Quantifying Hydromodification Impacts and Developing Mitigation Using a F o u r Factor Approach
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Transcript of Quantifying Hydromodification Impacts and Developing Mitigation Using a F o u r Factor Approach
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Quantifying Hydromodification Impacts and Developing Mitigation Using a Four Factor Approach
Judd [email protected]
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