Texas Colorado River Floodplain Coalition Locally Elected Officials Intro to Floodplains and Floodplain Management
December 17, 2018 Agenda 8AM-10AM - Session 1: Why it floods on the Colorado River……..Wes Birdwell 10AM-10:15 - Break 10:15-12N - Session 2: Where can I go for help?...... Wes Birdwell 12N-12:30 - Lunch (will be brought in) 12:30-3PM - Session 3: Intro to Floodplain Mgt ………Michael Segner, TWDB 3PM-3:15 - Break 3:15-5PM - Session 4: NFIP 101…..Michael Segner, TWDB “The picture’s pretty bleak, gentlemen … the world’s climates are changing, the mammals are taking over, and we all have a brain about the size of a walnut.” Session 1 Why it floods on the Colorado
It’s a big watershed It gets very heavy rainfall occasionally Hill country terrain is very steep Lower three counties are very flat Only one flood control reservoir Lower river can produce large floods
Why it floods on the Colorado River
Drainage Area Sq. mi. Lake Buchanan 6,500 Lake LBJ 5,000 Lake Travis 1,800 TOTAL 13,300 Lower River 3,600 The Texas Hill Country gets very big floods
Location of Maximum Discharge in USA Converging Weather Patterns Central Texas Flood History
1935 20”
2007 19”
October 2018 12” August 2017 60”
Colorado May 2015 25”
River Basin July 2002 45” August 2017 Hurricane Harvey Lower River
October 2018 Llano River 1935 Flood Austin TX Buchanan Dam July 1938 Flood Total Rainfall July 16-25, 1938 Location of Lower Colorado River Basin and Values of Flash Flood Magnitude Index
(adapted from Beard 1975) Location of Lower Colorado River Basin and Values of Flash Flood Magnitude Index (Adapted from Beard 1975) Colorado River Flow - Time Flood Waters Above Austin Colorado River Flow-Time Flood Waters Below Austin The Flood Protection “System” Structural and Non-Structural
DEED • STRUCTURAL My property • Dams • Levees • Channelization Floods are not the norm
Normal River Elevation Common Floods
Minor Flood Flood Warning Level
Normal River Elevation Occasional Flood
Moderate Flood Low Lying Structures
Normal River Elevation Rare floods
Major Flood Many buildings, homes, roads flooded Normal River Elevation River Flood Side View
Normal River Elevation River Flood Side View
Floodplain
Normal River Elevation
Flood Downstream = Flood Upstream Water Supply Dam
Dam
Water Supply
Flood Downstream < Flood Upstream Water Supply Dam
Flood Upstream
Dam
Flood Downstream Water Supply
Flood Downstream < Flood Upstream Multi-Purpose Dam
Dam
Water Supply Multi-Purpose Dam
Dam Flood Upstream
Control Point Controlled Flood Pool Floodgates Water Supply Flood Downstream
Flood Downstream < Flood Upstream Multi-Purpose Dam
Dam Flood Upstream
Uncontrolled Spillway Uncontrolled Flood Storage Control Controlled Flood Pool Point Floodgates Water Supply Flood Downstream
Flood Downstream < Flood Upstream Flood Mitigation Dam No Flood
Base Flow Upstream Dam
Flood Storage Base Flow Downstream
NO FLOOD Base Flow Upstream = Base Flow Downstream Flood Mitigation Dam In Flood
Flood Upstream Dam
Flood Storage Flood Downstream
IN FLOOD Flood Downstream < Flood Upstream Flood Inflow Volume Storage Flood Total Useable Minor Flood < Flood Time Release Flood Inflow Moderate Flood Total FloodVolume Useable storage volume Flood Time FloodPeak FloodgateRelease Mitigation Flood Flood Inflow Major Flood Time Total FloodVolume storagevolume UseableFlood FloodPeak Release Floodgate Mitigation Mitigation Flood The Highland Lakes System
Lake Buchanan Buchanan Dam Lake Inks Starcke Dam Lake LBJ Mansfield Dam
Lake Travis Lake Marble Inks Dam Falls
Lake Austin
Tom Miller Dam
Wirtz Dam Highland Lakes and Dams Highland Lakes Dams Designed Flood Operations Tools Buchanan 37 floodgates 0 floodpool Inks 0 floodgates 0 floodpool Wirtz 10 floodgates 0 floodpool Starcke 10 floodgates 0 floodpool Travis 24 floodgates 2 MAF floodpool Austin 9 floodgates 0 floodpool
Mansfield Dam Flood Control Plan
CORP OF ENGINEERS, MANSFIELD DAM AND RESERVOIR NORMAL FLOOD CONTROL REGULATION SCHEDULE
Reservoir Level Flood Control Release Control Points
3,000 cfs 30,000 cfs (20.5’)33’ @ Austin Forecast: 681-683 45,000 cfs (25.1’)27.2’ @ Bastrop 50,000 cfs (25.5’)35.5’ @ Columbus
30,000 cfs (20.5’)33’ @ Austin Forecast: 683-685 27.2’ 5,000 cfs 45,000 cfs (25.1’) @ Bastrop 50,000 cfs (25.5’)35.5’ @ Columbus
30,000 cfs (20.5’)33’ @ Austin Forecast: 685-691 5,000 cfs 45,000 cfs (25.1’@27.2’ Bastrop (a) during Jan, Feb, 50,000 cfs (25.5’)35.5’ @ Columbus Mar, Apr, Jul, Aug, Nov, Dec 30,000 cfs (20.5’)33’ @ Austin (b) during May, 30,000 cfs 45,000 cfs (25.1’)27.2’ @ Bastrop June, Sept, Oct 50,000 cfs (25.5’)35.5’ @ Columbus
30,000 cfs (20.5’)33’ @ Austin Forecast: 691-710 30,000 cfs 45,000 cfs (25.1’)27.2’ @ Bastrop 50,000 cfs (25.5’)35.5’ @ Columbus
----- 50,000 cfs 50,000 cfs (24.8’) @ Austin Forecast: 710-714 50,000 cfs (26.7’)----- @ Bastrop 50,000 cfs (25.5’)---- @ Columbus
Forecast: 714-722 90,000 cfs* No Controls The Bureau of Reclamation will specify the releases for Forecast: above 722 safety of the structure.
* Releases shall not exceed the associated peak flood reservoir inflow. Note: No curtailment of normal hydroelectric turbine releases shall be required due to flood control operations. EXHIBIT E-2 From Corps of Engineers Water Control Manual December 1979 The “System” Structural and Non-Structural
• NONSTRUCTURAL DEED • Hydromet NOAA Weather CurrentRadio Gages Coverage • Forecast Models in Central Texas • NOAA Radios • Local Permitting • Floodplains • Flood Easements
Current Hydromet System
LCRA Hydromet Streamflow Gauges (www.hydromet.lcra.org) LCRA Hydromet Rainfall Gauges Weather Radar Imagery Flood Warning Pre-1980’s
LCRA Upper Col R: LCRA Rainfall All Dams except Mansfield Observers Floodgate Operations are Report via based on lake elevation at telephone Buchanan the dam Inks Wirtz Llano R Mansfield Starcke LCRA USACE WCM: Western Tribs: Mansfield LCRA to fill LCRA Rainfall the flood pool, Observers Austin then evacuate Report via over time telephone Lower Col R: Observers Flood Warning 1980’s All Dams except Mansfield Floodgate Operations based NWS on lake elevation at the dam Upper Col R with increased reliance on NWS Forecast real time lake elevation and Rainfall Observers gaged streamflow
Buchanan Mansfield - LCRA LCRA Inks developed Western Tributaries forecast based on Llano R Wirtz Rainfall Observers Starcke Buchanan operations, lake 1. UT Austin Mansfield 2. Manual Fall-back elevation, and some gaged Austin streamflow USACE approve Real Time Streamflow Lower Col R: Gage NWS Forecast Flood Warning 1990’s All Dams except Mansfield Floodgate Operations Upper Col R: Forecast based on real time NWS Forecast Gaged lake elevation and Manual Fall-back Streamflow with some attempt to use real time gaged rainfall Buchanan Mansfield Inks LCRA develop Western Tribs: Llano R Wirtz forecast based LCRA Forecast Starcke on Buchanan 1. UT Austin Mansfield operations and 2. Manual gaged 3. LCRA (CFS) Austin streamflow USACE approve
Streamflow Gage w/rainfall Lower Col R: Rainfall Gage NWS Forecast Flood Warning 2000’s
Upper Col R: NWS Forecast Manual Fall-back All Dams except Mansfield LCRA Floodgate Operations based on real time gaged lake elevation and streamflow, with increased reliance on real time gaged rainfall
Buchanan Mansfield – Floodgate operations based on LCRA Inks developed forecast from gaged streamflow and rainfall, Llano R Wirtz Starcke USACE approve Western Tribs: Mansfield LCRA Forecast 1. CFS/cal-radar Austin 2. Manual Fall-back
Streamflow Gage w/rainfall Lower Col R: Rainfall Gage NWS Forecast Flood Warning Today
Upper Col R: NWS Real Time Streamflow Gage w/rainfall
Buchanan Inks Highland Lakes: LCRA
Llano R Wirtz Starcke Western Tribs: LCRA Mansfield Austin
Lower Col R: NWS FEMA’s Map Modernization Program 2004
Nationwide 5-Year Plan $ 1 Billion Budget Texas’ Map Mod Counties The Process..... No New Studies Information Search
Base Map
Terrain Data
Engineering Studies
Hydrology
Hydraulics Mapping Report Coordination /Outreach Floodplain Map Production
Flood Mapping Project Phases
Community Coordination Project Topographic Post & Mapping DFIRM Preliminary Scoping Data Preliminary Needs Production DFIRM Acquisition Processing Assessment
30 Days 30-90 Days 1-8 Months 12-18 Months 30 Days 12-24 Months
Time frames given are approximate and will vary from study to study New Floodplain Maps are Digital Layers
Base Map (Roads) New DFIRM Aerial Photography Terrain (Contours) Streams, lakes, rivers Old FIRM Engineered Flood Zones Zone A Refinement Zone A Refinement LIDAR Acquisition
LIght Detection and Ranging (LIDAR) Airborne Laser System Fixed Wing Helicopter Potential for High Level accuracy (1 foot range)
LIDAR: Affordable, Accurate Topo Main St. Main Bastrop County 2007 LiDAR
Highway 71
Bare Earth Points
Raw Surface Bare Earth Surface Point Density
►LiDAR
Photogrammetry Lake Travis Mapping Example – Gilleland Creek Detailed Engineering Study
Field Survey New Topo/Orthophoto Detailed Hydrologic and Hydraulic Modeling Floodway Computations Flood Profiles Calibrated to Historical Events Most Expensive Type of Study Field Surveys Engineering Study – Hydrology
HOW MUCH? Engineering Study - Hydraulics
HOW HIGH? Base + Terrain + Engineering Study
Digital Flood Insurance Rate = Map (DFIRM) Zone AE (Detailed Study) Redelineation Sample of Zone AE (Detailed Study) Stream
Sample Detail Study Stream Cross- Sections, BFEs, & Floodways Unstudied Streams
Flood Damages $$$$ Expensive Insurance $$$$ Flood Elevation???
Floodplain
Normal Elevation
River or stream -- cross-section Studied Streams
Existing Structures Flood Insurance (known risk) Base Flood Elevation Known
Known flooding depth Normal Elevation
River or stream -- cross-section Studied Stream With Improvement
Existing and New Structures No Flood Insurance Required
Base Flood Elevation
Excavated Areas
River or stream - cross-section Studied Streams Help Reduce Future Flood Risk
New Structures
Base Flood Elevation
Required Freeboard Normal Elevation
River or stream - cross-section Effects of Urbanization Structures in the 100-yr. Floodplain Are Increasing Around the Highland Lakes
6000 5254 5000 4000 2626 3000 2238 2000 1277
Structures 712 1000 361 663 608 2 115 273 124 0
Inks LBJ Travis Austin 1989 Buchanan Marble Falls 1998 Source: Phase 1 Contour Mapping Project, LCRA 1999 and Recon Study for Central Colorado River Watershed, USACE, 1989 Overall >180 % Increase in 10-yrs. 12000 10496 10000 8000 6000 3749
4000 Structures 2000 0 1989 1998
Source: Phase 1 Contour Mapping Project, LCRA 1999 and Recon Study for Central Colorado River Watershed, USACE, 1989 9% County-wide 15% 31% Percentage of Growth from 47% 51% 79% 1990-2000
41% 41% 51% 48% 9% 11%
3% 3%
Source: 2000 US Census Data 12% County-wide 15% 43% Percentage of Growth from 58% 87% 204% 1990-2010
78% 78% 95% 138% 23% 17%
3% 3%
Source: 2000 US Census Data 12% County-wide 15% 50% Percentage of Growth from 75% 104% 293% 1990-2018
100% 110% 124% 230% 25% 41%
7% 3%
Source: 2000 US Census Data LCRA/USACE Flood Damage Evaluation Project 1998-2004
Texas’ first comprehensive study of the potential for flooding on a major river Colorado River from San Saba to Matagorda Seamless 4 foot topography in the 500-Yr FEMA mapped floodplain Preliminary new 100 and 500-year flood information Estimated costs of flood damages Developed damage reduction alternatives along the Colorado River Evaluated costs and benefits Detailed Topo Map of 500 Yr Floodplain Developed topographic maps
Scale 1:2400 s ll a F le rb a M e k a L Model the Floods
82 Routed the floods Counted Structures in the Colorado River Floodplains Total Structures 100-year: 15,775 + 500-year: 28,229
TOTAL = 44,002
(Note: Bar graphs are cumulative) are graphs Bar (Note: NumberStructuresof
*Downstream of Mansfield Dam
84 Damages in Million $ Estimated Potential Flood Step Step 4
L. Buchanan $100 $150 $200 $50 $0 -
Inks L. $0.4 Estimate the Damages
L. MarbleL. Falls LBJ $5.4
$43.3 Damages
Travis Co.L. TravisBelow $16.1
$180.6 Bastrop Co.
$16.9 OverStructures 12,000 Flooded in million Damages $377 Colorado Co.
$6.2 Fayette Co. $22.2 Wharton Co.
Matagorda Co. $7.6
$62.3
$16.1 Lake Travis Structures at Four Flood Levels 2004
4,000 3,788
3,500 3,404 FEMA 3,000 2,698 Current Study 2,500 2,089 2,025 2,000 Reference Elevations Flood FEMA Study 1,500 1,308 500 Year 728.7 732.6 100 Year 716.2 722.0 1,000 50 Year 710.2 716.7 Number of Number Structures 424 500 10 Year 691.2 697.0 136 0 10 Year 50 Year 100 Year 500 Year
Note: Bar graphs are cumulative
86 Investigated alternatives for reducing flood damages
Do nothing Non-structural
Raise floodplain elevations and make insurance available
Buy and remove structures to reduce potential for damages
Modify Mansfield Dam operating rules for releasing floodwaters Structural
Construct new reservoir(s) upstream of Mansfield Dam
Build levees
Build local detention
Increase stream channel Combine one or more of the above 87 Lake Buchanan FDEP Profiles Inks Lake FDEP Profiles Inks Lake October Flood
Inks Lake peak 895’ October 18, 2018
Inks Lake at Buchanan Dam peak TW October 16, 2018 Lake LBJ FDEP Profiles Lake LBJ October 2018 Flood
El 836 @ Sandy Harbor El 837 @ RM1431 and Inks El 828 @ Wirtz Dam Gage Datum 970’
Llano Gage Datum...... 970’ October 16...... 40’ MSL……………1010’ Lake Marble Falls FDEP Profiles Lake Marble Falls October Flood
Lake Marble Falls El 753’ October 16, 2018
Lake Marble Falls @ Wirtz Dam El 769’ October 16, 2018 Lake Travis FDEP Profiles Lake Travis October Flood
Lake Travis elevation 704.4’ peak at noon on October 20, 2018
Starcke Dam peak TW El 721 October 16, 2018 Lake Austin FDEP Profiles Lake Austin FDEP Profiles
Lake Austin elevation 492.5’ peak at Tom Miller, 499’ at RR 620 Lady Bird Lake FDEP Profiles Bastrop County Colorado River FDEP Profiles
Bastrop Gage Datum....307.4’ Harvey……...... 25.9’ Oct 2018……....22.3’ MSL Harvey….333.3’ MSL Oct………329.7’
H Oct Upper Colorado County Colorado River FDEP Profiles Columbus and LaGrange Harvey HWM
LaGrange Gage Datum..210.04’ Harvey……..... 54.10’ MSL………….264.14’
Columbus Gage Datum...145.52’ Harvey……...... 48.09’ MSL………….193.61’ Lower Colorado County Colorado River FDEP Profiles Wharton County Colorado River FDEP Profiles Wharton County Harvey HWM
Gage Datum 52.42’ Harvey……...50.50’ MSL………..102.92’ Upper Matagorda County Colorado River FDEP Profiles
Gage Datum….0’ Harvey……...46.1’ MSL…………46.1’ Lower Matagorda County Colorado River FDEP Profiles Flood Damage on the Highland Lakes and Colorado River Average Annual Precipitation Texas
TP-40 24 hour 100 Year Rainfall
above Austin, but may be impacted below Austin TP 40 100-Yr Rainfall Probability in TCRFC district