Development of
an Alternative
Flow Control
Standard for the
Mill Creek Basin
Tim Kraft, PE, Otak, Inc.
Gary Wolff, PE, D.WRE,Otak, Inc.
Ryan Billen, PE, Otak, Inc.
Brett Jordan, PhD, HydroGeo Designs, LLC
Jeff Schnabel, Clark County Department of Environmental Services
Presentation Overview
 Background
 Approach
 Results
Photo
Battle
Ground
Vancouver
Background
 Clark County NPDES Phase I Permit requires
flow-duration standard for stormwater
discharges
 Default standard is to match pre-developed
(forested) flow-duration from 50% of 2-year
through the 50-year
 Default standard is based on research on PugetSound area watersheds (gravel bed streams)
 Ecology allows basin-specific standards tailored
to local conditions
Project Goal
 Development of alternative flow-control standard
for the Mill Creek Basin
 The standard will include:
• Recommended land cover for the predevelopment condition
• Recommended flow threshold
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Approach




Geomorphic Assessment
Hydrologic Modeling of Watershed
Hydraulic Analysis and Representative Reaches
Development of the alternative flow control
standards
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Selection of Study Sites
 Detailed study sites selected based on:
1. Geographic coverage
2. Variations in stream characteristics
3. Potential for changes in flow and sensitivity to
flow changes
4. Existing and potential anadramous fish habitat
5. Accessibility
Study Sites
Geomorphic Assessment
 Evaluation of current channel conditions
 Interpret historic and current trends in channel
stability
 Assessment based heavily on a Channel
Evolution Model
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Channel Evolution Model
Site 5
Site 5
Stage VI (Re-stabilizing)
Site 6
Site 6
Stage IV (Degrading, Widening)
Site 1
Site 1
Stage IV-V (Degrading, Widening)
Site 2
Site 2
Stage VI (Re-stabilizing)
Hydrologic Modeling
 Quantify the frequency and duration of
streamflows under current and pre-developed
land-use conditions
 Based on a calibrated HSPF model of the Mill
Creek Basin
 Results used in Hydraulic Analysis and in
interpretation of basin conditions
Existing Land Use
Hydrologic Results
Site 5 Flood Frequency
Return Period (years)
1.01
1.25
2
5
10
20
50
20
10
5
2
100 200 500
1,000
Discharge (cfs)
Existing
Forested
100
10
99
98
95
90
80
70
60
50
40
30
Exceedance Probability
1
0.5
0.2
Site 1 Flood Frequency
Return Period (years)
1.01
1.25
2
5
10
20
50
20
10
5
2
100 200
500
10,000
Discharge (cfs)
Existing
Forested
1,000
100
10
99
98
95
90
80
70
60
50
40
30
Exceedance Probability
1
0.5
0.2
Site 2 Flood Frequency
Return Period (years)
1.01
1.25
2
5
10
20
50
20
10
5
2
100 200 500
1,000
Discharge (cfs)
Existing
Forested
100
10
99
98
95
90
80
70
60
50
40
30
Exceedance Probability
1
0.5
0.2
Hydraulic Analysis
 Used to determine discharge required to initiate
bed material sediment transport at each site
 Based on HEC-RAS model set up for each site
 Calculation of critical discharge different for finegrained and coarse grained sites
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Critical Discharge
Determination
 Coarse Grained Sites:
• Sites 1 and 6
• Measured Bed Gradations
 c   *c ( S   ) D50
 Fine Grained Sites:
• Sites 2, 5 and 6
• In-situ jet testing of the cohesive bed material
 = k( 0 - c )
Results at Coarse Grained
Sites
Site
Critical
Discharge
(cfs)
1 (entire reach)
Existing Hydrology
Forested Hydrology
Return
Period
(years)
Percent of
2-Year Peak
Return
Period
(years)
Percent of
2-Year Peak
132
1.04
46%
1.19
60%
1 (downstream of log weir)
56
<1.01
19%
<1.01
26%
6
73
1.04
45%
1.4
74%
Results at Fine Grained Sites
Site
2
5
6
Critical Shear
Measurement
Used
Critical
Discharge
(cfs)
Median
Low Value
Median
Low Value
Median
Low Value
n/a*
2.1
261
237
166
72
*Critical discharge is greater than all modeled flows
**Critical discharge is too low for flood frequency
Existing Hydrology
Forested Hydrology
Return
Period
(years)
Percent of
2-Year Peak
Return
Period
(years)
Percent of
2-Year Peak
n/a*
n/a**
9.3
7.1
2.1
1.04
n/a*
3.4%
204%
185%
103%
45%
n/a*
n/a**
47
34
5.5
1.4
n/a*
4.9%
378%
343%
168%
73%
Assessment Results Summary
Site
1
2
5
6
Geomorphic
(Assessed Stability of
Reach)
Unstable
(CEM Stage IV-V)
Stable (re-stabilizing)
(CEM Stage VI)
Stable (re-stabilizing)
(CEM stage VI)
Unstable
(CEM stage IV)
Analysis Findings
Hydrologic
Hydraulic
(Flow increase due
(Critical flow recurrence interval)
to land cover
changes)
Low
19 percent of 2-year existing*
Moderate
Inconclusive
High
7-year existing
High
Half the 2-year existing
Proposed Alternative Flow
Control Standards
Land Cover for Pre-Developed Conditions:
• Forest for areas forested in 2002
• Forest for the area tributary to Site 6
• Field for all other areas
Flow Threshold:
• ½ the 2-year calculated
using the predeveloped land cover
condition
Questions?