Houston, TX

Study tries to understand how sediment affects urban streams

Marisol Gallagher

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HOUSTON — Flooding is an unavoidable reality for a city built on a floodplain. On top of that, proposals to mitigate it may not be realistic. A minimum of one-sixth the volume of Galveston Island of sediment was swept downstream during Hurricane Harvey in 2017. This problem has caused ongoing issues in the city.

“We need to step carefully,” said Andrew Stearns, a graduate student. He is currently working with UH geologist Julia Wellner, associate professor in the Department of Earth and Atmospheric Sciences, to study the phenomenon.

This problem affects policies as arguments are rising throughout the city about whether or not modifying stream flow to move flooding and associated sediments out of neighborhoods should be continued.

“When we move sediment and straighten streams to shorten their paths, we just push the problem to other neighborhoods further downstream, or we are essentially saying, this problem is for someone else to figure out,” said Stearns.

Using millions of dollars worth of commercially collected LiDAR data, Stearns studies the erosion and deposition of sediment caused by floodwaters from Harvey. His findings showed that sediment was stranded downstream on drainage banks, such as Buffalo Bayou Park near downtown Houston and in the Houston Ship Channel.

The tidal effect on the sediment movement was also one of the things people failed to consider. It extends up to five miles upriver of downtown. This tidal limit caused a drop in the stream velocity and was a secondary cause of an increase in sediment deposition downstream areas during the Harvey floods.

The Port of Houston spends more than $70 million every year only to dredge sediment from the ship channel. The port has also requested over $500 million to restore the ship channel to its normal depth after the Harvey Hurricane. Now, they are engaged in a $1 billion project to expand the largest port in the U.S., Stearns and Wellner believe that this information can help with sediment management.

“My team at UH studies sand grains and the properties of sand grains,” said Wellner. “It’s hard to believe that a pile of sand can grow from a single grain to a volume that costs millions of dollars to dredge and remove. It takes a project like this to come in and observe these piles of sand on a citywide scale and say: yes, we are free to straighten or pave channels, but that pile of sand, that sediment, grows especially after large floods and we have to understand the budget implications down the road or, more often, downstream.”

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