A Study of the Extremity of the October 1998 Rainfall Event

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Presentation transcript:

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

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

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

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

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

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

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

IDF Curves

Sample Rainfall Data

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 2.82 0.9099 11 2 hr 4.89 .9772 44 3 hr 5.63 .9789 47 6 hr 7.95 .9914 116 12 hr 9.40 .9903 104 24 hr 13.42 .9972 353

Putting it all together

Putting it all together

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...

Thanks for listening... Any questions?