AbstractsEarth & Environmental Science

For local protection of coastal stretches where buildings are situated in or form part of the coastal defense line the deep water reef concept has been developed. Two reef concepts were designed; a shore-parallel breakwater (of 3 km length) and a segmented breakwater (three blinds of 1 km length), both located in 10 m water depth at 1.5 km off the coast of Scheveningen. In the present research the hydrodynamics and long term morphodynamics of the two deep water reefs are analyzed with the physics-based numerical model Delft3D. The two reef concepts are modeled in 2DH mode with the Parallel Online morphological approach. The hydrodynamic processes at submerged breakwaters are different from those operating at emerged breakwaters. Wave transmission by the submerged breakwater crest creates two additional effects; 1) a water level set-up with a discharge over the reef and 2) reduced wave action at the landward area of the reef. The water level set-up drives two large-scale circulation cells off the reef tips, with velocities up to 1 m/s. In the surf zone the longshore currents are directed towards the shadow zone, where the second effect creates a sheltered area. The magnitude of these effects is for the parallel reef concept amongst other parameters dependent on the wave height and wave transmission, whereas for the blinds reef concept this is also dependent on the wave direction due to the orientation of the blinds. The longshore varying currents and reduced waves in the surf zone cause the coastline to development into a salient form. At the lee of the reef accretion occurs, whereas at the southward and northward sides of the sheltered area erosion occurs. The morphological development is similar for the two reef concepts and of the same order of magnitude. The blinds reef is the preferred alternative for construction, as less severe erosion is modeled to occur. Equilibrium will likely be attained in 15 to 25 years after construction.