Estimating Avoided Losses Rain Events in Harris …Estimating Avoided Losses Rain Events in Harris...
Transcript of Estimating Avoided Losses Rain Events in Harris …Estimating Avoided Losses Rain Events in Harris...
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