Optimizing Shoreface Nourishment Designs using Process Based Models

Abstract

Although shoreface nourishments are an increasingly interesting option for coastal managers to counteract coastal erosion, their design is often highly empirical. A better understanding of the way shoreface nourishments interact with the initial beach profile may help to reduce the degree of empiricism in nourishment design. Literature study has shown that numerical modelling is commonly used to design shoreface nourishments and to assess their development and efficiency. It has also shown that there are two dominant mechanism are to be expected from a shoreface nourishment to lower the coastal erosion. These mechanisms correspond to an increase in beach width and a lowering in significant wave height. Previous performed studies on shoreface nourishment design effects along the North-Holland coast (de Sonneville, 2012) concluded that the differentiating factors possibly influencing the counteraction of coastal erosion are; offshore distance, height and alongshore length. Therefore the objective of this study is to investigate how the significant wave height and beach width are affected by changing the; offshore distance, height and length of a shoreface nourishment using a process based modelling suite. These effects are divided in two different time scales: • First the long term shoreface nourishment effects are modelled in Delft3D to check how the beach width and wave height are affected after one year of morphological modelling. • The short term storm events effects of a shoreface nourishment are checked by modelling a storm event in XBeach. To see whether the effects per nourishment are alike for various beaches, two different cases are considered for the long term modelling approach. For the location of the two cases eyes fell on previous studies on shoreface nourishments in Noordwijk (NL) and Nags Head (NC,USA). Long term modelling results of both cases show that shoreface nourishments with low height and large alongshore length are best for lowering the significant wave height and increasing the beach width. Differences per case are found in the offshore distance of the nourishment. In Noordwijk a nourishment behind the outer breaker bar is regarded as the best design to counteract coastal erosion, where in the Nags Head case a nourishment on top of the breaker bar is preferred. As waves shape the lateral beach profile through a morphodynamic feedback loop, profiles tends to flatten as the wave height increases, thereby dissipating more wave energy. This transition between beach states and the response time needed to changes between these states implies a form of efficiency. While adding a shoreface nourishment to the lateral profile, its efficiency for changing between beach states should remain. Looking at the preferred designs per case, it shows that both designs follow the pre-nourishment profile the best, therefore making it easier for the beach to adapt to a new beach state. This concludes that shoreface nourishment with long alongshore length which follows the initial crossshore profile the best results in the lowest significant wave height and largest increase in beach width. The short term storm effects are checked my modelling the 1953 Watersnoordramp using the preferred shoreface nourishment in the Noordwijk case. This modelling approach yields in inconclusive results as the modelling duration is too short to observe differences between the scenario’s with or without the nourishment. However, the nourishment tends to help the reshaping of the profile towards a more dissipative storm state as more sediment is added in the coastal zone, implying more wave dissipation in the long term. More research has to be performed to confirm this hypothesis