Analysis: 2,300-year rain event caused October flood

AUSTIN (KXAN) -- Devastating flooding last month was caused by what may have been the most intense rainstorm ever recorded in Austin. Calculations by the City of Austin's Watershed Protection Department have determined the downpour on Rinard Creek was so unusual, it is considered a 1-in-2,300 year rain event. 12.7 inches fell in the creek basin in just six hours the morning of Oct. 30.

On Marble Creek in southeast Austin, the six hour total of 12.4 inches was a 2,100 year rainfall. And in Dry Creek, the foot of rain in six hours was determined to be a 1,700 year rain event.

The flooding along Dry Creek in Southeast Travis County killed three people. Hundreds of homes were damaged or destroyed.

Austin Floodplain Manager Kevin Shunk told KXAN First Warning Weather he continues to research the storm, but believes the 13 inches of rain in six hours near ABIA is likely the most rain ever recorded in Austin in such a short duration. Shunk says it is important for Austinites to understand that this was not a 2,000 year rain event for all of the city, just those specific creek basins in southeast Travis County.

What is called a "return period" of 2,300 years does not mean the event will only occur once in two millennia, but that there is a 1-in-2,300 chance of it occurring in any given year. Shunk says, "it could happen again tomorrow, but the likelihood of that amount of rain falling again in that duration is extremely small."

Both flood-producing storms in late October were likely related to the strong El Niño Pacific Ocean pattern.

Monday night on KXAN News at 10, Jim Spencer will take an in-depth look at what one NASA scientist is calling the "Godzilla" El Niño, and what it means for Central Texas weather is the weeks and months ahead.  

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