Influence of offshore breakwaters on coastal sediment transport rate and coastline morphology

Offshore breakwaters have been used in many coastal areas for coastal protection with greater or lesser success. However to date these have not been used along the Dutch coast for this purpose. This is probably due to the absence of accurate design tools for their functional design, coupled with the uncertainty of whether this method of coastal protection is economical, as compared to currently used methods such as beach nourishment. Additionally, it is rather difficult to predict the morphological response of the coastline to a designed structure due to the large number of variables involved, and their inter-dependence. The fact that there is also a considerable tidal range and a significant tidal current further complicates the matter, and makes the analysis even more difficult. This Study / Thesis presents firstly an assessment of the applicability of detached infinitely long offshore breakwaters for protection of sections of the Dutch coast from coastal erosion. Secondly it gives insight into the way in which the different boundary conditions, such as wave climate and geometry parameters of both the coastline and breakwater influence the coastal sediment transport and coastal morphology. Finally, it gives general guidelines for carrying out a functional design and for predicting the coastal response; i.e. whether the structure is likely to cause: (a) Negligible effect on the coast ie. limited shoreline response (b) Formation of salients; (c) Formation of Tombolos; In analyzing the likely response to the wave climate, the (one line model) programme Unibest was used. The results obtained from this model were compared to those obtained by using general design rules/tools for emerged breakwaters, developed in previous studies. The results are also used to develop and provide new design guidelines for the use of submerged breakwaters for coastal protection. A calibration of the model was first carried out using the results of previous studies and measured wave climate near the areas of interest. This calibrated model was then used to estimate the sediment transport rate with and without the influence of the breakwater. Having estimated these rates, an attempt was made to predict crudely the likely coastline response to breakwaters of different geometry parameters, assuming a wave climate similar to that which existed during the last few decades. From the analyses carried out herein it may be concluded that the use of offshore breakwaters for coastal protection appears to be technically feasible. Moreover, various coastline responses are possible, depending on the design parameters of the structure. The most important parameters affecting the breakwater's performance are breakwater length and height. Further it appears that the breakwater has to be submerged once the length is of the order 1000 m or more. Because of the variation in hydraulic boundary conditions (in particular tidal levels) along the Dutch coast, and the great influence of this parameter on the whole analysis, the deductions made from this study are only applicable to sites where these boundary conditions are similar.KWP-collectio