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Hydrograph Modification Management in Contra Costa County

Hydrograph Modification Management in Contra Costa County. Dan Cloak, P.E. Dan Cloak Environmental Consulting. Region 2 Requirements. NPDES permittees must propose a plan Manage increases in flow and volume where increases could: Increase erosion Generate silt pollution

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Hydrograph Modification Management in Contra Costa County

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  1. Hydrograph Modification Management in Contra Costa County Dan Cloak, P.E. Dan Cloak Environmental Consulting

  2. Region 2 Requirements • NPDES permittees must propose a plan • Manage increases in flow and volume where increases could: • Increase erosion • Generate silt pollution • Impact beneficial uses • Post-project runoff may not exceed pre-project rates and durations • Option: “Equivalent Limitation” • Account for expected stream change • Maintain or improve beneficial uses

  3. Contra Costa HMP • Ready to be implemented now • Succinct standards, with compliance options • Encourage Low Impact Development Integrated Management Practices (LID IMPs) • Allow proposals for stream restoration in lieu of flow control where benefits clearly outweigh potential impacts • No exemptions for: • Project size (>1 acre impervious area must comply) • Infill projects in highly developed watersheds • Project cost

  4. Four Compliance Options • Demonstrate project will not increase directly connected impervious area • Implement pre-designed hydrograph modification IMPs • Use a continuous simulation model to compare post- to pre-project flows • Demonstrate increased flows will not accelerate stream erosion

  5. Option 2: Use IMPs Program has designs, specs and sizing factors for: • Flow-through planter • In-ground planter • Vegetated/grassy swale • Bioretention basin • Dry well • Infiltration trench • Infiltration basin

  6. Why Use LID IMPs? • Treatment and hydromodification management • Integrate treatment facilities into landscaping, easements & setbacks • Aesthetically attractive • Low maintenance • No standing water

  7. Flow-through Planter

  8. Vegetated (“Dry”) Swale

  9. 9-acre, mixed use • Clay soils • Flat grades • Max. use • Storm drains • Setbacks Multi-family Residential Retail Restaurant Retail nursery

  10. Swale “C-2” • 6' to 10' width fits into setback • Underdrain/ overflow to storm drain below

  11. Area “C-2” • Follow roof peaks and grade breaks • Area size determined by site layout • Use valley gutters instead of catch basins

  12. 15 areas; 15 swales

  13. Flow-Through Planter: Flow Peak Control

  14. Flow-Through Planter: Flow Duration Control

  15. What is potential impact of underflow on streams? • Scenario 1: Partially built-out watershed • Only a small portion of watershed produces underflow • No potential impact • Scenario 2: Entire watershed equipped with IMPs • Avoid routing storm drains directly to streams • Use landscape buffers around riparian areas • Program has proposed further modeling of watershed-scale scenarios

  16. Four Compliance Options • Demonstrate project will not increase directly connected impervious area • Implement pre-designed hydrograph modification IMPs • Use a continuous simulation model to compare post- to pre-project flows • Demonstrate increased flows will not accelerate stream erosion

  17. Option 4: No Impact to Streams • Categorize development project as posing a high, medium, or low risk of accelerating stream erosion • “Low Risk” • Report showing all channels between project & Bay are hardened, tidal, or aggrading

  18. Option 4: No Impact to Streams • “Medium Risk” • Could be applied to streams where • Sensitivity of boundary shear stress to flow is low (e.g. high width-to-depth ratio) • Resistance of channel materials is high • Could be applied to smaller projects in partially built-out watersheds • Mitigation project plan and supporting analysis • Support for the mitigation project from regulatory agencies having jurisdiction

  19. Option 4: No Impact to Streams • “High Risk” • Presumed that increases in runoff flows will accelerate bed and bank erosion • Comprehensive analysis required to determine design objectives for channel restoration • Comprehensive program of in-stream measures to improve habitat functions while accommodating increaed flows • Requirements determined case-by-case in consultation with regulatory agencies

  20. Summary: Contra Costa’s Approach • Protect urban watersheds from ongoing hydromodification • Requirements apply to infill projects and projects as small as 1 acre—or less • Use IMPs for treatment and flow control • Assist applicants to comply • Provide designs and sizing factors • Solve existing stream problems in lieu of flow control where it makes sense to do so • Case-by-case approach to large projects

  21. Acknowledgements • Tom DalzielContra Costa Clean Water Program • Contra Costa Clean Water ProgramC.3 Technical Work Group • Jeff HaltinerPhilip Williams & Associates • Christie BeemanPhilip Williams & Associates • Steve AndersonBrown & Caldwell • Tony DubinBrown & Caldwell • More info, including the final HMP, at: www.cccleanwater.org/construction/nd.php

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