The PERLND block within the HSPF input file contains parameters

The PERLND block within the HSPF input file contains parameters
Sacramento Stormwater Quality Partnership
Hydromodification Management Plan
The PERLND block within the HSPF input file contains parameters that affect the vertical and lateral
movement of water moisture through a pervious land segment. Figure 6-1 shows a schematic view of
the PERLND water budget terms and key HSPF parameters. The schematic illustrates the movement of
water among interception storage, upper zone storage, lower zone storage, groundwater storage, and
deep/inactive groundwater storage. The schematic also illustrates flux terms, such as overland flow and
interflow.
The algorithms that control the movement among these storage layers are described thoroughly in the
HSPF Users Manual (EPA, 2001).
Table 6-2 contains brief descriptions of the HSPF parameters used to characterize pervious land
surfaces, along with the HSPF input parameters used for the Sacramento area. The parameters that
often affect stormwater runoff most (INFILT, LZSN) are listed in the table below. These highlighted
parameters were the focus of our investigation of the range and variation among statewide HSPF
studies (detailed after Table 6-2) and our testing of prospective parameters. The descriptions and
parameter ranges in the table were adapted from EPA BASINS Technical Note 6 – Estimating Hydrologic
and Hydraulic Parameters for HSPF, which is available from the EPA web site:
http://www.epa.gov/waterscience/basins/bsnsdocs.html
Table 6-2 Summary of HSPF Input Parameters for Sacramento County
PWAT-PARM1 – Flags
Parameter
Units
Description
CSNOFG
None
Flag to use snow simulation data
Value or Range
0
RTOPFG
None
Flag to select overland flow routing method. Set TOPFG=1; This method has
been subjected to more widespread application.
1
UZFG
None
Flag to select upper zone inflow computation method Set UZFG=1; This method
has been subjected to more widespread application.
1
VCSFG
None
Flag to select constant or monthly-variable interception storage capacity,
CEPSC. Monthly value can be varied to represent seasonal changes in foliage
cover
1
VUZFG
None
Flag to select constant or monthly-variable upper zone nominal soil moisture
storage, UZSN.
0
VMNFG
None
Flag to select constant or monthly-variable Mannings n for overland flow plane,
NSUR. .
0
VIFWFG
None
Flag to select constant or monthly-variable interflow inflow parameter, INTFW.
Monthly values are not often used.
0
VIRCFG
None
Flag to select constant or monthly varied interflow recession parameter, IRC.
Monthly values are not often used.
0
VLEFG
None
Flag to select constant or monthly varied lower zone evapotranspiration (ET)
parameter, LZETP.
0
PWAT-PARM2
Parameter
Units
Description
FOREST
None
Fraction of land covered by forest that will continue to transpire in winter (i.e.
coniferous). This is only relevant if snow is being considered (i.e., CSNOFG=1 in
PWATER-PARM1).
Range of Values
0
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Sacramento Stormwater Quality Partnership
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LZSN
Inches
Lower zone nominal soil moisture storage. This parameter affects the
proportion of water going to surface runoff, interflow and active groundwater
4.2 to 5.5
INFILT
in/hr
INFILT is the parameter that controls the overall division of the available
moisture from precipitation (after interception) into surface runoff. This is
NOT equivalent to a field-measured infiltration rate.
LSUR
Feet
Length of assumed overland flow plane. LSUR approximates the average length
of travel for water to reach any drainage path such as streams, swales, ditches,
etc.
120 to 200
SLSUR
ft/ft
Average slope of assumed overland flow path. Average SLSUR values for each
land use being simulated can often be estimated directly with GIS capabilities.
0.05 to 0.10
KVARY
1/inches
Groundwater recession flow parameter used to describe non-linear
groundwater recession rate
AGWRC
None
Groundwater recession rate, or ratio of current groundwater discharge to that
from 24 hours earlier
Parameter
Units
Description
PETMAX
Deg F
Temperature below which ET will be reduced to 50% of that in the input time
series
35
PETMIN
Deg F
Temperature at and below which ET will be zero. PETMIN represents the
temperature threshold where plant transpiration is effectively suspended
30
INFEXP
None
Exponent that determines how much a deviation from nominal lower zone
storage affects the infiltration rate. This parameter is commonly set to a value
of 2.
2
INFILD
None
Ratio of maximum and mean soil infiltration capacities. This parameter is
commonly set to a value of 2.
2
DEEPFR
None
The fraction of infiltrating water that is lost to deep/inactive aquifers with the
remaining fraction assigned to active groundwater storage that contributes
base flow to the stream.
0.4
BASETP
None
ET by riparian vegetation as active groundwater enters streambed; specified as
a fraction of potential ET, which is fulfilled only as outflow exists.
0.05
AGEWTP
None
Fraction of PERLND that is subject to direct evaporation from groundwater
storage, e.g. wetlands or marsh areas.
0.05
Parameter
Units
Description
CEPSC
inches
Amount of rainfall, in inches, which is retained by vegetation, never reaches the
land surface, and is eventually evaporated.
0.08
UZSN
inches
Nominal upper zone soil moisture storage. UZSN is related to land surface
characteristics, topography, and LZSN.
0.60
NSUR
None
Manning’s friction coefficient, n, for overland flow plane.
0.20
INTFW
None
Coefficient that determines the amount of water that enters the ground from
surface detention storage and becomes interflow
1.5
IRC
None
Interflow recession coefficient IRC is the ratio of the current daily interflow
discharge to the interflow discharge on the previous day.
0.7
LZETP
None
Index to lower zone evapotranspiration LZETP affects ET from the lower zone,
which represents the primary soil moisture storage and root zone of the soil
profile.
0.5
0.025 to 0.11
3
0.92
PWAT-PARM3
Range of Values
PWAT-PARM4
Range of Values
The parameter descriptions were obtained from the EPA BASINS Technical Note 6.
96
Notes: Figure adapted from the HSPF User’s Manual (EPA, 2001): http://www.epa.gov/waterscience/BASINS/b3docs/HSPF.pdf Sacramento Stormwater Quality Partnership Hydromodification Management Plan HSPF PERLND Water Moisture Schematic Figure 6‐1 Sacramento Stormwater Quality Partnership
Hydromodification Management Plan
The Consultant team collected and examined published studies that used HSPF to perform hydrologic
modeling. Whenever possible, the HSPF input files that were used in these studies were also collected
and reviewed. The following models and study areas were reviewed:
EPA BASINS Technical Note 6 - The EPA publication (July 2000) is a very useful guide that describes key
HSPF parameters and suggests initial values. This technical note provides BASINS users with guidance in
how to estimate the input parameters in the ATEMP, SNOW, PWATER, IWATER, HYDR, and ADCALC
portions of the HSPF model.
Calabazas Creek - In 1997, Aqua Terra Consultants used HSPF to study multipurpose design of detention
facilities in Calabazas Creek watershed for the Santa Clara Valley Water District.
Santa Monica Bay Watershed - The Southern California Coastal Water Research Project (SCCWRP) and
Tetra Tech created HSPF models to simulate hydrologic processes and pollutant loadings to Santa
Monica Bay. The specific parameter values were selected by calibrating an HSPF model to flow
monitoring data in the Santa Monica Bay watershed, specifically on Malibu Creek. The values represent
a composite of the various upstream soils and land uses.
Calleguas Creek - This project was a pilot study to evaluate the use of HSPF as a management tool for
comprehensive watershed assessment within the climatic, physiographic, and topographic conditions of
Ventura County. The Calleguas Creek model, developed by Aqua Terra Consultants, simulates watershed
hydrology using a combination of six different land use categories, topographic data and soils data.
Bay Area Hydrology Model (BAHM) - The Bay Area Hydrology Model (BAHM) uses a graphical user
interface and pre-selected HSPF parameters to simulate stormwater runoff from project sites and size
stormwater control facilities to mitigate the impacts of land use changes. BAHM includes HSPF
parameters for common soil and land use combinations.
6.2.7 Modeling the Hydraulic Response of BMPs
The consultant team has constructed representations of each LID BMP to be included in the BMP Sizing
Calculator in HSPF. For example, a stormwater planter with underdrain is represented with separate
surface ponding, growing medium, storage layers, an overflow relief outlet, a restricted underdrain
outlet (as appropriate), and transmissivity of underlying soils. The configuration of these BMP elements
and associated hydraulic characteristics can be varied to determine the configuration that provides the
best performance in the least amount of space. The HSPF method for representing storage facilities is
called an F-TABLE, and is described in Section 6.3.4 of this chapter.
6.2.8 Modeling Approach Limitations
For LID facility design, the maximum contributing drainage area to each facility is generally limited to a
maximum of 5 acres (most contributing drainage areas to LID facilities will be smaller than 5 acres).
However, multiple LID facilities can be incorporated into a given project site, potentially resulting in full
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Sacramento Stormwater Quality Partnership
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hydromodification compliance using LID approaches for a large project site. For project sites
incorporating the use of extended detention facilities for hydromodification flow control, the maximum
contributing drainage area to a detention basin is 200 acres. Projects with contributing drainage areas
(to an extended detention basin) greater than 200 acres shall develop a separate continuous simulation
runoff model to quantify the hydromodification flow control effects.
6.3 Hydromodification Control – LID BMPs
6.3.1 LID Sizing Factor Development
To compute sizing factors for each of the LID BMPs included in the sizing calculator, the impervious
runoff time series is routed through the BMP to develop a post-project “mitigated” runoff time series.
Each BMP mitigates post-project runoff by providing infiltration and/or reduction of discharge rates to
the drainage system. The post-project mitigated time series is then compared to the pre-project runoff
time series to assess BMP performance. The BMP size (for example, surface area) is varied over the
course of multiple model iterations until a size is identified that adequately matches post-project to preproject runoff. The runoff comparison is performed both for peak rates and durations. The following
standards, which are consistent with flow rate and duration control standards presented elsewhere in
this HMP, are applied to assess BMP performance:
•
Flow duration control - For flow rates ranging from 25% or 45% (as described in Chapter 5) of
the pre-project 2-year recurrence interval event (0.25Q2 or 0.45Q2) to the pre-project 10-year
runoff event (Q10), the post-project discharge rates and durations shall not deviate above the
pre-project rates and durations by more than 10% over and more than 10% of the length of the
flow duration curve. The specific lower flow threshold will depend on results from the channel
susceptibility assessment.
•
Peak flow control - For flow rates ranging from the lower flow threshold to Q5, the post-project
peak flows shall not exceed pre-project peak flows. For flow rates from Q5 to Q10, post-project
peak flows may exceed pre-project flows by up to 10% for a 1-year frequency interval. For
example, post-project flows could exceed pre-project flows by up to 10% for the interval from
Q9 to Q10 or from Q5.5 to Q6.5, but not from Q8 to Q10.
6.3.2 Scenarios Modeled
The HSPF software program is used to characterize pre-project runoff scenarios corresponding to the
previously discussed rainfall gauges, four (4) soil types, and two (2) ranges of slopes.
Table 6-3 summarizes the pre-project pervious area runoff scenarios included in the analyses.
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Table 6-3 Pervious Area Runoff Scenarios
Rainfall Gauge
Soil Type
Natomas
A
Natomas
A
Natomas
A
Natomas
A
Natomas
B
Natomas
B
Natomas
B
Natomas
B
Natomas
C
Natomas
C
Natomas
C
Natomas
C
Natomas
D
Natomas
D
Natomas
D
Natomas
D
Elk Grove
A
Elk Grove
A
Elk Grove
A
Elk Grove
A
Elk Grove
B
Elk Grove
B
Elk Grove
B
Elk Grove
B
Elk Grove
C
Elk Grove
C
Elk Grove
C
Elk Grove
C
Elk Grove
D
Elk Grove
D
Elk Grove
D
Elk Grove
D
Rancho Cordova
A
Rancho Cordova
A
Rancho Cordova
A
Rancho Cordova
A
Rancho Cordova
B
Rancho Cordova
B
Rancho Cordova
B
Rancho Cordova
B
Rancho Cordova
C
Rancho Cordova
C
Rancho Cordova
C
Watershed Slope
Non-Steep
Non-Steep
Steep
Steep
Non-Steep
Non-Steep
Steep
Steep
Non-Steep
Non-Steep
Steep
Steep
Non-Steep
Non-Steep
Steep
Steep
Non-Steep
Non-Steep
Steep
Steep
Non-Steep
Non-Steep
Steep
Steep
Non-Steep
Non-Steep
Steep
Steep
Non-Steep
Non-Steep
Steep
Steep
Non-Steep
Non-Steep
Steep
Steep
Non-Steep
Non-Steep
Steep
Steep
Non-Steep
Non-Steep
Steep
Land Cover
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
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Sacramento Stormwater Quality Partnership
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Rancho Cordova
Rancho Cordova
Rancho Cordova
Rancho Cordova
Rancho Cordova
Orangevale
Orangevale
Orangevale
Orangevale
Orangevale
Orangevale
Orangevale
Orangevale
Orangevale
Orangevale
Orangevale
Orangevale
Orangevale
Orangevale
Orangevale
Orangevale
C
D
D
D
D
A
A
A
A
B
B
B
B
C
C
C
C
D
D
D
D
Steep
Non-Steep
Non-Steep
Steep
Steep
Non-Steep
Non-Steep
Steep
Steep
Non-Steep
Non-Steep
Steep
Steep
Non-Steep
Non-Steep
Steep
Steep
Non-Steep
Non-Steep
Steep
Steep
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
Grassland
Agriculture
6.3.3 General BMP Characteristics
This section describes the technical approach used to represent BMPs in the HSPF model. The discussion
focuses on the key physical aspects of BMP performance (i.e., how a BMP routes water through its
different layers) and how these physical processes are represented in HSPF. This section also describes
key hydraulic and modeling assumptions and how these assumptions impact both the modeling process
and the accuracy of the results across the full range of flow conditions.
The flow control BMP designs selected for the sizing calculator all include some combination of
detention storage and water quality treatment media. For example, the stormwater planter BMP
includes (in order of vertical routing) a surface ponding layer, a growing medium layer, and a storage
layer. Each layer has its configuration, porosity, volume, and hydraulic conditions that influence the rate
of flow to the next layer.
HSPF uses stage-storage-discharge tables to represent the hydraulic behavior of devices that detain and
discharge water (e.g., all of the LID BMPs to be included in the sizing calculator). The stage represents
depth of water in the facility, the storage represents the volume of water stored in the facility for that
stage, and the discharge is the calculated outflow for that stage. Outflow may be via an orifice,
infiltration, evaporation, or any other mechanism for which a relationship to stage or storage can be
defined.
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The following general hydraulic assumptions are applied to all modeled BMPs:
•
•
•
•
•
•
Inflow is uniformly distributed over the surface area of the BMP (i.e. level-pool ponding).
Infiltration and soil water movement is a 1-dimensional flux in the vertical direction (neglecting
lateral flows is a conservative assumption).
Soil moisture within a homogeneous growing medium layer is assumed to be evenly distributed
throughout the growing medium layer both vertically and horizontally. This assumes an
engineered BMP will be free of macropores.
Percolation from the growing medium layer to the storage layer is computed based on
unsaturated or saturated hydraulic equations, based on the amount of moisture contained in
the growing medium during each model time step.
Water flows out the bottom of the BMP into the surrounding soil at the rate of saturated
hydraulic conductivity.
The sandy loam soil used for the growing medium has an effective porosity of 0.412, based on
Table 5.3.2 in the Handbook of Hydrology (Maidment, 1993). A sensitivity analyses conducted to
determine the effect of porosity on BMP performance determined that porosity has little
influence on the required sizing factor.
6.3.4 Stormwater Planter BMP HSPF Representation
The Stormwater Planter is one of the treatment control measures included in the Stormwater Quality
Design Manual (Design Manual), (Partnership, 2007). This BMP option has various configurations such as
stormwater planter with open bottom, stormwater planter constructed with underdrains stormwater
planter configured in a concrete structure (or with full lining) with underdrains, and a stormwater
planter in series with an underground vault. A stormwater planter option without an underdrain is also
commonly known as a bioretention facility which allows in-situ infiltration. Sections 6.3.6, 6.3.7, 6.3.8
and 6.3.9 provide detailed discussion of the stormwater planter configurations.
An example stormwater planter BMP is modeled using two FTABLEs. The first FTABLE represents the
surface ponding layer, growing medium layer, and overflow outlet. The second FTABLE represents the
storage layer, exfiltration to surrounding soils, and underdrain outflow, if applicable. Percolation from
the growing medium to the storage layer is modeled as an outflow from the first FTABLE and inflow to
the second FTABLE.
6.3.4.1 FTABLE 1: Upper Growing medium layer, Ponding Storage and Overflow Outlet
Storm water routed from impervious surfaces first enters the upper layer of a stormwater planter area,
represented by FTABLE 1 (Figure 6-2). The HSPF model assumes that all inflow will infiltrate if the layer is
not saturated. This is a reasonable assumption based on the anticipated range of inflows. The growing
medium layer is represented by depths from 0 to 1.5 feet. The volume of storage at 1.5 ft is equal to the
storage within the growing medium layer at saturation. Above this depth water is stored in the ponding
reservoir.
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Water contained in the upper growing medium layer is stored as soil moisture. Although there are
depths indicated in the first column of the FTABLE, the soil water is considered to be evenly distributed
throughout the growing medium layer (e.g. a soil depth of 0.5 feet in FTABLE 1 corresponds to one-third
saturated, not water filling the bottom 0.5 feet of the upper growing medium layer). Above 1.5 ft, water
ponds on the planter surface, and the FTABLE 1 depth column corresponds to the actual water surface.
The fourth column in FTABLE 1 lists the rate of soil water percolation out the bottom of the upper
growing medium layer and into the lower gravel layer. This column is calculated using Darcy’s Law and
the van Genuchten relations. Percolation does not occur unless the soil water content exceeds the
holding capacity of the soil (i.e. the gravitational head is greater than the suction or matric head within
the soil pores). The percolation rate calculations assume a free surface at the interface with the lower
layer. However, the percolation rate is limited if the lower layer reaches capacity and becomes
saturated. In this case the percolation rate through the upper layer is limited to the percolation rate
through the lower layer, which in itself is limited by the total outflow from the lower layer through the
underdrain orifice and percolation to the surrounding soil. Thus, the percolation rate through the upper
layer is limited to underdrain outflow rate plus a small amount of percolation to the surrounding soil
when the bioretention facility reaches capacity.
The fifth column in the FTABLE is the outflow through the overflow pipe, which is calculated using a weir
equation. Outflow through the overflow pipe does not occur until the depth of storage in the ponding
reservoir is above the pipe inlet.
6.3.4.2 FTABLE 2: Lower Gravel Layer, Percolation to Surrounding Soils, Underdrain Outlet
The second FTABLE represents the lower gravel layer and the underdrain. Percolation outflow from the
first FTABLE is routed as inflow to the second FTABLE (Figure 6-3). This FTABLE represents the lower
gravel layer, which has a depth of 1.5 ft. Water is stored as volumetric water content with a maximum
storage limited to saturation of the gravel medium. The percolation rate out the bottom of the lower
layer is limited by the hydraulic conductivity of the surrounding soil, which is a conservative assumption
(percolation will actually be faster when native soils will be unsaturated).
When an underdrain is included in the configuration, the ‘Q Outlet’ column is included in the FTABLE for
the outflow rate. This rate is calculated using the orifice equation so that the underdrain flow will match
lower flow control rate (0.25Q2 or 0.45Q2) when the lower gravel layer is fully saturated.
6.3.5 Iterative BMP Sizing Steps
Once the geometric characteristics of the BMP are represented in FTABLEs, as described above in
Section 6.3.4, the sizing factors are computed using an iterative process involving multiple HSPF
simulations and statistical analyses. The process involves varying the surface area until peak flow and
flow duration control meet the BMP performance criteria set forth in Section 6.3.1.
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The ability of the BMP to achieve peak flow and flow duration control is evaluated by generating and
comparing partial duration series statistics and flow duration statistics for (a) the pre-project runoff
from a pervious land surface and (b) the post-project outflow from the BMP serving an equivalent area
that has been converted to an impervious surface. A 24-hour inter-event period (as defined by 24 hours
with BMP outflow less than 0.05 cfs/ac) is used to separate storm events in the partial duration series.
The footprint of the BMP is included in the calculations to preserve equivalence between the pre-project
and post-project analysis (i.e. Pre-project Area = Impervious Area + BMP Area). The HSPF model allows
for direct rainfall to the BMP surface area.
BMP surface area is increased incrementally until flow and duration control are achieved. Flow and
duration control are considered to be achieved when the mitigated post-project peak flows and flow
durations are less than or equal to the pre-project flows and durations, as defined by the performance
criteria in Section 6.3.1.
6.3.6 LID Option – Stormwater Planter with Underdrain
The Permittee’s Design Manual (Partnership, 2007) contains information regarding the design of
stormwater planter facilities. Since the manual criteria were developed for water quality treatment
purposes only, some modification to the criteria are required to make the BMP option an integrated
solution for hydromodification flow control as well.
The stormwater planter facility consists of a surface ponding layer, a growing medium layer, and a below
ground storage layer (Figure 6-4). The stormwater planter with underdrain BMP captures water in the
ponding layer, filters it through a growing medium that consists of soil and plant roots, percolates water
from the growing medium into a storage layer, and then slowly discharges treated stormwater via
exfiltration to surrounding native soils and regulated discharge through an underdrain pipe to the local
storm water drainage system.
For the LID sizing factor analysis, the following stormwater planter layers are modeled:
•
•
•
Ponding layer: depth of active storage, depth of freeboard above overflow relief
Growing medium: depth of soil at specified porosity
Storage layer: depth of gravel at specified porosity
As described above, the plan surface area of the BMP is iteratively sized until the BMP controls outflows
and durations to levels that are less than or equal to pre-project conditions across flow rates ranging
from the lower flow control limit (0.25Q2 or 0.45Q2) to the upper flow control limit (Q10). The sizes of the
ponding layer and storage layer are converted into volumes, so that the project designer can flexibly
configure the ponding layer and storage layer based on the site constraints. For example, the design
engineer could configure the ponding layer with half the depth but twice the plan surface area called for
by the sizing factor if this fits the project site. Additionally, the designer could use commerciallyavailable storage vessels to meet the volume requirements instead of using gravel.
104
FTABLE
1
rows cols
31
***
5
Depth
(ft)
Area
Volume
(acres) (acre-ft)
Q Perc
Q Over
***
(cfs)
(cfs)
***
0.00
0.03
0.0000
0.0000
0.000
0.10
0.03
0.0012
0.0000
0.000
0.20
0.03
0.0024
0.0000
0.000
1.40
0.03
0.0168
0.0132
0.000
1.50
1.60
0.03
0.03
0.0180
0.0210
0.0707
0.0760
0.000
0.000
2.40
0.03
0.0495
0.1957
0.100
2.50
0.03
0.0525
0.1957
0.312
END FTABLE1
Notes: Sacramento Stormwater Quality Partnership Hydromodification Management Plan Example FTABLE Describing Upper Layer of Stormwater Planter Created By: TD Figure 6‐2 FTABLE
rows cols
Depth
(ft)
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1.10
1.20
1.30
1.40
1.50
2
***
16
5
Area
Volume
Q Perc Q Outlet
***
(acres) (acre-ft)
(cfs)
(cfs)
***
0.03
0.0000
0.0000
0.000
0.03
0.0012
0.0001
0.000
0.03
0.0025
0.0007
0.001
0.03
0.0037
0.0007
0.005
0.03
0.0050
0.0007
0.018
0.03
0.0062
0.0007
0.047
0.03
0.0075
0.0007
0.104
0.03
0.0087
0.0007
0.133
0.03
0.0100
0.0007
0.142
0.03
0.0112
0.0007
0.151
0.03
0.0125
0.0007
0.159
0.03
0.0137
0.0007
0.167
0.03
0.0149
0.0007
0.174
0.03
0.0162
0.0007
0.181
0.03
0.0174
0.0007
0.190
0.03
0.0187
0.0007
0.195
END FTABLE2 Notes: Sacramento Stormwater Quality Partnership Hydromodification Management Plan Example FTABLE Describing Lower Gravel Layer of Stormwater Planter Created By: TD Figure 6‐3 Notes: Sacramento Stormwater Quality Partnership Hydromodification Management Plan Stormwater Planter with Underdrain Created By: JC Figure 6‐4 Sacramento Stormwater Quality Partnership
Hydromodification Management Plan
6.3.7 LID Option – Stormwater Planter without Underdrain
For applications with well-draining native soils (e.g., NRCS hydrologic group A or B soils), an underdrain
pipe will not be included with the Stormwater Planter BMP option, as depicted in Figure 6-5.
6.3.8 LID Option – Stormwater Planter with Full Lining
Stormwater Planters with full lining will treat and detain runoff without allowing seepage into the
underlying soil. These types of planters are typically used in areas where infiltration is not permitted or
desired, such as next to buildings, or on steep slopes, or in areas where groundwater contamination is a
concern. Such planter systems typically receive runoff via downspouts leading from the roofs of adjacent
buildings. However, they can also be set in-ground and receive sheet flow from adjacent paved areas.
Runoff passes through the growing medium layer and is collected in an underlying storage layer (Figure
6-6). A perforated-pipe underdrain is typically connected to a storm drain or other discharge point. An
overflow inlet conveys flows which exceed the capacity of the planter. This BMP option shall only be
used in Group C or D soil applications.
6.3.9 LID Option – Stormwater Planter in Series with Underground Vault
This BMP configuration routes storm water through a stormwater planter for water quality treatment,
and then discharges runoff to a nearby vault for detention and hydromodification flow control release
(Figure 6-7). The vault contains a lower orifice to restrict outflows to meet the HMP’s flow control
requirements. The vault portion of the BMP could be located below, adjacent to, or farther away from
the stormwater planter portion of the BMP. This BMP might be a preferred measure in commercial
applications where distributed water quality treatment outflows could be collected into a single vault
for flow control underneath a parking lot. There is not a water quality treatment-only option for this
scenario.
For the HMP, sizing factor simulations are performed based on the following key assumptions:
•
•
•
Stormwater Planter configuration: The stormwater planter portion of this BMP is designed
similar to the stormwater planter BMP, except that the storage layer is only deep enough to
contain a perforated underdrain pipe that will convey treated runoff to the vault portion of the
BMP.
Vault configuration: The vault contains concrete side walls and top, as well as an access hatch
for inspection and maintenance. The bottom of the vault is open to allow infiltration to the
surrounding soils. The vault is simulated as a 4-foot deep chamber, but the designer could select
other configurations with similar or lesser depths.
Vault outlets: The vault contains two outlets. The lower outlet is a flow control orifice that
releases water at a maximum rate equal to the lower flow control limit (0.25Q2 or 0.45Q2). The
upper outlet from the vault is located at 80 percent of the vault’s height and has a capacity to
108
Sacramento Stormwater Quality Partnership
Hydromodification Management Plan
convey the peak anticipated BMP system inflow. The overflow relief shall be located no lower
than the elevation of the vault’s inlet pipe.
6.3.10 Sizing Factor Tables
Table 6-4 through Table 6-9 provide a summary of the sizing factors developed for each of the lower
flow thresholds (0.25Q2 and 0.45Q2), four rain gauges, and grassland versus agricultural land cover. The
tables are specific to each of the four BMP options described above and are provided for both steep and
non-steep terrain.
109
Notes: Sacramento Stormwater Quality Partnership Hydromodification Management Plan Stormwater Planter without Underdrain Created By: JC Figure 6‐5 Notes: Sacramento Stormwater Quality Partnership Hydromodification Management Plan Stormwater Planter with Full Lining Created By: JC Figure 6‐6 Notes: Sacramento Stormwater Quality Partnership Hydromodification Management Plan Stormwater Planter in Series with Underground Vault Created By: JC Figure 6‐7 Sacramento Stormwater Quality Partnership
Hydromodification Management Plan
Table 6-4 Sizing Factors – Stormwater Planter (with and without an underdrain) – Steep Slope
Grass Cover Condition
Soil
Soil
Group A
Group B
Soil
Group C
Soil
Group D
Agricultural Cover Condition
Soil
Soil
Soil
Group A
Group B
Group C
Soil
Group D
A
0.045
0.090
0.075
0.055
0.045
0.095
0.080
0.060
V1
0.038
0.075
0.063
0.046
0.038
0.079
0.067
0.050
V2
0.027
0.054
0.045
0.033
0.027
0.057
0.048
0.036
A
0.045
0.090
0.055
0.040
0.045
0.095
0.060
0.045
V1
0.038
0.075
0.046
0.033
0.038
0.079
0.050
0.038
V2
0.027
0.054
0.033
0.024
0.027
0.057
0.036
0.027
A
0.060
0.105
0.095
0.065
0.065
0.110
0.100
0.070
V1
0.050
0.088
0.079
0.054
0.054
0.092
0.083
0.058
V2
0.036
0.063
0.057
0.039
0.039
0.066
0.060
0.042
A
0.060
0.105
0.070
0.055
0.065
0.110
0.085
0.065
V1
0.050
0.088
0.058
0.046
0.054
0.092
0.071
0.054
V2
0.036
0.063
0.042
0.033
0.039
0.066
0.051
0.039
A
0.045
0.085
0.060
0.045
0.045
0.085
0.065
0.050
V1
0.038
0.071
0.050
0.038
0.038
0.071
0.054
0.042
V2
0.027
0.051
0.036
0.027
0.027
0.051
0.039
0.030
A
0.045
0.080
0.050
0.040
0.045
0.085
0.050
0.040
V1
0.038
0.067
0.042
0.033
0.038
0.071
0.042
0.033
V2
0.027
0.048
0.030
0.024
0.027
0.051
0.030
0.024
A
0.050
0.085
0.070
0.050
0.050
0.090
0.075
0.055
V1
0.042
0.071
0.058
0.042
0.042
0.075
0.063
0.046
V2
0.030
0.051
0.042
0.030
0.030
0.054
0.045
0.033
A
0.050
0.085
0.055
0.045
0.050
0.090
0.055
0.045
V1
0.042
0.071
0.046
0.038
0.042
0.075
0.046
0.038
V2
0.030
0.051
0.033
0.027
0.030
0.054
0.033
0.027
Facility
Elk Grove Gauge
0.25Q2 – Q10
0.45Q2 – Q10
Natomas Gauge
0.25Q2 – Q10
0.45Q2 – Q10
Orangevale Gauge
0.25Q2 – Q10
0.45Q2 – Q10
Rancho Cordova Gauge
0.25Q2 – Q10
0.45Q2 – Q10
Q2 = 2-year pre-project flow rate based upon partial duration analysis of long-term hourly rainfall records
Q10 = 10-year pre-project flow rate based upon partial duration analysis of long-term hourly rainfall records
A = Surface area sizing factor, V1 = Surface volume sizing factor, V2 = Subsurface volume sizing factor
Notes: The stormwater planter for soil groups A and B does not have an underdrain; the stormwater planter for soil groups C and D has an
underdrain.
113
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Table 6-5 Sizing Factors – Stormwater Planter (with and without an underdrain) – Non-Steep Slope
Grass Cover Condition
Soil
Soil
Group A
Group B
Soil
Group C
Soil
Group D
Agricultural Cover Condition
Soil
Soil
Soil
Group A
Group B
Group C
Soil
Group D
A
0.050
0.100
0.080
0.060
0.050
0.105
0.090
0.065
V1
0.042
0.083
0.067
0.050
0.042
0.088
0.075
0.054
V2
0.030
0.060
0.048
0.036
0.030
0.063
0.054
0.039
A
0.050
0.100
0.065
0.045
0.050
0.105
0.070
0.050
V1
0.042
0.083
0.054
0.038
0.042
0.088
0.058
0.042
V2
0.030
0.060
0.039
0.027
0.030
0.063
0.042
0.030
A
0.065
0.110
0.100
0.070
0.065
0.115
0.105
0.080
V1
0.054
0.092
0.083
0.058
0.054
0.096
0.088
0.067
V2
0.039
0.066
0.060
0.042
0.039
0.069
0.063
0.048
A
0.065
0.110
0.080
0.060
0.065
0.115
0.090
0.065
V1
0.054
0.092
0.067
0.050
0.054
0.096
0.075
0.054
V2
0.039
0.066
0.048
0.036
0.039
0.069
0.054
0.039
A
0.050
0.090
0.065
0.050
0.050
0.095
0.070
0.055
V1
0.042
0.075
0.054
0.042
0.042
0.079
0.058
0.046
V2
0.030
0.054
0.039
0.030
0.030
0.057
0.042
0.033
A
0.050
0.090
0.060
0.040
0.050
0.095
0.060
0.045
V1
0.042
0.075
0.050
0.033
0.042
0.079
0.050
0.038
V2
0.030
0.054
0.036
0.024
0.030
0.057
0.036
0.027
A
0.055
0.095
0.075
0.055
0.055
0.095
0.080
0.060
V1
0.046
0.079
0.063
0.046
0.046
0.079
0.067
0.050
V2
0.033
0.057
0.045
0.033
0.033
0.057
0.048
0.036
A
0.050
0.095
0.060
0.045
0.055
0.095
0.065
0.050
V1
0.042
0.079
0.050
0.038
0.046
0.079
0.054
0.042
V2
0.030
0.057
0.036
0.027
0.033
0.057
0.039
0.030
Facility
Elk Grove Gauge
0.25Q2 – Q10
0.45Q2 – Q10
Natomas Gauge
0.25Q2 – Q10
0.45Q2 – Q10
Orangevale Gauge
0.25Q2 – Q10
0.45Q2 – Q10
Rancho Cordova Gauge
0.25Q2 – Q10
0.45Q2 – Q10
Q2 = 2-year pre-project flow rate based upon partial duration analysis of long-term hourly rainfall records
Q10 = 10-year pre-project flow rate based upon partial duration analysis of long-term hourly rainfall records
A = Surface area sizing factor, V1 = Surface volume sizing factor, V2 = Subsurface volume sizing factor
Notes: The stormwater planter for soil groups A and B does not have an underdrain; the stormwater planter for soil groups C and D has an
underdrain.
114
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Table 6-6 Sizing Factors – Stormwater Planter with Full Lining – Steep Slope
Grass Cover Condition
Soil
Soil
Group A
Group B
Soil
Group C
A
N/A
N/A
0.080
0.055
N/A
N/A
0.090
0.060
V1
N/A
N/A
0.067
0.046
N/A
N/A
0.075
0.050
V2
N/A
N/A
0.048
0.033
N/A
N/A
0.054
0.036
A
N/A
N/A
0.060
0.045
N/A
N/A
0.065
0.045
V1
N/A
N/A
0.050
0.038
N/A
N/A
0.054
0.038
V2
N/A
N/A
0.036
0.027
N/A
N/A
0.039
0.027
A
N/A
N/A
0.100
0.070
N/A
N/A
0.105
0.075
V1
N/A
N/A
0.083
0.058
N/A
N/A
0.088
0.063
V2
N/A
N/A
0.060
0.042
N/A
N/A
0.063
0.045
A
N/A
N/A
0.080
0.060
N/A
N/A
0.090
0.065
V1
N/A
N/A
0.067
0.050
N/A
N/A
0.075
0.054
V2
N/A
N/A
0.048
0.036
N/A
N/A
0.054
0.039
A
N/A
N/A
0.065
0.050
N/A
N/A
0.070
0.050
V1
N/A
N/A
0.054
0.042
N/A
N/A
0.058
0.042
V2
N/A
N/A
0.039
0.030
N/A
N/A
0.042
0.030
A
N/A
N/A
0.050
0.040
N/A
N/A
0.055
0.040
V1
N/A
N/A
0.042
0.033
N/A
N/A
0.046
0.033
V2
N/A
N/A
0.030
0.024
N/A
N/A
0.033
0.024
A
N/A
N/A
0.075
0.055
N/A
N/A
0.085
0.060
V1
N/A
N/A
0.063
0.046
N/A
N/A
0.071
0.050
V2
N/A
N/A
0.045
0.033
N/A
N/A
0.051
0.036
A
N/A
N/A
0.055
0.045
N/A
N/A
0.060
0.050
V1
N/A
N/A
0.046
0.038
N/A
N/A
0.050
0.042
V2
N/A
N/A
0.033
0.027
N/A
N/A
0.036
0.030
Facility
Soil
Group D
Agricultural Cover Condition
Soil
Soil
Soil
Group A
Group B
Group C
Soil
Group D
Elk Grove Gauge
0.25Q2 – Q10
0.45Q2 – Q10
Natomas Gauge
0.25Q2 – Q10
0.45Q2 – Q10
Orangevale Gauge
0.25Q2 – Q10
0.45Q2 – Q10
Rancho Cordova Gauge
0.25Q2 – Q10
0.45Q2 – Q10
Q2 = 2-year pre-project flow rate based upon partial duration analysis of long-term hourly rainfall records
Q10 = 10-year pre-project flow rate based upon partial duration analysis of long-term hourly rainfall records
A = Surface area sizing factor, V1 = Surface volume sizing factor, V2 = Subsurface volume sizing factor
115
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Table 6-7 Sizing Factors – Stormwater Planter with Full Lining – Non-Steep Slope
Grass Cover Condition
Soil
Soil
Group A
Group B
Soil
Group C
A
N/A
N/A
0.090
0.060
N/A
N/A
0.105
0.065
V1
N/A
N/A
0.075
0.050
N/A
N/A
0.088
0.054
V2
N/A
N/A
0.054
0.036
N/A
N/A
0.063
0.039
A
N/A
N/A
0.065
0.045
N/A
N/A
0.070
0.050
V1
N/A
N/A
0.054
0.038
N/A
N/A
0.058
0.042
V2
N/A
N/A
0.039
0.027
N/A
N/A
0.042
0.030
A
N/A
N/A
0.110
0.075
N/A
N/A
0.110
0.085
V1
N/A
N/A
0.092
0.063
N/A
N/A
0.092
0.071
V2
N/A
N/A
0.066
0.045
N/A
N/A
0.066
0.051
A
N/A
N/A
0.090
0.060
N/A
N/A
0.100
0.070
V1
N/A
N/A
0.075
0.050
N/A
N/A
0.083
0.058
V2
N/A
N/A
0.054
0.036
N/A
N/A
0.060
0.042
A
N/A
N/A
0.070
0.050
N/A
N/A
0.080
0.055
V1
N/A
N/A
0.058
0.042
N/A
N/A
0.067
0.046
V2
N/A
N/A
0.042
0.030
N/A
N/A
0.048
0.033
A
N/A
N/A
0.060
0.040
N/A
N/A
0.060
0.045
V1
N/A
N/A
0.050
0.033
N/A
N/A
0.050
0.038
V2
N/A
N/A
0.036
0.024
N/A
N/A
0.036
0.027
A
N/A
N/A
0.085
0.060
N/A
N/A
0.090
0.065
V1
N/A
N/A
0.071
0.050
N/A
N/A
0.075
0.054
V2
N/A
N/A
0.051
0.036
N/A
N/A
0.054
0.039
A
N/A
N/A
0.060
0.050
N/A
N/A
0.065
0.050
V1
N/A
N/A
0.050
0.042
N/A
N/A
0.054
0.042
V2
N/A
N/A
0.036
0.030
N/A
N/A
0.039
0.030
Facility
Soil
Group D
Agricultural Cover Condition
Soil
Soil
Soil
Group A
Group B
Group C
Soil
Group D
Elk Grove Gauge
0.25Q2 – Q10
0.45Q2 – Q10
Natomas Gauge
0.25Q2 – Q10
0.45Q2 – Q10
Orangevale Gauge
0.25Q2 – Q10
0.45Q2 – Q10
Rancho Cordova Gauge
0.25Q2 – Q10
0.45Q2 – Q10
Q2 = 2-year pre-project flow rate based upon partial duration analysis of long-term hourly rainfall records
Q10 = 10-year pre-project flow rate based upon partial duration analysis of long-term hourly rainfall records
A = Surface area sizing factor, V1 = Surface volume sizing factor, V2 = Subsurface volume sizing factor.
116
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Table 6-8 Sizing Factors – Stormwater Planter in Series with Underground Vault – Steep Slope
Grass Cover Condition
Soil
Soil
Group A
Group B
Soil
Group C
Soil
Group D
Agricultural Cover Condition
Soil
Soil
Soil
Group A
Group B
Group C
Soil
Group D
A
0.040
0.040
0.040
0.040
0.040
0.040
0.040
0.040
V1
0.033
0.033
0.033
0.033
0.033
0.033
0.033
0.033
V2
0.088
0.160
0.128
0.096
0.088
0.180
0.144
0.104
A
0.040
0.040
0.040
0.040
0.040
0.040
0.040
0.040
V1
0.033
0.033
0.033
0.033
0.033
0.033
0.033
0.033
V2
0.088
0.140
0.104
0.080
0.088
0.160
0.104
0.080
A
0.040
0.040
0.040
0.040
0.040
0.040
0.040
0.040
V1
0.033
0.033
0.033
0.033
0.033
0.033
0.033
0.033
V2
0.096
0.180
0.160
0.120
0.096
0.180
0.168
0.144
A
0.040
0.040
0.040
0.040
0.040
0.040
0.040
0.040
V1
0.033
0.033
0.033
0.033
0.033
0.033
0.033
0.033
V2
0.096
0.160
0.144
0.096
0.096
0.160
0.152
0.120
A
0.040
0.040
0.040
0.040
0.040
0.040
0.040
0.040
V1
0.033
0.033
0.033
0.033
0.033
0.033
0.033
0.033
V2
0.080
0.140
0.120
0.088
0.080
0.160
0.128
0.088
A
0.040
0.040
0.040
0.040
0.040
0.040
0.040
0.040
V1
0.033
0.033
0.033
0.033
0.033
0.033
0.033
0.033
V2
0.080
0.120
0.088
0.072
0.080
0.120
0.096
0.080
A
0.040
0.040
0.040
0.040
0.040
0.040
0.040
0.040
V1
0.033
0.033
0.033
0.033
0.033
0.033
0.033
0.033
V2
0.080
0.140
0.128
0.096
0.088
0.160
0.144
0.104
A
0.040
0.040
0.040
0.040
0.040
0.040
0.040
0.040
V1
0.033
0.033
0.033
0.033
0.033
0.033
0.033
0.033
V2
0.080
0.120
0.104
0.080
0.088
0.140
0.112
0.088
Facility
Elk Grove Gauge
0.25Q2 – Q10
0.45Q2 – Q10
Natomas Gauge
0.25Q2 – Q10
0.45Q2 – Q10
Orangevale Gauge
0.25Q2 – Q10
0.45Q2 – Q10
Rancho Cordova Gauge
0.25Q2 – Q10
0.45Q2 – Q10
Q2 = 2-year pre-project flow rate based upon partial duration analysis of long-term hourly rainfall records
Q10 = 10-year pre-project flow rate based upon partial duration analysis of long-term hourly rainfall records
A = Surface area sizing factor, V1 = Surface volume sizing factor, V2 = Subsurface volume sizing factor
117
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Table 6-9 Sizing Factors – Stormwater Planter in Series with Underground Vault – Non-Steep Slope
Grass Cover Condition
Soil
Soil
Group A
Group B
Soil
Group C
Soil
Group D
Agricultural Cover Condition
Soil
Soil
Soil
Group A
Group B
Group C
Soil
Group D
A
0.040
0.040
0.040
0.040
0.040
0.040
0.040
0.040
V1
0.033
0.033
0.033
0.033
0.033
0.033
0.033
0.033
V2
0.096
0.180
0.144
0.104
0.096
0.200
0.160
0.120
A
0.040
0.040
0.040
0.040
0.040
0.040
0.040
0.040
V1
0.033
0.033
0.033
0.033
0.033
0.033
0.033
0.033
V2
0.096
0.160
0.104
0.080
0.096
0.180
0.120
0.088
A
0.040
0.040
0.040
0.040
0.040
0.040
0.040
0.040
V1
0.033
0.033
0.033
0.033
0.033
0.033
0.033
0.033
V2
0.104
0.180
0.168
0.144
0.104
0.200
0.176
0.152
A
0.040
0.040
0.040
0.040
0.040
0.040
0.040
0.040
V1
0.033
0.033
0.033
0.033
0.033
0.033
0.033
0.033
V2
0.104
0.160
0.152
0.120
0.104
0.180
0.152
0.144
A
0.040
0.040
0.040
0.040
0.040
0.040
0.040
0.040
V1
0.033
0.033
0.033
0.033
0.033
0.033
0.033
0.033
V2
0.088
0.160
0.128
0.096
0.096
0.180
0.144
0.104
A
0.040
0.040
0.040
0.040
0.040
0.040
0.040
0.040
V1
0.033
0.033
0.033
0.033
0.033
0.033
0.033
0.033
V2
0.088
0.140
0.096
0.080
0.096
0.140
0.104
0.080
A
0.040
0.040
0.040
0.040
0.040
0.040
0.040
0.040
V1
0.033
0.033
0.033
0.033
0.033
0.033
0.033
0.033
V2
0.088
0.160
0.144
0.104
0.096
0.180
0.152
0.120
A
0.040
0.040
0.040
0.040
0.040
0.040
0.040
0.040
V1
0.033
0.033
0.033
0.033
0.033
0.033
0.033
0.033
V2
0.088
0.140
0.112
0.088
0.096
0.160
0.120
0.096
Facility
Elk Grove Gauge
0.25Q2 – Q10
0.45Q2 – Q10
Natomas Gauge
0.25Q2 – Q10
0.45Q2 – Q10
Orangevale Gauge
0.25Q2 – Q10
0.45Q2 – Q10
Rancho Cordova Gauge
0.25Q2 – Q10
0.45Q2 – Q10
Q2 = 2-year pre-project flow rate based upon partial duration analysis of long-term hourly rainfall records
Q10 = 10-year pre-project flow rate based upon partial duration analysis of long-term hourly rainfall records
A = Surface area sizing factor, V1 = Surface volume sizing factor, V2 = Subsurface volume sizing factor.
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6.4
Hydromodification Control – Extended Detention BMPs
6.4.1 Automated Pond Sizing for Extended Detention Basins
LID-based facilities have specific design configurations (depths of planting mediums, depths of gravel
layers, overflow heights, etc.), which allows for facility sizing to be tied directly to the contributing
watershed impervious area. Detention facilities, however, have variable design variables including pond
depth, pond side slopes, and outlet orifice sizes and locations above the basin floor. Therefore, an
automated detention sizing routine is required to perform sizing given the user’s basic input design
parameters.
The intended purpose of the automated detention sizing tool is to provide project applicants with a
simplified approach to design detention facilities to meet hydromodification peak flow and flow
duration control requirements. The general process will work as follows:
1. Enter information summarizing project site drainage conditions. Specifically, a proposed project
site is divided into individual drainage areas, or drainage management areas (DMAs).
2. Enter hydrologic characteristics for each DMA, including the contributing drainage area, soil type
(Group A, B, C or D), rainfall station, pre- and post-project land cover information (e.g.,
grassland, impervious, etc.), and DMA slope (average longitudinal slope across the DMA).
3. Based on the inputs for DMAs draining to a detention facility, the automated detention sizing
tool constructs pre-project and post-project (unmitigated, without detention routing) long-term
runoff time series. This is accomplished using a hydrograph database containing per-unit-area
runoff rates for a full range of site conditions. This hydrograph database is created by running a
series of long-term runoff simulations in HSPF.
4. Enter an initial configuration for the detention facility, including surface area, depth to riser
overflow, and side slopes. The user will determine whether to hold either pond area or pond
depth constant while iterating on the other. For design of the outlet control structure, the
automated sizing algorithm will use a pre-defined configuration that includes two flow control
orifices and an overflow weir. Generally, a low flow orifice is placed at an elevation coincident
with the bottom of the basin, a mid-level orifice is placed halfway up the riser, and an overflow
weir is located just below the riser overflow elevation.
5. Post-project, unmitigated, long-term runoff time series will be routed through detention pond
scenarios using a level-pool (Modified Puls) computational routing technique. The reservoir
routing routine computes hourly values for detention basin inflow, basin ponding depth, basin
exfiltration, and outflow through the outlet control structure. Basin outflows form the
“mitigated post-project” time series that are compared to the pre-project conditions.
6. The software (automatic pond sizer) compares the mitigated post-project peak flows and flow
durations with pre-project results within the geomorphically-significant flow range (between the
lower flow threshold and the 10-year flow rate). If the mitigated post-project results are less
than or equal to the pre-project flow (allowing for a 10-percent variance, per HMP BMP
performance criteria), then the pond sizing is deemed complete and HMP performance criteria
is satisfied.
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7. If the current configuration does not meet the HMP performance requirements, the automated
detention sizing procedure continues to iterate and perform the reservoir routing and statistical
post-processing calculations until the pond is properly sized.
Input data for the automated pond sizer is generated as follows:
1. Lower and upper thresholds are pre-calculated based upon susceptibility analysis results
entered by the user. Specific associated flow values are calculated based upon the pre-project
site area, slope, soils and rainfall gauge. Post-project information related to site area, slope, soils
and cover are also entered to define the post-development hydrologic condition.
2. The user chooses to iterate on either area or depth for sizing the pond. The user specifies the
constant value of the sizing parameters that is not being iteratively adjusted.
3. Side slopes for the detention pond are specified in accordance with criteria set forth in Design
Manual, 2007.
4. Outlet structure dimensions are automatically created based on the pond depth and lower flow
threshold. The current assumptions are as follows:
a. A low-level orifice is located at the bottom of the pond, and the diameter of the orifice
is calculated based on matching the maximum discharge (when the pond is full) with the
lower flow threshold (0.25Q2 or 0.45Q2).
b. A mid-level orifice is located at the middle depth of the pond, and the diameter of the
orifice is calculated based on matching the maximum discharge (when the pond is full)
with the upper flow threshold (i.e., Q10).
c. The upper-level overflow weir is located 1 foot below the top of the pond, which
provides 1 foot of freeboard.
Note that when the depth of the pond changes, the orifice sizes will be recalculated due to the variation
in the head over the orifice.
Flow durations are calculated by analyzing an input times series and calculating the total number of time
steps for which the listed values fall within a specified set of ranges, or bins. For the purposes of this
analysis, the full flow range of interest is between the lower flow threshold (0.25Q2 or 0.45Q2) and the
upper flow threshold (Q10). The pond sizer subroutine is executed using both the pre-project time series
and the post-project-mitigated time series hydrographs.
Results for pre-project and post-project-mitigated time series are compared based on the durations
(total time) calculated for each of the flow bins. A “pass” or “fail” result is generated for each bin based
on whether the post-project duration is less than or equal to the pre-project duration. The comparison
takes into account the following HMP criteria variances detailed in Section 6.3.1.
If the results indicate that the pond is not adequately sized, then the size is automatically adjusted until
HMP criteria is met. Figure 6-8 summarizes the pond sizing routine.
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Notes: Sacramento Stormwater Quality Partnership Hydromodification Management Plan Pond Sizing Routine Flow Chart Created By: JC Figure 6‐8 Sacramento Stormwater Quality Partnership
Hydromodification Management Plan
6.4.2 Extended Detention Basin Options
Extended detention basin configuration options conform to specifications detailed in the Design Manual
(Partnership, 2007). This HMP will include the following extended detention basin option in the BMP
Sizing Calculator:
•
Dry extended detention basins
The BMP Sizing Calculator includes a crediting mechanism that reduces the required pond volume if one
or more of the pond’s tributary DMAs is mitigated by LID facilities. The crediting system works by
assuming the runoff produced by these “LID mitigated” DMAs is equivalent to the pre-project conditions
(rather than the post-project conditions). The resulting inflow hydrograph to the pond is lowered, which
reduces the total pond volume needed to match pre-project peak flows and flow durations.
The Design Manual will be updated to reflect any required criteria changes to make the extended
detention facilities an integrated solution for both water quality treatment and hydromodification flow
control. Specific criteria, such as minimum orifice size and maintenance requirements, will be
documented in the Design Manual.
6.5
Hydromodification Control – Runoff Reduction BMPs
Runoff reduction BMPs reduce runoff from the conveyance system near the source, before the runoff
reaches a project site discharge location or BMP. Examples of runoff reduction BMPs include porous
pavement, disconnected pavement, alternate driveways, disconnected roof drains, interceptor trees,
and self-retaining areas. For purposes of the HMP, runoff reduction is quantified based upon design
criteria set forth in the Design Manual (Partnership, 2007). Porous pavement, self-retaining areas and
disconnected roof drains will be integrated into the sizing calculator.
6.5.1 Porous Pavement
Per the Design Manual, “porous pavement allows stormwater runoff to infiltrate into the ground
through voids in the pavement materials. There are many types of porous pavement, including pervious
concrete and asphalt, modular block, reinforced grass, cobblestone block and gravel.”
The effects of porous pavement in the reduction of stormwater runoff in a drainage management area
(DMA) are quantified using runoff coefficients derived from the porous pavement efficient multiplier
(Design Manual, Appendix D, Table D-2a).
6.5.2 Self-Retaining Area
Self-retaining areas accept runoff from small impervious surface areas and retains the runoff without
discharge to the receiving storm conveyance system. Specific criteria are included in the Contra Costa C3
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Stormwater Guidebook (Contra Costa Clean Water Program, 2010) as well as the San Diego Model
SUSMP (San Diego Copermittees, 2009).
Where a landscaped area is upslope from or surrounded by paved areas, a self-retaining area (also
called a zero-discharge area) may be created. Self-retaining areas are designed to retain the first oneinch of rainfall without producing any runoff. The technique works best on flat, heavily landscaped sites.
It may be used on mild slopes if there is a reasonable expectation that the first inch of rainfall will
produce no runoff.
A self-retaining turf and landscape area is created by berming the area or depressing the grade into a
concave cross-section so that the area retains the first inch of rainfall. Inlets of area drains, if any, shall
be set 3 inches above the low point to allow ponding.
Drainage from roofs and pavement areas can be directed to self-retaining areas and allowed to infiltrate
into the soil. The maximum allowable ratio is 2 parts impervious to 1 part pervious. The specific design
criteria will be incorporated into the BMP Sizing Calculator and the updated Design Manual.
6.5.3 Disconnected Roof Drains
The Permittees will consider adding disconnected roof drains in the BMP Sizing Calculator. The following
description is from the Design Manual for this runoff reduction option:
•
6.6
Disconnected roof drains, “roof drains can be disconnected from the storm drain system by
directing the roof runoff across vegetation or into subsurface infiltration devices where it is
filtered or infiltrates into the ground. The roof runoff may be directed across lawns, through
dense groundcover, ….” The effects of disconnected roof drains in the reduction of stormwater
runoff in a DMA are quantified in worksheets provided in Appendix D of the Design Manual.
BMP Sizing Calculator Software
6.6.1 Sizing Calculator Development
The sizing factors and detention routing methods detailed above will be incorporated into a Sizing
Calculator that engineers and municipal plan review staff can use to describe site hydrology, compute
pre- and post-project runoff rates, and size BMPs.
During the site design process, the project applicant’s engineer will divide a project site into separate
drainage management areas (DMAs) that drain to individual BMPs. Based on the type of LID BMP
selected, the amount of impervious and pervious tributary land, local soil type and site slope, the sizing
calculator looks up the appropriate value derived from the HSPF modeling analysis. For extended
detention BMPs, the appropriate pre-project time series are loaded into the software for the flow
duration comparisons.
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The Sizing Calculator approach is based upon sizing tools previously developed for Contra Costa and San
Diego Counties in California.
6.6.2 User Manual
A complete user manual for the Sacramento BMP Sizing Calculator will be developed along with the final
version of the software tool. Information entered into the software is grouped into one of three main
modules. These include the following:
•
•
•
Basin Manager: This module contains basic site information and data used to determine the
required flow threshold range for the project’s receiving waters (project could have one or more
receiving waters for which flow thresholds are determined).
LID Sizing: This module contains information regarding Drainage Management Areas (DMA) and
specifics regarding the sizing of LID BMPs as well as runoff reduction BMPs.
Detention Pond Sizing: This module contains information regarding pre and post-project time
series data and specifics regarding the automated detention pond sizing tool.
6.6.2.1 Basin Manager Module
The Basin Manager module defines the project site and is subdivided into three data entry forms:
•
List of Project Storm Water Discharge Locations - Data entry includes basic site information
specific to each project discharge location and planned BMP to mitigate hydromodification and
storm water quality impacts.
•
Details of Receiving Channel Analysis - Data entry includes receiving channel susceptibility
ratings (lateral and vertical susceptibility) used to determine the lower flow thresholds for each
discharge location from the project site.
6.6.2.2 LID Sizing Module
The LID Sizing module defines LID BMP and runoff reduction sizing options and is subdivided into two
forms.
•
Drainage Management Area List - Data entry includes BMP or runoff reduction type, soils, land
cover and drainage area information.
•
LID Sizing Analysis - This is a results page detailing the minimum surface area and volume
requirements required so that the LID BMP or runoff reduction options meets HMP and
treatment control criteria.
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6.6.2.3 Detention Pond Sizing Module
•
Drainage Management Area List - Data entry includes BMP type, soils, land cover and drainage
area information.
•
Detention Pond Sizing - This page details the minimum storage volume and outlet orifice sizes
required so that the detention pond option meets HMP and treatment control criteria. Required
input includes the detention pond depth and basin side slopes.
•
Drawdown - This tab calculates the time for a full detention facility to fully dewater.
6.7 References
EPA. 2000. EPA BASINS Technical Note 6 – Estimating Hydrologic and Hydraulic Parameters for HSPF Technical Note 6 (2000) http://www.epa.gov/waterscience/basins/bsnsdocs.html
EPA. 2001 HYDROLOGICAL SIMULATION PROGRAM – FORTRAN (HSPF) User’s Manual
http://www.epa.gov/waterscience/BASINS/b3docs/HSPF.pdf
Maidment. 1993. Handbook of Hydrology, McGraw-Hill, 1424pp.
Partnership. 2007. Stormwater Quality Design Manual for the Sacramento and South Placer Regions,
Sacramento Stormwater Quality Partnership.
Contra Costa. 2010. Contra Costa C3 Stormwater Guidebook – 5th Edition, Contra Costa Clean Water
Program.
San Diego Copermittees. 2009. San Diego Model Standard Urban Stormwater Mitigation Plan (SUSMP).
San Diego. 2009. San Diego County Permittees. Final Hydromodification Management Plan. December
29, 2009.
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7 ALTERNATIVE DESIGN APPROACHES FOR HYDROMODIFICATION
MANAGEMENT
The Partnership recognizes there may be some urban development projects for which it is not feasible
or desirable to use the approaches described previously in this plan to manage hydromodification. This
chapter describes two alternative compliance approaches for use in such cases.
7.1 In-Stream Control
An applicant may choose to pursue an in-stream solution for addressing hydromodification impacts from
the urban development project, rather than constructing the controls on-site. Or the in-stream
control(s) could be used in combination with on-site controls. If this option is chosen, such projects will
require continuous simulation modeling and collection of field data; therefore requiring significant time
and expense. Also, the documented results will be subject to rigorous review by the permitting agencies
(local, state and federal agencies) responsible for ultimate approval of the alternative criteria. Such
approval is not guaranteed.
The goal of in-stream (hydromodification mitigation) control is to modify a receiving channel such that it
supports the beneficial uses and physical and ecological functions of the channel to the same extent or
greater than it did prior to the proposed development. For example, if the existing condition is an
incised channel with little ecological value due to historic impacts, there is little value in stabilizing the
creek in this condition to accommodate higher future flows, and an alternate goal might be to restore
the channel to a previous condition that is more stable. More specifically, the applicant shall ensure that
the project:
•
•
Be in geomorphic dynamic equilibrium (it is desirable that it shall have small amounts of local
scour and deposition to support biological processes, but it shall not experience significant net
erosion or deposition of sediment over the entire reach over a sustained period of several
years).
Provide the appropriate physical processes and forms to sustainably support the flora and fauna
that existed prior to development.
If an in-stream channel design approach is chosen as hydromodification mitigation for the project site,
then the following items must be addressed:
•
•
Show that projected increases in runoff peaks and/or durations, along with sediment reductions
associated with development, will not accelerate degradation or erosion of rehabilitated
receiving stream reaches.
A proposed stream rehabilitation mitigation measure can accommodate additional runoff from
a proposed project, the project proponent may consider implementation of planning measures
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•
•
•
such as buffers and restoration activities, revegetation, and use of less-impacting facilities at the
point of discharge in lieu of implementation of storm water flow controls.
Stream rehabilitation mitigation measures could include the modification of the channel
gradient, cross section, or boundary materials to achieve stable conditions in the altered flow
regime. Implementation of such measures will require a geomorphic analysis to show that the
proposed changes to the stream channel cross sections, vegetation, discharge rates, velocities,
and durations will not have adverse impact to the receiving channel’s beneficial uses.
Such measures could not include concrete.
Such measures must be designed considering the ultimate condition 100-year flows (as well as
lower return frequency events) to the rehabilitated channel segment.
Further details on design approaches to be used for in-stream hydromodification mitigation can be
found in Appendix C – NRCS, 2007, Part 654 Stream Restoration Design National Engineering Handbook.
Chapter 8: Threshold Channel Design. National Resource Conservation Service, US Department of
Agriculture, August 2007.
7.2 In-Lieu Program
A project proponent may qualify for coverage under an alternative compliance "in lieu" program,
whereby actions will be taken or funds will be provided for addressing hydromodification in another
area of the watershed, in lieu of constructing controls on-site. Such a program has not yet been
developed for the Sacramento HMP area and each agency may handle this differently. The in-lieu
program requires approval from the Regional Water Board.
In recognition of the dynamic and evolving nature of the field of hydromodification management,
additional alternative compliance scenarios may be proposed by the Partnership in the future, subject to
review and approval of the Regional Water Board.
7.3 References
Brown and Caldwell. 2008. Using Continuous Simulation to Size Storm Water Control Facilities. May,
2008.
San Diego. 2009. San Diego County Permittees. Final Hydromodification Management Plan. December
29, 2009.
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8 COORDINATION, REVIEW AND OUTREACH
As part of the development of this HMP, extensive coordination, review and outreach was conducted.
This chapter provides an overview of the Partnership’s HMP work group, the technical advisory group
meetings that were conducted to review technical components of the HMP as they were developed, and
the public outreach that was conducted to disseminate information regarding the HMP to interested
parties.
8.1 Partnership HMP Work Group
The Permittees formed the Sacramento HMP work group and the representatives from each Permittee
were involved in developing the HMP. During the development of this HMP, Partnership held regular
progress meeting to discuss key issues – applicability criteria, susceptibility assessment, BMP sizing
tools.
The Partnership also held an internal workshop for the agency staff on November 18, 2010. This internal
workshop provided the agency staff with the project background and the technical components in the
HMP.
In addition to the HMP work group progress meetings, City and County of Sacramento staff kept close
communications with all work group members to ensure that the HMP work products were reviewed by
all work group members. This HMP is a collaborative effort of the Sacramento Stormwater Quality
Partnership to meet the Permit requirement on Hydromodification Management.
8.2 Technical Advisory Committee Meetings
Members of the Technical Advisory Committee (TAC) included the following technical experts:
•
•
Dr. Eric Stein, Principal Scientist – Biology Department, Southern California Coastal Water
Research Project, Costa Mesa, California.
Mr. Holger Fuerst, Mackay & Somps Civil Engineers, Roseville, California.
Two TAC meetings were held on January 12, 2010 and October 25, 2010. The TAC reviewed the
Partnership’s applicability criteria and mapping, susceptibility assessment results as well as the BMP
sizing tool approach. TAC members provided feedback on the applicability criteria and susceptibility
assessment approach. Mr. Eric Berntsen, Stormwater Unit, State Water Resources Control Board also
attended the October 2010 meeting and provided feedback on the issues listed above.
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8.3 Public Outreach
In order to receive input from all stakeholders on the HMP, the Partnership conducted the following
outreach activities.
Development Community
The Partnership met with key members of the North State Building Industry Association (BIA) on
November 16, 2010 to provide an overview of the draft HMP before it was posted on the web site for
public review and to receive feedback. The Partnership presented the HMP background, applicability
criteria and the technical component of exemption criteria, stream susceptibility assessment and BMP
sizing tools. The group discussed the hydromodification management applicability criteria, Low Impact
Development – concerns of implementation and the relationship of LID, HMP and flood control
standards.
Environmental Community
The Partnership met with representatives of the environmental community in meetings hosted by the
Sacramento Area Creek Council on December 6, 2010 and January 4, 2011. The Partnership presented
the HMP background, applicability criteria and the technical component of exemption criteria, stream
susceptibility assessment and BMP sizing tools. The group discussed the applicability criteria. The
environmental group also stressed the importance of the stream rehabilitation and monitoring needs
for protecting the local waterways. Two follow-up meeting were also held to clarify the specific
applicability criteria with the environmental group.
Public workshops
The Partnership held two public workshops related to the Sacramento HMP on December 9th and 14th,
2010. The workshops were announced/advertised through the following organizations:
•
•
•
•
•
•
•
•
•
North State Building Industry Association (BIA)
American Council of Engineering Companies (ACEC-CA)
ASCE Sacramento Section
Sacramento Area Creeks Councils
American Society of Landscape Architects (ASLA)
Sacramento Area Council of Governments (SACOG)
Urban Land Institute (ULI) Sacramento District
Institute of Transportation Engineers, Western District
Regional Water Board stakeholder list
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Over 65 people attended the workshops to learn about the proposed HMP components and provide
feedback. Appendix G includes the public workshop flyer, agenda and attendance records.
The Partnership also provided partial draft HMP documents to the public for review from 12/8/2010 to
1/7/2011. The Partnership received two comment letters. The comment letters and the replies to the
comments are included in Appendix H.
8.4 Regional Water Board Comments and Reply
The Regional Water Board reviewed the January 28 HMP submitted by the Partnership and provided
comments on April 29, 2011. Upon receiving the April 29 Regional Water Board comment letter, the
Partnership facilitated three meetings with the Regional Water Board and stakeholders to answer
questions and resolve differences. The communications and the reply letter to the April 29 comments
are included in Appendix I.
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APPENDICES
Appendix A - CASQA. 2009. California Stormwater Quality Association. White Paper: Introduction to
Hydromodification. May 20, 2009.
Appendix B - OEHHA. 2009. Hydromodification: Principles, Problems, and Solutions, prepared by the
Office of Environmental Health Hazard Assessment and the State Water Resources Control Board,
2009.
Appendix C - NRCS. 2007. Part 654 Stream Restoration Design National Engineering Handbook. Chapter
8: Threshold Channel Design. National Resource Conservation Service, US Department of
Agriculture, August 2007.
Appendix D - Bledsoe, B., R. Hawley, E.D. Stein and D.B. Booth. 2010. Hydromodification Screening
Tools: Field Manual for Assessing Channel Susceptibility. Technical Report 606. Southern California
Coastal Water Research Project. Costa Mesa, CA.
Appendix E – Addendum to Susceptibility Assessment Field Sheets.
Appendix F – Guidance notes on continuous simulation modeling.
Appendix G – Public Workshops Documentation.
Appendix H – Public comments and replies on Partial Public Review Draft Document – Sacramento
Stormwater Quality Partnership HMP.
Appendix I – Reply to Regional Water Board April 29, 2011 Comments on Sacramento Hydromodification
Management Plan
Appendix J - Memo: Sacramento Stormwater Quality Partnership HMP – Sensitivity Analysis of 3%
Impervious Cover Change for Highly Developed Area
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