Effects of Hurricane Harvey's extraordinary rain event on sedimentation at the tidal inlets of Galveston Bay, Texas

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Tropical cyclones cause significant changes in coastal sediments and shorelines. While there is an understanding that storm surges are important, the effects of fresh water surges from high rainfall events are less well studied and understood. Hurricane Harvey dumped ~34 trillion gallons of rainwater on the Texas-Louisiana Gulf of Mexico coast, the most of any tropical storm in U.S. history. The Houston-Galveston area was at the bullseye of the rainfall, creating an extraordinary 11-day fresh water surge into Galveston Bay, the largest Texas estuary. Water flow volumes were so high and outflow currents so strong that sand deposits were left all along the banks of Houston's normally slow-flowing bayous (streams and rivers), indicating that large volumes of coarse sediment were mobilized by the storm, much of which likely ended up in Galveston Bay. Just weeks prior to the hurricane, The University of Houston completed geophysical sonar surveys of the two natural inlets to Galveston Bay: Bolivar Roads and San Luis Pass. These surveys gathered geophysical data that resulted in the production of high-resolution depth maps and acoustic images of the inlet floors which showed sediment types, thickness, and vertical layering and bottom features, among other things. This research repeats those surveys, post-storm, and collects sediment cores to document storm erosion and deposition at Bolivar Roads and San Luis Pass. A comparison of the pre- and post-storm surveys provides an unprecedented opportunity to learn about the erosion and sediment transport and deposition of a storm the magnitude of Hurricane Harvey. Broader impacts of the project include data and reports that will be shared with the public and relevant government agencies to aid in management and decision making because Bolivar Roads is not only one of the busiest shipping channels in the nation but is also the proposed location for the gates of a coastal barrier that has been proposed to protect Galveston Bay. Information from a comparison of the two sets of sonar surveys will be critical for planning and managing any proposed flood/storm surge structure and results of the study will be made available to the public and appropriate government and planning officials. An additional impact will be the training of students who will participate in the post-storm marine geophysical survey. Because the University of Houston has a diverse student body and is a Hispanic-serving institution, efforts will be made to engage students from minorities under-represented in the sciences.This research repeats marine geophysical surveys (side scan, swath bathymetry, and CHIRP) at Bolivar Roads and San Luis Pass, the two major inlets to Galveston Bay in coastal Texas where identical surveys were taken just a few weeks prior to Hurricane Harvey. This repeat survey will document changes to the inlets and estuary resulting from the unprecedented rainfall event associated with Hurricane Harvey, which dumped 34 trillion gallons of rain on the Houston/Galveston area. Sediment cores from the estuary and inlets will also be collected. Results of the repeat survey and analyses of the cores will be compared to the results of the survey taken pre-storm to get a better understanding of sediment erosion and deposition and redistribution. It will also examine changes in estuary/inlet seafloor morphology. The San Luis Pass survey will use a CHIRP sonar (2-16 kHz) to image subbottom layers over a network of ~50 km of track lines near the pass because the water in this area is too shallow for swath sonars. The Bolivar Roads survey will cover a 2 x 6.5 km section of the inlet using the CHIRP sonar and a 548 kHz side-scan sonar/interferometric bathymetry instrument. Soundings from the latter system will be used to make a high-resolution bathymetry map, which will show changes in depth caused by storm erosion and deposition. This sonar also provides acoustic backscatter images of the water bottom, showing topography, sediment type, bedforms, and anthropogenic debris. A third cruise will collect ~25 gravity cores, that will be used for ground-truth of the sonar images and to look for a storm event layer. Comparison of pre- and post-storm surveys will show how the strong currents and heavy sediment loads of the rainfall surge affected the inlets. This information is important for complete coastal sedimentation models as well as for agencies tasked with coastal management, especially if the planned coastal barrier to protect Galveston is built across Bolivar Roads.