Process-based modelling of morphological response to submerged breakwaters

Abstract

Submerged breakwaters (SBWs) are becoming increasingly popular as alternative coastal defence system due to the lack of impact on beach amenity and aesthetics compared to common emerged beach protection measures. However, the recent significant amount of failing SBWs resulting in additional shoreline erosion reported in [Ranasinghe and Turner 2006], indicates the importance of understanding the driving processes of salient development before routinely adopting SBWs in practice. The main objective of this thesis is to gain more insight into single shore-parallel detached SBW induced hydrodynamic processes driving morphological changes. In order to study SBW induced hydrodynamic conditions resulting in morphological response, a depth-averaged Delft3D model is used. By online coupling of Delft3D-FLOW and SWAN, the wave-current interaction is accounted. To exclude site-specific conditions, an idealized approach is used, including an alongshore uniform beach profile and shore normal short wave forcing. For this idealized situation, a sensitivity analysis of numerical parameters is performed, as well as a validation on individual SBW induced processes based on published literature. By examining the cross- and alongshore momentum balance for a variety of results from numerical simulations only changing alongshore length and offshore distance of the SBW, dominant SBW induced alongshore differences in water level and resulting currents are explained in detail. In addition, SBW design parameters are studied using the same momentum balances. Besides offshore distance, alongshore length of the SBW and directionality of the incoming waves, these include the crest width, crest height, incoming wave height and breakwater roughness. To confirm the findings from the hydrodynamic analysis as the important driving processes of SBW induced morphological changes, additional morphological simulations are included and morphological SBW induced response is compared to initial hydrodynamic conditions. As a result, a computationally efficient depth-averaged Delft3D model is obtained, which is capable of simulating SBW induced processes accurately compared to published literature. From the idealized simulations, more insight is given in two distinct SBW induced processes driving morphological response. These processes reducing nearshore water level set-up are the spatial distribution of wave forcing (commonly referred as wave sheltering effect) and the momentum balance between wave forcing and bottom stresses over the SBW. In addition to the parameters presented in [Ranasinghe et al. 2010], the breakwater roughness and directional spreading of waves are important parameters to take into account when constructing SBWs. Morphological simulations confirm the relation between the hydrodynamic processes described and the morphological response to SBWs. The ability of Delft3D to simulate morphological response to SBWs, enables a powerful numerical tool for future studies on SBW induced (morphological) processes.