Comparing wave transformation of a hard, impermeable structure with various biogenic, permeable structures
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Intertidal areas worldwide are threatened by man-made basin alterations, boat-wakes, and sea-level rise, resulting in changed sediment dynamics and a process called sediment starvation. Traditionally, hard and impermeable structures are constructed on intertidal foreshores to attenuate wave energy and restore the disbalance in sediment dynamics. However, wave reflection and scouring in front of the structure are reasons why there is a growing consensus toward more permeable and biogenic structures in coastal defence schemes. The assessment of the interaction of waves with these permeable structures is limited in the literature. This flume study quantified and compared wave attenuation, reflection, and scouring potential of different-sized gabions filled with empty oyster shells, empty mussel shells, loose brushwood, and bundled brushwood to a hard brick stone structure under varying hydrodynamical conditions. The results show that consistent differences in wave attenuation were hardly observed between hard and biogenic materials. The emerged mussel structure even attenuated wave energy best for low submergence ratios. Emerged hard structures with low submergence ratios did generate up to 46.2% more wave reflection than the various biogenic structures for incident short-period waves. There was also a higher bed shear stress under wave action measured just before the emerged hard structure. Additionally, the correlation between wave reflection/attenuation and relative submergence showed a large spread, highlighting the importance of incident wave characteristics in describing this correlation. The findings demonstrate why there is increasing attention to using biogenic structures to protect intertidal areas from sediment starvation and can be used as guidelines for implementation under natural conditions.