coordinated solutions for floodplain management and risk ...• watershed-wide, structure-level...
TRANSCRIPT
KC Urban Stormwater Conference
February 4, 2019
Coordinated Solutions for
Floodplain Management
and Risk Reduction
Agenda
1. Project Background
2. Project Approach
3. Assumptions
4. Methodology
5. Results
Project Background
Project BackgroundShort description or sentence can go here.
Sub title
•
•
•
More Tips
•
•
Walnut Creek Watershed
53,000-acre HUC 12 Watershed
Project Background
Walnut Creek Watershed
53,000-acre HUC 12 Watershed
400+ acres developed annually
Project Background
2,200 parcels impacted by floodplain
628 parcels in Clive impacted by floodplain
1% Steams banks are stable
57% Steams have moderate to severe erosion
Project Background
• Goals:• Large Scale, Structure-Specific
Risk Assessment
• Evaluation of Current and Future
Flood Risk
• Mitigation Options
• Public engagement
Walnut Creek Mitigation Benefits
Estimator (MBE) Analysis
FEMA/STARR II
USACE
Walnut Creek WMA
Mitigation Benefits Estimator
Mitigation Benefits Estimator (MBE)• Automate the FEMA Benefit-Cost Analysis
(BCA) Toolkit
• Analyze structure-specific risk (residential and
non-residential)
• Estimate benefits/costs for various mitigation
options
• Elevation
• Acquisition
• Basement In-Fill
MBE Results
• Geodatabase
• Structure-specific results
• Flood risk
• Damages ($) – recurrence interval and
annualized
• Depth of flooding (ft)
• Mitigation Option Identification
• Cost and benefits
– Elevation
– Acquisition
– Basement fill-ins
Project Approach
Project Approach
• Phase I – Data Collection
• Stakeholder Group Formation
• Assumption Confirmation
• Phase II
• Flood Hazard Data Development
• Phase III
• MBE Assessment - Risk Assessment and
Mitigation Options Analysis
• Phase IV
• Reporting of Results
Data Collection
Stantec USACE Local
Structure Data
Building Value n n
Building Footprints n
First Floor Elevation n n
Replacement Value n
Flood Data
Existing Conditions n
Future Conditions n
Depth Damage Functions
Residential n
Non-Residential n
Mitigation Option Information
Elevation n n
Acquisition n n
Basement In-Fill n
Assumptions
Building & Damage Assumptions
FFE
• Building replacement value
• Cost per square foot: Craftsman 2018 Nation
Building Cost Manual
• Building Content value
• 1.0 (100% of building replacement value) per
USACE residential and non-residential contend
depth damage functions
• First floor elevation (FFE)
• Where the FFE was unknown a highest adjacent
grade plus one foot was used.
• Depth damage functions
• USACE Economic Guidance Memorandum 04-017
Flood Hazard Data Development
Walnut Creek at NW 100th Street
%Chance Exceedance Return Year
Existing Conditions Model (CFS)
FEMA Flood Insurance Study Discharges (CFS)
2018 USACE Future –Atlas 14 (cfs)
0.1 1000 35,660 41,400
0.2 500 23,280 23,245 35,300
0.5 200 18,410 28,000
1 100 16,600 16,570 23,000
2 50 13,410 13,340 18,400
4 25 10,450 14,400
10 10 7,110 7,105 9,900
20 5 4,990 7,000
Mitigation Project
Assumptions
• Elevation
• Acquisition
• Basement Infill
MBE Methodology & Analysis
Methodology
Water Surface
ElevationsFlood Depth
Percent Damage
Losses ($)
Annualized Loss
($)
Project Benefits
&
Costs
($)
Mitigation Benefit Ratio
Risk Assessment Methodology
• Methodology: based on FEMA’s BCA Toolkit (probabilistic)
• Probabilistic Damages – damages calculated at designated recurrence
intervals
Analysis
MBE Results
Walnut Creek Watershed MBE Results
• 13,079 Total Properties Analyzed
• 787 Properties With Risk > $0
• $4.5m Annualized Risk Existing
Condition
• $11.6m Annualized Risk Future
Condition
• 75% Residential Properties
City NameStructures Analyzed
At Risk Structures
Clive 3,044 396
Dallas Center 21 --
Des Moines 1,442 60
Grimes 334 4
Johnston 47 --
Urbandale 5,224 120
Waukee 184 --
West Des Moines 1,881 16
Windsor Heights 868 179
Other 34 12
City of Clive
100 Yr Approx
100 Yr Detailed
Floodway
500 Yr
City of Clive Annualized Risk Assessment Values
100 Yr Approx
100 Yr Detailed
Floodway
500 Yr
$14k
$23,440k
City of Clive Annualized Risk Assessment Values
100 Yr Approx
100 Yr Detailed
Floodway
500 Yr
$14k
$23,440k
Acquisition – Mitigation Benefit Ratio
> 1.00 (Max 5.1)
0.75 – 1.00
0.50 – 0.75
0.25 – 0.50
< 0.25
MBR Values
Properties Costs Benefits MBR
26 $24m $35m > 0.75
17 $19m $31m > 1.00
Elevation – Mitigation Benefit Ratio
MBR Values
> 1.00 (Max 8.7)
0.75 – 1.00
0.50 – 0.75
0.25 – 0.50
< 0.25
Properties Costs Benefits MBR
75 $15m $39m > 0.75
63 $8m $34m > 1.00
Basement Fill – Mitigation Benefit Ratio
MBR Values
> 1.00 (Max 2.1)
0.75 – 1.00
0.50 – 0.75
0.25 – 0.50
< 0.25
Properties Costs Benefits MBR
7 $201k $231k > 0.75
4 $113k $154k > 1.00
Specific Structure Example – NW 78th St, Clive
100 Yr Approx
100 Yr Detailed
Floodway
500 Yr
Specific Structure Example – Building Information
Structure Information
First Floor Elevation 830.8 ft
Building Type/Use Residential
Building Replacement Value $88,783
Content Value $44,392
Flood Damages ($) & Flood Depth – Pre Mitigation
Structure Damage Content Damages
Total Damages (structure +
Content) Flood Depth (ft)
5-year -- -- --
10-year $16,166 $5,342 $21,508 0.07
25-year $28,588 $7913 $36,501 3.06
50-year $43,466 $11,235 $54,701 6.42
100-year $88,783 $11,575 $100,358 6.91
200-year $88,783 $12,375 $101,158 7.49
500-year $88,783 $13,066 $101,849 8.13
1,000-year $88,783 $13,714 $102,497 8.69
Annualized Loss $3,504 $819 $4323 --
Flood Damages ($) & Flood Depth – Pre Mitigation
Benefits Costs MBR
Acquisition
$87,000 $127,000 0.67
Elevation
$77,000 $49,000 1.58
Basement Fill
$34,000 $17,000 2.07
Walnut Creek Watershed MBE Results
Existing Conditions
MBE Results
Future Conditions
MBE Results
Elevation Total Mitigation Cost
Needed (MBR>1.0)$10.6 million $32.2 million
Elevation Total Estimated Benefits
(MBR>1.0)$41.1 million $122.1 million
Acquisition Total Mitigation Cost
Needed (MBR>1.0)$20.4 million $38.7 million
Acquisition Total Estimated Benefits
(MBR>1.0)$34.4 million $96.8 million
Basement In-Fill Total Estimated
Benefits (MBR>1.0)$0.2 million $0.8 million
Basement In-Fill Total Estimated
Benefits (MBR>1.0)$0.3 million $1.5 million
Walnut Creek Watershed MBE Benefits
• Watershed-wide, structure-level flood risk
assessment – existing and future
• Identification of financial resources necessary to
mitigate flood risk
• Consistent risk communication across
organizations
• Opportunity to incorporate results into local HMPs
• Organization of HMA grant application inputs
• Preliminary evaluation of mitigation projects
• Mechanism to prioritize funding
Questions