Effect of an artificial oyster reef on wave attenuation

Abstract

An increasing amount of the worlds coastal regions suffer from structural erosion due to anthropogenic influences. A prime example for this phenomenon is the Oosterschelde, Netherlands, where the partial closure of an estuary lead to a decline of the local intertidal flats, threatening the valuable functions they provide as an ecosystem and for shore protection. A sustainable measure to slow down the decline, is the usage of wave disrupting oyster reefs, which reduce the wave energy on the flat, thus reducing erosion. As of now, the conditionality and extend of the wave attenuation effect of such reefs is still widely unknown. This is thwarting a more extensive usage of oyster reefs for erosion management, since the actual effects of such a reef are highly uncertain prior to construction. This thesis was designed to investigate and quantify wave attenuation capabilities of an artificial oyster reef in a macrotidal environment, improving the understanding of the impact of such structures on their physical environment. A wave analysis was carried out on a reef using field data, allowing for the quantification of attenuation, the identification of processes, a parameterization of the results and an insight on the predictability of attenuation. Extensive field data was acquired on seven locations on a transect over the investigated reef, including pressure and velocity data. The reef was built using shell filled wire gabions and is orientated perpendicular to the main wind direction. It has dimensions of approximately 120 m length, 8 m width and 0.8 m height. The reef crest is located 0.5 m above mean low water level and 2.3 m below mean high water level, being emerged approximately 30 % of the time. Incoming waves are characterized by maximum significant wave heights of 0.8 m and maximum mean periods of 3 seconds. Wave attenuation was quantified by comparing the energy flux in front of (seaward) and behind (shoreward) the reef under different hydrodynamic conditions. The conditionality of attenuation was found to be best represented by the relative submergence ds /Hm0,i , which represents the ratio of the freeboard on top of the reef and the incoming wave height. Attenuation was found until a ds /Hm0,i ratio of approximately 4. Compared to the bare tidal flat in front of the reef, the reef attenuated energy up to 40 times more effectively per distance. The spatial footprint of the reef was found to vary strongly with different ds /Hm0,i ratios, but being roughly of the same order of magnitude as the reef length. Reflection, overtopping, wave breaking and friction were identified as the main mechanisms behind energy attenuation. An attempt to parameterize these was done using dimensionless numbers, which can be used to predict attenuation by using incoming wave, and reef characteristics. The determined processes were best represented by the parameters ds /Hm0,i and L01 /ds. Three empirical relations describing wave attenuation over breakwaters were used to investigate whether existing formulas can predict wave attenuation over the observed oyster reef. The measured and calculated results show a strong correlation of R ~ 0.9 but need to be tuned to reach reasonable root mean square errors. This implies that such existing formulas can be adjusted to represent oyster reefs, making them a possible tool to predict oyster reef performance in the future. The findings show that the present oyster reef is effective in attenuating waves and contribute to the overall understanding of how oyster reefs can reduce erosion. A better insight on the quantity of attenuation and the spatial footprint of the reef's effects was achieved. The footprint of the reef's effect is hereby limited to an area close to it, meaning that the reefs impact on erosion should not be overestimated. Such a reef should thus rather be seen as a complementary measure against erosion, stabilising the local area. As a next step, setting up a numerical model is recommended, where the wave attenuation can be linked to actual erosion quantities. Quantitative research on additional oyster gabions can further be used to refine existing empirical relations, creating some predictability of transmission values over oyster reefs.