A GIS-Based Flood Risk Assessment for

the City of Rockford

University of Illinois

Prairie Research Institute

Illinois State Water Survey

Lisa Graff, CFM, GISP

Clayton Ballerine, CFM

Brad McVay, CFM, GISP

Project Partners • Illinois State Water Survey (ISWS)

• Illinois Department of Natural Resources, Office of Water Resources (IDNR/OWR)

• City of Rockford

• FEMA

City of Rockford, IL

Rockford Flooding

Flood Studies

Map Change

Impacts

6

Scope of Work

• ISWS

▫ Multi-frequency Flood Depth Grids

▫ Percent 30-year Flooding Chance Grid

▫ HAZUS Level 2 Risk Assessment

▫ Database

▫ Maps of Priority Areas for Mitigation Action

Data Deliverables

• Detailed, structure-by-structure flood risk data for each building in the floodplain.

▫ Survey Data

▫ Building Characteristics

▫ Flood Risk Information

What is Risk?

Risk

What is Hazus?

• Program designed by FEMA for the purpose of providing communities with the means to identify and reduce risk from natural hazards

• Program elements include:

– Hazus-MH

– User Groups

– Education Program

– Other resources

• Available from FEMA free of charge (requires ArcGIS license)

Input

detailed engineering

data

Combination of local and default hazard,

building, and damage data

Default hazard, inventory, and damage information

Levels of analysis

Required

user effort and

sophistication

Level 3

Level 2

Level 1

Level 1 Level 2 Level 3

Hazard •USGS NED

•Stream centerlines based

on topography

•USGS regional regression

equations and gage records

will be used to determine

discharge frequency

curves.

•User prepares hazard data

using the FIT

•User will supply improved

DEM

•User supplies flood

modeling results including

stream cross-sections

attributed with elevations,

or lines of

BFE

•A flood boundary of some

form is required.

•User will likely work with

hydraulic models outside

of the flood model and the

FIT

Levels of analysis

• What you need:

▫ Best Available Flood Inundation Data

▫ GIS Structure Inventory

▫ Assessors Information

Data for Analysis

• Depth Grids for 5 flood frequencies

Flood Inundation Data

• Effective FEMA Floodplain Data

Structure Data & Assessors Data

Structure Identification: Cont.

Structure Identification: Cont.

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Survey Data

Collection

20

21

Survey Data Collection

Level 1 Level 2 Level 3

Inventory Hazus default data. Allocation of census block data via statistical analysis, and broad assumptions for first floor elevation. General land use, Lifelines, Agriculture, Vehicles inventory, Essential facilities.

•User supplied inventory data, such as Tax Assessor data. •Users enhance the first floor height and other parameters

High quality data regarding building values, flood vulnerabilities, contents, occupancies, etc., extended to industrial and other high-value facilities.

Levels of analysis

User Defined Facilities (UDF)

• Hazus method for site specific analysis

• Inputs:

▫ Building inventory

Table containing values for each structure

Very specific format for upload

Lat Long values to represent location

▫ User Defined Depth Grids

10%, 4%, 2%, 1%, and 0.2% annual chance floods

UDF versus Census Block GBS • GBS data is aggregated to the Census Block

▫ Assumes structures are

distributed evenly over

the census block

• 1% Annual Chance Flood

▫ Building losses

GBS = $1,474,000

UDF = $0

UDF versus Census Block GBS • GBS tends to overestimate

economic losses

• 1% Annual Chance Flood

▫ Building losses

GBS = $249,000

UDF = $131,000

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Filling in the gaps

• Tax exempt structures

• Missing attributes

▫ Occupancy Class

▫ Replacement Cost

▫ Square Footage

▫ Number of Stories

▫ Year Built

▫ Design Level

▫ Foundation Type

▫ First Floor Height

Filling in the Gaps

• Sources:

▫ Zillow/Realtor/Trulia

Square footage, Year Built, Foundation Type, Number of Stories

▫ Streetview

Foundation Type, Number of Stories

▫ Orthophotos

Square footage (estimated)

Filling in the gaps

• Hazus Technical Manuals

▫ Replacement cost RS Means 2006

$ values per square foot for different occupancy classes

▫ Default values Square footage, First floor height, etc.

• Document methodology/sources

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Occupancy Class

29

Residential

Commercial

Industrial

Agricultural

Exempt

Level 1 Level 2 Level 3

Damage Curves

•Broad regional default curves based on available FIA or USACE depth damage curves. •Library of curves available for user selection.

•User specifically modifies the existing curve library for local practices.

•User-input curves based on detailed building surveys, specific crop conditions etc.

Levels of analysis

Depth-Damage Curves • Percentage of damage is calculated for each structure based on the depth of flooding.

▫ Based on Occupancy, foundation type

▫ For buildings, first floor height

▫ For equipment, height above first floor

• Damage percentage is used to determine loss in USD

Depth of

flooding

Depth used for damage estimation

Analysis Output

• Building Loss

• Content Loss

• Business Inventory Loss

• Percent Damaged for Structures

• Average Annualized Loss (AAL)

Average Annualized Loss

• Estimated long-term value of losses averaged on an annual basis

• Useful for estimating potential flood losses over a defined period of time

• Formula

▫ (10% – 4%) *(Loss 10% + Loss 4%) / 2 +

▫ (4% – 2%) * (Loss 4% + Loss 2%) / 2 +

▫ (2% – 1%) * (Loss 2% + Loss 1%) / 2 +

▫ (1% – 0.2%) * (Loss 1% +Loss 0.2%) / 2 +

▫ 0.2% * Loss 0.2%

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Presenting Results – Best Practice

• Results are ESTIMATES and should be treated as such

• Include a disclaimer

• Results should be rounded

• Results can be summed for an area

or focused on individual structures

depending on need.

Presentindg Results

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Presenting

Results

Presenting Results

Keith Creek Mitigation

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2007 2008 2009 2010 2011 2012 Total

#Properties Acquired

0 98 14 0 0 0 112

#Properties Demolished

0 12 0 37 39 0 88

1998

2016

10% Annual Chance Depth Grid

4% Annual Chance Depth Grid

2% Annual Chance Depth Grid

1% Annual Chance Depth Grid

0.2% Annual Chance Depth Grid

Thank You For Listening!

44

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