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Bastrop County's Alum Creek study will aid search for future flooding fixes

Austin American-Statesman logo Austin American-Statesman 5/9/2019 By Brandon Mulder, Austin American-Statesman
a bridge over a river in a forest: The Alum Creek Bridge washed out during the Memorial Day weekend storm in 2016. It was replaced through a federal bridge program and completed and opened in March 2017. CONTRIBUTED © Austin American-Statesman/Austin American-Statesman/TNS The Alum Creek Bridge washed out during the Memorial Day weekend storm in 2016. It was replaced through a federal bridge program and completed and opened in March 2017. CONTRIBUTED

May 09--Bastrop County engineering consultants have launched a study of one of the county's many creeks that flow into the Colorado River, another step in a years-long effort to quantify and analyze the county's flood prone nature.

HALFF Associates is taking a close look at the Alum Creek watershed, an area that encompasses 56 square miles and 72 stream miles and runs southward from the northern portion of Bastrop State Park to the Colorado River. Alum Creek will be the eighth creek the consultants will analyze after completing studies last year of the Willow-Gazley Creek near Smithville and Gills Branch and Piney Creek in Bastrop.

Taken together, the studies will help the county and cities guide future development away from flood-prone areas.

"The whole point of this is to help reduce flood risk," said HALFF Associates project manager Paul Morales. "With the best available data, it helps a municipality or a county manage the flood plain risk and reduce it. So as development occurs, we can push it outside of the 100 year flood plain and reduce flood risk."

Already, the creek has posed flood threats to existing roads, bridges and structures in the watershed, county officials said. During heavy storms in May 2016, the creek's high waters destroyed a bridge that spanned a tributary, washing out its abutments until it collapsed.

"It blew out a bridge that had been there since 1950 and was structurally fine," Bastrop County Precinct 2 Commissioner Clara Beckett said.

The county's biggest motivator behind the Alum Creek study is trying to ascertain how the area has changed following the 2011 Complex Fire, whose burn area nearly matches the boundaries of the watershed. Without the vegetation, and with root systems wiped out by the blaze, holding the area's sandy soils in place, erosion has likely altered the topography in the watershed, the consultant said. Until vegetation can fully grow back and lock the earth into place again, the flow rates of streams have increased.

"Those roots that were holding soil aren't infiltrating the water anymore," Morales said. "Everything's just running off, so (the water) is getting to the creeks quicker and then your peak flows in the creeks go up, which results in a wider flood plain."

The flood plain maps issued by the Federal Emergency Management Agency decades ago are outdated and inaccurate, county officials said. The data that is collected by the engineers will not only serve the county's permitting process, but will also be turned over to FEMA in order to update the county's flood plain maps.

The data will also be used to determine solutions for flooding issues where development already exists.

"We're going to come up with some solutions to try and mitigate the existing flooding that's occurring, whether it's on roadways or homes," Morales said. "The solution may not solve all the problems, but it will help to minimize the flooding."

After the study, which is scheduled to be completed in May 2020, the county will look for grant money from federal and state sources to fund flood mitigation projects, officials said.

Other creeks in Bastrop County that have previously been studied by HALFF Associates include Cedar Creek, Walnut Creek, Sandy Creek, Willow Creek, Gazley Creek, Gills Branch and Piney Creek.

The Gills Branch study, which was ordered by the city of Bastrop, called for channel improvements to Gills Branch as well as stormwater flow enhancements at three road crossings. Those improvements are estimated to cost $4.3 million.

"It's a long process, but it's a process that needs to be done in order to reduce risk and save lives," Morales said.

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