Houston was inundated with sediment due to the impact of Hurricane Harvey.
In 2017, Hurricane Harvey hit the Gulf Coast of Texas and caused the most significant amount of precipitation ever recorded in the United States. Some areas of the state received over 60 inches (152 centimeters) of rain in just one week. As a result, an exceptionally large amount of sediment was also moved. A recent study published in Geology estimated that approximately 27 million cubic meters of sand, silt, and other debris were carried through Houston during and immediately after the storm. This is equivalent to up to 51 times the amount of sediment typically entering Galveston Bay in a year.
Julia Wellner, a sedimentologist from the University of Houston and coauthor of the study, stated that everything turned into a sediment source.
Sediment is not typically a focal point in disaster coverage, yet it presents a significant obstacle in the aftermath. Streets become covered in mud, sewers become blocked, reservoirs lose their capacity, and streams fill with sand. In response to Hurricane Harvey, the federal government allocated approximately $351 million to remove sediment from navigational channels in Texas.
However, despite the clear influence of the sediment, the extent of the storm’s agitation was not known until recently. This data is not just a point of interest; it holds practical significance for how cities can effectively plan for future calamities.
The color of Galveston Bay has changed to beige.
Using openly accessible digital elevation models (DEMs), the scientists examined the elevation of land prior to and following the storm to monitor the movement of sediment due to erosion and deposition. By including the reported dredging volume from the Houston Ship Channel and Buffalo Bayou Park, as well as an additional 100 million metric tons estimated by other researchers to have entered downstream Galveston Bay (which appeared beige in satellite imagery during the storm), the team was able to determine the total amount of sediment that flowed through the two flood control reservoirs and 12 primary waterways that drain the Houston metro region.
On August 31, 2017, NASA’s Earth Observatory captured an image of Houston from space, revealing sediment-filled waters in Galveston Bay just 6 days after Hurricane Harvey hit the area. The image was taken by Jesse Allen.
Andrew Stearns, a graduate student in sedimentology at the University of Houston, emphasized that the estimated amount of shifted sediment is significant but also a conservative estimate. He stated that the actual total is substantially larger due to a significant amount of sediment being moved underwater, which could not be observed through imagery.
“When land is developed, it becomes more susceptible to erosion of sediment.”
This basic approach is limited to the sediment above the water, but it’s still intriguing, said Nina Stark, a coastal geotechnical engineer at the University of Florida who was not involved in the study. “By keeping the methods a little bit on the simpler side, that maybe creates larger uncertainty, but that likely doesn’t change the message,” she said.
Stark stated that due to the simplicity of the approach, it can easily be implemented in other locations. The lessons learned from the hurricane can serve as a warning for other cities. Houston, the fourth largest city in the nation, experienced a 63% increase in its urban footprint in the 20 years leading up to Hurricane Harvey. This rapid expansion has resulted in a significant amount of pavement and other impermeable surfaces, which can accelerate the flow of water through the city. As a result, water can penetrate deeper into the soil at a faster rate.
The team also discovered that the rivers and creeks in Houston that were most altered with concrete had the highest sediment movement.
According to Stearns, when land is developed, it becomes more susceptible to erosion due to increased sediment.
Filling the Reservoir
As the amount of sediment in a waterway increases, the space available for water to flow decreases, resulting in more severe flooding.
The excessive amount of sediment in Houston will have lasting impacts, specifically in the Addicks and Barker Reservoirs. These reservoirs were constructed by the city in the 1940s as a means of flood control, and they are located approximately 20 miles (32 kilometers) west of downtown. However, over time, the reservoir watersheds have become significantly more developed, with only 1% of the land developed before construction and now 40% developed. This has resulted in approximately 20,000 homes being located near the reservoir dams. The sediment brought by Hurricane Harvey has further increased their risk of flooding.
Approximately 26% of the sediment that was transported ultimately accumulated in only two reservoirs, causing their holding capacities to decrease by approximately 1.2% and 1.6%, according to the research. This may not seem like a significant amount, but it is enough to potentially impact the effectiveness of future flood events, as noted by Wellner. Even a storm slightly smaller than Harvey could now have similarly destructive outcomes.
To accurately forecast reservoir water levels, a thorough understanding of the processes that occur during these events is crucial.
The writers suggest that it is necessary for cities to consider the movement of sediment within their watersheds.
“In the past 20 years, I don’t recall any mentioned instances of flooding where sediment was identified as a significant issue,” stated Stearns. This is likely because it is not easily visible and is submerged. Due to the small size of sand particles and the large size of basins, it is often overlooked. However, understanding the movement of sediment can greatly benefit policy and planning efforts.
Torbjörn Törnqvist, a geologist at Tulane University who was not part of the study, emphasized the importance of comprehending the mechanisms that occur during these occurrences in order to accurately forecast reservoir levels.
Törnqvist concurs with Stearns’ assertion that only a small number of urban planners are considering sediment levels in their development plans. “While there is a greater focus on managing water during extreme events, addressing sediment remains a relatively unexplored territory,” he stated.
“J. Besl, a science writer, tweeted this.”
Citation: Besl, J. (2023), Hurricane Harvey filled Houston with sediment, Eos, 104, https://doi.org/10.1029/2023EO230380. Published on 6 October 2023.
Text © 2023. The authors. CC BY-NC-ND 3.0
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