Transcript Slide 1

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
Reservoir,
12" min. depth
Reverse bend
trap or hooded
overflow
Building
exterior wall
Downspout
Cobbles or
splash block
18" sandy loam,
minimum
infiltration rate
5" per hour
Filter fabric
Concrete or other
structural planter wall with
waterproof membrane
12" open-graded
gravel, approx.
½" dia.
Perforated pipe
Additional
waterproofing on
building as
needed
Drain to storm drain or discharge;
bottom-out or side-out options
Vegetated (“Dry”) Swale
18" sandy loam,
min. infiltration rate 5"/hr
grasses or landscape plant
1% min
4
1
6" min. depth
12" curb cut
native soil; no
compaction
6“ perforated pipe
18" x 12"; ½" gravel
or drain rock
6' min. overall
9-acre, mixed use
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Multi-family
Residential
Clay soils
Flat grades
Max. use
Storm drains
Setbacks
Retail
Retail
nursery
Restaurant
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
0.80
IMP Outflow
Pervious Area Runoff
0.5Q2
0.70
Peak Flow (cfs)
0.60
0.50
0.40
0.30
0.20
0.10
0.00
0
1
2
3
4
5
6
Recurrence Interval (years)
7
8
9
10
Flow-Through Planter:
Flow Duration Control
0.80
IMP Outflow
Pervious Area Runoff
0.5Q2
Q10
0.70
0.60
Flow (cfs)
0.50
0.40
0.30
0.20
0.10
0.00
0.00%
0.02%
0.04%
0.06%
0.08%
0.10%
0.12%
% Time Exceeded
0.14%
0.16%
0.18%
0.20%
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:
www.cccleanwater.org/construction/nd.php