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 ● 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

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

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: Flow-through planter In-ground planter Vegetated/grassy swale Bioretention basin Dry well Infiltration trench Infiltration basin

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

Flow-through Planter

Vegetated (“Dry”) Swale

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

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

Area “C-2” Follow roof peaks and grade breaks Area size determined by site layout 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? 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

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 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

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

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

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

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