Assessment of the influence of permeable pile groins on nearshore hydraulics

Abstract

One of the most economically and ecologically important issues related to coastal engineering is the preservation of the coastal areas. The mean to do so, is the good understanding of the sediment transport mechanisms that are involved in the transformation of coasts. In order to protect these valuable areas, man-made structures have been developed, such as permeable pile groins which are among the oldest, but also to the most interesting coastal structures. They constitute of wooden piles driven into the seabed perpendicular to the shore and thus they allow both water and sediment to pass through them, acting as a hydraulic roughness on the longshore current leading to a reduced littoral current velocity in their vicinity. However, although permeable pile groins have been constructed along many world’s coastlines through the years, the research done on how they affect the coastal system is still limited. The aim of this study is to bridge the existing knowledge gap and provide a clearer view on the factors that influence the nearshore hydraulics at Domburg coast. The evaluation of permeable pile groins is going to be done by means of a process-based model in SWASH. The SWASH model will be used for the examination of several hydraulic conditions, and will be afterwards validated using the results from the experiments that Hulsbergen carried out. The validation of the model is done in several steps, in order to make the understanding of the influence of each parameter easier. First, only the wave induced current is examined, followed by the tide induced current. The combination of wave and tide induced current follows and last but not least two (2) different types of permeable piles groins are implemented. By this method, not only the influence of permeable pile groins is examined, but also the capability of SWASH itself to capture the real phenomena and give accurate results. The results of the present study revealed that SWASH model is very capable of capturing the real behavior of the current, since high correlation with the experimental results can be observed for all the test-cases examined. The results also showed a clear retardation of the current due to the presence of the pile groins which in turn leads to a reduction in the capacity of the current to transport sediment.