coordinated solutions for floodplain management and risk ...• watershed-wide, structure-level...

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

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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