Quantifying
Hydromodification
Impacts and
Developing Mitigation
Using a Four Factor
Approach
Judd Goodman
jgoodman@geosyntec.com
CASQA Conference
November 2 , 2010
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Objectives
What is geomorphology?
Why should I care about it?
How are geomorphic
impacts modeled?
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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 Management
fix an existing
geomorphic impact
prevent a future
geomorphic impact
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Why care about geomorphology?
Example of geomorphic impact:
Judd
flow
flow
flow
~290 acre
watershed
300-ft
Pre-Development
Images from GoogleEarth
17-ft
flow
140-ft
Post-Development
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How are geomorphic impacts modeled?
Qualitative:
Lane (1955)
Source: Rosgen (1996), From Lane, 1955. Reprinted with permissions
Quantitative:
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Hydrologic Models are applied to simulate the hydrologic response of catchments
under pre- and post-developed conditions for a continuous period of record.
Hydrologic Inputs:
•Rainfall
•Catchment Delineation
•Soils
•% Imperviousness
•Lag Time
•In-stream Infiltration
•Evapotranspiration
Discharge
flow
PostUrban
Pre-Urban
Time
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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.
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HYDROLOGY
FACTOR
ATTRIBUTE
NATURAL
CONDITION
DEVELOPED
CONDITION
Drainage Area
~286 acres
~296 acres
Average 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
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Cross-sections and longitudinal profiles of the active channel are surveyed
at strategic locations during the geomorphic field assessment.
flow
Plan
View
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CHANNEL GEOMETRY
FACTOR
ATTRIBUTE
NATURAL
CONDITION
DEVELOPED CONDITION
Longitudinal Slope
3.0%
2.1%
Plan Form
Single incised channel
cutoff from the floodplain.
Multiple braided
Severe meanders at the
channels
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
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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
Critical Shear Stress
Bank Material Type
(tc lbs/ft2)
ASCE Manual No. 77
Hardpans, Duripans
0.67
Compacted Clays
0.50
Graded Loams with
Cobble
Stiff Clays
0.38
0.32
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FACTOR
ATTRIBUTE
BED AND BANK MATERIAL
NATURAL CONDITION
DEVELOPED CONDITION
Median Grain
Size of Bed
(D50)
1 to 2 mm
Bed Material
Sand bed with abundant
Sand bed with
granitic cobble and gravel.
interspersed gravel and
Cobble and gravel is absent
cobble.
further downstream.
Bank Material
Consolidated sandy silt
Consolidated sandy silt banks
banks embedded with
with embedded gravel and
gravel and cobble.
cobble. Little vegetation.
Heavily vegetated.
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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
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SEDIMENT
SUPPLY
FACTOR ATTRIBUTE
Sediment
Yield
NATURAL CONDITION
DEVELOPED CONDITION
0.5 to 1.0
ac-ft/sq-mi/yr
~0
ac-ft/sq-mi/yr
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Model Summary
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Estimating Geomorphic Impact
Stage, effective shear stress, and flow velocity are computed
using
and
data as inputs to
a hydraulic model.
Stage, effective shear stress, flow velocity, and
are then input into the applicable
work or sediment transport equation and summed over the
period of record.
Work for Consolidated
Material:
n
W   t i  t c   V  ti
i 1
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Estimating Geomorphic Impact
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
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Estimating Geomorphic Impact
is accounted for by reducing the baseline
Ratio of Transport by the Ratio of Sediment Supply to that
computation point.
Target Ratio of Transport =
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)
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Statistical
Relationship
40 Cross Sections:
 Thompson Creek
 Ross Creek
Ep
(Existing / Pre-Urban)
Santa Clara
Valley HMP
100.0
10.0
1.0
 San Tomas Creek
0.1
Stable/Low
Med/High
Field Designated Erosion Condition
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Out-of-Stream Mitigation
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Out-of-Stream Mitigation
Route post-development runoff through flow duration control facilities to
mimic pre-development hydrology.
Detention
Basin
Bio-Retention
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In-Stream Mitigation
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In-Stream Mitigation
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?
(
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)
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Watershed Scale Longitudinal Profile
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