Design and Effectiveness of Coastal Protection Structures: Case Studies and Modelling Approaches

Beach groynes are structures for erosion protection along sandy coasts near inlets and can reduce the coastal erosion substantially, but open groynes cannot stop erosion completely because sand can be removed from the groyne compartments by cross-shore processes. Beach groynes should be designed with sufficient bypassing of sand to minimise erosion. Regular beach maintenance is required to keep a sufficient beach width for recreational purposes. The effectiveness of groyne compartments can be significantly improved by using T-head groynes or by using a submerged sill or breakwater in between the groynes. An economic evaluation shows that the beach maintenance costs over 50 years may be substantially higher than the construction costs of a submerged breakwater. An important parameter to be studied is the longshore transport, which requires detailed information of the wave climate, preferably based on measured data (offshore buoys) in combination with deep water wave modelling. Various models have been used to determine the net longshore sand transport and coastline changes. The design of groynes to reduce coastal erosion is illustrated by three field cases (Atlantic coast near Soulac, France; Lagos coast, Nigeria and Black Sea coast, Romania). These example cases show that beach groynes are effective structures, but sufficient bypassing of longshore sand transport is essential to minimise erosion. Regular beach fills in the groyne compartments may be required at high-energy (exposed) coasts. A submerged or emerged breakwater can be built between the groynes to protect the beach in the groyne compartments against erosion by cross-shore processes.