Initial morphological evolution of a mega nourishment

Abstract

The coastal protection system of the Dutch low-lying hinterland is under increasing pressure due to growing population and high economic value combined with climate change and a rising sea level. Supplying sand to dunes, beach and shoreface by means of nourishments is a widely applied technique to increase coastal safety. Climate change and a rising sea level lead to higher water levels and higher and longer waves at the North Sea. These increased loads cause a heavier impact on the Dutch coast and therefore, wave boundary conditions have been revised by the Dutch Government. After a reassessment of the coastal protection system in 2003, several parts of the Dutch coast did not comply with the safety standards and were appointed 'Weak Links'. The Hondsbossche and Pettemer sea defence was one of the weak links. The Hondsbossche Dunes nourishment provided the desired solution to improve coastal safety and at the same time maintain, and where possible, improve environmental quality. The design of the nourishment is of a magnitude that is unprecedented. It covers a stretch of approximately twelve kilometres of coast. In contrast to traditional nourishments that typically consist of about 1-2 million m3 of sand, the Hondsbossche Dunes nourishment contains a volume of 35.6 million m3 of sand. The implementation of nourishments modifies the topography of the bed and impacts the nearshore coastal regime. After the nourishment is constructed, the cross-shore profile evolves towards an equilibrium shape, and the shoreline planform adjusts via alongshore spreading. As a result, increased erosion rates are often observed in the first period after completion of nourishments. To improve the design of future nourishments, it is essential to understand and quantify the processes that govern the initial morphological evolution of recently completed nourishment projects. Twelve months after completion of the nourishment, sand losses at the Hondsbossche Dunes nourishment appeared to be large. This study focuses on understanding the processes that govern these large initial volume losses. Bathymetric data obtained from monthly monitoring surveys during and after construction, combined with wave forcing data are used to analyse morphological evolution and hydrodynamic forcing. Furthermore, a process-based cross-shore sediment transport model is applied to assess the relative contribution of several hydro- and morphodynamic factors on the initial morphological development of the nourishment in more detail. The results show that after twelve months 2.5% of the initially nourished sediment volume is lost from the project area. The high losses are mainly due to energetic wave forcing in the first twelve months after construction, with an average net northward transport potential that is 2.5 times higher compared to the long term yearly average. Morphological response is an event-driven process, such that large morphological changes coincide with months with energetic waves. About 60-80% of the volume changes occur over a period of only three months, during the Northern Hemisphere winter. Compared to long term natural variation in wave forcing, relatively more and higher waves approach from South-Western direction during the first year after construction. This results in severe erosion at the southern shoulder of the nourishment and considerable accretion at the northern transition zone with the adjacent coastline. The adaptation of steep cross-shore profiles on similar timescales as the volume changes, suggests a relation between steep slopes and volume losses. However, the interaction with other morphological parameters (e.g. alongshore gradients and variations in grain size) and the presence of subtidal bars prevent the establishment of a direct relation. These insights can provide guidance for other large-scale sandy strategies. The findings indicate that wave forcing dominates the initial morphological response of a recently completed mega nourishment. The stochastic character of temporal wave forcing implies that a considerable uncertainty bandwidth is introduced in the design of nourishments. The effect of other parameters, such as profile shape and sediment size, is less pronounced in morphological development. Also these parameters are more controllable, and as a result they introduce relatively less uncertainty in the prediction of initial sediment losses at recently completed nourishments.