1. Doug Beyerlein, P.E., Clear Creek Solutions, Inc.
The Bay Area Hydrology Model: A Tool for Analyzing Hydromodification Effects of Development Projects and Sizing Solutions
Jill C. Bicknell, P.E., EOA , Inc. Santa Clara Valley Urban Runoff Program
Doug Beyerlein, P.E., Clear Creek Solutions, Inc.

2. The Bay Area Hydrology Model: A Tool for Analyzing Hydromodification Effects of Development Projects and Sizing Solutions
This presentation given by Jill Bicknell and Doug Beyerlein at the September 2006 CASQA Conference in Sacramento, CA.

3. Acknowledgements Funding Agencies Arleen Feng (ACCWP)
Santa Clara Valley Urban Runoff Pollution Prevention Program (SCVURPPP)
Alameda Countywide Clean Water Program (ACCWP)
San Mateo Countywide Stormwater Pollution Prevention Program (STOPPP)
Arleen Feng (ACCWP)
Tony Donigian (Aqua Terra Consultants)
Gary Palhegyi (GeoSyntec Consultants)

4. Presentation Overview
Background / Permit Requirements
Technical Approach to Control of Hydromodification
Design Challenges / Need for BAHM
Overview of the BAHM
BAHM Parameter Development
BAHM Application
BAHM = Bay Area Hydrology Model

5. What is Hydromodification?
Change in the runoff hydrograph (flow pattern) from an area due to development
Impacts of land development:
Increase in impervious surface
Decrease in amount of vegetation
Grading and compaction of soils
Construction of drainage facilities

6. What is Hydromodification? (continued)
Effects of land development on the site runoff hydrograph:
Less infiltration / evapotranspiration
More surface runoff (increased volume)
Runoff leaves the site faster (increased peak flows)
Runoff occurs more often (increased duration)
Runoff conveyed directly to creek (increased connectivity)

7. Hydromodification Control Requirements
SF Bay Area Phase 1 MS4 Permits
Permit Provision C.3.f.i.:
Increases in runoff peak flow, volume, and duration shall be managed for all Group 1 Projects*, where such increased flow and/or volume can cause increased erosion of creek beds and banks…
* Group 1 = > 1 acre impervious surface

8. Hydromodification Control Requirements
Permit Provision C.3.f.i. (continued):
Post-project runoff shall not exceed estimated pre-project rates and/or durations, where the increased storm water discharge rates and durations will result in increased potential for erosion…
Must develop and implement Hydro-Modification Management Plans (HMPs)

9. Hydromodification Control Requirements
Permit Provision C.3.f.ii. (Exemptions):
Projects that discharge to tidal area, channel continuously hardened to the Bay, or directly to the Bay
Projects that are “infill projects” in “highly developed watersheds”
Other projects where potential for increased erosion is minimal (e.g., no increase in impervious area)

10. Stable stream, bed close to old tree
Segment 5
Stable stream, bed close to old tree

11. Segment 4
Channel incision on Yerba Buena Creek

12. Undermining outfall protection structure
Segment 1
Undermining outfall protection structure

13. Pre- and post-project runoff characteristics
All Events Flood Frequency
Flow Duration
Critical flow (Qc=15 cfs)
2-year
Frequency of events and duration

14. Control Strategies
Peak flow control - not effective for erosion control (low flows matter)
Single event/design storm approaches – not adequate hydromod control
Flow duration control - recommended
Maintain magnitude and duration of post-project flows same as pre-project
Considers multi-year discharge record
Site design (LID) measures – effective in reducing flow, use to supplement flow duration control facility

15. Flow Duration Histograms

16. Flow Duration Curve Matching

17. Range of Storms to Manage (Set of Design Storms)
Qc to the 10-year event

18. Tale of Five HMPs Santa Clara Valley Contra Costa County
Submitted Final HMP Report April 2005
RWQCB adopted July 2005
Contra Costa County
Submitted Final HMP May 2005
RWQCB adopted July 2006
San Mateo & Alameda Counties, Fairfield-Suisun
Submitted final HMPs, under review by RWQCB
Adopt December 2006?

19. Performance Criteria
Applicable projects with on-site flow controls that are designed to provide flow duration control to the pre-project condition are considered to comply with the HMP.
Flow duration controls shall be designed to match pre- and post-project flow rates and durations from 10% of the pre-project 2-year peak flow to the pre-project 10-year peak flow.

20. On-Site Options
Use site design techniques to reduce runoff flow and volume
Decrease impervious surface area
Disconnect impervious areas
Promote infiltration
Select treatment BMPs that reduce volume
swales, detention areas, bioretention, green roofs

21. On-Site Options, continued
Construct flow control structures
Retention/detention basins
Underground vaults/tanks
Combine flow control with flood control and/or treatment facilities
Examples: detention basin, wet pond, constructed wetlands

22. * Schematic Flow Duration Control Pond
Overflow provision for peak events (storage may also be increased to meet flood control requirements)
Inflows: site runoff after reductions from site design, infiltration or other retention measures C D A
E - released at maximum discharge rate in pipe
Dead storage (optional)
Discharge to stream
Bottom infiltration where applicable * B
Point of Compliance: drainage point for comparing post-project and pre-project Flow Duration curves
Legend: A) outlet pipe riser; B) low flow orifice; C) intermediate orifice (1 shown); D) weir notch (V-type shown); E) freeboard above riser (typically 1 foot).
Schematic Flow Duration Control Pond

23. Integrating Flow Duration Control (FDC) with Other BMPs
Urban Runoff
FDC Basin
Bio-infiltration Swale
FDC Basin
Stream
FDC Vault
Bio-infiltration Swale
On-Site BMPs
LID

24. Process for Evaluating Flow Control Requirements
Hydrologic Analysis
Generate pre- vs. post-project flow duration curves using hydrologic model
Continuous simulation required
Available models:
Corps of Engineers’ HEC-HMS
EPA HSPF
EPA SWMM
Western Washington Hydrology Model

25. Design Challenges Challenge #1: Flow duration control design
Requires use of continuous simulation hydrologic model
Use of these models is data intensive and time consuming
Lack of knowledge and experience
Challenge #2: Integrating flow controls with site design and treatment controls
How to estimate flow reduction benefits of other BMPs
How to estimate treatment capability of flow control facility

26. BAHM based on WWHM
BAHM (Bay Area Hydrology Model) uses the EPA HSPF computational engine and WWHM (Western Washington Hydrology Model) software platform.
BAHM hydrology parameter values are derived from locally calibrated watersheds.

27. WWHM Developed for the Washington State Department of Ecology.
Used in the 19 counties of Western Washington.

28. BAHM Designed for the San Francisco Bay Area Alameda County
Santa Clara County
San Mateo County

29. BAHM Components user-friendly graphical interface
automatically loads appropriate parameter values and meteorological data based on project location
uses long-term (30+ year) local precip records and scales precip based on ratio of project site MAP and precip gage MAP

30. BAHM
Graphical interface: Project Site

31. BAHM Graphical interface: Pre-Project Land Use
Based on soil, vegetation, land slope, impervious area

32. BAHM Graphical interface: Project Land Use
Based on soil, vegetation, land slope, impervious area

33. BAHM Graphical interface: HMP Facility
Pond, tank, vault, gravel trench bed, bioretention

34. BAHM Graphical interface: Runoff Analysis
AutoPond optimizes (minimizes) pond dimensions to meet HMP flow duration criteria

35. BAHM Calibration of local hydrologic parameter values
Castro Valley Creek watershed
Alameda Creek watershed

36. BAHM
Comparison of simulated and observed Castro Valley Creek streamflow

37. BAHM
Comparison of simulated and observed Alameda Creek streamflow

38. WWHM Application Examples
Commercial site: Costco store, Woodinville, WA (14.38 acres)

39. WWHM Application Examples
Commercial site: Costco store, Woodinville, WA (6 acre-feet of underground storage)

40. WWHM Application Examples
Planned community: Snoqualmie Ridge, King County, WA (1,343 acres)

41. WWHM Application Examples
Planned community: Snoqualmie Ridge, King County, WA (10 stormwater ponds ranging in size from 2 to 20 acre-feet)

42. Conclusion
BAHM will facilitate compliance with HMP and design of flow control facilities in the Bay Area by providing:
An easier, standardized way to do continuous simulation modeling
A means to compute flow control benefits of site design/LID and treatment measures
Standardized reporting to assist municipal staff in design review

43. Click on “New and Redevelopment Related Products and Reports”
For More Information...
Visit our website at:
Click on “New and Redevelopment Related Products and Reports”
Work products will be included on this site. Currently the work plan and HMP workplan are available for downloading.
That’s all, folks.