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
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
Forecast @ 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 homesProperty damages ~ $1 billion454 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
Tc 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?
