Numerical modelling of erosion rates, life span and maintenance volumes of mega nourishments

Mega-nourishments, aiming at providing long-term coastal safety, nature qualities and recreational space, have been applied recently at the Holland coast and are considered at various other places in the world. Methods to quickly evaluate the potential and lifetime of these coastal mega nourishments are therefore very much desired, which is the main objective of this research. Two types of mega nourishments can be distinguished: feeder-type mega nourishments may erode freely to feed adjacent coasts for a more natural, dynamic dune growth while permanent mega-nourishments are designed to preserve safety levels and need to maintain their size and shape and thus needs to be nourished themselves. The design and impact assessment studies for both types of mega nourishments require detailed morphological studies to determine the morphological evolution. In this paper 2DH (Delft3D) and 1D (UNIBEST-CL+ and LONGMOR) numerical models were calibrated using data of the Sand Motor mega-nourishment and were then applied to model a series of mega-nourishments with various width over length ratios and volumes in order to derive relations and design graphs for erosion rates, life span and maintenance volumes. These relations and design graphs can be used in project initiation phases and feasibility studies. The magnitude of the modelled wave-driven longshore sediment transport rates in 1D coastline models depend on the representation of wave refraction on the lower shoreface, since a distinction should be made between the non-rotating lower shoreface and active surfzone. It was shown that the life time of nourishments is mainly determined by the dimensions of the nourishment and incoming wave energy.