Hydrograph Modification Management in Contra Costa County

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

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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 (gt1 acre impervious area must
  comply)
• Infill projects in highly developed watersheds
• Project cost

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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

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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

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Why Use LID IMPs?

• Treatment and hydromodification management
• Integrate treatment facilities into landscaping,
  easements setbacks
• Aesthetically attractive
• Low maintenance
• No standing water

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Flow-through Planter
8
Vegetated (Dry) Swale
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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

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Area C-2

• Follow roof peaks and grade breaks
• Area size determined by site layout
• Use valley gutters instead of catch basins

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15 areas 15 swales
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Flow-Through Planter Flow Peak Control
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Flow-Through Planter Flow Duration Control
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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

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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

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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

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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

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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

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Summary Contra Costas Approach

• Protect urban watersheds from ongoing
  hydromodification
• Requirements apply to infill projects and
  projects as small as 1 acreor 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

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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