Developing a design criterion for the shoreline response to multiple submerged breakwaters

Abstract

Shore-parallel submerged breakwaters (SBWs) appear as an attractive form of coastal protection. Compared to their emergent counterpart there is no impact on the beach amenity and aesthetics. In addition, there is a possibility for recreational purposes. However, the actual application of SBWs is often discouraged because the complexity of the hydrodynamics in the vicinity of SBWs makes the shoreline response hard to predict. This complexity further increases in the case of multiple SBWs. Added processes such as the distribution of the return flow and the (hydrodynamic) interaction between the separate SBWs induce a complex wave and flow pattern in the lee of the SBWs. The objective of this thesis is to develop a design criterion predicting the shoreline response to multiple SBWs. Studies showed that the initial mode of the shoreline response can be linked to the flow pattern observed in the lee of the SBW. A 2-cell flow pattern causes a divergent flow at the shoreline and therefore local erosion, whereas a 4-cell flow pattern causes convergent flow at the shoreline with local accretion as a result. These patterns have comparable results in cases with a single SBW and cases with multiple SBWs. There exist multiple methods to predict if a certain SBW system will cause a 2-cell or 4-cell flow pattern. This thesis will look into the existing criteria based on the physical parameters of the system. Because the design parameters can be used directly to predict a shoreline response via this criterion, it is deemed to be better suited for the objective of this thesis. However, in its current form, the criterion is only valid for single SBW systems. The criterion to predict the mode of shoreline response (accretive versus erosive) to a single SBW is made applicable to multiple SBW cases by adding a term accounting for the added processes. The extra term is based on a theoretical analysis and numerical modelling (Delft3D) of the impact of the lateral confinement ratio. This impact is made visible by analysing the availability of sediment and the different degree in wave sheltering with respect to a single SBW system. Although the criterion cannot be used for a quantitative prediction of the shoreline response, it can function as a tool for the first assessment of the mode of shoreline response in cases of designing a coastal protection system based on multiple SBWs. The criterion can be used for preliminary designing advice or as a basis for further research regarding the utilization of SBWs as a form of coastal protection. Further research can include a continuation of the research of the applicability of the criterion outside of idealized models and the necessity of treating certain aspects separately (e.g. a large tidal amplitude). The criterion would benefit from validation with field measurements and physical modelling. Additionally, research is needed on the sensitivity to the barrier width and roughness as these parameters are, as of now, not included in the criterion.