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 attractiveLow maintenanceNo standing water

Flow-through PlanterReservoir, 12" min. depth

Reverse bend trap or hooded overflow

18" sandy loam, minimum infiltration rate 5" per hour

12" open-graded gravel, approx. ½" dia.

Perforated pipe

Downspout

Building exterior wall

Cobbles or splash block

Filter fabric

Concrete or other structural planter wall with waterproof membrane

Additional waterproofing on building as needed

Drain to storm drain or discharge; bottom-out or side-out options

Vegetated (“Dry”) Swale

6" min. depth

41

12" curb cut

1% min

6' min. overall

grasses or landscape plantings

18" sandy loam, min. infiltration rate 5"/ hr.

native soil; no compaction

18" x 12"; ½" gravel or drain rock

6“ perforated pipe

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

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0 1 2 3 4 5 6 7 8 9 10

Recurrence Interval (years)

Pea

k F

low

(cf

s)

IMP OutflowPervious Area Runoff0.5Q2

Flow-Through Planter: Flow Duration Control

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.00% 0.02% 0.04% 0.06% 0.08% 0.10% 0.12% 0.14% 0.16% 0.18% 0.20%

% Time Exceeded

Flo

w (

cfs)

IMP OutflowPervious Area Runoff0.5Q2Q10

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 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