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Hydromodification Compliance in the Bay Area

Hydromodification Compliance in the Bay Area. Presented by Norman Gonsalves Caltrans District 4 July 2010. What is HYDROMODIFICATION?. The alteration of natural stream hydrology by human activity. Why could HM be a problem?. Stream bank and stream bed erosion Sedimentation

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Hydromodification Compliance in the Bay Area

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  1. Hydromodification Compliancein theBay 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

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