Beargrass Creek Case Study Description of the Study Area Hydrology & Hydraulics Economic Analysis...
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Transcript of Beargrass Creek Case Study Description of the Study Area Hydrology & Hydraulics Economic Analysis...
Beargrass Creek Case Study
• Description of the Study Area
• Hydrology & Hydraulics
• Economic Analysis
• Project Planning
• Assessment of the Risk Based Analysis Methodology
Beargrass Creek Study Area
North Fork
Middle Fork
South Fork
Buechel Br
Ohio River
61 mi2
Drainage Area
Levee on the Ohio River
Pump Station at the Levee(Capacity 7800 cfs!)
Concrete-Lined Channel
Detention Pond
Inlet Weir
Beargrass Creek at the Detention Pond
Pond Outlet Pipe
1
2
3
4
5
6 7 8
10
1112
13
1415
12
34
5
Buechel
Branch
(2.2 m
iles)
South Fork Beargrass C
reek (12 miles)
Damage Reaches
9 Example Reach SF-9
Beargrass Creek Case Study
• Description of the Study Area
• Hydrology & Hydraulics
• Economic Analysis
• Project Planning
• Assessment of the Risk Based Analysis Methodology
Flood Frequency Curve (SF-9)Separate curve for each reach and each plan
Uncertainty in Frequency CurveReach SF-9, Without Plan Conditions
Prob Mean
(cfs)
Mean +2 SD
Mean -2 SD
Log10 (SD)
0.01 4310 3008 6176 0.0781
0.5 1220 1098 1356 0.0229
QKQQ10log1010 *loglog
1
2
3
4
5
6 7 8
10
1112
13
1415
12
34
5
Buechel
Branch
(61 cr
oss-sec
ts)
South Fork Beargrass C
reek (202 cross-
sects)Water Surface Profiles
9
Water Surface Profiles
Uncertainty in Stage-Discharge
SD= 0.5 ft at 100 yr flow
ConstantReduces prop.to depth
Beargrass Creek Case Study
• Description of the Study Area
• Hydrology & Hydraulics
• Economic Analysis
• Project Planning
• Assessment of the Risk Based Analysis Methodology
Computation of Expected Annual Damage (EAD)
Stage (H)
Dis
char
ge (
Q)
Exceedance Probability (p)
Dis
char
ge (
Q)
Stage (H)
Dam
age
(D)
Exceedance Probability (p)D
amag
e (D
)
1
0
)( dppDEAD
Damage Categories
• Single-family residential• Multi-family residential• Commercial buildings• Public buildings• Automobiles• Cemeteries• Traffic disruption• Utilities
p=0.999
p=0.1p=0.01p=0.002
Structures
Index Location
• Each damage reach has an index location
• All structures are assumed to exist there
• First floor elevation adjusted to reflect the change in location within the reach Rm 9.960
Rm 10.363
Rm 10.124
Index for SF-9
Invert
p=0.01
p=0.1p=0.5
Building Damage
• Value of the structure, V• Value of the contents,
C = kV • k=V/C, contents to value
ratio (~40%)• Damage is a function of
depth of flooding, expressed as ratio,r(h), of value
First Floor Elevation
h
ChrVhrD )(21
Depth, h r1(h) r2(h)
3ft 27% 35%
6ft 40% 45%
Uncertainty in Building Damage• Value of structure,
– SD=10% of V for residential
– Commercial distribution described by
• Value of contents (SD of k in C=kV)
• Uncertainty in first floor elevation, SD=0.2ft
• Uncertainty in damage ratios, r(h)
First Floor Elevation
h
ChrVhrD )(21
Stage-Damage Curve
Multi-family Residential, Reach SF-9
Stage-Damage Curves
• Each structure is treated individually
• Stage-damage curve with uncertainty is produced for each damage category for each reach
• Added together to give the total stage-damage curve for the reach(?)
Beargrass Creek Case Study
• Description of the Study Area
• Hydrology & Hydraulics
• Economic Analysis
• Project Planning
• Assessment of the Risk Based Analysis Methodology
Planning Team
• Three key people:– Planner: formulates project alternatives, works
with local sponsor– Hydraulic Engineer: determines discharge and
stage data– Economist: estimates damage, costs, benefits
and does the risk analysis
Planning Methodology
• Identify potential project components (detention ponds, levees, …)– 22 initially proposed, 11 on Beargrass Creek, and 11 on
Buechel Branch
• Evaluate them all individually to see if net benefits are positive– 8 components on Buechel Branch eliminated
• Combine components into plans, incrementally – 10 components in NED plan: 8 detention ponds,
1 floodwall, 1 channel improvement
1
2
3
4
5
6 7 8
10
1112
13
1415
12
34
5
Buechel
Branch
Three Plan Development Reaches
932
1
Risk of Flooding
• Establish a target stage at each damage reach index point
• Find annual probability of exceeding that stage
• Find reliability of passing design floods
Target Stage
Assessment of Engineering Risk
• Conditional probability– Assumes a particular flood
severity
• Annual probability– Integrates over all flood
severities
• Risk measures actually used– Annual exceedance probability
– Conditional nonexceedance probability
Target Stage H
F(h)
0
1
Nonexceedance probability
Exceedance probability
Computation of Engineering Risk Measuresfrom the Stage-Frequency Curve
Annual exceedance probability– Find pe for target stage at each Monte
Carlo replicate– Get expected value and median of pe
values over all simulations– Get long term risk as 1-(1-pe)n
Conditional nonexceedance probability– Find H* for given p* at each
replicate– Find % of replicates for which
H* < Target stage
Q
Q*
f2(H|Q)
H*
p
f1(Q|p)
p*
Q*H
p
f3(H|p)
p*
H*
H
pe
Target Stage
Q
Beargrass Creek Case Study
• Description of the Study Area
• Hydrology & Hydraulics
• Economic Analysis
• Project Planning
• Assessment of the Risk Based Analysis Methodology
Overall Assessment
• The core methodology is solid and is an advance in engineering practice of flood risk assessment
• Focus is completely on damage reaches considered as statistically independent entities
• Whole project risk and 25%,50%,75% damage values cannot be built up this way
• Can specification of standard deviations of analysis variables be improved?
Beargrass Creek 100 year Flood Plain Map
Middle Fork
South Fork
Spatial Subdivision of the Region
Spatial Unit Used for
Whole River Expected Annual Damage (EAD), Benefit-Cost analysis
3 Main River Reaches Incremental analysis to get NED plan
22 Damage Reaches Basic unit for analysis using HEC-FDA
263 Hydraulic Cross-sections
Water surface elevation profile computation
2150 Structures Structure inventory
Whole Project Risk Assessment
• Take a flood of severity, p, and integrate the damage along the reach– Without any plan (o)– With a plan (w)– Benefit of plan is B = Do - Dw
• Randomize the flood discharge and stage for the whole project rather than for each reach
• Compute project-based damage values for each randomization and use them to get B25, B75 values