IMPACTS AND VEGETATION-INDUCED ATTENUATION OF WIND- AND VESSEL-GENERATED WAVES
Date
2019-05
Authors
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Publisher
University of Delaware
Abstract
Rising sea levels due to climate change combined with increasing economic activity along coastal areas necessitate effective coastal defenses. Ships laden with heavy cargo can generate large wave groups that may erode shorelines. Pea Patch Island, the home of Fort Delaware and a major wading bird nesting site located on the Delaware River, suffers from beach erosion and wetlands loss. The Delaware River attracts an array of cargo ships which generate large ship wakes. Riprap installed on part of the island’s shoreline absorbs and reflects wave energy, but increases erosion at the ends of the riprap structure. On shorelines without riprap, vegetation attenuates wave energy by inducing drag on the water, thus reducing erosion rates. Two studies were conducted simultaneously from June 6 to July 9, 2018 on the island to determine the effects of ship wakes on the east and west sides of the island and how vegetation in the retreating western wetlands attenuates ship wakes. Ultrasonic distance meters, pressure sensors, and current meters were deployed in cross-shore transects to obtain water depths, and cross-shore and alongshore water velocities. Time-lapse cameras recorded daytime imagery of ship passages and wakes. At the western marshy site. the vegetation patch was composed of Phragmites australis and Schoenoplectus pungens. Measured plant characteristics and site elevation profiles were used to explore potential impacts on hydrodynamics. Total wind wave energy exceeded total ship wake energy during the month-long deployment, but the ship wake energy was more heavily concentrated. The relative contributions of wind-generated waves increased landward along the transect. Vegetation more effectively attenuated high frequency significant wave heights than low frequency significant wave heights. Phragmites australis was shown to effectively attenuate wave energy flux by at least 30%.
Description
Keywords
Civil engineering, Attenuation of wind and waves