Experiment on wave overtopping and overwash of bare and vegetated dunes

Kobayashi, NobuhisaThe observed increase of population in coastal areas worldwide and the rise in sea level highlight the importance of protecting the coast. It is commonly known that a beach with a dune provides storm protection. However, rapid profile changes and destruction of sand dunes may be caused by wave-induced overwash and increase the flood risk landward of the dunes. This investigation examines a possible non-intrusive form of shore protection. Vegetation has been used to stabilize dunes against wind and wave attenuation by vegetation has been investigated to predict wind waves propagating across inundated vegetated areas. However, the effects of vegetation on dune erosion and overwash during storm events has never been studied. A laboratory experiment encompassing five tests on the effects of woody plants on erosion and overwash of high and low dunes was performed. While fordune scarp-ing occurred for the three high dune tests, it did not occur for the two low dune tests. In comparison to the respective bare dune, wave overtopping and overwash was not reduced by a narrow vegetation zone on a steep backdune of a high dune. However, a wide vegetation zone covering the high dune reduced foredune scarping, prevented wave overtopping initially and reduced sand overwash after the initiation of wave over-topping. By retarding wave uprush and reducing wave overtopping and overwash, a wide vegetation zone covering an entire low dune reduced dune erosion. The data set obtained will be applied in future to extend the time-and depth averaged, process based numerical model CSHORE to include the vegetation effects on wave overtopping and overwash of vegetated dunes. The extended CSHORE may be applied to prototype dunes.University of Delaware, Department of Civil EngineeringM.C.E