soil Type pulldown menu (usually gravel).

Soil Layer 2 (ft): Planter gravel layer depth.

Underdrain Diameter (ft): Planter underdrain pipe diameter (set to zero if no underdrain is included).

Orifice Diameter (in): Planter underdrain pipe orifice diameter (set to zero if no underdrain is included).

Riser Height Above Planter Surface (ft): Height of planter overflow pipe above planter soil surface.

Riser Diameter (in): Planter overflow pipe diameter.

Native Infiltration: Yes (infiltration into the underlying native soil)

Measured Infiltration Rate (in/hr): Native soil infiltration rate.

If infiltration is used then the user should consult the Infiltration discussion on page 116.

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SDHM 3.0 includes automated sizing of the planter box based on a user-set target infiltration percentage. After the target percentage is set then the user can click on the

Size Infiltration Planter button. SDHM 3.0 will iterate to determine the planter length and width needed to meet the target infiltration percentage.

The Quick Planter button simply fills in the Infiltration Planter element input boxes without attempting to size the infiltration planter.

The infiltration planter provides water quality treatment when the runoff moves vertically through the infiltration planter’s engineered soil mix (layers 1-2). The total percent treated is reported as the Water Quality Filtered value.

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FLOW-THROUGH PLANTER ELEMENT

NOTE: To access the Flow-Through Planter element click on the LID Toolbox bar.

A flow-through planter is similar to the infiltration planter, except that water is not allowed to infiltrate into the native soil underlying the gravel layer of the planter. As with the infiltration planter, stormwater enters the planter above ground and then infiltrate through the soil and gravel storage layers before exiting through a discharge pipe.

Flow-through Planter

For the purpose of flow control the discharge from the pipe should not exceed the

Predeveloped discharge from the project site for the flow duration range specified by

SDHM 3.0.

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The flow-through planter dimensions and parameters are:

Planter Length (ft): Length of planter box.

Planter Bottom Width (ft): Width of planter box.

Freeboard: Additional storage height above top of riser.

Effective Total Depth (ft): Planter height from bottom of planter to top of riser plus freeboard.

Soil Layer 1 Type: Select from Soil Type pulldown menu.

Soil Layer 1 (ft): Planter soil layer depth.

Soil Layer 2 Type: Select from Soil Type pulldown menu (usually gravel).

Soil Layer 2 (ft): Planter gravel layer depth.

Underdrain Diameter (ft): Planter underdrain pipe diameter (must be greater than zero).

Orifice Diameter (in): Planter underdrain pipe orifice diameter (less than or equal to the underdrain diameter).

Riser Height Above Planter Surface (ft): Height of planter overflow pipe above planter soil surface.

Riser Diameter (in): Planter overflow pipe diameter.

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The flow-through planter can be sized to meet water quality criteria or water quantity

(flow duration) criteria.

The Quick Planter button simply fills in the Flow-Through Planter element input boxes without attempting to size the flow-through planter facility for either water quality or water quantity.

The Size Water Quality button sizes the flow-through planter to meet the water quality criteria for treating a minimum percent of the total runoff volume that enters the flowthrough planter. The flow-through planter provides water quality treatment when the runoff moves vertically through the flow-through planter engineered soil mix (layers 1-

2). The total percent treated is reported as the Water Quality Filtered value.

The Size Water Quantity button sizes the flow-through planter to meet the flow duration criteria for not increasing flows between the lower and upper thresholds specified in the

Duration Criteria screen. This includes the sum of the flow through the underdrain and the flow through the riser. For this purpose Outlet 1 of the Flow-Through Planter element should be used for the Point of Compliance (POC). The underdrain orifice diameter will be adjusted to limit the Mitigated POC flows for the flow-through planter to meet the flow duration criteria. The dimensions of the flow-through planter may also be changed, if needed.

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POROUS PAVEMENT

Porous pavement LID options include porous asphalt or concrete and grid/lattice systems

(non-concrete) and paving blocks. The use of any of these LID options requires that certain minimum standards and requirements are met related to subgrade, geotextile material, separation or bottom filter layer, base material, wearing layer, drainage conveyance, acceptance testing, and surface maintenance.

NOTE: Porous pavement can be used in place of conventional pavement for roadways, sidewalks, driveways, and parking lots. Check with Appendix C or the local municipal permitting agency to find out under what conditions porous pavement is allowed.

Porous pavement can be represented by the porous pavement element in SDHM 3.0 if the following three conditions are met:

1. The infiltration rate of the porous pavement is greater than the peak rainfall rate.

2. The infiltration rate of the porous pavement is greater than the underlying native soil.

3. There is subgrade layer of crushed rock/gravel between the porous pavement and the native soil.

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The Porous Pavement element is an impervious land use basin element that drains directly to storage layer similar to a gravel trench bed. .

The Porous Pavement element dimensions and parameters are:

Pavement Length (ft): Roadway length.

Pavement Bottom Width (ft): Roadway width.

Effective Total Depth (ft): Height from bottom of porous pavement subgrade to top of pavement plus at least 0.5 feet extra.

Bottom Slope (ft/ft): Gravel layer slope or grade.

The effective volume factor is a value between zero and 1.00. It is only used when the bottom slope is greater than 2%. The effective volume factor is the fraction ratio of the average maximum water depth behind a check dam in the gravel layer (Sublayer 1) compared to the maximum gravel layer depth (Sublayer 1). For example, if the average maximum water height is 6 inches and the gravel depth is 9 inches then the Effective

Volume Factor = 0.67 (6/9). The effective volume factor is multiplied by the Sublayer 1 storage volume to determine the actual maximum volume available for stormwater storage before the check dam is overtopped and the water in the gravel layer depth

(Sublayer 1) proceeds to a downstream conveyance facility.

Pavement Thickness (ft): Porous pavement layer depth.

Pavement Porosity: Porous pavement porosity.

Layer 1 Thickness (ft): Subgrade gravel layer depth.

Layer 1 Porosity: Subgrade gravel porosity.

Layer 2 Thickness (ft): Sand layer depth (if appropriate).

Layer 2 Porosity: Sand porosity.

Ponding Depth Above Pavement (ft): Height at which sheet flow occurs on the pavement.

Underdrain Diameter (in): Set to zero if there is no underdrain.

Underdrain Height (ft): Height of the bottom of the underdrain above the bottom layer.

Native Infiltration: Yes (infiltration into the underlying native soil)

Measured Infiltration Rate (in/hr): Native soil infiltration rate.

Infiltration Reduction Factor: 1/Native soil infiltration rate safety factor (see page 116).

If infiltration is used then the user should consult the Infiltration discussion on page 116.

The porous pavement layers represent the pavement layer and two subgrade layers and their design characteristics (thickness and porosity). The subgrade layers (Sublayer 1 and

Sublayer 2) are available to provide storage prior to discharge through infiltration to the native soil or discharge via an underdrain.

Quick Pavement will create a porous pavement feature with default values without checking it for compliancy with flow duration standards.

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The porous pavement surface area automatically receives rainfall and produces evapotranspiration. Due to this automatic application, input of the porous pavement surface area should be excluded from the land use basin element’s total surface area.

If ponding is not allowed then the ponding depth above pavement value should be set to zero.

NOTE: Check with Appendix C or the local municipal permitting agency to find out if ponding on the surface of the pavement is allowed.

NOTE: See Appendix C or consult with the local municipal permitting agency for additional considerations regarding infiltration and determination of the appropriate infiltration reduction factor.

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DISPERSION

LID Dispersion practices can include roof runoff dispersion onto adjacent yard area, parking lot runoff onto adjacent lawn area, and reverse slope sidewalks draining onto adjacent vegetated areas.

NOTE: Specific minimum requirements and standards must be met to allow dispersion

(see Appendix C and the local municipal permitting agency for details).

Dispersion is represented in SDHM

3.0 with lateral flow basin elements.

Information on the pervious and impervious lateral flow basin elements is presented following this discussion of dispersion modeling.

Roof Dispersion Runoff Example

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The impervious lateral basin (Lateral I Basin 1 in the above scenario) is connected to the pervious lateral basin (Lateral Basin 1). All of the runoff generated by impervious roof

Lateral I Basin 1 is distributed onto pervious urban Lateral Basin 1 before routing to a stormwater control facility (pond, vault, etc.).

The lateral basin dimensions and parameters to adjust to represent dispersion are:

Impervious (IMPLND) type: select flat or moderate slope.

Soil (PERLND) type: select one of the 60 different pervious land types based on soil, land cover, and slope. A and B soils will provide more dispersion benefits than C or D soils because of their ability to infiltrate more runoff.

Lateral Area: size of contributing or receiving area (acres).

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Dispersion will decrease the total runoff, but probably will not totally eliminate the need for a stormwater control facility. A vault (in this example) or other stormwater control facility can be connected to the discharge from the pervious lateral basin to provide the final required mitigation.

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LATERAL BASIN ELEMENT (Pervious)

Runoff dispersion from impervious surfaces onto adjacent pervious land can be modeled using pervious and impervious lateral land use basins. For example, runoff from an impervious parking lot can sheet flow onto an adjacent lawn prior to draining into a stormwater conveyance system. This action slows the runoff and allows for some limited infiltration into the pervious lawn soil prior to discharging into a conveyance system.

The pervious lateral land use basin is similar to the standard land use basin except that the runoff from the lateral basin goes to another adjacent lateral basin (impervious or pervious) rather than directly to a conveyance system or stormwater facility. By definition, the pervious lateral basin contains only a single pervious land type.

Impervious area is handled separately with the impervious lateral land use basin (Lateral

I Basin).

The user selects the pervious lateral basin land type by checking the appropriate box on the Available Soil Types Tools screen. This information is automatically placed in the

Soil (PERLND) Type box above. Once entered, the land type can be changed by clicking on the Change button on the right.

The user enters the number of acres represented by the lateral basin land type.

If the lateral basin contains two or more pervious land use types then the user should create a separate lateral basin for each.

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LATERAL I BASIN ELEMENT (Impervious)

The impervious lateral land use basin is similar to the standard land use basin except that the surface runoff from the lateral impervious basin goes to another adjacent lateral basin

(impervious or pervious) rather than directly to a conveyance system or stormwater facility. By definition, the impervious lateral land use basin contains only impervious land types. Pervious area is handled separately with the pervious lateral land use basin

(Lateral Basin).

The user selects the impervious lateral basin land type by checking the appropriate box on the Available Impervious Coverages screen. This information is automatically placed in the Impervious (IMPLND) Type box above. Once entered, the land type can be changed by clicking on the Change button on the right.

The user enters the number of acres represented by the lateral impervious basin land type.

If the lateral impervious basin contains two or more impervious land use types then the user should create a separate lateral I basin for each.

To model parking lot runoff dispersion onto adjacent lawn connect the Lateral I Basin (the parking lot) to the Lateral Basin (the lawn). In the model’s calculations surface runoff from the parking lot is added to the surface of the lawn (urban vegetation). The total runoff will then directed to a stormwater conveyance system by the user.

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DRY WELL ELEMENT

A dry well is similar to the in-ground

(infiltration) planter, except that there is no bottom discharge pipe or underdrain. Water must infiltrate into the native soil underlying the gravel layer of the planter. The native soil must have sufficient infiltration capacity to infiltrate all of the stormwater.

In SDHM 3.0 the dry well is represented by a specialized application of the gravel trench element available in the LID Toolbox. To access the elements in the LID Toolbox menu click on the LID Toolbox bar.

Dry Well

The dry well dimensions and parameters:

Dry Well Length (ft): Length of well.

Dry Well Width (ft): Width of well.

Reservoir Thickness (ft): Depth of open water storage.

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Top Soil Layer Thickness (ft): Dry well soil layer depth.

Top Soil Layer Porosity: Dry well soil porosity.

Gravel/Sand Layer Thickness (ft): Dry well gravel layer depth.

Gravel/Sand Layer Porosity: Dry well gravel porosity.

Native Infiltration: Yes (infiltration into the underlying native soil)

Measured Infiltration Rate (in/hr): Native soil infiltration rate.

If infiltration is used then the user should consult the Infiltration discussion on page 116.

SDHM 3.0 includes automated sizing of the dry well based on a user-set target infiltration percentage. After the target percentage is set then the user can click on the

Size Dry Well button. SDHM 3.0 will iterate to determine the dry well length and width needed to meet the target infiltration percentage.

NOTE: See Appendix C or consult with the local municipal permitting agency for additional considerations regarding infiltration and determination of the appropriate infiltration reduction factor.

Note that the dry well is covered; there is no precipitation on or evaporation from the dry well.

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INFILTRATION TRENCH ELEMENT

An infiltration trench is similar to the dry well. There is no bottom discharge pipe or underdrain. Water must infiltrate into the native soil underlying the gravel layer of the planter. The native soil must have sufficient infiltration capacity to infiltrate all of the stormwater.

In SDHM 3.0 the infiltration trench is

Infiltration Trench represented by a specialized application of the gravel trench element available in the LID Toolbox. To access the elements in the

LID Toolbox menu click on the LID Toolbox bar.

The infiltration trench dimensions and parameters are:

Trench Length (ft): Infiltration trench length.

Trench Bottom Width (ft): Infiltration trench width.

Berm Height (ft): Height above top of trench at which overflow occurs (one foot above riser height).

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Layer 1 Thickness (ft): Infiltration trench soil layer depth.

Layer 1 Porosity: Infiltration trench soil porosity.

Riser Height (ft): Height of infiltration trench overflow pipe above trench soil surface. If a weir is preferred instead of a riser then set the riser height to the weir height and set the riser diameter to the weir length.

Riser Diameter (in): Infiltration trench overflow pipe diameter.

Native Infiltration: Yes (infiltration into the underlying native soil)

Measured Infiltration Rate (in/hr): Native soil infiltration rate.

If infiltration is used then the user should consult the Infiltration discussion on page 116.

The infiltration trench does not include an underdrain. If an underdrain is required then use the gravel trench element (page 64) instead and set the underdrain height and orifice diameter using the orifice input (the orifice height is defined as from the bottom of the lowest layer in the trench).

SDHM 3.0 includes automated sizing of the infiltration trench based on a user-set target infiltration percentage. After the target percentage is set then the user can click on the

Size Infiltration Trench button. SDHM 3.0 will iterate to determine the infiltration trench length and width needed to meet the target infiltration percentage.

NOTE: See Appendix C or consult with the local municipal permitting agency for additional considerations regarding infiltration and determination of the appropriate infiltration reduction factor.

Note that, unlike the dry well, the infiltration trench receives precipitation on and evaporation from the trench surface.

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INFILTRATION BASIN ELEMENT

An infiltration basin/pond allows stormwater to enter the basin/pond above ground and then infiltrate through the bottom of the basin/pond before exiting through a discharge pipe. Water can also infiltrate into the native soil beneath the basin/pond.

For the purpose of flow control the discharge from the pipe should not exceed the Predeveloped discharge from the project site for the flow duration range specified by the local jurisdiction.

Infiltration Basin/Pond

In SDHM 3.0 the infiltration basin/pond is represented by a specialized application of the trapezoidal pond element available in the LID Toolbox. To access the elements in the

LID Toolbox menu click on the LID Toolbox bar.

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The infiltration basin/pond dimensions and parameters are:

Bottom Length (ft): Infiltration basin/pond length.

Bottom Width (ft): Infiltration basin/pond width.

Effective Depth (ft): Infiltration basin height from basin/pond bottom to top of riser plus at least 0.5 feet extra.

Left Side Slope (H/V): 0 (zero) for vertical infiltration basin/pond sides.

Bottom Side Slope (H/V): 0 (zero) for vertical infiltration basin/pond sides.

Right Side Slope (H/V): 0 (zero) for vertical infiltration basin/pond sides.

Top Side Slope (H/V): 0 (zero) for vertical infiltration basin/pond sides.

Riser Height (ft): Height of infiltration basin/pond overflow pipe above basin/pond soil surface.

Riser Diameter (in): Infiltration basin/pond overflow pipe diameter.

Infiltration: Yes (infiltration into the underlying native soil)

Measured Infiltration Rate (in/hr): Native soil infiltration rate.

Use Wetted Surface Area (sidewalls): Yes, if infiltration through the basin/pond side slopes is allowed.

If infiltration is used then the user should consult the Infiltration discussion on page 116.

SDHM 3.0 includes automated sizing of the infiltration basin/pond based on a user-set target infiltration percentage. After the target percentage is set then the user can click on the Size Infiltration Basin button. SDHM 3.0 will iterate to determine the infiltration basin/pond length and width needed to meet the target infiltration percentage.

NOTE: See Appendix C or consult with the local municipal permitting agency for additional considerations regarding infiltration and determination of the appropriate infiltration reduction factor.

An infiltration basin/pond receives precipitation on and evaporation from the basin/pond

surface.

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GREEN ROOF ELEMENT

A green roof is roof covered with vegetation and a growing medium (typically an engineered soil mix). Green roofs are not always green and are also known as vegetated roofs or eco-roofs.

The advantage of a green roof is its ability to store some runoff on the plants’ surfaces and in the growing medium. Evapotranspiration by the plants and growing medium reduces the total runoff. Runoff movement through the growing medium slows down the runoff and reduces peak discharge during storm events.

A green roof is represented by the SDHM 3.0 green roof element.

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The dimensions and parameters to adjust to represent a green roof are:

Green Area (ac): Size of the green roof.

Depth of Material (in): Growing media/soil depth.

Slope of Rooftop (ft/ft): Roof surface slope.

Vegetative Cover: Type of vegetation on green roof (choices are: ground cover, shrubs, or trees).

Length of rooftop (ft): Length of the longest runoff path to reach a roof drain.

Default input values are automatically included with the element. They should be changed to reflect actual roof conditions.

The green roof surface area automatically receives rainfall and produces evapotranspiration. Due to this model input the green roof surface area should be excluded from the basin element’s total surface area.

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If the green roof is connected to a downstream element or is selected as a point of compliance the user should make sure that the groundwater runoff is included. Unlike the other drainage area elements (land use basin element, etc.), the green roof groundwater always contributes to the total runoff. The green roof groundwater has nowhere else to go.

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RAINWATER HARVESTING

Rainwater harvesting involves water collection, storage, and reuse for residential outdoor use. The LID credit is pretty simple: the drainage area for which there is 100% capture does not have to be included in the SDHM 3.0 Mitigated land use scenario.

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ADDITIONAL INFORMATION

The following pages contain additional information about these features:

 Outlet Structure Configurations

 Infiltration

 Auto Pond, Auto Vault, Auto Tank

 Stage-Storage-Discharge Table

 Point of Compliance

 Connecting Elements

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OUTLET STRUCTURE CONFIGURATIONS

The trapezoidal pond, vault, tank, irregular pond, gravel trench bed, and sand filter all use a riser for the outlet structure to control discharge from the facility.

The riser is a vertical pipe with a height above pond bottom (typically one foot less than the effective depth). The user specifies the riser height and diameter.

The riser can have up to three round orifices. The bottom orifice is usually located at the bottom of the pond and/or above any dead storage in the facility. The user can set the diameter and height of each orifice.

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The user specifies the riser type as either flat or notched. The weir notch can be either rectangular, V-notch, or a Sutro weir. The shape of each type of weir is shown below.

Rectangular Notch V-Notch Sutro

By selecting the appropriate notch type the user is then given the option to enter the appropriate notch type dimensions.

Riser and orifice equations used in SDHM are provided below.

Headr = the water height over the notch/orifice bottom. q = discharge

Riser Head Discharge:

Head = water level above riser q = 9.739 * Riser Diameter * Head ^ 1.5

Orifice Equation: q = 3.782 * (Orifice Diameter) ^ 2 * SQRT(Headr)

Rectangular Notch: b = NotchWidth *- (1- 0.2 * Headr) where b >= 0.8

q = 3.33 * b * Headr ^ 1.5

Sutro:

Hb = Notch Height

Hc = Riser Height – Notch Height

Wb = Bottom Width

Q = Cd * Wb * (h – Hb/3) * SQRT(2g * Hb)

V-Notch:

Notch Bottom = height from bottom of riser to bottom of notch

Theta = Notch Angle

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b = -0.48875 + 0.003843 * Theta - 0.000092124 * Theta ^2

c = 0.3392 - 0.0024318 * Theta + 0.00004715 * Theta ^2

YoverH = Headr / (NotchBottom + Headr)

Coef = a + b * Headr + c * Headr ^2 q = (Coef * Tan(Theta / 2)) * (Headr ^ (5 / 2))

These equations are provided from the Washington State Department of Ecology’s 2014

Stormwater Management Manual for Western Washington. The outlet designs are shown below. They have been reproduced from Volume III of the Stormwater Management

Manual for Western Washington which has more information on the subject.

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The physical configuration of the outlet structure should include protection for the riser and orifices to prevent clogging of the outlet from debris or sediment. Various outlet configurations are shown below. They have been reproduced from Volume III of the

Stormwater Management Manual for Western Washington which has more information on the subject.

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Riser protection structures. Diagrams courtesy of Washington State

Department of Ecology.

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INFILTRATION

Infiltration of stormwater runoff is a recommended solution if certain conditions are met.

These conditions include: a soils report, testing, groundwater protection, pre-settling, and appropriate construction techniques.

NOTE: See Appendix C or consult with the local municipal permitting agency for additional considerations regarding infiltration and determination of the appropriate infiltration reduction factor.

The user clicks on the

Infiltration option arrow to change infiltration from NO to

YES. This activates the infiltration input options: measured infiltration rate, infiltration reduction factor, and whether or not to allow infiltration through the wetted side slopes/walls.

The infiltration reduction factor is a multiplier for the measured infiltration rate and should be less than one. It is the same as the inverse of a safety factor. For example, a safety factor of 2 is equal to a reduction factor of 0.5.

Infiltration occurs only through the bottom of the facility if the wetted surface area option is turned off. Otherwise the entire wetted surface area is used for infiltration.

After the model is run and flow is routed through the infiltration facility the total volume infiltrated, total volume through the riser, total volume through the facility, and percent infiltrated are reported on the screen. If the percent infiltrated is 100% then there is no surface discharge from the facility. The percent infiltrated can be less than 100% as long as the surface discharge does not exceed the flow duration criteria.

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AUTO POND, AUTO VAULT, AUTO TANK

Auto Pond, Auto Vault, and Auto Tank all work the same. Each optimizing routine automatically creates a pond, vault, or tank size and designs the outlet structure to meet the flow duration criteria. The user can either create a pond, vault, or tank from scratch or optimize an existing design.

The following information applies to all three optimizing routines (Auto Pond, Auto

Vault, and Auto Tank), but for the purposes of simplifying the following documentation the term “Auto Pond” applies equally to Auto Vault and Auto Tank.

Auto Pond requires that the Predeveloped and Mitigated basins be defined prior to using

Auto Pond. Clicking on the Auto Pond button brings up the Auto Pond window and the associated Auto Pond controls.

Auto Pond controls:

Automatic Pond Adjuster: The slider at the top of the Auto Pond window allows the user to decide how thoroughly the pond will be designed for efficiency. The lowest setting (0-

1 min) at the left constructs an initial pond without checking the flow duration criteria.

The second setting to the right creates and sizes a pond to pass the flow duration criteria; however, the pond is not necessarily optimized. The higher settings increase the amount

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Pond Depth: Pond depth is the total depth of the pond and should include at least one foot of freeboard (above the riser). The pond’s original depth will be used when optimizing an existing pond; changing the value in the Pond Depth text box will override any previous set depth value. The default depth is 4 feet.

Pond Length to Width Ratio: This bottom length to width ratio will be maintained regardless of the pond size or orientation. The default ratio value is 1.0

Pond Side Slopes: Auto Pond assumes that all of the pond’s sides have the same side slope. The side slope is defined as the horizontal distance divided by the vertical. A typical side slope is 3 (3 feet horizontal to every 1 foot vertical). The default side slope value is 3.

Choose Outlet Structure: The user has the choice of either 1 orifice and rectangular notch or 3 orifices. If the user wants to select another outlet structure option then the pond must be manually sized.

Create Pond: This button creates a pond when the user does not input any pond dimensions or outlet structure information. Any previously input pond information will be deleted.

Optimize Pond: This button optimizes an existing pond. It cannot be used if the user has not already created a pond.

Accept Pond: This button will stop the Auto Pond routine at the last pond size and discharge characteristics that produce a pond that passes the flow duration criteria. Auto

Pond will not stop immediately if the flow duration criteria have not yet been met.

The bottom length and width and volume at riser head will be computed by Auto Pond; they cannot be input by the user.

Auto Vault and Auto Tank operate the same way as Auto Pond.

There are some situations where Auto Pond (or Auto Vault) will not work. These situations occur when complex routing conditions upstream of the pond make it difficult or impossible for Auto Pond to determine which land use will be contributing runoff to the pond. For these situations the pond will have to be manually sized. Go to page 53 to find information on how to manually size a pond or other HMP facility.

NOTE: If Auto Pond selects a bottom orifice diameter smaller than the smallest diameter allowed by the local municipal permitting agency then additional mitigating BMPs may be required to meet local hydromodification control requirements. Please see Appendix C or consult with local municipal permitting agency for more details. For manual sizing information see page 53.

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STAGE-STORAGE-DISCHARGE TABLE

The stage-storage-discharge table hydraulically represents any facility that requires stormwater routing. The table is automatically generated by SDHM 3.0 when the user inputs storage facility dimensions and outlet structure information. SDHM 3.0 generates

91 lines of stage, surface area, storage, surface discharge, and infiltration values starting at a stage value of zero (facility bottom height) and increasing in equal increments to the maximum stage value (facility effective depth).

When the user or SDHM 3.0 changes a facility dimension (for example, bottom length) or an orifice diameter or height the model immediately recalculates the stage-storagedischarge table.

The user can input to SDHM 3.0 a stage-storage-discharge table created outside of

SDHM 3.0. To use a stage-storage-discharge table created out of SDHM 3.0 the SSD

Table element is required. See the SSD Table element description below for more information on how to load such a table to SDHM 3.0 program.

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POINT OF COMPLIANCE

SDHM 3.0 allows for multiple points of compliance (maximum of 50) in a single project.

A point of compliance is defined as the location at which the Predeveloped and Mitigated flows will analyzed for compliance with the flow control standard.

The point of compliance is selected by right clicking on the element at which the compliance analysis will be made. In the example above, the point of compliance analysis will be conducted at the outlet of the trapezoidal pond.

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A Point of Compliance (POC) box will allow the user to specify which outlet (if there is more than one) to connect to the point of compliance and corresponding number for the

POC.

In the above example, there are two outlets shown (Outlet 1 and Outlet 2). Outlet 1 represents the surface discharge through the outlet structure. Outlet 2 is the infiltration discharge through the bottom of the facility. Only the surface discharge (Outlet 1) should be connected to the POC.

When an underdrain is included in the facility (bioretention or porous pavement) the discharge from the underdrain will be considered part of the Outlet 1 total discharge.

A sand filter should include the discharge from both Outlet 1 (surface discharge not filtered by the sand filter) and Outlet 2 (surface discharge filtered by the sand filter) connected to the POC, assuming that the POC is immediately downstream of the sand filter.

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Once the point of compliance has been selected the element is modified on the Schematic screen to include a small box with the POC number (1 in this example) and the letter “A” (for

Analysis) in the lower right corner. This identifies the outlet from this element as a point of compliance.

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CONNECTING ELEMENTS

Elements are connected by right clicking on the upstream element (in this example Basin

1) and selecting and then left clicking on the Connect To Element option. By doing so

SDHM 3.0 extends a line from the upstream element to wherever the user wants to connect that element.

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The user extends the connection line to the downstream element (in this example, a pond) and left clicks on the destination element. This action brings up the From Basin to

Conveyance box that allows the user to specify which runoff components to route to the downstream element.

Stormwater runoff is defined as surface flow + interflow. Both boxes should be checked. Groundwater should not be checked for the standard land development mitigation analysis. Groundwater should only be checked when there is observed and documented base flow occurring from the upstream basin.

After the appropriate boxes have been checked click the OK button.

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The final screen will look like the above screen. The land use basin information screen on the right will show that Basin 1 surface and interflow flows to Trapezoidal Pond 1

(groundwater is not connected).

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ANALYSIS SCREEN

The Analysis tool bar button (third from the left) brings up the Analysis screen where the user can look at the results of the Predeveloped and Mitigated scenarios. The Analysis screen allows the user to analyze and compare flow durations, flow frequency, drawdown times, and hydrographs.

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The user can analyze all time series datasets or just flow, stage, precipitation, evaporation, or point of compliance (POC) flows by selecting the appropriate tab below the list of the different datasets available for analysis.

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FLOW DURATION

Flow duration at the point of compliance (POC 1) is the most common analysis. A plot of the flow duration values is shown on the left, the flow values on the right.

The flow duration flow range is from the lower threshold flow frequency value (in this example 10% of the 2-year value) to the 10-year value. As shown in the flow duration table to the right of the flow duration curves, this flow range is divided into approximately 100 levels (flow values). For each flow level/value SDHM 3.0 counts the number of times that the flow at the Point of Compliance for the Predevelopment scenario (Predev) exceeds that specific flow level/value. It does the same count for the

Mitigated scenario flow (Mit). The total number of counts is the number of simulated hourly intervals that the flow exceeds that specific flow level/value.

The Percentage column is the ratio of the Dev count to the Predev count. This ratio must be less than or equal to 110% for flow levels/values between the lower threshold value and the 10-year value (the upper limit). If the percentage value does not exceed this maximum ratio (110% for the lower threshold value to the 10-year value) then the

Pass/Fail column shows a Pass for that flow level. If they are exceeded then a Fail is shown. One Fail and the facility fails the flow duration criteria. The facility overall

Pass/Fail is listed at the top of the flow duration table.

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FLOW FREQUENCY

Flow frequency plots are shown on the left and the 2-, 5-, 10-, and 25-year frequency values are on the right. Flow frequency calculations are based on selecting partial duration flow values and ranking them by their Weibull Plotting Position.

The Weibull Plotting Position formula is:

Tr = (N+a)/(m-b) where Tr = return period (years) m = rank (largest event, m = 1)

N = number of simulation years a = 1.0 b = 0.0

Probability = 1/Tr

The return period value, Tr, is used in SDHM to determine the 2-year, 5-year, 10-year, and 25-year peak flow values. If necessary, the 2-year, 5-year, 10-year, and 25-year values are interpolated from the Tr values generated by Weibull.

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DRAWDOWN

The drawdown screen is used to compute pond stages (water depths). These stages are summarized and reported in terms of drain/retention time (in days).

For this example, the 10-year stage (based on the 10-year flow) is 3.20 feet.

This 10-year stage has a drawdown time of less than 1 day.

Ponds may have drain times in excess of the allowed maximum of 96 hours (4 days).

This can occur when a pond has a small bottom orifice. If this is not acceptable then the user needs to change the pond outlet configuration, manually run the Mitigated scenario, and repeat the analyze stage computations. A situation may occur where it is not possible to have both an acceptable pond drawdown/ retention time and meet the flow duration criteria.

NOTE: The flow duration criteria take precedence unless the user is instructed otherwise by Appendix C or the local municipal permitting agency.

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HYDROGRAPHS

The user can graph/plot any or all time series data by selecting the Hydrograph tab.

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The Create Graph screen is shown and the user can select the time series to plot, the time interval (yearly, monthly, daily, or hourly), and type of data (peaks, average, or volume).

The following numbering system is used for the flow time series:

500-599: Predeveloped flow (Predeveloped scenario)

700-799: Inflow to the POC (Mitigated runoff entering the BMP facility)

800-899: POC flow (Mitigated flow exiting the BMP facility)

1000-1999: Other POC related time series (flow and stage)

The selected time series are shown. To graph the selected time series the user clicks on the Graph button.

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The hydrograph shows the monthly maximum/peak flow values for each time series for the entire simulation period (in this example, from 1968 through 2004).

The graph can be either saved or printed.

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REPORTS SCREEN

Click on the Reports tool bar button (fourth from the left) to generate a project report.

The project report is a separate file that contains all of the user-provided input and a summary of the model output. The project report file can be saved or printed.

The user has the option of producing the report file in a number of different formats.

Click on “Text Report” button to generate the report file in WordPad RTF format.

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Scroll down the WordPad screen to see all of the results.

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Click on “PDF Report” button to generate the report file in PDF format.

Scroll down the PDF screen to see all of the results or select specific sections using the bookmarks at the left.

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Click on “Landuse Report” button to generate a listing of the input basin land use data in

WordPad RTF format.

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Click on “Parameter Report” button to generate a listing of the HSPF input parameter values that have been changed from their default values. The listing is in WordPad RTF format. If the listing is blank then there have been no HSPF input parameter value changes made.

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TOOLS SCREEN

The Tools screen is accessed with the Tools tool bar (second from the right). The two purposes of the Tools screen are:

(1) To allow users to import HSPF PERLND parameter values from existing HSPF UCI files and/or view and edit SDHM 3.0 PERLND parameter values.

(2) To allow users to export time series datasets.

To export a time series dataset click on the Export Dataset box.

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The list of available time series datasets will be shown. The user can select the start and end dates for the data they want to export.

The time step (hourly, daily, monthly, yearly) can also be specified. If the user wants daily, monthly, or yearly data the user is given the choice of either selecting the maximum, minimum, or the sum of the hourly values.

Click the Export button.

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The user provides a file name and the format or type of file. The file type can be ASCII text, comma delimited, Access database, recharge, SWMM, or WWHM.

Click Save to save the exported time series file.

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LID ANALYSIS SCREEN

The LID tool bar button (farthest on the right) brings up the Low Impact Development

Scenario Generator screen.

The LID scenario generator can be used to compare the amount of runoff from different land types and combinations. The user can quickly see how changing the land use affects surface runoff, interflow, groundwater, and evapotranspiration.

NOTE: The LID scenario generator works only in the Mitigated scenario.

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The easiest way to compare different land use scenarios is to place all of them on the same Schematic Editor screen grid. Each basin can then represent a different land use scenario. Because the LID scenario generator only compares runoff volume there is no need to do any routing through a conveyance system or stormwater facility.

For this example the three basins are assigned the following land uses:

Basin 1: 1 acre A, Dirt, Flat

Basin 2: 1 acre C, NatVeg, Steep

Basin 3: 1 acre Impervious, Flat

Each basin is assigned a different POC (point of compliance) for the LID analysis.

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Click on the Compute LID Base Data button to generate the LID analysis data and summarize the surface runoff, interflow, groundwater, precipitation, evaporation, and total runoff for all of the basins. The results will be shown for each basin based on its

POC number.

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For Basin 1 (1 acre of A, Dirt, Flat) the distribution of the precipitation is:

Surface runoff = 0.047 inches per year

Interflow = 0.114 inches per year

Groundwater = 0.825 inches per year

Evaporation = 9.231 inches per year

The sum of the surface runoff + interflow + groundwater + evaporation equals 10.217 inches per year. The precipitation at this site equals 10.206 inches per year. The difference is the initial storage in the soil column.

To look at the other basins click on the Select POC To arrow and select the basin of interest.

The LID analysis results can be presented in terms of either inches per year or acre-feet per year by checking the appropriate box in the lower right portion of the LID analysis screen.

To compare the different scenarios side-by-side in a graphical format click on the Water

Balance Chart button.

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The water balance chart graphically displays the runoff distribution for all three land use scenarios side-by-side.

The bottom red is the surface runoff. Above in yellow is interflow; then green for groundwater and blue for evaporation. Basin 1 (Scenario 1) is an A soil with dirt land cover on a flat slope and produces the least amount of surface runoff and interflow (the sum of surface and interflow is the total stormwater runoff). Basin 2 is a C soil with natural vegetation land cover on a steep slope; it produces more surface runoff and interflow than Basin 1. Basin 3 is impervious and produces the largest amount of surface runoff and interflow and the smallest amount of evaporation.

A maximum of seven scenarios can be graphed at one time.

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OPTIONS

Options can be accessed by going to View, Options. This will bring up the Options screen and the ability to modify the built-in default duration criteria for flow duration matching and scaling factors for climate variables.

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DURATION CRITERIA

The flow duration criterion is:

1. If the post-development flow duration values exceed any of the Predevelopment flow levels between the lower threshold (x% of the two-year and 100% of the tenyear Predevelopment peak flow values more than 10 percent of the time (110

Percent Threshold) then the flow duration standard has not been met.

The duration criteria in SDHM can be modified by the user if appropriate and the local municipal permitting agency allows (see NOTE below).

The user can conduct the duration analysis using either (1) durations based on

Predevelopment (Weibull calculated) flow frequency, or (2) durations based on user defined flow values.

If using durations based on Predevelopment flow frequency the percent of the lower limit can be changed from the default of the 2-year flow event to a higher or lower percent

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San Diego Hydrology Model 3.0 Interim Version User Manual – May 2016 value. The lower and upper flow frequency limits (2-year and 10-year) also can be changed.

If using durations based on user defined flow values click on that option and input the lower and upper flow values.

The user has the option of changing the lower and upper thresholds based on the following USGS regional regression equations:

Q2 = 3.60*(A^0.672)*(P^0.753)

Q10 = 6.56*(A^0.783)*(P^1.07)

Where A = drainage area (sq. miles)

P = mean annual precipitation (inches)

The lower threshold equals 0.10Q2.

Mean annual precipitation values for the standard 14 San Diego County rain gages are shown below.

Bonita

Descanso

Encinitas

Fallbrook

Fashion Valley

Granite Hills

9.1 inches

20.5

9.3

13.9

10.4

12.8

Kearny Mesa

Lake Henshaw

Lake Wolford

Morena Lake

Oceanside

Poway

Ramona

10.8

22.6

16.8

16.6

11.3

11.6

13.0

Santa Ysabel 21.1

Note that when changing the default threshold value(s) this must be done for each point of compliance if there is more than one point of compliance.

The default pass/fail threshold is 110%. This value can be changed by the user.

The duration criteria can be changed for a single point of compliance. Click on the

Update button once all of the changes have been made. To return to the default values click on the Restore Defaults button.

NOTE: Any change(s) to the default duration criteria must be approved by the appropriate local municipal permitting agency or specified in Appendix C.

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SCALING FACTORS

The user can change the scaling factors for precipitation (minimum and maximum) and pan evaporation.

NOTE: Any change in default scaling factors requires approval by the local municipal permitting agency or Appendix C.

Click on the Update button once all of the changes have been made. To return to the default values click on the Restore Defaults button.

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APPENDIX A: DEFAULT SDHM 3.0 HSPF PERVIOUS

PARAMETER VALUES

The default SDHM 3.0 HSPF pervious parameter values are found in SDHM 3.0 file defaultpers.uci.

HSPF parameter documentation is found in the document:

Bicknell, B.R., J.C. Imhoff, J.L. Kittle Jr, T.H. Jobes, and A.S. Donigian Jr. 2001.

Hydrological Simulation Program – Fortran, User’s Manual for Version 12. AQUA

TERRA Consultants. Mountain View, CA.

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Table 1. SDHM Pervious Land Types

PERLND No.  Soil Type 

1  A 

2  A 

Land Cover 

Natural Vegetation 

Land Slope 

Flat (0‐5%) 

Natural Vegetation  Moderate (5‐15%) 

31 

32 

33 

34 

27 

28 

29 

30 

23 

24 

25 

26 

19 

20 

21 

22 

35 

36 

37 

38 

39 

40 

15 

16 

17 

18 

11 

12 

13 

14 

10 

Natural Vegetation 

Dirt 

Dirt 

Dirt 

Rock 

Rock 

Rock 

Natural Vegetation 

Steep (>15%) 

Flat (0‐5%) 

Moderate (5‐15%) 

Steep (>15%) 

Flat (0‐5%) 

Moderate (5‐15%) 

Steep (>15%) 

Flat (0‐5%) 

Natural Vegetation  Moderate (5‐15%) 

Natural Vegetation  Steep (>15%) 

Dirt 

Dirt 

Flat (0‐5%) 

Moderate (5‐15%) 

Dirt 

Rock 

Rock 

Rock 

Steep (>15%) 

Flat (0‐5%) 

Moderate (5‐15%) 

Steep (>15%) 

Natural Vegetation  Flat (0‐5%) 

Natural Vegetation  Moderate (5‐15%) 

Natural Vegetation 

Dirt 

Steep (>15%) 

Flat (0‐5%) 

Dirt 

Dirt 

Rock 

Rock 

Moderate (5‐15%) 

Steep (>15%) 

Flat (0‐5%) 

Moderate (5‐15%) 

Rock 

Natural Vegetation 

Steep (>15%) 

Flat (0‐5%) 

Natural Vegetation  Moderate (5‐15%) 

Natural Vegetation  Steep (>15%) 

Dirt 

Dirt 

Dirt 

Rock 

Flat (0‐5%) 

Moderate (5‐15%) 

Steep (>15%) 

Flat (0‐5%) 

Rock 

Rock 

Urban 

Urban 

Urban 

Urban 

Moderate (5‐15%) 

Steep (>15%) 

Flat (0‐5%) 

Moderate (5‐15%) 

Steep (>15%) 

Flat (0‐5%) 

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53 

54 

55 

56 

49 

50 

51 

52 

57 

58 

59 

60 

45 

46 

47 

48 

41 

42 

43 

44 

Urban 

Urban 

Urban 

Urban 

Urban 

Urban 

Urban 

Urban 

Moderate (5‐15%) 

Steep (>15%) 

Flat (0‐5%) 

Moderate (5‐15%) 

Steep (>15%) 

Flat (0‐5%) 

Moderate (5‐15%) 

Steep (>15%) 

Urban, No Irrigation  Flat (0‐5%) 

Urban, No Irrigation  Moderate (5‐15%) 

Urban, No Irrigation 

Urban, No Irrigation 

Steep (>15%) 

Flat (0‐5%) 

Urban, No Irrigation  Moderate (5‐15%) 

Urban, No Irrigation  Steep (>15%) 

Urban, No Irrigation  Flat (0‐5%) 

Urban, No Irrigation  Moderate (5‐15%) 

Urban, No Irrigation 

Urban, No Irrigation 

Steep (>15%) 

Flat (0‐5%) 

Urban, No Irrigation  Moderate (5‐15%) 

Urban, No Irrigation  Steep (>15%) 

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Table 2. SDHM HSPF Pervious Parameter Values – Part I

PLS  NAME  LZSN  INFILT  LSUR  SLSUR  KVARY  AGWRC 

1  A,NatVeg,Flat  5.20  0.090  200  0.050  2.50  0.915 

2.20 

2.10 

4.80 

4.50 

4.20 

4.80 

4.50 

4.20 

2.20 

4.80 

4.50 

4.20 

4.80 

4.50 

4.20 

2.40 

2.40 

2.20 

2.10 

5.20 

4.80 

4.50 

5.00 

4.70 

5.00 

4.70 

4.40 

5.00 

4.70 

4.40 

2.50 

2.30 

4.80 

4.50 

5.20 

4.80 

4.50 

2.60 

2.40 

2.20 

2  A,NatVeg,Mod 

3  A,NatVeg,Steep 

4  A,Dirt,Flat 

5  A,Dirt,Mod 

6  A,Dirt,Steep 

7  A,Rock,Flat 

8  A,Rock,Mod 

9  A,Rock,Steep 

10  B,NatVeg,Flat 

11  B,NatVeg,Mod 

12  B,NatVeg,Steep 

13  B,Dirt,Flat 

14  B,Dirt,Mod 

15  B,Dirt,Steep 

16  B,Rock,Flat 

17  B,Rock,Mod 

18  B,Rock,Steep 

19  C,NatVeg,Flat 

20  C,NatVeg,Mod 

21  C,NatVeg,Steep 

22  C,Dirt,Flat 

23  C,Dirt,Mod 

24  C,Dirt,Steep 

25  C,Rock,Flat 

26  C,Rock,Mod 

27  C,Rock,Steep 

28  D,NatVeg,Flat 

29  D,NatVeg,Mod 

30  D,NatVeg,Steep 

31  D,Dirt,Flat 

32  D,Dirt,Mod 

33  D,Dirt,Steep 

34  D,Rock,Flat 

35  D,Rock,Mod 

36  D,Rock,Steep 

37  A,Urban,Flat 

38  A,Urban,Mod 

39  A,Urban,Steep 

40  B,Urban,Flat 

41  B,Urban,Mod 

0.020 

0.015 

0.040 

0.030 

0.020 

0.045 

0.040 

0.030 

0.020 

0.050 

0.040 

0.030 

0.045 

0.040 

0.030 

0.022 

0.022 

0.020 

0.015 

0.090 

0.070 

0.045 

0.070 

0.055 

0.070 

0.055 

0.040 

0.070 

0.055 

0.040 

0.035 

0.028 

0.070 

0.045 

0.090 

0.070 

0.045 

0.045 

0.035 

0.022 

175 

150 

200 

175 

150 

200 

175 

150 

150 

200 

175 

150 

200 

175 

150 

200 

200 

175 

150 

200 

175 

150 

200 

175 

200 

175 

150 

200 

175 

150 

200 

175 

175 

150 

200 

175 

150 

200 

175 

150 

0.100 

0.150 

0.050 

0.100 

0.150 

0.050 

0.100 

0.150 

0.150 

0.050 

0.100 

0.150 

0.050 

0.100 

0.150 

0.050 

0.050 

0.100 

0.150 

0.050 

0.100 

0.150 

0.050 

0.100 

0.050 

0.100 

0.150 

0.050 

0.100 

0.150 

0.050 

0.100 

0.100 

0.150 

0.050 

0.100 

0.150 

0.050 

0.100 

0.150 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

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42  B,Urban,Steep 

43  C,Urban,Flat 

44  C,Urban,Mod 

45  C,Urban,Steep 

46  D,Urban,Flat 

47  D,Urban,Mod 

48  D,Urban,Steep 

49  A,UrbNoIrr,Flat 

50  A,UrbNoIrr,Mod 

51  A,UrbNoIrr,Steep 

52  B,UrbNoIrr,Flat 

53  B,UrbNoIrr,Mod 

54  B,UrbNoIrr,Steep 

55  C,UrbNoIrr,Flat 

56  C,UrbNoIrr,Mod 

57  C,UrbNoIrr,Steep 

0.070 

0.045 

0.070 

0.055 

0.040 

0.050 

0.040 

0.030 

0.040 

0.050 

0.040 

0.030 

0.040 

0.030 

0.020 

0.090 

4.80 

4.50 

5.00 

4.70 

4.40 

4.80 

4.50 

4.20 

4.40 

4.80 

4.50 

4.20 

4.80 

4.50 

4.20 

5.20 

58  D,UrbNoIrr,Flat 

59  D,UrbNoIrr,Mod 

60  D,UrbNoIrr,Steep 

4.80 

4.50 

4.20 

0.040 

0.030 

0.020 

200 

175 

150 

0.050 

0.100 

0.150 

61  Green Roof  1.00  0.050  50  0.001 

LZSN: Lower Zone Storage Nominal (inches)

INFILT: Infiltration (inches per hour)

LSUR: Length of surface flow path (feet)

SLSUR: Slope of surface flow path (feet/feet)

KVARY: Variable groundwater recession

AGWRC: Active Groundwater Recession Constant (per day)

175 

150 

200 

175 

150 

200 

175 

150 

150 

200 

175 

150 

200 

175 

150 

200 

0.100 

0.150 

0.050 

0.100 

0.150 

0.050 

0.100 

0.150 

0.150 

0.050 

0.100 

0.150 

0.050 

0.100 

0.150 

0.050 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.915 

0.100 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

2.50 

0.50 

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2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

Table 3. SDHM HSPF Pervious Parameter Values – Part II

PLS  NAME  INFEXP  INFILD  DEEPFR  BASETP  AGWETP 

1  A,NatVeg,Flat  2.00  2.00  0.00  0.050  0.050 

2  A,NatVeg,Mod 

3  A,NatVeg,Steep 

4  A,Dirt,Flat 

5  A,Dirt,Mod 

6  A,Dirt,Steep 

7  A,Rock,Flat 

8  A,Rock,Mod 

9  A,Rock,Steep 

10  B,NatVeg,Flat 

11  B,NatVeg,Mod 

12  B,NatVeg,Steep 

13  B,Dirt,Flat 

14  B,Dirt,Mod 

15  B,Dirt,Steep 

16  B,Rock,Flat 

17  B,Rock,Mod 

18  B,Rock,Steep 

19  C,NatVeg,Flat 

20  C,NatVeg,Mod 

21  C,NatVeg,Steep 

22  C,Dirt,Flat 

23  C,Dirt,Mod 

24  C,Dirt,Steep 

25  C,Rock,Flat 

26  C,Rock,Mod 

27  C,Rock,Steep 

28  D,NatVeg,Flat 

29  D,NatVeg,Mod 

30  D,NatVeg,Steep 

31  D,Dirt,Flat 

32  D,Dirt,Mod 

33  D,Dirt,Steep 

34  D,Rock,Flat 

35  D,Rock,Mod 

36  D,Rock,Steep 

37  A,Urban,Flat 

38  A,Urban,Mod 

39  A,Urban,Steep 

40  B,Urban,Flat 

41  B,Urban,Mod 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

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42  B,Urban,Steep 

43  C,Urban,Flat 

44  C,Urban,Mod 

45  C,Urban,Steep 

46  D,Urban,Flat 

47  D,Urban,Mod 

48  D,Urban,Steep 

49  A,UrbNoIrr,Flat 

50  A,UrbNoIrr,Mod 

51  A,UrbNoIrr,Steep 

52  B,UrbNoIrr,Flat 

53  B,UrbNoIrr,Mod 

54  B,UrbNoIrr,Steep 

55  C,UrbNoIrr,Flat 

56  C,UrbNoIrr,Mod 

57  C,UrbNoIrr,Steep 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

58  D,UrbNoIrr,Flat 

59  D,UrbNoIrr,Mod 

60  D,UrbNoIrr,Steep 

2.00 

2.00 

2.00 

2.00 

2.00 

2.00 

0.00 

0.00 

0.00 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

61  Green Roof  2.00  2.00  0.00  0.150 

INFEXP: Infiltration Exponent

INFILD: Infiltration ratio (maximum to mean)

DEEPFR: Fraction of groundwater to deep aquifer or inactive storage

BASETP: Base flow (from groundwater) Evapotranspiration fraction

AGWETP: Active Groundwater Evapotranspiration fraction

0.800 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

0.050 

161

San Diego Hydrology Model 3.0 Interim Version User Manual – May 2016

Table 4. SDHM HSPF Pervious Parameter Values – Part III

PLS  NAME  CEPSC  UZSN  NSUR  INTFW 

1  A,NatVeg,Flat  See Table 5  0.600  0.200  1.50 

2  A,NatVeg,Mod 

3  A,NatVeg,Steep 

4  A,Dirt,Flat 

5  A,Dirt,Mod 

6  A,Dirt,Steep 

7  A,Rock,Flat 

8  A,Rock,Mod 

9  A,Rock,Steep 

10  B,NatVeg,Flat 

11  B,NatVeg,Mod 

12  B,NatVeg,Steep 

13  B,Dirt,Flat 

14  B,Dirt,Mod 

15  B,Dirt,Steep 

16  B,Rock,Flat 

17  B,Rock,Mod 

18  B,Rock,Steep 

19  C,NatVeg,Flat 

20  C,NatVeg,Mod 

21  C,NatVeg,Steep 

22  C,Dirt,Flat 

23  C,Dirt,Mod 

24  C,Dirt,Steep 

25  C,Rock,Flat 

26  C,Rock,Mod 

27  C,Rock,Steep 

28  D,NatVeg,Flat 

29  D,NatVeg,Mod 

30  D,NatVeg,Steep 

31  D,Dirt,Flat 

32  D,Dirt,Mod 

33  D,Dirt,Steep 

34  D,Rock,Flat 

35  D,Rock,Mod 

36  D,Rock,Steep 

37  A,Urban,Flat 

38  A,Urban,Mod 

39  A,Urban,Steep 

40  B,Urban,Flat 

41  B,Urban,Mod 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

162

IRC  LZETP 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

San Diego Hydrology Model 3.0 Interim Version User Manual – May 2016

42  B,Urban,Steep 

43  C,Urban,Flat 

44  C,Urban,Mod 

45  C,Urban,Steep 

46  D,Urban,Flat 

47  D,Urban,Mod 

48  D,Urban,Steep 

49  A,UrbNoIrr,Flat 

50  A,UrbNoIrr,Mod 

51  A,UrbNoIrr,Steep 

52  B,UrbNoIrr,Flat 

53  B,UrbNoIrr,Mod 

54  B,UrbNoIrr,Steep 

55  C,UrbNoIrr,Flat 

56  C,UrbNoIrr,Mod 

57  C,UrbNoIrr,Steep 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

58  D,UrbNoIrr,Flat 

59  D,UrbNoIrr,Mod 

60  D,UrbNoIrr,Steep 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

See Table 5  0.600  0.200 

61  Green Roof  See Table 5  0.100  0.550 

CEPSC: Interception storage (inches)

UZSN: Upper Zone Storage Nominal (inches)

NSUR: Surface roughness (Manning’s n)

INTFW: Interflow index

IRC: Interflow Recession Constant (per day)

LZETP: Lower Zone Evapotranspiration fraction

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.50 

1.00 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.700  see Table 6 

0.100  see Table 6 

163

San Diego Hydrology Model 3.0 Interim Version User Manual – May 2016

Table 5. SDHM HSPF Pervious Parameter Values: Monthly Interception Storage (inches)

PLS  NAME 

1  A,NatVeg,Flat 

2  A,NatVeg,Mod 

JAN 

0.10 

0.10 

FEB 

0.10 

0.10 

MAR 

0.10 

0.10 

APR 

0.10 

0.10 

MAY 

0.06 

0.06 

JUN 

0.06 

0.06 

JUL 

0.06 

0.06 

3  A,NatVeg,Steep 

4  A,Dirt,Flat 

5  A,Dirt,Mod 

6  A,Dirt,Steep 

7  A,Rock,Flat 

8  A,Rock,Mod 

9  A,Rock,Steep 

10  B,NatVeg,Flat 

11  B,NatVeg,Mod 

12  B,NatVeg,Steep 

13  B,Dirt,Flat 

14  B,Dirt,Mod 

15  B,Dirt,Steep 

16  B,Rock,Flat 

17  B,Rock,Mod 

18  B,Rock,Steep 

19  C,NatVeg,Flat 

20  C,NatVeg,Mod 

21  C,NatVeg,Steep 

22  C,Dirt,Flat 

23  C,Dirt,Mod 

24  C,Dirt,Steep 

25  C,Rock,Flat 

26  C,Rock,Mod 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

AUG 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

SEP 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

NOV 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

OCT 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

DEC 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

164

San Diego Hydrology Model 3.0 Interim Version User Manual – May 2016

27  C,Rock,Steep 

28  D,NatVeg,Flat 

29  D,NatVeg,Mod 

30  D,NatVeg,Steep 

31  D,Dirt,Flat 

32  D,Dirt,Mod 

33  D,Dirt,Steep 

34  D,Rock,Flat 

35  D,Rock,Mod 

36  D,Rock,Steep 

37  A,Urban,Flat 

38  A,Urban,Mod 

39  A,Urban,Steep 

40  B,Urban,Flat 

41  B,Urban,Mod 

42  B,Urban,Steep 

43  C,Urban,Flat 

44  C,Urban,Mod 

45  C,Urban,Steep 

46  D,Urban,Flat 

47  D,Urban,Mod 

48  D,Urban,Steep 

49  A,UrbNoIrr,Flat 

50  A,UrbNoIrr,Mod 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

51  A,UrbNoIrr,Steep  0.10 

52  B,UrbNoIrr,Flat  0.10 

53  B,UrbNoIrr,Mod  0.10 

54  B,UrbNoIrr,Steep  0.10 

55  C,UrbNoIrr,Flat  0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

165

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

San Diego Hydrology Model 3.0 Interim Version User Manual – May 2016

56  C,UrbNoIrr,Mod  0.10 

57  C,UrbNoIrr,Steep  0.10 

58  D,UrbNoIrr,Flat 

59  D,UrbNoIrr,Mod 

0.10 

0.10 

60  D,UrbNoIrr,Steep  0.10 

61  Green Roof  0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.06 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

0.10 

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Table 6. SDHM HSPF Pervious Parameter Values: Monthly Lower Zone Evapotranspiration

PLS  NAME 

1  A,NatVeg,Flat 

2  A,NatVeg,Mod 

JAN 

0.40 

0.40 

FEB 

0.40 

0.40 

MAR 

0.40 

0.40 

APR 

0.40 

0.40 

MAY 

0.60 

0.60 

JUN 

0.60 

0.60 

JUL 

0.60 

0.60 

3  A,NatVeg,Steep 

4  A,Dirt,Flat 

5  A,Dirt,Mod 

6  A,Dirt,Steep 

7  A,Rock,Flat 

8  A,Rock,Mod 

9  A,Rock,Steep 

10  B,NatVeg,Flat 

11  B,NatVeg,Mod 

12  B,NatVeg,Steep 

13  B,Dirt,Flat 

14  B,Dirt,Mod 

15  B,Dirt,Steep 

16  B,Rock,Flat 

17  B,Rock,Mod 

18  B,Rock,Steep 

19  C,NatVeg,Flat 

20  C,NatVeg,Mod 

21  C,NatVeg,Steep 

22  C,Dirt,Flat 

23  C,Dirt,Mod 

24  C,Dirt,Steep 

25  C,Rock,Flat 

26  C,Rock,Mod 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

AUG 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

SEP 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

NOV 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

OCT 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

DEC 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

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27  C,Rock,Steep 

28  D,NatVeg,Flat 

29  D,NatVeg,Mod 

30  D,NatVeg,Steep 

31  D,Dirt,Flat 

32  D,Dirt,Mod 

33  D,Dirt,Steep 

34  D,Rock,Flat 

35  D,Rock,Mod 

36  D,Rock,Steep 

37  A,Urban,Flat 

38  A,Urban,Mod 

39  A,Urban,Steep 

40  B,Urban,Flat 

41  B,Urban,Mod 

42  B,Urban,Steep 

43  C,Urban,Flat 

44  C,Urban,Mod 

45  C,Urban,Steep 

46  D,Urban,Flat 

47  D,Urban,Mod 

48  D,Urban,Steep 

49  A,UrbNoIrr,Flat 

50  A,UrbNoIrr,Mod 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

51  A,UrbNoIrr,Steep  0.40 

52  B,UrbNoIrr,Flat  0.40 

53  B,UrbNoIrr,Mod  0.40 

54  B,UrbNoIrr,Steep  0.40 

55  C,UrbNoIrr,Flat  0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

168

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

San Diego Hydrology Model 3.0 Interim Version User Manual – May 2016

56  C,UrbNoIrr,Mod  0.40 

57  C,UrbNoIrr,Steep  0.40 

58  D,UrbNoIrr,Flat 

59  D,UrbNoIrr,Mod 

0.40 

0.40 

60  D,UrbNoIrr,Steep  0.40 

61  Green Roof  0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.60 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

0.40 

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San Diego Hydrology Model 3.0 Interim Version User Manual – May 2016

APPENDIX B: DEFAULT SDHM 3.0 HSPF IMPERVIOUS

PARAMETER VALUES

The default SDHM 3.0 HSPF impervious parameter values are found in SDHM file defaultpers.uci.

HSPF parameter documentation is found in the document:

Bicknell, B.R., J.C. Imhoff, J.L. Kittle Jr, T.H. Jobes, and A.S. Donigian Jr. 2001.

Hydrological Simulation Program – Fortran, User’s Manual for Version 12. AQUA

TERRA Consultants. Mountain View, CA.

Table 1. SDHM Impervious Land Types

IMPLND No. Surface

1 Impervious

Slope

Flat (0-5%)

2

3

Impervious

Impervious

Moderate (5-15%)

Steep (>15%)

Table 2. SDHM HSPF Impervious Parameter Values – Part I

IMPLND No. LSUR SLSUR NSUR RETSC

1 100 0.05 0.05 0.10

2 100 0.10 0.05 0.08

3 100 0.15 0.05 0.05

LSUR: Length of surface flow path (feet) for impervious area

SLSUR: Slope of surface flow path (feet/feet) for impervious area

NSUR: Surface roughness (Manning’s n) for impervious area

RETSC: Surface retention storage (inches) for impervious area

Table 3. SDHM HSPF Impervious Parameter Values – Part II

IMPLND No.

1

2

RETS SURS

0.00 0.00

0.00 0.00

3 0.00 0.00

RETSC: Initial surface retention storage (inches) for impervious area

SURS: Initial surface runoff (inches) for impervious area

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San Diego Hydrology Model 3.0 Interim Version User Manual – May 2016

APPENDIX C: ADDITIONAL GUIDANCE FOR USING

SDHM 3.0

Scope and Purpose: This appendix includes guidance and background information that are not incorporated into the SDHM 3.0 software, but which the user needs to know in order to use SDHM 3.0 for designing projects in the participating jurisdictions. The three main topic areas in this appendix are flagged in the main user manual text by specially formatted notes under the SDHM 3.0 elements or software features to which they are related:

Appendix C Topic Relevant Sections in User Manual

Infiltration Reduction Factor

Flow Duration Outlet Structures

(includes sizing of low-flow orifice and alternative configurations)

Infiltration, page 116; applicable when specifying characteristics of a facility (pond, vault, tank, some LID elements) if “yes” is selected as the Infiltration option.

Outlet Structure Configurations, pages 110-

115; applicable when specifying characteristics of a flow duration facility.

Drawdown (drain) time for flow duration facilities

Drawdown (Analysis screen), page 131.

This guidance was originally created by the stormwater programs of Alameda, Santa

Clara, and San Mateo counties. Please consult with the local municipal permitting agency for additional considerations.

Additional guidance and references are also discussed at the end of this appendix.

Infiltration Reduction Factor

The Western Washington Hydrology Model included this factor to reflect the requirement in the Stormwater Management Manual for Western Washington

(SMMWW), to incorporate a Correction Factor (CF) to determine long-term infiltration rates; the inverse of the CF is the Infiltration Reduction Factor in SDHM 3.0. The

SMMWW gives three methods for determining CF: 1) a table providing empirical correlations between long-term infiltration rates and USDA Soil Textural Classification;

2) ASTM gradation testing at full-scale infiltration facilities; or 3) In-situ infiltration tests, preferably using a Pilot Infiltration Test specified in an appendix of the SMMWW.

Application of a CF or safety factor attempts to account for clogging and the reduction in infiltration over time, which might apply to the bottom of a flow duration pond or the top layer of a bioretention facility. However, a safety factor is also used to account for uncertainties in the available estimate of in-situ infiltration rates. The SMMWW notes that its suggested CF values, which range from 2 to 4, “represent an average degree of long-term facility maintenance, TSS reduction through pretreatment, and site variability in the subsurface conditions”, and that increases or decreases to these factors should be considered for unusual situations.

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San Diego Hydrology Model 3.0 Interim Version User Manual – May 2016

Suggested safety factors in other texts and guidance generally range from 1 to 4. San

Diego stormwater permits may require some form of tracking and verification for treatment and hydromodification facilities. In addition, designers should not be overly conservative in selecting a very high safety factor, since this might lead to overcontrolled (lower) development flows and an increase risk of causing impacts from deposition or sedimentation in the receiving channels. In the absence of other guidance, it is suggested that the SDHM 3.0 Infiltration Reduction Factor not be less than 0.25 or greater than 0.5.

Note: San Diego stormwater programs may also restrict the use of infiltration for treatment purposes in certain conditions; since the flow duration facilities are also performing some treatment, designers should discuss treatment measure design with the applicable jurisdiction.

Flow Duration Outlet Structures: Practical Design

Considerations

Low-flow Orifice Sizing

The diameter of the low-flow (bottom) orifice is an important design parameter for flow duration facilities, since flows discharged through this outlet should be at or below the project threshold for controlled flows (Qcp). However maintenance and/or other practical considerations may dictate a practical limit to how small this orifice may be, which may be larger than the optimal theoretical diameter determined by Auto Pond. As an example, the SWMMWW specifies a minimum orifice diameter of 0.5 inches, for flow restrictor assemblies that are within protective enclosures that screen out large particles and also have 1-2 ft of sump below the orifice to allow for some sediment accumulation.

While the user can manually set a minimum size for the low-flow orifice, doing so before running Auto Pond is not recommended as this may impair the program’s ability to optimize the pond configuration. The following general approach is suggested for designing a pond when there is a small value for the low end of the flow matching range:

1. First estimate the minimum pond volume allowing Auto Pond to freely determine the diameter and placement of all orifices.

2. Then manually accept all of the pond settings except low-flow orifice diameter.

Set the low-flow orifice to the desired minimum size, after consulting the local municipal permitting agency.

3. Manually run the mitigated scenario as described on page 53 and review the

Analysis screen to check if the revised mitigated flow still passes the flowduration criteria for curve matching. If so, proceed with the pond design using the revised outlet.

4. If the revised design shows Fail scoring at one or more flow levels, excess flow durations may be reduced somewhat by reducing the depth of the pond which lowers the head above the orifice (SWMMWW recognizes a practical minimum

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San Diego Hydrology Model 3.0 Interim Version User Manual – May 2016 of 3 feet of live storage if pond shallowing is required at the minimum orifice size). As an alternative, further mitigation can be applied to the low-flow orifice flow by adding an additional infiltration measure downstream. This can be sized either approximately by estimating an average excess flow from the orifice or with the help of SDHM 3.0 by returning to the screen for the Pond characteristics and specifying a different Downstream Connection for the bottom orifice, which is then connected to an additional element. With this revision to the post project scenario, the Point of Compliance for the system would then be located at the downstream end of the additional low-flow mitigation.

Alternative Outlet Configurations

SDHM 3.0 has two default types of outlet configurations (multiple orifice or orifice plus weir notch) based on a standpipe riser structure detailed in the SMMWW. The entire standpipe is usually within a cylindrical enclosure or manhole to exclude trash and larger particles that could clog the outlet. The SMMWW notes that orifices can also be placed on a tee section or a vertical baffle within the same type of enclosure. An alternative configuration is a flat headwall with orifices and or notches, protected by racks or gratings. This may be fabricated from a large steel plate, similar in construction to the extended detention outlets specified in the Denver (Colorado) manual referenced below.

This alternative outlet can be simulated in the SDHM 3.0 as a very large diameter standpipe, where the width of the top notch is equal to the overflow width at the top of the plate between its supports.

Drawdown time and treatment/vector considerations

Flow duration control facilities are designed to detain stormwater on-site for an extended period of time. The drawdown time is a concern to designers in relation to three areas of design besides hydromodification management:

1. Standing water for extended periods provides a potential habitat in which mosquitoes can breed. San Diego stormwater programs work with their local mosquito abatement or vector control agencies to develop guidelines for stormwater facility design; these generally recommend that design detention times not exceed 96 hours. Provisions for access and inspection by vector control personnel are also required. Contact the local permitting agency for details of local vector control provisions, which apply to both treatment measures and flow duration facilities.

2. Stormwater that is detained also undergoes water quality treatment through settling and/or infiltration of pollutants. The focus of water quality management is reducing mean annual loads and typical concentrations of pollutants in receiving waters, so treatment design focuses on typical storms which contain the bulk of annual runoff volume. Stormwater permits and guidance documents describe the local design criteria for volume based treatment measures, which apply to a wider range of projects than the hydromodification management requirements. Recommended drawdown times for detention structures are typically at least 48 hours, but not to exceed 96 hours.

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3. Flood control design is intended to control peak flows for large sized storms (with expected recurrence intervals such as 25, 50 or 100 years). Flood control facilities typically require capture and detention of a specified volume of stormwater, which then is discharged out at flows that can be safely conveyed by downstream channels without undue risk of flooding. Flood control facilities usually are required to drain within 24 hours after the end of the design storm in order to be empty for the next storm event. This concern that flood control storage remain available for large events has led flood control agencies to require that any storage volume for water quality not be credited for flood control, a feature that is sometimes referred to as “dead storage”.

Although many factors affect the drawdown time, the suggestions below may help

SDHM 3.0 users in evaluating these other requirements. If flow duration control is required for a project site, it is recommended that the design process start with by using

SDHM 3.0 to obtain a preliminary design for the flow duration pond, vault, or tank.

Then check the performance of the facility for vector control concerns, and against treatment and/or flood control design criteria as appropriate. The latter are both based on the concept of a single empirical “design storm” which does not directly correspond to the flow duration approach using frequency analysis in a long-term simulation.

Stormwater treatment design requires the use of volume-based runoff coefficients, which although similar in concept to runoff coefficients used for flood control, are determined differently. Runoff coefficients used for flood control were derived for large storms with some conservatism built-in to estimates of peak flow rates and water surface elevations.

Runoff coefficients for stormwater treatment have been adjusted to reflect runoff from small storms where a greater percentage of the rainfall is held within the catchment.

Vector Management

If the 4-day (96-hour) drawdown is seldom or never exceeded over the simulation period, then likelihood of mosquito breeding in the facility is very low and the design for the pond, vault or tank does not need to be modified. If a 4-day drawdown time is exceeded then the system may need to be redesigned to reduce the drawdown time. The designer should consider additional reductions in impervious area and/or LID elements to help reduce the facility size.

To evaluate the frequency and distribution of larger events in more detail, use the

Hydrograph tool (page 132) to plot monthly peaks for several years at a time of the mitigated (development) scenario to get an idea of how often the discharge that corresponds to the 4-day drain time would be exceeded during warmer months, when mosquito development times are shortest.

Treatment Credit

Use the applicable design criteria to determine the minimum treatment volume for the development scenario. Look at the pond volume representing a 2-day drawdown in the

SDHM’s flow duration drawdown table. If this is larger than the calculated treatment volume, no further treatment design is needed. If the pond volume is less than the

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80% of the runoff from the site. Infiltration loss for each pond stage is shown in the

Stage-Storage-Discharge table, accessed by selecting the “Open Table” option at the bottom of the main Pond screen.

Flood Control Detention

Local flood control design criteria must be obtained from the appropriate agency, as well as any other policies or restrictions that may apply to drainage design. A single design storm event can be imported as a time series (page 139) and applied to the development scenario instead of the simulated precipitation record. If additional live storage is needed, it may be added to upper levels of the same facility or provided elsewhere on the site.

Guidance by Other Agencies

Some agencies in other parts of the United States have developed extensive guidance for design of stormwater management measures. Two manuals are discussed below that provide detailed discussions or examples that may be helpful to users of SDHM 3.0, although the suitability of these recommendations for San Diego conditions has not been verified. These documents can help provide context and ideas for users for SDHM 3.0, but adapting these ideas requires the exercise of professional engineering judgment.

Mention of the procedures and details in these documents does not imply any endorsement or guarantee that they will be appropriate for addressing the

Hydromodification Management Standards in San Diego jurisdictions.

Stormwater Management Manual for Western Washington (SMMWW) was prepared by the Washington Department of Ecology for implementation in 19 counties of Western

Washington. The latest (2014) edition in 5 volumes is on the Web at: https://fortress.wa.gov/ecy/publications/SummaryPages/1410055.html

Design recommendations from this manual were the basis for many features of the

WWHM that have been carried over into SDHM 3.0. Portions of Volume 3 (Hydrology) that may be of interest to project designers include:

 Pages 3-2 through 3-18 illustrate several types of roof downspout controls, simple pre-engineered designs for infiltrating and/or dispersing runoff from roof areas in order to reduce runoff volume and/or increase potential groundwater recharge.

 Pages 3-50 to 3-63 discuss outlet control structures, their maintenance and source equations modeled into WWHM and SDHM

 Pages 3-75 to 3-93 regarding Infiltration Reduction Factor

Urban Storm Drain Criteria Manual by the Denver Urban Drainage and Flood Control

District is on the Web at: http://www.udfcd.org/downloads/down_critmanual.htm

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Volume 3 covers design of stormwater treatment measures, including extended detention basins on pages S-66 through S-77 and structural details shown on pages SD-1 to SD-16.

Although these designs are not presented for hydromodification management control, the perforated plate design concept allows fine-tuning of drawdown times and is adaptable for use in flow duration facilities.

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APPENDIX D: BIORETENTION MODELING

METHODOLOGY

Water Movement Through The Soil Column

Water movement through the soil column is dependent on soil layer characteristics and saturation rates for different discharge conditions.

Consider a simple two-layered bioretention facility designed with two soil layers with different characteristics. As water enters the facility at the top, it infiltrates into the soil based on the modified Green Ampt equation (Equation 1). The water then moves through the top soil layer at the computed rate, determined by Darcy’s and Van

Genuchten’s equations. As the soil approaches field capacity (i.e., gravity head is greater than matric head), we can determine when water will begin to infiltrate into the second layer (lower layer) of the soil column. This occurs when the matric head is less than the gravity head in the first layer (top layer).

Since the two layers have different soil characteristics, water will move through the two layers at different rates. Once both layers have achieved field capacity then the layer that first becomes saturated is determined by which layer is more restrictive. This is determined by using Darcy’s equation to compute flux for each layer at the current level of saturation. The layer with the more restrictive flux is the layer that becomes saturated for that time step. The next time step the same comparison is made.

The rate and location of water discharging from the soil layer is determined by the discharge conditions selected by the user.

There are four possible combinations of discharge conditions:

1. There is no discharge from the subsurface layers (except for evapotranspiration).

This means that there is no underdrain and there is no infiltration into the native soil.

Which this discharge condition is unlikely, we still need to be able to model it.

2. There is an underdrain, but no native infiltration. Discharge from the underdrain is computed based on head conditions for the underdrain. The underdrain is configured to have an orifice. (It is possible for the orifice to be the same diameter as the underdrain.) With a maximum of three soil layers determining head conditions for the orifice is complicated. Each modeled layer must overcome matric head before flow through the underdrain can begin. Once matric head is overcome by gravity head for all of the layers then the underdrain begins to flow. The flow rate is determined based on the ability of the water to move through the soil layers and by the discharge from the orifice, whichever is smaller. Head conditions are determined by computing the saturation level of the lowest soil layer first. Once the lowest soil layer is saturated and flow begins then the gravity head is considered to be at the saturation level of the lowest soil layer. Once the lowest soil layer is saturated completely then the head will include the gravity head from the next soil layer above

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3. There is native infiltration but no underdrain. Discharge (infiltration) into the native soil is computed based a user entered infiltration rate in units of inches per hour.

Specific head conditions are not used in determining infiltration into the native soil.

Any impact due to head on the infiltration rate is considered to be part of the determination of the native soil infiltration rate. Because it is possible to have a maximum of three soil layers, each modeled layer must overcome matric head before infiltration to the native soil can begin. Once matric head is overcome by gravity head for all modeled layers then infiltration begins at a maximum rate determined either by the ability of the water to move through the soil layers or by the ability of the water to infiltrate into the native soil, whichever is limiting.

4. There is both an underdrain and native infiltration. Underdrain flow and native infiltration are computed as discussed above. However, there is one other limitation to consider. In the case where the flow through the soil layer is less than the sum of the discharge through the underdrain and the native infiltration then the flow through the soil layer becomes the limiting flow and must be divided between the native infiltration and the underdrain. This division is done based on the relative discharge rates of each.

Note that wetted surface area can be included in the discharge calculations by adding the infiltration through the wetted surface area to the lower soil layer and the upper surface layer individually. This is done by computing the portion of the wetted surface area that is part of the upper surface layer and computing the infiltration independently from the portion of the wetted surface area that is part of the lower soil layers.

Water Movement Equations

There are several equations used to determine water movement from the surface of the bioretention facility, through the soil layers, and into an underdrain or native infiltration.

The water movement process can be divided into three different zones:

1) Surface ponding and infiltration into the top soil layer (soil layer 1)

2) Percolation through the subsurface layers

3) Underdrain flow and native infiltration

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Surface ponding and infiltration into the top subsurface layer

The modified Green Ampt equation (Equation 1) controls the infiltration rate into the top soil layer:

f

K

1

(

)(

d

F

)

(Equation 1; Ref: Rossman, 2009)

f = soil surface infiltration rate (cm/hr)

 soil porosity of top soil layer

 soil moisture

 suction content of top head at the wetting soil front layer

(cm)

F= soil moisture content of the top soil layer (cm)

d= surface ponding depth (cm)

K= hydraulic conductivity based on saturation of top soil layer (cm/hr)

K (relative hydraulic conductivity) can be computed using the following Van Genuchten approximation equation:

(Equation 2; Ref:

Blum et al, 2001)

A few issues arise when dealing with multiple subsurface soil layers. The K value used in Equation 1 must be computed from the top soil layer. Infiltration into the upper soil layer must not exceed the lesser of the maximum percolation rates for each of the soil layers. Finally, the rate of percolation of the top layer may be reduced because the layer or layers beneath the top layer cannot accept the percolation flux because of existing saturation levels.

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Percolation through the subsurface layers

Water storage and movement through the three subsurface layers will be computed using

Darcy’s equation as shown below:

q

 

K

h

z

(Equation 3)

Where:

q = Darcy flux (cm/hr)

K = hydraulic conductivity of the porous medium (cm/hr)

h = total hydraulic head (cm)

z = elevation (cm)

The total head, h, is the sum of the matric head,

, and the gravity head, z:

h

z

.

(Equation 4)

Substituting for h yields:

q

 

K d

(

z

)

.

dz

(Equation 5)

Hydraulic conductivity and matric head vary with soil moisture content. These values can be computed by solving the Van Genuchten’s equation (Equation 6) for both values.

Note that



 0 when the soil is saturated.

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(Equation 6; Ref:

Blum et al, 2001)

Effective saturation (SE) can be computed using the following Van Genuchten equation:

(Equation 7; Ref:

Blum et al, 2001)

Ignoring z (elevation head) results in h = hm (matric head).

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Evapotranspiration from the Soil Column

Evapotranspiration is an important component of the bioretention facility’s hydrologic processes. Evapotranspiration removes water from bioretention surface ponding and the soil column during non-storm periods. The routine will satisfy potential evapotranspiration (PET) demands in the same sequence as implemented in HSPF:

1. Water available from vegetation interception storage

2. Water available from surface ponding

3. Water available from the bioretention soil layers (top layer first)

Water will be removed from vegetation interception storage and surface ponding and the bioretention soil layers (starting at the top layer) down to the rooting depth at the potential rate. Water is taken from the soil layers below the rooting depth based on a percentage factor to be determined. Without this factor there will be no way to remove water from below the rooting depth once it becomes completely saturated.

References

Blum, V.S., S. Israel, and S.P. Larson. 2001. Adapting MODFLOW to Simulate Water

Movement in the Unsaturated Zone. MODFLOW 2001 and Other Modeling Odysseys,

International Groundwater Modeling Center (IGWMC), Colorado School of Mines,

Golden, Colorado, September 11-14, 2001. In MODFLOW 2001 and Other Modeling

Odysseys, Proceedings. pp.60-65.

Rossman, L.A. 2009. Modeling Low Impact Development Alternatives with SWMM.

Presented at CHI International Stormwater and Urban Water Systems Conference,

Toronto, Ontario, Canada, February 20, 2009.

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APPENDIX E: SDHM 3.0 COMPLEX MODEL EXAMPLE

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This is an example project that has also been used in a San Diego SWMM training class by Rick Engineering.

In this particular example the project site is 1.33 acres in total area. The exact project site location is unknown, but the precipitation record used for the modeling was the Poway rain gage.

The soil type was specified to be D soil. The Predeveloped land use was labeled “massgraded lot”. We will assume that this is the same as the “Dirt” land cover category in

SDHM 3.0. The land slope is 3%. In SDHM 3.0 this slope falls within the Flat slope category of 0-5%. Therefore, the Predeveloped area will consist of 1.33 acres of D, Dirt,

Flat.

The proposed development consists of buildings (roofs), courtyard, and parking, plus landscaped graded slopes and a biofiltration basin. The biofiltration basin will provide hydromod mitigation for the development.

Table 1 shows a breakdown of the proposed development area:

Table 1. Proposed Development

DMA  Description  Total (ac)  % Impervious  Imp (ac)  Perv (ac) 

1a 

1b  building, courtyard, parking  graded slope (steep) 

1c  biofiltration basin 

Total    

1.08 

0.19 

0.06 

1.33 

73% 

0% 

0% 

  

0.79 

0.00 

0.00 

0.79 

0.29 

0.19 

0.06 

0.54 

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Note that most of the graded slope is the side slope to the biofiltration basin. In SDHM

3.0 we can include it as the side slopes in the area represented by the biofiltration basin

(SDHM bioretention element). Therefore, we can adjust the proposed development area breakdown, as shown in Table 2.

Table 2. Proposed Development Adjusted

DMA  Description  Total (ac)  % Impervious  Imp (ac)  Perv (ac) 

1a  building, courtyard, parking 

1b*  graded slope (steep) 

1c  biofiltration basin bottom 

1c**  biofiltration basin sides 

1.08 

0.05 

0.06 

0.14 

73% 

0% 

0% 

0% 

0.79 

0.00 

0.00 

0.00 

0.29 

0.05 

0.06 

0.14 

Total     1.33     0.79  0.54 

* Slopes not draining directly to biofiltration basin.

** Slopes draining directly to biofiltration basin.

Based on the information in Table 2, the Mitigated scenario developed area will be input as shown in Table 3.

Table 3. SDHM 3.0 Land Input

Description  Area (ac) 

D, UrbNoIrr, Flat  0.29 

D, UrbNoIrr, Steep 

Bioretention Total Area 

Impervous, Flat 

0.05 

0.20 

0.79 

Total  1.33 

Note: the pervious areas are assumed to be urban vegetation with no irrigation

(UrbNoIrr).

For this site the hydromod low threshold is 0.1Q2 (10% of the 2-year peak flow), as calculated by SDHM 3.0.

With the above information we can now set up our project in SDHM 3.0.

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We open SDHM 3.0 and using our mouse we select our project site on the San Diego

County map by clicking on the project location. We can use the map controls in the lower left corner to enlarge or shrink the map. We select the appropriate nearby rain gage

(Poway). The rain gage and precipitation multiplication factor (1.000) are shown in the upper right side of the screen.

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We go to View, Options and input the lower and upper thresholds based on the following

USGS regional regression equations:

Q2 = 3.60*(A^0.672)*(P^0.753)

Q10 = 6.56*(A^0.783)*(P^1.07)

Where A = drainage area (sq. miles)

P = mean annual precipitation (inches)

The lower threshold equals 0.10Q2.

The drainage area is 1.33 acres (0.002078125 square miles). The mean annual precipitation for Poway is 11.6 inches.

Q2 = 0.359cfs

Q10 = 0.717 cfs

The lower threshold is 0.10Q2 = 0.0359 cfs.

The upper threshold is Q10 = 0.717 cfs.

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Next we go to the General Project Information screen and for the Predeveloped scenario select the land use basin element (default name: Basin 1).

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In the Subbasin Name box the default name of Basin 1 can be changed to a different name of the user’s choosing. In this example we change the name to DMA 1.

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For DMA 1 we input 1.33 acres of D, Dirt, Flat.

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We need to connect DMA 1 to the Point of Compliance (POC). To do this we right click on the DMA 1 element and then select “Connect to Point of Compliance”.

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We use the default settings of connecting surface flow and interflow to POC 1.

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We can now run HSPF to calculate the stormwater runoff for the Predeveloped scenario by clicking on the “Run Scenario” button.

We are finished with the Predeveloped scenario. Next is the Mitigated scenario.

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We select the Mitigated scenario, add a land use basin element, and rename it DMA 1.

Note: there is no requirement that the Predeveloped and mitigated scenario elements have the same name; we only change the name for our convenience.

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We input the DMA 1 developed area from Table 3, except for the area associated with the bioretention element (the bioretention element is rained on directly and its area should

NOT be included in the land use element area) :

Table 3. SDHM 3.0 Land Input

Description 

D, UrbNoIrr, Flat 

Area (ac) 

0.29 

D, UrbNoIrr, Steep 

Bioretention Total Area 

Impervous, Flat 

0.05 

0.20 

0.79 

Total  1.33 

Note: the pervious areas are assumed to be urban vegetation with no irrigation

(UrbNoIrr).

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We add a bioretention element (default name: Bio Swale 1).

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We rename it Biofiltration.

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We right click on DMA 1 to connect it to another element (in this example we are connecting it to Biofiltration).

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We use our mouse to move the green “Connect To” box to the grid square in which the

Biofiltration element is located and then click on the Biofiltration element.

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We then get a pop-up box with the basin to conveyance connection options. We select the default option of connecting the DMA 1 element surface flow and interflow to the

Biofiltration element and click OK.

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Next we want to connect the outflow from the Biofiltration element to the Point of

Compliance (POC 1) by first right clicking on the Biofiltration element and then selecting

“Connect to Point of Compliance”.

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We are connecting Outlet 1 (all surface discharge including flow through the underdrain) to POC 1 (the same POC as in the Predeveloped scenario).

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We are now ready to input information for the biofiltration basin.

The biofiltration basin has a bottom footprint of 0.06 acres. That is equal to 2613.6 square feet. We don’t know the actual dimensions of the bottom footprint, but from the drawing it appears to be long and narrow. If we assume an average bottom width of 15 feet then the bottom length is approximately 174 feet. Those are the dimensions that we will use in the model.

On the surface there is one foot of ponding prior to overflowing into a riser. There is an additional one foot of freeboard above the riser height.

The biofiltration basin has two soil layers. The top layer (Layer 1) is 2 feet thick. Its composition is not specified. For the purposes of this example we will assume that it is an amended soil with an infiltration rate of 15 inches per hour (check with the local municipal permitting agency to find out what soil mix is required for the top layer). The lower layer (Layer 2) is gravel and is one foot thick.

At the bottom of the gravel layer is an underdrain with a perforated pipe diameter of 4 inches (0.33 feet). It drains to an outlet control structure that contains a 0.625-inch diameter orifice. This orifice will control all flow from the two soil layers. It will not control the flow through the surface riser.

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We select side slopes of 7 feet horizontal to 1 foot vertical to produce a total biofiltration basin surface area of 0.20 acres.

To check the biofiltration basin bottom and top surface areas we click on the down arrow to the right of the “Show Swale Table” Open Table label.

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The top row of the Sub-surface Table (Stage 0.00 feet) lists the footprint surface area for the bottom of the biofiltration basin. Based on a width of 15 feet and length of 174 feet the bottom surface area is 0.059917 (or 0.60) acres.

The bottom row of the Surface Table (Stage 5.00 feet) lists the surface area at the top of the freeboard. This stage of 5.00 feet is the sum of Layer 1 depth of 2 feet, Layer 2 depth of 1 foot, riser height of 1 foot, and freeboard height of 1 foot. The surface area for this maximum stage value is 0.199403 (or 0.20) acres.

Both the bottom area and top areas are correct. The biofiltration basin looks like:

Our Calculations

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The side walls are vertical for the two soil layers and then sloping for the ponding area.

We check the “Sidewall Invert Location” box to specify vertical side walls for the two soil layers.

We also check the “Use simple swale” box to speed up the HSPF calculations. This slightly under estimates the effectiveness of the biofiltration basin, but makes the model run about 10 times faster.

We can now run the Mitigated scenario.

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After clicking on Run Scenario the percent of total runoff volume through the underdrain for the entire simulation period (1963-2004) is reported on the biofiltration element screen. For this biofiltration basin approximately 86 percent of the total runoff volume when through the two soil layers and was discharged by the underdrain. The remaining

14 percent overtopped the riser and was discharged without going through the underdrain control orifice.

The percent through the underdrain does not tell us whether or not the project meets the hydromod flow duration requirements, but it gives us an idea of how much of the runoff is controlled.

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We go to the Analysis window by clicking on the third icon from the right at the top of the screen to check the hydromod flow duration results.

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We click on the POC 1 tab and the flow duration results are plotted and summarized in a table.

The plotted results show the red line (Mitigated flows) to the right of the blue line

(Predeveloped flows) for the range of flows from 0.1Q2 (0.0359 cfs) to Q10 (0.717 cfs).

The flow duration table on the right shows that for each flow level the percentage

(Mitigated/Predeveloped) exceeds the maximum allowed 110% and fails for that level. It only takes one failure for the hydromod standard to not be met.

The results demonstrate that the biofiltration basin is not large enough.

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The biofiltration basin bottom footprint needs to be increased while the total biofiltration area remains 0.20 acres. This can be accomplished by increasing the bottom width from

15 to 30 feet and making the side slopes steeper (4 to 1 instead of 7 to 1).

The basin bottom area is now 0.12 acres, while the top surface area still remains at 0.20 acres.

The model is then rerun and the durations re-analyzed.

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The new results show that the lower levels fail, but the higher levels pass.

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The underdrain orifice diameter is increased from 0.625 inches to 0.90 inches. This allows more water to discharge through the biofiltration soil column (increasing the percentage filtered to 97%). More water is discharged below the lower hydromod threshold of 0.0359 cfs than previously and less water ponds on the surface and discharges through the riser. The biofiltration basin now passes at all flow duration levels. This is all done while staying within the 0.20 acre footprint.

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Above are the final biofiltration basin dimensions.

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The Weibull-calculated flow frequency results can be observed by going to the Analysis window, selecting Flow Frequency, and then clicking on the POC 1 tab.

The flow frequency table shows that the Mitigated frequency values are smaller than the corresponding Predevelopment frequency values.

The Report file can now be generated and the project saved and submitted to the local municipal permitting agency.

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