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?