Beach development behind detached breakwater.

Abstract

Concurrent wave and morphology data were collected around a coastal protection scheme on the U. K. south coast. The scheme consists of eight detached breakwaters protecting a renourished sand and shingle beach, and is situated in a strongly macro-tidal environment. The development of the beach morphology is described. The beach trapped sand and shingle moving eastwards into it, and lost material from the eastern end. While the beach was designed to maintain a shingle beach, it was found that the scheme was most effective at trapping sand, which led to tombolo formation behind the updrift breakwaters. Current engineering design methods for describing beach development were applied to the scheme. Empirical techniques were found to be poor predictors of the salient length, although the simplest methods were reasonable guides to the scheme response over a variety of tidal levels. The US Army Corps of Engineers one-line model GENESIS (Hanson, 1989) was applied to the scheme. Using observed values of beach, structure and wave conditions, it was necessary to exaggerate transport due to longshore gradients in wave height relative to transport due to oblique wave approach to correctly describe salient formation. While it was possible to reduce model calibration errors, model validation was not successful. This was due to the inability of the model to allow tombolo formation, and also due to the lack of a 'constant! beach profile, due to the different behaviour of the sand and shingle. Empiricalo rthogonafl unctiona nalysisw as carriedo ut on the beachs urveyd ata.F rom the limited records available, it was clear that the scheme reduced profile variance behind it, compared to the updrift and downdrift shorelinesT. he schemea lso led to morec omplex3 D seasonaml ovementso f beachm aterial,i n contrastto the predominantly2 D responseu pdrift.