Flood Event Analysis to Estimate the Avoided Damages Due to Flood Improvement Projects & Voluntary Buyout Program Ataul Hannan, P.E., CFM Planning Division Director Harris County Flood Control District ASFPM 2016 Annual Conference Grand Rapids, MI, June 19-24, 2016
Overview Harris County Estimating Avoided Losses 2015 Rain Events Due to Buyouts Results Due to Projects Results 2016 Tax Day Rain Events
Harris County
Harris County Terrain
The Flood Control District Harris County Open Channel Network 2,500 Miles of Bayous and Creeks Area = 1,756 Square Miles ± 1,500 Channels ± 2,500 Miles of Channels Population = 5 Million (County) 2.7 Million (Houston)
Floodplain Map
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
OUR MISSION Provide flood damage reduction projects that work, with appropriate regard for community and natural values.
Flood Damage Reduction Tools Widen and deepen bayous and creeks Flood Damage Reduction Tools Relocate residents out of the floodplain Excavate large stormwater detention basins
Rain Events of 2015 9 PM 10 PM
May 2015 Rainfall Ten Wettest Months in Texas History (1895-Present) May 2015 June 2004 May 1914 September 1936 May 1929 May 1935 September 1974 October 1919 April 1957 November 2004 6.66 6.62 6.55 6.45 6.38 6.32 6.26 6.19 6.18 8.93 Rainfall (in) May 2015 wettest month ever since 1895. Surpassed previous record by 2.27 inches! May 2015 rainfall = 35 trillion gallons across TX!
Rainfall Heat Maps
White Oak Bayou Memorial Day Event I-45 & N. Main Heights Blvd
Property Damage
Rainfall Heat Maps
Concentrated House Flooding Areas Total flooded homes likely between 3000 and 6000
Losses Avoided Due to Buyouts
Voluntary Buyouts Millions spent on voluntary buyouts $207M for over 2,000 structures via FEMA grants 75% Federal Share More than 900 other properties purchased with District funds
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 Structures inside the 0.2% (500-year) floodplain for studied streams in the County Data is stored in a shapefile FF Elev. River Station WSELs Economic Damage Estimate Buyout Status
Stream Gage Network 133 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
Event WSEL Profiles
Buyout Avoided Damages Profile
Avoided Damages Due to Buyouts Memorial Day 2015 Flood Event
Result Highlights
Flood Reduction Projects
Rainfall Heat Maps
Sims Bayou Project 19.3 miles of Channel Improvement 3 Regional Detention Basin 22 Bridge Replacement Federal Cost to Date $263M HCFCD Cost to Date $129M
Sims Bayou Project
White Oak Bayou Project 15.3 miles of Channel Improvement 4 Regional Detention Basin 12 miles of park/trail Replacement Federal Cost to Date $70M HCFCD Cost to Date $54M
White Oak Bayou Project Jones Rd Fairbanks-N. Houston W. Little York Hollister Rd.
Project Brays 21.2 miles of Channel Improvement 4 Regional Detention Basin 30 Bridge Replacement Federal Cost to Date $167M HCFCD Cost to Date $141M
Eldridge Project Brays 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 Brays Bayou Channels Holcombe Chimney Rock South Rice
Brays Bayou Memorial Day Event Eldridge looking east Arthur Storey Park looking south Mike Driscoll Park/ Old Westheimer looking north
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
Modeling Method I Approximate Method Gaged Floodplain VS Effective Combination Floodplain Constructed components of Project are indirectly included in the analysis for the Gaged Floodplain
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 1.4 Z = 300R radar Calibration of radar data based on trend between radar reflectivity values and hourly rainfall totals at each gauge 1.3 Z = 249R radar
Radar Rainfall
Method II Stream Profiles
Method I Results
Method II Results
Sims Bayou Results
White Oak Bayou Results
White Oak Bayou Results
Avoided Damages Due to Projects May 25 October 25 October 31 Brays Bayou 7,000 200 200 Sims Bayou 1,700 --- 1,700 White Oak Bayou 1,600 350 350
April 17-18 Radar 1 st Round 7 pm to 1 am 11 pm 2 nd Round 1 am to 8 am 3 am
Highest Rainfall Measured 5 min 1.0 Cypress Crk at SH 249 15 min 2.2 Greens Bayou at North Belt 30 min 3.6 Cypress Crk at SH 249 1 hour 4.7 Greens Bayou at Knobcrest 2 hour 7.3 Langham Crk at Longenbaugh 3 hour 8.3 Little Mound Crk at Mathis Rd 6 hour 13.9 Little Mound Crk at Mathis Rd 12 hour 16.7 Little Mound Crk at Mathis Rd 24 hour 17.4 Little Mound Crk at Mathis Rd 23.5 in 14.5 hours in Pattison, Waller County
Rainfall Comparison Recent Flood Events Date 1-hr 3-hr 6-hr 12-hr 24-hr 6/8/01 (Allison) 6.3 13.5 21.2 28.3 28.5 4/18/09 6.9 9.2 9.9 10.0 11.0 5/26/15 (Memorial Day) 4.8 8.0 10.1 11.0 11.0 4/18/16 (Tax Day) 4.7 8.3 13.9 16.7 17.4
3 Largest Rainfalls in 37 Years July 1979 (TS Claudette): 43.0 inches (24-hrs) US 24-hr rainfall record Alvin (Brazoria County) June 2001 (TS Allison): 28.5 inches (12-hrs) Greens Bayou (NE Harris County) Tax Day 2016: 23.5 inches (12-hrs) S Waller County Harris County PMP vs. Historic Events Rainfall Duration 1979 2001 2016 38.7 12-hr N/A 74% 61% 47.1 24-hr 91% N/A N/A
12 Hr Peak Rainfall Isohyetals, Affected Areas
12 Hour Peak Rainfall Frequency
Cypress Creek Record flood levels set at six bridges. 11 bridges closed; 3 open between I-45 & US 290 Barker Cypress Huffmeister Grant Champions Forest
Preliminary House Flooding Locations ~60% flooded structures within 1% floodplain
FEMA Partnered Home Buyouts That Would Have Flooded 505 Greens Bayou 400 White Oak Bayou 220 Cypress Creek 180 San Jacinto River 150 Halls Bayou 35 Brays Bayou 10 Other Watersheds ~ 1,500 ~2,830 home buyouts in these watersheds since 1989
DRAFT EDIT 5.31.2013
DRAFT EDIT 5.31.2013
QUESTIONS???? Reduce the risk and rediscover the beauty