Hydrograph Modification Management in Contra Costa County - PowerPoint PPT Presentation

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Hydrograph Modification Management in Contra Costa County

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Flow-through Planter. Vegetated ('Dry') Swale. 9-acre, mixed ... Flow-Through Planter: Flow Duration Control. What is potential impact of underflow on streams? ... – PowerPoint PPT presentation

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Title: 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

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 (gt1 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 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

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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