Andres A. Salazar
Walter P Moore and Associates, Inc.
Presentation(s):

Wednesday, June 6
4:00 PM – 5:30 PM
Stormwater Symposium
Oral
Andres Salazar
Walter P Moore and Associates, Inc.
Rainfall Depth Duration Frequencies (DDF) curves are used to determine design storms for storm water and flood control projects. Common sources for DDF curves include NOAA atlases (Atlas 14, TP40, etc) or local studies. These curves are derived from statistical analysis of historical rainfall data that typically cover several decades. Extreme events obtained from these curves are usually the 100year storm (1% annual exceedance chance) and 500year (0.2% annual exceedance).
But what happens when the rainfall event to be analyzed exceeds the maximum value calculated in the DDF tables? That question was posed when Hurricane Harvey dropped unprecedented amounts of rain over the Houston Metropolitan area between August 25 and 29, 2017. Almost every rain gauge in Harris County exceeded by a large margin the published 500year rainfall totals for the two and fourday durations.
A method to calculate the frequency or return periods of such extreme events is presented along with the results applied to Hurricane Harvey. The proposed method is based on extrapolation of the LMoment probability distribution. Rainfall totals across Harris County are analyzed for three hydrologic regions and summarized by gage and region. The method appears to prove that a rainfall like Harvey is extremely rare, something that is expected to occur in average about once every 30,000 to 50,000 years.
The method offers a quick means to better understand the rarity of Harveylike rainfalls using existing data, but it should be used with caution. Some limitations of the method are recognized and presented including the short period of analysis, climate variations, and the lower confidence interval for extreme events. Last, although a mathematical answer for the return period is found, Harvey now serves as a new benchmark of meteorological possibilities.
Walter P Moore and Associates, Inc.
Wednesday, June 6
4:00 PM – 5:30 PM
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