Estimating Avoided Losses

Rain Events in Harris County2016 TFMA Spring Conference

Houston, TX

Todd Ward, P.E., CFM

Brian Edmondson, P.E., CFM

Nasiha Alibegovic, EIT, CFM

Harris County Flood Control District

A special purpose district created in 1937 by the

Texas legislature in response to floods that

devastated the Houston-area in 1929 and 1935

•Serves as a local partner to leverage federal tax

dollars for flood damage reduction

•Harris County – where we are

•Flood Control – what we do

•District – who we are

•Harris County Commissioners Court serves as

our board of directors or governing body

Provide flood damage

reduction projects that

work, with appropriate

regard for community

and natural values.

OUR MISSION

Flood Damage Reduction Tools Widen and deepen bayous and creeks

Excavate large stormwater detention basins

Relocate residents out of the floodplain

Flood

Damage

Reduction

Tools

Overview

Purpose of Estimating Avoided Losses

2015 Rain Events

HCFCD Flood Reduction Projects

Tools for Estimating Avoided Losses

Due to Buyouts

• Results

Tools for Estimating Avoided Losses

Due to Projects

• Results

Purpose of Estimating Avoided Losses

Millions spent on massive flood reduction projects

Millions spent on voluntary buyouts

• $207M for over 2,000 structures via FEMA grants – 75% Federal Share

• More than 900 other properties purchased w/ District funds

Translate these efforts into tangible numbers to

describe benefits to the communities

HCFCD USACE TOTAL

Brays Bayou $240M $240M $480M

Sims Bayou $124M $265M $389M

White Oak Bayou $54M $70M $124M

Losses Avoided Due to Buyouts

• Homes hopelessly deep (several feet) in floodplain

• HOUSE is the problem – NOT the stream

• Riverine flooding (bayous, creeks, and streams

coming out of banks)

Ideal Voluntary Home Buyout Properties

Rain Events of 2015

9 PM

10 PM

Rainfall Heat Maps

Data and Tools Utilized

• Most Up-To Date HEC-RAS Models

(FEMA Effective or newer)

• Structural Inventory Database

• Stream Gage Network (HCFCD Flood

Warning System)

Structural

Inventory Database

• Data is stored in a shapefile

Structures inside the

0.2% (500-year)

floodplain for

studied streams in

the County

• FF Elev.

• River Station

WSELs

• Economic Damage

Estimate

• Buyout Status

Stream Gage

Network

142 Gages along HC

creeks, bayous and

tributaries

• Sensors transmit

rainfall depths and

water levels in real

time

• Data publicly

available ww.harriscountyfws.org

GIS Data

Linearly Interpolated WSE Profile

Avoided Damages Due to Buyouts

Memorial Day 2015 Flood Event

WSEL VS Annual Chance Relationship

Log-Linear Relationship for Determining Percent Annual Chance Flood EventFEMA’s Guidance for Flood Risk Analysis and Mapping – Flood Depth and Analysis Grids (May 2014)

Wat

er

Surf

ace

Ele

vati

on

(Li

ne

ar)

Percent Annual Chance of Flooding (Log)

Event WSEL Profiles

• Determine peak WSEL at each stream gage

• Calculate % annual chance flood event

• Linearly interpolate % annual chance of

flooding at each XS between gages

(extrapolate U/S and D/S of most U/S and

D/S gages)

• Calculate estimated WSEL associated with

estimated annual chance of flooding at

each XS based on known frequency WSELs

Event WSEL Profiles

Buyout Avoided Damages Profile

Result HighlightsOctober 24-25 October 31

Greens

Bayou

Greens

Bayou

Hunting

Bayou

Approximate

Event Frequency5 to 7-Year 10-Year

10 to 100-

Year

Estimated

Flooded Structures80 264 271

Estimated Damage $1.63M $9.57M $7.11M

Buyouts 913 913 108

Avoided

Structural Flooding48 105 29

Avoided Damage $0.63M $1.83M $1.00M

Flood Reduction Projects

Sims Bayou Project

Sims Bayou Project

Sims Bayou Project

Sims Bayou Project

Sims Risk Map 1% Floodplain Change

FEDERAL BRIEFING March 2015 | Washington, D.C.

White Oak Bayou Project

Jones Rd Fairbanks-N. Houston

W. Little York Hollister Rd.

White Oak Bayou Project

Project Brays

Project BraysEldridge Mike Driscoll Park/ Old Westheimer

Arthur Storey Park Willow Waterhole

Project Brays

Over 11 “Astrodomes” of New Detention

New Channel Conveyance Longer than Loop 610

State Hwy 288 Holcombe

Chimney Rock South Rice

Brays Bayou Channels

Arthur Storey Park looking south

Eldridge looking east

Mike Driscoll Park/ Old Westheimer looking north

Brays Bayou Memorial Day Event

• Record flood levels - Loop 610 to US 59

• 10% (10-yr) and 2% (50-yr) frequency range

Peak: 52.2 ft

Low Houses Finish Floor

Gage Reading At South Rice

Str

ea

m E

leva

tio

n (

ft)

Time (Latest Reading: Wednesday, May 27, 2015 6:49 AM)

Brays Bayou Memorial Day Event

Event Date HWM

Allison (2001) 45.5’

05/26/15 40.2’

03/04/92 40.1’

Frances (1998) 37.6’

Ike (2008) 36.3’

Str

ea

m E

leva

tio

n (

ft)

Time (Latest Reading: Wednesday, May 27, 2015 6:49 AM)

Gage Reading At Heights Boulevard

Peak: 38.1 ft

• Downtown to Heights – just above 10-Year

White Oak Bayou Memorial Day Event

I-45 & N. Main

Heights Blvd

White Oak Bayou Memorial Day Event

Areas of Concentrated House Flooding

Property Damage

Losses Avoided Due to Projects

Available Data

• Gage Rainfall (HCFWS)

• Stream Water Surface Elevations

• Surveyed High Water Marks

• FEMA Effective Models and Profiles

• Updated Models and Profiles

• 2008 1-foot LiDAR

• Building Footprints with FF Elev.

• Rainfall Heat Maps

• Approximate Method

• Gaged Floodplain VS Effective

Combination Floodplain

• Constructed components of Project

Brays are indirectly included in the

analysis for the Gaged Floodplain

Modeling Method I

Method I Results

Modeling Method II

• HWM Floodplain VS Modeled Event

Floodplain

• Used over 40 surveyed HWM

• HWM Floodplain was field-verified

• May 25th Rainfall was modeled in

HEC-HMS and HEC-RAS using effective

models to estimate potential flood damage

pre-Project Brays

Gage Rainfall

Radar Rainfall

• NEXRAD Digital Precipitation Array (DPA)

• One-hour increments

• May 25 11:00 PM to May 26 2:00 PM

• Standard reflectivity-rainfall relationship

𝐙 = 𝟑𝟎𝟎𝐑𝑟𝑎𝑑𝑎𝑟𝟏.𝟒

• Calibration of radar data based on trend

between radar reflectivity values and hourly

rainfall totals at each gauge

𝐙 = 𝟐𝟒𝟗𝐑𝑟𝑎𝑑𝑎𝑟𝟏.𝟑

Radar Rainfall

Method II Stream Profiles

Method II Results

Sims Bayou Profiles

Sims Bayou Results

White Oak Bayou Profiles

White Oak Bayou Results

White Oak Bayou Results

Avoided Damages Due to Projects

May 25 October 25 October 31

Brays Bayou 7,300 210 210

Sims Bayou 1,700 --- 1,700

White Oak Bayou 1,600 350 350

QUESTIONS