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Quantifying Hydromodification Impacts and Developing Mitigation Using a Four Factor Approach

Judd Goodmanjgoodman@geosyntec.com

CASQA ConferenceNovember 2 , 2010

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Objectives

How are geomorphic impacts modeled?

Δhydrology, Δchannel geometry,Δbed & bank material strength,Δsediment supplyf( )Geomorphi

cImpact

=

What is geomorphology?

Why should I care about it?

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What is geomorphology?

Geomorphology = the scientific study of landforms and the processes that shape them

Fluvial = of, relating to, or occurring in a river or stream

Geomorphic Impact = Changes in landforms and the processes that shape them. Often caused by land use change.

Restoration vs. Hydromod Managementfix an existing

geomorphic impactprevent a future

geomorphic impact

Pre-Development

flow flow

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Why care about geomorphology?Example of geomorphic impact:

flow

140-ft

flow

Images from

GoogleEarth

Post-Development

17-ft

Judd

300-ft~290 acre watershed

Hydromo

d!

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Qs D50 α Qw SQualitative:

Δhydrology, Δchannel geometry,Δbed & bank material strength,Δsediment supply

f( )GeomorphicImpact

=Quantitative:

Lane (1955)

Source: Rosgen (1996), From Lane, 1955. Reprinted with permissions

How are geomorphic impacts modeled?

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ΔhydrologyHydrologic Models are applied to simulate the hydrologic response of catchments under pre- and post-developed conditions for a continuous period of record.

Pre-Urban

Post-Urban

Time

Disc

harg

e

Hydrologic Inputs:• Rainfall• Catchment

Delineation• Soils• % Imperviousness• Lag Time• In-stream

Infiltration• Evapotranspiration

flow

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ΔhydrologyFlow output from hydrologic model is used to generate flow duration curves.

Currently Hydromodification Management focuses on

State of the practice is flow-duration matching.

Δhydrology

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FACTOR ATTRIBUTE NATURAL

CONDITIONDEVELOPED CONDITION

HYDROLO

GY

Drainage Area ~286 acres ~296 acresAverage Annual

Rainfall 18.4 inches

Hydrologic Soil Group 88% Type B, 10% Type C, 2% Type A

Imperviousness 0 to 1% 33%Peak Flow 600 cfs 731 cfs

Runoff Coefficient 0.033 0.349

Δhydrology: SEVERE

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Cross-sections and longitudinal profiles of the active channel are surveyed at strategic locations during the geomorphic field assessment.

Δchannel geometry

flow

Plan View

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FACTOR ATTRIBUTE NATURAL CONDITION

DEVELOPED CONDITION

CHANNEL

GEOMETRY

Longitudinal Slope 3.0% 2.1%

Plan FormMultiple braided channels

Single incised channel cutoff from the

floodplain. Severe meanders at the

downstream end cut into hill slope toe.

Channel Depth 0.5 to 2 feet 12 to 18 feet

Channel Width 5 to 30 feet 50 to 140 feet

Hydraulic Roughness

Unvegetated: n = 0.035

Vegetated: n = 0.06

Unvegetated: n = 0.035

Vegetated: n=0.05

Δchannel geometry: SEVERE

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Δbed & bank material strengthFor each cross-section surveyed, a measure of critical shear

stress is obtained on the bed and bank material. • Non-cohesive bed:

Wolman Pebble Count and/or Sieve Analysis

• Cohesive bed and bank: Jet Test or Characterize using Tables

• Vegetated bank:Characterize using Tables

Bank Material TypeCritical Shear Stress

(tc lbs/ft2)

ASCE Manual No. 77

Hardpans, Duripans 0.67

Compacted Clays 0.50

Graded Loams with Cobble

0.38

Stiff Clays 0.32

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FACTOR ATTRIBUTE NATURAL CONDITION DEVELOPED CONDITION

BED AND

BANK MATERIAL

Median Grain Size of Bed

(D50)1 to 2 mm

Bed MaterialSand bed with

interspersed gravel and cobble.

Sand bed with abundant granitic cobble and

gravel. Cobble and gravel is absent further

downstream.

Bank Material

Consolidated sandy silt banks embedded

with gravel and cobble. Heavily

vegetated.

Consolidated sandy silt banks with embedded

gravel and cobble. Little vegetation.

Δbed & bank material strength: SLIGHT

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Δsediment supplySediment yields are calculated using a GIS raster based analysis. Sediment generated from developed land and areas tributary to detention facilities are removed in the post-project condition.

Sediment Yield Map

% Sed Reduction =[ Yield (pre) - Yield (post) ]

Yield (pre)

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FACTOR

ATTRIBUTE

NATURAL CONDITION

DEVELOPED CONDITION

SEDIMENT SUPP

LY

Sediment Yield

0.5 to 1.0ac-ft/sq-mi/yr

~0ac-ft/sq-mi/yr

Δsediment supply: SEVERE

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Model SummaryHydrologic

Model

Flow Duration Curve

HydraulicModel

Stage,Shear stress,Velocity

Channel Geometry

Transport ModelBed &

Bank Material Strength

Work/ Transport

Ratio of Transpor

tSediment Supply Reduction

Ratio of Sed

Supply

Statistical

Model

Probability of Geomorphic Impact

Rainfall,Catchment Delineation,Soils,% Imperviousness,Lag Time,In-stream Infiltration,Evapotranspiration

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Stage, effective shear stress, and flow velocity are computed using flow duration and channel geometry data as inputs to a hydraulic model.

Step 1:

n

iici tVW

1

tt

Work for Consolidated Material:

Stage, effective shear stress, flow velocity, and critical bed / bank material strength are then input into the applicable work or sediment transport equation and summed over the period of record.

Estimating Geomorphic Impact

Step 2:

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Step 3: Ratio of Transport is calculated by comparing relative changes in total work/transport capacity in the pre- and post-development conditions:

Transport postTransport pre

Estimating Geomorphic Impact

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Step 4: Sediment supply loss is accounted for by reducing the baseline Ratio of Transport by the Ratio of Sediment Supply to that computation point.

Sediment Supply post

Step 5: For each cross-section location the Ratio of Transport is compared to the Ratio of Sediment Supply (Target Ratio) to get a Probability of Instability.

Estimating Geomorphic Impact

Sediment Supply pre

Target Ratio of Transport =

Ratio of Transport (Target = 1.0)

Study in Bay Area:

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0.1

1.0

10.0

100.0

Stable/Low Med/High

Ep(E

xist

ing

/ Pre

-Urb

an)

Field Designated Erosion Condition

Statistical Relationship

40 Cross Sections: Thompson Creek Ross Creek San Tomas Creek

Santa Clara Valley HMP

Rati

o of

Tr

ansp

ort

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Out-of-Stream Mitigation Hydrologic

Model

Flow Duration Curve

HydraulicModel

Stage,Shear stress,Velocity

Channel Geometry

Transport ModelBed &

Bank Material Strength

Work/ TransportSediment Supply Reduction

StatisticalModel

Probability of Geomorphic Impact

Rainfall,Catchment Delineation,Soils,% Imperviousness,Lag Time,In-stream Infiltration,Evapotranspiration

Ratio of Transpor

tRatio

of Sed

Supply

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Out-of-Stream Mitigation Route post-development runoff through flow duration control facilities to mimic pre-development hydrology.

DetentionBasin

Bio-Retention

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In-Stream MitigationHydrologic

Model

Flow Duration Curve

HydraulicModel

Stage,Shear stress,Velocity

Channel Geometry

Transport ModelBed &

Bank Material Strength

Work/ TransportSediment Supply Reduction

StatisticalModel

Probability of Geomorphic Impact

Rainfall,Catchment Delineation,Soils,% Imperviousness,Lag Time,In-stream Infiltration,Evapotranspiration

Ratio of Transpor

tRatio

of Sed

Supply

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In-Stream Mitigation

Goal:Conserve Transport

Reduce longitudinal slope with in-stream grade control structures to mimic pre-development work/sediment transport.

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Project Solution?• Equilibrium Slope = 0.2% may not be feasible to construct.• Outfall structure needs to be retrofit to properly dissipate

energy.• Opportunity for combined flow control and grade control

mitigation.

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Thank You!Questions?

HydrologicModel

Flow Duration Curve

HydraulicModel

Stage,Shear stress,Velocity

Channel Geometry

Transport ModelBed &

Bank Material Strength

Work/ TransportSediment Supply Reduction

StatisticalModel

Probability of Geomorphic Impact

Rainfall,Catchment Delineation,Soils,% Imperviousness,Lag Time,In-stream Infiltration,Evapotranspiration

Δhydrology, Δchannel geometry,Δbed & bank material strength,Δsediment supplyf( )Geomorphic

Impact =

Ratio of Transpor

tRatio

of Sed

Supply

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Watershed Scale Longitudinal Profile