1. Hydromodification Compliance in the Bay Area
Presented by
Norman Gonsalves
Caltrans District 4
July 2010

2. What is HYDROMODIFICATION?
The alteration of natural stream hydrology by human activity.

3. Why could HM be a problem?
Stream bank and stream bed erosion
Sedimentation
Increased pollutant delivery to receiving waters
Decreased salinity in estuaries and coastal areas

4. Stream bank erosion

5. Stream bed erosion

6. Sedimentation

7. Flooding (sedimentation)

8. What causes HM? Increased runoff volume from upland areas
Channel lining and armoring
Channel entrainment and straightening
Flow impounding

9. Increased runoff volume

10. Channel lining

11. Channel armoring

12. Channel entrainment (levee)

13. Who considers HM a problem?
The USEPA, but leaves regulation to states.
States include Washington, California, others.
California counties regulating HM include:
Alameda (west)
Contra Costa (all)
Santa Clara (west)
San Mateo (all)
San Diego (all)
Los Angeles (all)

14. What are the requirements?
For areas with HM regulations:
Do not exceed pre-project flow peak flows
Do no exceed pre-project flow durations, from 0.1*Q2 to Q10
Applies to projects discharging to susceptible receiving waters
Applies to projects with 1 acre or more of added impervious surfaces
Applies to Caltrans projects requiring 401 Certification

15. Flow-duration curves

16. Susceptible receiving waters
Water bodies that are NOT susceptible include:
Bays, oceans, large lakes and reservoirs;
Tidal reaches;
Channels lined, armored or piped for their entire lengths all the way to (1) or (2) above;
Aggrading channels

17. Exempted projects
Projects with less than 1 acre of added impervious area
Projects discharging to non-susceptible water bodies
Infill projects (> 65% impervious area) in highly developed watersheds (< 10% vacant parcels)

18. Example of exempt areas
Susceptibility map.

19. Basic ways to meet HM requirement
Infiltrate excess volume created by added impervious areas
Discharge excess volume below erosive flow rates of receiving waters (0.1Q2 pre-project), using storage structures

20. Storage structures for HM management
Storage structures, including:
Basins (lined or unlined),
Underground storage pipes,
Ditches (lined or unlined),
Sand filters,
Bio-retention systems

21. Infiltration devices for HM management
Infiltration basins,
Infiltration trenches,
Bio-retention systems,
Unlined detention basins,
Unlined ditches,
Biostrips,
Embankment side slopes,
Other pervious surfaces

22. Design tools for HM management
Bay Area Hydrology Model (BAHM) (proprietary, but free version available)
Hydrological Simulation Program – Fortran (HSPF) (EPA, free)
Storm Water Management Model (SWMM) (EPA, free)

23. Tools compared HSPF – powerful but hard to use. No built-in data.
BAHM – based on HSPF, user-friendly, set up specifically for HM, built-in data for 4 Bay Area counties, but unstable and buggy.
SWMM – can calculate HM devices as part of entire drainage system, fairly user-friendly, no built-in data, less capable infiltration model.

24. BAHM – Select Project Site

25. BAHM – Add A Watershed

26. BAHM – Add A Detention Pond

27. BAHM – Review Results

28. BAHM – Produce Report

29. BAHM – Pond Elements

30. BAHM Tank and Channel Elements

31. BAHM – More Devices

32. BAHM – Outlet Structures

33. SWMM – User interface

34. SWMM – LID + Drainage Calcs

35. SWMM – Drainage Profile