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Flash Flood Prediction

Flash Flood Prediction. James McDonald 4/29/08. Introduction - Relevance. 90% of all national disasters are weather and flood related Central Texas – “Flash Flood Alley” Project Goal – To compare flash flood prediction on multiple scales (NWS, LCRA, City of Austin).

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Flash Flood Prediction

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  1. Flash Flood Prediction James McDonald 4/29/08

  2. Introduction - Relevance • 90% of all national disasters are weather and flood related • Central Texas – “Flash Flood Alley” • Project Goal – To compare flash flood prediction on multiple scales (NWS, LCRA, City of Austin)

  3. Agencies – The main players • NWS – ultimately responsible for issuing flood warning • LCRA – manages flood releases from the Highland Lakes • City of Austin – flood monitoring and emergency management

  4. NWS – West Gulf River Forecasting Center • 1 of 13 NWS River Forecasting Centers • Also includes in house Meteorological forecasting

  5. NWS River Forecast Procedure

  6. NexRAD Radar • 4 km x 4 km grid cell • Rain gages across state used to calibrate radar • Radar - mean areal precipitation (per hour)

  7. Sub-basins • Delineated by USGS streamgage • Total QPF (Quantitative Precip. Forecast) • 6 hr. UH time step • Stage height by USGS rating curve

  8. NWSRFS - (NWS River Forecast System) • Similar to HEC-HMS (good for design) • NWSRFS – good for real time flood forecasting • Sacramento Soil Moisture Accounting Method (SAC-SMA) • Able to adjust soil moisture to tweak model

  9. Human Adjustment

  10. Distributive Modeling • The Future • Finer rainfall distribution instead of mean areal across entire sub-basin • Rainfall variation - GIS

  11. LCRA - Lower Colorado River Authority

  12. The Forecast Spreadsheet – early 1980’s • Inputs: • Colorado River Forecast @ San Saba • Time-lagged tributary hydrographs • WSEL of Lake Buchanan & Lake Travis • Reservoir releases

  13. The Spreadsheet - Forecasting

  14. Catchment Forecast System (CFS) • Post 1997 flood • Integrates HEC programs: • PRECIP • HEC1 • UNET

  15. Hydromet: Hydrometeorological Data Acquisition System

  16. HEC’s Corps Water Management System (CWMS) • Integrates the following models: • HEC-HMS, Hydrologic Modeling System • 289 sub-basins • 205 routing reaches • 18,340 mi^2 • HEC-ResSim, Reservoir Evaluation System Simulation • 7 reservoirs • HEC-RAS, River Analysis System • 864 cross-sections • 347 river miles

  17. History of Austin flooding November 15, 2001: 8” of rain in 6 hr. (reported up to 15”) October 17, 1998: 31 deaths 7,000 people evacuated from homes Property damages ~ $1 billion 454 Austin homes damaged December 20, 1991: 200 homes in Travis and Bastrop counties were completely under water May 24, 1981: 13 drowned, $36 million in damages, Shoal Creek 90 gpm -> 6 million gpm

  18. City of Austin – Am I flooding now? • Flood Early Warning System • Emergency Operations Center

  19. Austin Watersheds – Time is not our friend • 7,000 homes in Austin floodplain • Tc range 30min. – 6 hr. • Lowering arm – low water crossing – marginal success • 20 roads close with 5-yr storm

  20. Flood Early Warning System • Components include: • ~40 creek and lake gauges • ~80 rain gauges • Automated data transfer every 15 minutes to the National Weather Service • High Hazard Dam Action Plan

  21. Conclusions – Final Comparisons

  22. Special thanks to: • Mike Shultz, National Weather Service, Ft. Worth, TX • Matt Ables & Melinda Luna, LCRA • Susan Janek, City of Austin, FEWS Questions?

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