Flash Flood Prediction

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
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Distributive Modeling

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

11
LCRA - Lower Colorado River Authority
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The Forecast Spreadsheet early 1980s

• Inputs
• Colorado River Forecast _at_ 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
HECs Corps Water Management System (CWMS)

• Integrates the following models
• HEC-HMS, Hydrologic Modeling System
• 289 sub-basins
• 205 routing reaches
• 18,340 mi2
• 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 -gt 6 million gpm
18
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
• 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

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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?