1. A Study of the Extremity of the October 1998 Rainfall Event
in South-Central and Eastern Texas
A Term Project for CE 394K: Surface Water Hydrology
Presented by Teresa Hart

2. Background Severe Flooding on October 17-18, 1998
18 Counties in South-Central and Eastern Texas affected
Highest streamflow on
record at many locations
At least 13 die in flood-
related accidents
Severe and costly damage
resulted

3. Objectives
Determine the return-period at various time-durations for the storm event at selected rain gages
Produce a series of contour maps of iso-frequency lines for the various time-durations

4. Sources of Rainfall Data
National Climatic Data Center
Hourly data from major airports (Austin, San Antonio)
Cooperator network; Canyon Lake gages
Lower Colorado River Authority
Hourly data from Colorado River Basin
18 gages with at least 5 inches of precipitation
U.S. Geological Survey
Daily precipitation values for 7 gages with at least 7 inches

5. Intensity-Duration-Frequency (IDF)
Technical Paper 40 (US Weather Bureau, 1964)
Isohyetal maps for durations of 30 minutes to 24 hours
and return periods of 2 to 100 years
Based on historical data and Extreme Value Type 1 Distribution

6. Texas DDF Curves WRI Report 98-4044 (US Geological Survey, 1998)
Maps of equal parameter lines for durations of 15 minutes to 7 days
and any given return period
Based on historical data and Generalized Logistic Distribution

7. Generalized Logistic Distribution (GLO)
where, Xd(F) = precipitation depth for a given frequency
F = frequency
Three-parameter distribution
Location Parameter, x
Scale Parameter, a
Shape Parameter, k
Example: Find 12-hour, 25-yr depth
= 3.2
= 0.78
= -0.23

8. IDF Curves

9. Sample Rainfall Data

10. Determination of Return Period
GLO Distribution:
Solve for F and convert to return period:
For San Antonio Airport,
Time Interval Depth, in Frequency Return Period, yr
1 hr
2 hr
3 hr
6 hr
12 hr
24 hr

11. Putting it all together

12. Putting it all together

13. Things to think about New Frequency Curves for Texas--Good stuff
Be Sure to Empty your Rain Gages during Record Storms
Storms Don’t Follow IDF Curves
New Frequency Curves for Texas--Good stuff
GIS just might not be so imposing after all...

14. Thanks for listening...
Any questions?