Cross-shore sediment transports on a cut profile

People always want to live near water and all over the world the areas near the sea get more and more built with houses, hotels and infrastructure. For this reason there is a trend to reclaim land by making an artificial island near or against the existing coast. Examples are the airport 'Chek Lap Kok' in Hong Kong and the 'Plan Waterman', a plan to extend the Dutch coastline between Hook of Holland and Scheveningen, also known as the 'Plan New Holland'. Mostly this extension is made by dredging material from relatively deep water near the coast and dumping the material in the nearshore section. The safest way is to shift the existing cross-shore profile over the distance of the land reclamation. This way there will not be very large changes in cross shore sediment transports, because the beach profile keeps the same shape. However, to make a land reclamation this way, a lot of material has to be dredged, which makes the land reclamation expensive. There is also another way to make the land reclamation. On a certain level beneath mean sea level a cut-off can be applied. This means, that the cross-shore profile will not be shifted completely over a certain distance. In the near shore area the design profile will have the same shape as the existing beach profile. Then, from a certain level beneath mean sea level, the design profile will have a constant and relatively steep slope until it reaches the existing beach profile. By doing this, a lot of material is saved when making the land reclamation. During this study the "Plan Waterman" is used for computations with the numerical model UNIBEST-TC, developed by DELFT HYDRAULICS. The model has been calibrated by using a beach profile at Ter Heijde and wave measurements from the EURO-O-platform. From initial computations on several cut profiles it is concluded, that it should be avoided to let waves break due to the cut-off. Because this sudden breaking of waves large seaward directed sediment transports occur which result in fast bed changes. A less steep slope of the cut-off can reduce the sudden breaking of waves. For several cut profiles morphological computations have been made over a period of 10 years and from a comparison of the volumes in the model it is concluded, that the cut-off should be placed at at least NAP -14 m, when a profile development is wanted, which comes close to the development of a land reclamation without a cutoff being applied. However, when looking from an economical point of view, a cut-off at small depth is still a good alternative. By applying a cut-off at small depth, a lot of material and thus money will be saved. The interest money of this saved money is much more than the costs of frequent nourishments necessary for maintaining the beach profile. Up to f4.000,-- per meter of coastline can be saved each year when applying a cut-off at NAP -6 m. However, a cut-off at NAP -6 m will give a decline of the waterline.Hydraulic EngineeringCivil Engineering and Geoscience