Hydrograph Modification Management in Contra Costa County Dan

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

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

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

Four Compliance Options 1. Demonstrate project will not increase directly connected impervious area 2. Implement pre-designed hydrograph modification IMPs 3. Use a continuous simulation model to compare post- to pre-project flows 4. Demonstrate increased flows will not accelerate stream erosion

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

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

Flow-through Planter

Vegetated (“Dry”) Swale

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

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

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

15 areas; 15 swales

Flow-Through Planter: Flow Peak Control

Flow-Through Planter: Flow Duration Control

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

Four Compliance Options 1. Demonstrate project will not increase directly connected impervious area 2. Implement pre-designed hydrograph modification IMPs 3. Use a continuous simulation model to compare post- to pre-project flows 4. Demonstrate increased flows will not accelerate stream erosion

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

Option 4: No Impact to Streams n “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 builtout watersheds ● Mitigation project plan and supporting analysis ● Support for the mitigation project from regulatory agencies having jurisdiction

Option 4: No Impact to Streams n “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

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

Acknowledgements n Tom Dalziel Contra Costa Clean Water Program n Contra Costa Clean Water Program C. 3 Technical Work Group n Jeff Haltiner Philip Williams & Associates n Christie Beeman Philip Williams & Associates n Steve Anderson Brown & Caldwell n Tony Dubin Brown & Caldwell n More info, including the final HMP, at: www. cccleanwater. org/construction/nd. php