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)

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

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

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

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

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

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

Human Adjustment

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

LCRA - Lower Colorado River Authority

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

The Spreadsheet - Forecasting

Catchment Forecast System (CFS)  Post 1997 flood  Integrates HEC programs:  PRECIP  HEC1  UNET

Hydromet: Hydrometeorological Data Acquisition System

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

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

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

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

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

Conclusions – Final Comparisons

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