Determining the efficacy of dune enhancement and beach nourishment practices to mitigate storm washover on a low-lying barrier island




Pollard, Claire Rydman


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Tropical cyclones that enter or form in the Gulf of Mexico generate storm surge and large waves that impact low-lying coastlines. Along much of the Gulf Coast, barrier islands are the primary line of defense against these powerful forces. Galveston Island, located 70 km south-southeast of Houston at the mouth of Galveston Bay, TX, is a major tourist and commercial center that has endured numerous hurricanes. Hurricane Ike is the most recent hurricane to make landfall on Galveston Island in September of 2008, causing dramatic changes to the coastal landscape. Discontinuous and densely vegetated foredunes less than 3-m tall and 30-m wide were the primary protection for 30 km of developed coastline on Galveston Island. The purpose of this study is to investigate the protective function of a foredune and determine if a larger dune system would have mitigated coastal erosion and flooding during Hurricane Ike.
A coupled hydrodynamic and morphodynamic numerical model, called XBeach, is used to simulate erosion and deposition induced by Hurricane Ike on a 2.6-km long portion of West Beach on Galveston Island. Six different simulated topographic scenarios were incorporated into the XBeach model to test the efficacy of dune enhancement and beach nourishment strategies. Results show that XBeach is a useful tool in simulating the effect of Hurricane Ike from the nearshore to the back barrier flats. Model results are assessed by comparing the post-storm computed surface to lidar data collected over the island three months after Hurricane Ike made landfall. XBeach displayed an excellent Brier skill score of 0.67 up to 0.92 within the foredune zone alone, and described erosion and deposition patterns well. Dune enhancement testing results indicate an unrealistically high foredune (6.5-m tall and 37-m wide) is required to prevent overwash by the 3.14-m high surge. Enhancing the dune system without nourishing the beach leaves the dune line exposed to direct wave attack, and consequently, the lower dunes are eroded and overwashed. However, by increasing the width of the beach and adding sand to the nearshore, wave and surge energy was further dissipated resulting in less erosion of the beach and foredune zones. A lower foredune (4.5-m tall by 37-m wide) in conjunction with a 25-m wide beach nourishment provided the greatest degree of protection for the study area and was the best use of sand. This research effort is intended to inform coastal managers of the best use of sand resources to protect the island from a future Ike.



barrier island, beach nourishment, dune enhancement, erosion, Hurricane Ike, XBeach



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