andrew collison, christie beeman and mike liquori philip williams & associates
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Andrew Collison, Christie Beeman and Mike Liquori
Philip Williams & Associatesa.collison@pwa-ltd.com
Assessing stream vulnerability to HydroMod
Contra Costa Clean Water Program
Outline
• Origin and description of low, medium, and high risk streams
• Classification rationale• Basic Geomorphic Assessment
classification examples• Comprehensive Assessment• Examples of mitigation projects vs.
comprehensive analysis and restoration
Compare pre- to post-project imperviousness
Existing Impervious Area & conveyance efficiency
Proposed Impervious Area& conveyance efficiency
NIncrease ?
Y
No Action
Project Proposal
Site Design
Go to next Step
(Option 1)
Site planning and
evaluation
Select and size on-site IMPs
using guidebookSizing Tool(Option 2)
Develop alternative on-site mitigation plan and analysis
(Option 3)
No Mitigation
Partial Mitigation
100% Mitigation
Stop
AssessStream
Vulnerability
Develop and evaluate BMPs
(Option 4)
Exempt from HMPLowYes
Risk of Increased Erosion and
Impacts
Management action:
allows in-stream measures in lieu
of on-site mitigation
HighBasic geomorphic
assessmentNo
Medium
Assess stream vulnerability to erosion
Project larger than 20 acres?
Is channel continuously
hardened, tidal ordepositional
between outletand bay?
No
Detailed analysis; In-stream mitigation plan
Yes Comprehensive geomorphic assessment
Comprehensive analysis; possible restoration plan
Field Evaluation & Review
Of Available
Data
High
Medium
Low
Risk of Increased Erosion and Impacts
Require basic on-site BMPs
(to MEP)
Defining ‘High, medium and low’ vulnerability
Armored flood control channels, channels within the tidal zone (C.3.f.ii. guidelines)
Develop a scientific basis for discriminating medium and high. Must be objective, repeatable, easy for applicant to implement and for permit grantor to check.
Conceptual approach for basic assessment
Stream erosion is most likely to follow HydroMod where:
• Small increases in flow lead to large increases in shear stress (shear stress sensitivity)
• Small increases in shear stress lead to large increases in erosion (low channel resistance)
Wide shallow channel – little increase in shear stress with Q. Q2 dissipates over floodplain
Narrow deep channel – large increase in shear stress with Q. Q2 confined in channel.
Conceptual approach for basic assessment
Incre
asin
g v
uln
era
bility
Coarse sediment and vegetated channel less erosion-prone
Fine sediment and unvegetated channel more erosion-prone
Increasing channel vulnerability
Conceptual approach for basic assessment
resistant sediment, highly entrenched
non resistant sediment, highly entrenched
Incre
asin
g v
uln
era
bility
Increasing channel vulnerability
Turning concepts into measurable attributes
resistant sediment, not very entrenched
non resistant sediment, not very entrenched
Identifying parameters and threshold values
• Assessed 20 stream sites in Contra Costa County
• Use best professional judgment to make initial risk assessment
• Measured numerous relevant field parameters
• Identified type and thresholds of field data that objectively led to same results as the professional judgment
Reconnaissance: test streams
Marsh Creek near OakleyLow gradient flood channel
Low Risk
Note however: channel misclassified as riprap in GIS (applicants will need to ground truth)
Reconnaissance: test streams
Marsh Creek near Marsh Creek reservoirLow-moderate gradient, natural channel, eroding outside bends
Medium Risk
Some excess energy can be expended on floodplain and vegetation, but limited potential for lateral erosion
Reconnaissance: test streams
Upper Marsh Creekmedium gradient, confined channel
High Risk
Excess energy directed to eroding bank
Examples of High Risk
Entrenchment Ratio = (Floodprone Area Width*) / (Bankfull Width)
Floodprone width = width at 2 x bankull depthNote: Rosgen definitions of degree of entrenchment differ from those used here
Turning concept into measurable attributes
ER > 1.6 – channel is non entrenchedBankfull width
Floodprone width
Bankfull width
Floodprone widthER < 1.6 – channel is entrenched
Bankfull depth
Bankfull depth
Turning concept into measurable attributes
Entrainment ratio = critical diameter for entrainment/bed diameter
If dc > size class of bed, channel is non-resistantIf dc < size class of bed, channel is resistant
Critical bed diameter for entrainment, dc = 13.7 x depth x slope
Primary and secondary vulnerability criteria
Primary Criteria
Vulnerability Medium High
Entrenchment Ratio > 1.6 < 1.6
Entrainment Ratio < 1.0 > 1.0
Secondary Criteria
Confinement Class UC WC or MC
Active Bank Erosion Class Low Moderate or High
Active Sedimentation Class varies varies
Width to Depth Ratio > 12 < 12
Schumm State Class 1, 5 & 6 2, 3 & 4
If both primary criteria indicate the same vulnerability class, that class is adopted.
If primary criteria disagree, use preponderance of secondary criteria.
In 2/3rds of cases (n=20) this initial test led to a decisive result that was in agreement with the field judgment
Primary and secondary vulnerability criteria
Primary Criteria
Vulnerability Medium High
Entrenchment Ratio > 1.6 < 1.6
Entrainment Ratio < 1.0 > 1.0
Secondary Criteria
Confinement Class UC WC or MC
Active Bank Erosion Class Low Moderate or High
Active Sedimentation Class varies varies
Width to Depth Ratio > 12 < 12
Schumm State Class 1, 5 & 6 2, 3 & 4
Example field sheets
Example field sheets
Comprehensive geomorphic assessment
• Less standardized to meet site specifics
• A detailed assessment should identify those geomorphic and hydrologic processes that contribute to channel stability, evaluate how the project will affect these processes, and assess how changes in these processes will affect long-term channel response
• Likely to involve historic assessment, hydrology and hydraulic assessment, possible sediment transport modeling of pre and post-development erosion processes
840 860 880 900 920 940 960 980 1000608
610
612
614
616
618
620
Jacques Gulch Site 1- 3-30-06 Plan: Plan 03 4/5/2006
Station (ft)
Ele
vatio
n (f
t)
Legend
WS 2
Ground
Bank Sta
.04 .03 .04
Comprehensive geomorphic assessment
Example – prediction of post HydroMod channel gradient
e.g. for 3 mile watershed stable channel would flatten from 0.015 to 0.008
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0
Area (sq. mi)
Eq
uil
ibri
um
Slo
pe
Notes. Envelopes show 95% confidence boundaries for regression data. Data shown for watersheds smaller than 10 square milesSource: PWA measured channel slopes above grade control structures
Equlibrium Slope for Urban and Rural Watersheds
f i g u r e 1
PWA Ref 1820
Newhall Ranch
Rural, no HydroModified channels
Urban, HydroModified channels
Determine post HydroMod stresses and develop appropriate stabilization
Source:
http://www.wes.army.mil/el/emrrp/tnotes2.html
Mitigation on ‘medium’ and ‘high’ risk streams
• Basic assessment can provide some guidance on mitigation, but more assessment and design analysis will be needed
• Modify channel so that attributes indicate greater stability – e.g. lower floodplain to increase entrainment ratio, - e.g. increase sinuosity to reduce entrainment ratio
Create floodplain to reduce shear stress sensitivity and increase habitat function
Mitigation on a ‘high risk’ stream
Immediately after installation
Three years after installation
Mitigation on a ‘high risk’ stream
Grade controls prevent incision and reduce shear
stress sensitivity. Lower gradient reduced
entrainment ratio.
Floodplain inset bench lowering – reduces stress and creates habitat
Mitigation on a ‘medium risk’ stream
Root wad revetment – increases resistance, reduces stress downstream and creates habitat Combination of
LWD revetment and willow mattress
Mitigation on a ‘medium risk’ stream
Summary• For small developments relatively simple field
indicators can be used to quickly classify the majority of streams into risk categories
• Larger developments or more complex stream systems require more sophisticated predictive approaches
• Mitigation should address the underlying cause of erosion, avoid tendency to harden as mitigation
• However, there are not good long term studies of creeks before and after HydroMod to validate any of the approaches out there
• Especially difficult assessing erosion risk of cohesive channels as erodibility is hard to measure
Questions?
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