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Quantifying Hydromodification Impacts and Developing Mitigation Using a F o u r Factor Approach. Judd Goodman jgoodman@geosyntec.com CASQA Conference November 2 , 2010. Objectives. What is geomorphology ?. Why should I care about it ?. How are geomorphic impacts modeled ?. ). f (.

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slide1

Quantifying Hydromodification Impacts and Developing Mitigation Using a FourFactor Approach

Judd Goodman

jgoodman@geosyntec.com

CASQA Conference

November 2 , 2010

slide2

Objectives

What is geomorphology?

Why should I care about it?

How are geomorphic impacts modeled?

)

f(

Δhydrology,

Δchannel geometry,

Δbed & bank material strength,

Δsediment supply

=

Geomorphic

Impact

slide3

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

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

Restoration vs. Hydromod Management

prevent a future geomorphic impact

fix an existing

geomorphic impact

slide4

Why care about geomorphology?

Example of geomorphic impact:

Judd

flow

flow

flow

flow

17-ft

140-ft

~290 acre

watershed

Hydromod!

300-ft

Post-Development

Pre-Development

4

Images from GoogleEarth

Include watershed changes and outfall undermining

slide5

How are geomorphic impacts modeled?

Qualitative:

QsD50αQwS

Lane (1955)

)

f(

Δhydrology,

Δchannel geometry,

Δbed & bank material strength,

Δsediment supply

Quantitative:

Geomorphic

Impact

=

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

slide6

Δhydrology

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

flow

  • Hydrologic Inputs:
  • Rainfall
  • Catchment Delineation
  • Soils
  • % Imperviousness
  • Lag Time
  • In-stream Infiltration
  • Evapotranspiration

Post-Urban

Discharge

Pre-Urban

Time

slide7

Δhydrology

Flow 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

slide9

Δchannel geometry

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

flow

Plan View

slide11

Δbed & bank material strength

For 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
slide13

Δsediment supply

Sediment 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.

% Sed Reduction =

[ Yield (pre) - Yield (post) ]

Yield (pre)

Sediment Yield Map

slide15

Model Summary

Rainfall,

Catchment Delineation,

Soils,

% Imperviousness,

Lag Time,

In-stream Infiltration,

Evapotranspiration

Hydrologic

Model

Flow Duration Curve

Hydraulic

Model

Channel Geometry

Stage,

Shear stress,

Velocity

Transport Model

Bed & Bank Material Strength

Ratio of Transport

Work/ Transport

Statistical

Model

Sediment Supply Reduction

Ratio of Sed Supply

Probability of Geomorphic Impact

slide16

Estimating Geomorphic Impact

Step 1:

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

Work for Consolidated Material:

Step 2:

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

slide17

Estimating Geomorphic Impact

Step 3:

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

Transport post

Transport pre

slide18

Estimating Geomorphic Impact

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

Target Ratio of Transport =

Sediment Supply pre

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.

Study in Bay Area:

Ratio of Transport (Target = 1.0)

slide19

Statistical Relationship

Santa Clara Valley HMP

Ratio of Transport

  • 40 Cross Sections:
  • Thompson Creek
  • Ross Creek
  • San Tomas Creek
slide20

Out-of-Stream Mitigation

Rainfall,

Catchment Delineation,

Soils,

% Imperviousness,

Lag Time,

In-stream Infiltration,

Evapotranspiration

Hydrologic

Model

Flow Duration Curve

Hydraulic

Model

Channel Geometry

Stage,

Shear stress,

Velocity

Transport Model

Bed & Bank Material Strength

Ratio of Transport

Work/ Transport

Statistical

Model

Sediment Supply Reduction

Ratio of Sed Supply

Probability of Geomorphic Impact

slide21

Out-of-Stream Mitigation

Route post-development runoff through flow duration control facilities to mimic pre-development hydrology.

Detention

Basin

Bio-Retention

slide22

In-Stream Mitigation

Rainfall,

Catchment Delineation,

Soils,

% Imperviousness,

Lag Time,

In-stream Infiltration,

Evapotranspiration

Hydrologic

Model

Flow Duration Curve

Hydraulic

Model

Channel Geometry

Stage,

Shear stress,

Velocity

Transport Model

Bed & Bank Material Strength

Ratio of Transport

Work/ Transport

Statistical

Model

Ratio of Sed Supply

Sediment Supply Reduction

Probability of Geomorphic Impact

slide23

In-Stream Mitigation

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

Goal:

Conserve Transport

slide24

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.
slide25

Rainfall,

Catchment Delineation,

Soils,

% Imperviousness,

Lag Time,

In-stream Infiltration,

Evapotranspiration

Hydrologic

Model

Thank You!

Questions?

Flow Duration Curve

Hydraulic

Model

Channel Geometry

Stage,

Shear stress,

Velocity

Transport Model

Bed & Bank Material Strength

Ratio of Transport

Work/ Transport

Statistical

Model

Ratio of Sed Supply

Sediment Supply Reduction

Probability of Geomorphic Impact

f(

)

Δhydrology,

Δchannel geometry,

Δbed & bank material strength,

Δsediment supply

=

Geomorphic

Impact