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FLASH FLOOD PREDICTION James McDonald 4/29/08
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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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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
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NWS – West Gulf River Forecasting Center 1 of 13 NWS River Forecasting Centers Also includes in house Meteorological forecasting
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NWS River Forecast Procedure NexRAD Radar Rain gauge calibration Rainfall Sacramento Soil Moisture Accounting Method Model Lag + K routing Stage height from rating curve Forecast
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NexRAD Radar 4 km x 4 km grid cell Rain gages across state used to calibrate radar Radar - mean areal precipitation (per hour)
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Sub-basins Delineated by USGS streamgage Total QPF (Quantitative Precip. Forecast) 6 hr. UH time step Stage height by USGS rating curve
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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
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Human Adjustment
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Distributive Modeling The Future Finer rainfall distribution instead of mean areal across entire sub-basin Rainfall variation - GIS
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LCRA - Lower Colorado River Authority
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The Forecast Spreadsheet – early 1980’s Inputs: Colorado River Forecast @ San Saba Time-lagged tributary hydrographs WSEL of Lake Buchanan & Lake Travis Reservoir releases
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The Spreadsheet - Forecasting
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Catchment Forecast System (CFS) Post 1997 flood Integrates HEC programs: PRECIP HEC1 UNET
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Hydromet: Hydrometeorological Data Acquisition System
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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
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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
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City of Austin – Am I flooding now? Flood Early Warning System Emergency Operations Center
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Austin Watersheds – Time is not our friend 7,000 homes in Austin floodplain T c range 30min. – 6 hr. Lowering arm – low water crossing – marginal success 20 roads close with 5-yr storm
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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
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Conclusions – Final Comparisons
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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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