Contra Costa IMP Review Memo

Contra Costa IMP Review Memo
CLEAR CREEK SOLUTIONS, INC.
15800 Village Green Drive #3
Mill Creek, WA 98012
425.892.6454
www.clearcreeksolutions.com
DATE:
2 April 2007
TO:
File
CC:
FROM:
Douglas Beyerlein, P.E.
SUBJECT:
Comparison of Contra Costa IMP and BAHM/WWHM3/HSPF
The purpose of this memo is to compare the Contra Costa Integrated Management Practice (IMP)
stormwater sizing tool and the Bay Area Hydrology Model (BAHM). Both are designed to meet HMP
requirements for jurisdictions in the San Francisco Bay Area. In addition, information on the Western
Washington Hydrology Model version 3 (WWHM3) and HSPF is included.
SUMMARY: IMP has an easy-to-use interface for the sizing of HMP facilities that make it
attractive to use. However, the limitations in the IMP options provided to the user to accurately
size and design their HMP facilities together with some questionable HSPF parameter values
limits IMP’s usefulness. It should be used only on very small sites (1-2 acres maximum) where
HMP sizing errors will not have a major impact on the surrounding aquatic resources.
Disclaimer: The opinions expressed in this memo are those of Doug Beyerlein, Principal Engineer,
Clear Creek Solutions, Inc., and are not necessarily those of any of the clients of Clear Creek Solutions.
Clear Creek Solutions is the developer of WWHM3 and BAHM. For more information contact Doug
Beyerlein at the address above or go to www.clearcreeksolutions.com.
Three sets of comparisons are presented in this memo:
1. basic features
2. specific model features
3. HSPF parameter values
The basic features of the four software systems are presented in Table 1.
Stormwater ~ Hydrologic Modeling ~ Software Development
Table 1. Basic Features Comparison
Model
IMP
Counties
Contra Costa
BAHM
WWHM3
HSPF
Alameda
19 counties of
world-wide
Santa Clara
Western Washington
San Mateo
In Use Since
2005
2007
2001
1979
Computational Engine
HSPF
HSPF
HSPF
HSPF
Model Interface
Windows-based
Windows-based
Windows-based
text-based
Recommended Project Size
small sites
all sites
all sites
all sites
Easy of Use
very easy
moderately easy
moderately easy
very difficult
Training Required
1 hour
4 hours
4 hours
40 hours
Flexibility of Use
many limitations
few limitations
few limitations
no limitations
Computations
static
dynamic
dynamic
dynamic
Length of Rainfall Record
35 years
35-50 years
35-50 years
user selected
Predevelopment Conditions
existing
existing
forest
user defined
user defined
Runoff Increase from Native to
Urban Pervious?
No
Yes
Yes
Facility Sizing Options
limited to pre-selected values
unlimited
unlimited
unlimited
Flow Duration Lower Limit
0.5Q2
0.1Q2
0.5Q2
user defined
Flow Duration Upper Limit
Q10
Q10
Q50
user defined
WQ Treatment Standard
80%
91% (preliminary)
91%
user defined
The significant difference in the basic features comparison between IMP and BAHM is that all of the
runoff (both pre- and post-development) has been precomputed in IMP. In contrast, BAHM dynamically
runs HSPF in the background after the user selects land use and HMP facility features.
The advantages of using IMP are quick computational time and a very simple and easy-to-use user
interface. The disadvantages are related to the limitations in selecting HMP facility options, as these
options have been pre-selected for the user. For example, infiltration trench depth options are limited
to 3, 4, or 5 feet; the riser height and diameter are fixed and cannot be changed by the user.
IMP assumes that there is no increase in runoff when the land use changes from native soil and
vegetation to constructed pervious areas consisting of compacted soil from construction activities, the
replacement of native vegetation with urban vegetation, and the addition of irrigation. BAHM computes
the existing pervious runoff and the developed pervious runoff based on different HSPF parameter
values for each land condition and includes an irrigation time series for urban vegetation. IMP does
not.
The specific model features of the four software systems are presented in Table 2.
Stormwater ~ Hydrologic Modeling ~ Software Development
Table 2. Specific Model Features Comparison
Model
IMP
Types of HMP Facilities
Soil Groups Modeled
Vegetation Groups
Modeled
BAHM
WWHM3
HSPF
Stormwater Pond
Stormwater Pond
user defined
Stormwater Vault
Stormwater Vault
Stormwater Tank
Stormwater Tank
In-Ground Planter
In-Ground Planter (1)
In-Ground Planter (1)
Flow-Through Planter
Flow-Through Planter (1)
Flow-Through Planter (1)
Vegetated Swale
Vegetated Swale (1)
Vegetated Swale (1)
Bioretention Area
Bioretention Area (1)
Bioretention Area (1)
Dry Well
Dry Well (2)
Dry Well (2)
Infiltration Trench
Infiltration Trench (3)
Infiltration Trench (3)
Infiltration Basin
Infiltration Basin (2)
Infiltration Basin (2)
Rain Garden (1)
Rain Garden (1)
Green Roof (4)
Green Roof (4)
User defined (5)
User defined (5)
A
A
A
B (1)
B
B (6)
C (2)
C
C
D
D
D/Saturated
Shrub
Forest
Forest
Shrub
Pasture
Grass (native)
Lawn
user defined
user defined
Urban
Land Slopes Modeled
10%
5%
5%
10%
10%
15%
20%
user defined
25%
User can model:
treatment plus flow control
Yes
Yes
Yes
N/A
treatment only
Yes
Yes
Yes
N/A
open channels
No
Yes
Yes
Yes
pipes/culverts
No
Yes (4)
Yes (4)
Yes
flow splitters
No
Yes
Yes
Yes
urban irrigation
No
Yes
Yes
Yes
wetlands
No
Yes
Yes
Yes
frequency results
No
Yes
Yes
N/A
duration match
No
Yes
Yes
N/A
hydrographs
No
Yes
Yes
Yes
report file
Yes
Yes
Yes
N/A
flow duration criteria
No
Yes
Yes
N/A
WQ treatment criteria
No
Yes
Yes
N/A
HSPF parameter values
No
Yes
Yes
Yes
precipitation time series
No
Yes
Yes
Yes
evaporation time series
No
Yes
Yes
Yes
computational time step
No
Yes (4)
Yes (4)
Yes
User can view:
User can change:
User can link model to:
input flow time series
No
Yes
Yes
Yes
HY8
No
Yes (4)
Yes (4)
No
GIS
No
Yes (4)
Yes (4)
No
SWMM
No
Yes (4)
Yes (4)
No
HEC-RAS
No
Yes (4)
Yes (4)
No
Stormwater ~ Hydrologic Modeling ~ Software Development
Notes
1
2
3
4
5
6
IMP
same as A soils
same as D soils
BAHM
Bioretention Swale
Pond with infiltration
Gravel Trench Bed
only in PRO version
SSD Table
WWHM
Bioretention Swale
Pond with infiltration
Gravel Trench Bed
only in PRO version
SSD Table
same as A soils
IMP provides small site HMP solutions: planters, vegetated swales, bioretention areas, and infiltration
facilities (dry wells, infiltration trenches, and infiltration basins). BAHM provides comparable solutions
plus stormwater ponds, vaults, and tanks.
BAHM also includes more soil types, types of vegetation, and slope categories. BAHM also allows the
user to model open channels, culverts, flow splitters, and wetlands, if needed.
Both IMP and BAHM allow the user to create and view a report file. BAHM also allows the user to view
frequency results, flow duration matches, and hydrographs. The user can also change the flow
duration criteria, water quality treatment criteria, HSPF parameter values, precipitation time series,
evaporation time series, and computational time step, if appropriate and approved by the reviewing
agency.
Users can input land use information through BAHM’s GIS Import interface. BAHM also allows the user
to link the HSPF-generated output to other software hydraulic routing packages (HY8, SWMM, and
HEC-RAS) to take advantage of their special features. These are special features available only in
BAHM PRO packages.
HSPF parameter values used in IMP and BAHM are shown in Table 3.
Table 3. HSPF Parameter Value Comparison
Model
IMP
BAHM
WWHM3
HSPF
Parameter values based on
Calabazas Cr, Santa Clara Co
Castro Valley Cr, Alameda Co
King Co &
user selected
WWHM3 values
Alameda Cr, Alameda Co
Snohomish Co
Ross Cr, Santa Clara Co
watersheds
Thompson Cr, Santa Clara Co
Pre-development land use
Shrub
Shrub
Forest
user selected
Land slope
all
moderate
moderate
user selected
INFILT A
0.70
0.07
2.00
user selected
INFILT D
0.03
0.04
0.08
user selected
LZSN A
7.0
4.8
5.0
user selected
LZSN D
7.0
4.5
4.5
user selected
INTFW A
0.4
3.2
0.0
user selected
INTFW D
0.4
1.2
6.0
user selected
UZSN A
0.5
0.7
0.5
user selected
UZSN D
0.5
0.7
0.5
user selected
IRC A
0.30
0.45
0.70
user selected
IRC D
0.30
0.45
0.50
user selected
CEPSC A
0.06-0.10
0.13-0.15
0.20
user selected
CEPSC D
0.08-0.15
0.13-0.15
0.20
user selected
LZETP A
0.4-0.6
0.50-0.65
0.70
user selected
LZETP D
0.5-0.7
0.50-0.65
0.70
user selected
No Contra Costa watershed was used to calibrate the HSPF parameter values for IMP.
Stormwater ~ Hydrologic Modeling ~ Software Development
A comparison of IMP and BAHM HSPF parameter values shows that IMP provides more stormwater
runoff from D soils than BAHM. In particular, the D soil IMP INTFW (interflow) value is very low (0.4).
This is probably why the IMP D soil produces peak runoff rates that are approximately 70 to 90 percent
of the impervious surface peak runoff rates (Brown and Caldwell memo dated 12 May 2005, p. 23). In
the experience of this reviewer, pervious peaks will not equal 70 to 90 percent of impervious peaks,
even for D soils. The computed pervious peaks appear to be too large.
Based on the differences in IMP and BAHM HSPF parameter values it is expected that IMP will
compute higher predevelopment/existing peak flows than BAHM. This will produce smaller-sized HMP
facilities than BAHM.
SUMMARY: IMP has an easy-to-use interface for the sizing of HMP facilities that make it
attractive to use. However, the limitations in the IMP options provided to the user to accurately
size and design their HMP facilities together with some questionable HSPF parameter values
limits IMP’s usefulness. It should be used only on very small sites (1-2 acres maximum) where
HMP sizing errors will not have a major impact on the surrounding aquatic resources.
References:
Bay Area Hydrology Model Draft User Manual. Clear Creek Solutions. November 2006.
Hydrological Simulation Program – Fortran User’s Manual for Version 11. EPA/600/R-97/080.
AQUA TERRA Consultants. August 1997.
IMP Sizing Tool Documentation is available at
http://www.cccleanwater.org/construction/Publications/SizingCalculator_12-18-06/
IMP%20Sizing%20Tool%20Documentation-Help-12-18-06.pdf
Stormwater C.3 Guidebook. 2006. Contra Costa Clean Water Program. Stormwater Quality
Requirements for Development Applications. Third Edition. October 2006. Appendix I: Sizing
Integrated Management Practices.
Stormwater C.3 Guidebook. 2006. Contra Costa Clean Water Program. Stormwater Quality
Requirements for Development Applications. Third Edition. October 2006. Appendix I: Attachment 2 –
Brown and Caldwell Memorandum. May 12, 2005. Including Appendix A: HSPF Parameters for
Pervious Land Surfaces: Parameter Values and Descriptions; Appendix B: Assumed Water Movement
Hydraulics for Modeling IMPs; and Appendix C: Summary of Sensitivity Analysis for the HSPF Modeling
and IMP Sizing.
Stormwater C.3 Guidebook. 2006. Contra Costa Clean Water Program. Stormwater Quality
Requirements for Development Applications. Third Edition. October 2006. Appendix I: Attachment 3 –
Brown and Caldwell Memorandum. May 4, 2005.
Western Washington Hydrology Model Version 3.0 User Manual. Clear Creek Solutions. August 2006.
WWHM3 Project Book. Clear Creek Solutions. August 2006.
Stormwater ~ Hydrologic Modeling ~ Software Development
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