hydrograph modification management in contra costa county dan cloak, p.e. dan cloak environmental...
TRANSCRIPT
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