More Onion Creek floods likely, hydrology expert concludes

A study of the Onion Creek Halloween flood performed by a University of Texas expert showed the flood was not a unique event for the area and that the city should be ready for similar events.

David R. Maidment, a UT professor of hydrology, gave a presentation that showed while the flood was severe — with storm surges equivalent to a Category 3 hurricane — flooding that damaged hundreds of homes fell within normal statistical predictions.

“This was not something that was an act of God,” Maidment said.

Maidment also said there may have been some misplaced blame on faulty flood gauges that malfunctioned as the creek crested. Greater use of other measurement tools outside of the city of Austin could have shown a flood was coming, he said.

His analysis pegged the flood as one so severe it should happen about every 50 years. But Maidment and officials at Monday’s Public Safety Commission noted that his estimation was only based on averages and showed it should not be a shock to see similar flooding once every 15 years.

Maidment examined the uniqueness of the flood, whether development exacerbated the situation and future risk to the area as well as characteristics of the Onion Creek Watershed.

The watershed is larger than the entire city of Austin, stretching from Dripping Springs to Buda to Austin-Bergstrom International Airport. Within that area rainwater flows from small tributaries to the meandering portion of Onion Creek where the flooding occurred and eventually to the Colorado River east of Del Valle.

Maidment concluded that development did not increase to the severity of the flood. The heaviest rains fell far upstream from the Onion Creek neighborhood. But he did not rule out it having an affect in any future event where the worst rain might be falling in the area of the flood.

Commissioners unanimously voted in support of Maidment’s suggestion that the city participate in a larger national conversation about water issues and that the Halloween flood be submitted as a case study to federal agencies.

“I don’t think this is Austin’s problem to solve alone,” he said.

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