Bastrop County 2007 Lidar
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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 Minor Flood Flood Inflow Flood Total Useable Flood < Flood Volume Storage Release Time Moderate Flood Flood Peak Useable Flood Flood Mitigation storage volume Flood Inflow Flood Floodgate Release Total Flood Volume Time Major Flood Flood Peak Flood Useable Flood Mitigation storage volume Floodgate Release Flood Inflow Flood Total Flood Volume Time 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