Efectes de les variacions del nivell del mar en la dinàmica a llarg termini del Zandmotor

The Dutch coast is profoundly threatened by sand erosion and sea level rise due to climate change consequences. In order to keep the country save, the Dutch government is highly implicated in the renourishment of beaches and dunes, and the construction of groynes and dykes, so that the shoreline is maintained at its 1990 position. In this framework, a sand mega-nourishment was constructed in the Delfland coast in 2011, the Zandmotor (ZM). With this work we aim at studying the long-term evolution of the ZM, under the effects of wave-driven sediment transport but also taking into account the consequences of sea level variations, including the daily variability associated with astronomical, barometric and wind stress tides, and the slow rise due to global warming. The fact that the ZM is the first of its kind in the world makes it necessary to accomplish this study by means of computational simulations. Simulations have been done using the Q2D-morfo model, starting at 2012, one year after construction, up to 2100, in order to properly assess the effects of climate change. The model has been forced with wave and sea level series constructed from historical data and including mean sea level rise (MSLR) under different IPCC scenarios of global warming. Before performing the long-term simulations, the model has been calibrated with measured bathymetries of the first 3 years. Results show an adverse evolution of the SW coast (located at 6 – 10 km from the centre of the ZM) compared to the NE one. The SW coastline suffers an important retreat under MSLR situations. Yet, the NE coast is maintained at the same position from 2015. This situation yields a feeding asymmetry, being the feeding towards the NE coast 25% larger than in the SW coast by 2100. Further, as a consequence of this behaviour, the dunes located at the SW coast are in great danger. Simulations indicate that by 2100 the coastline will almost have reached the dune base at high tide and, under a extreme inundation event, the dune base would already be reached by 2050. Also, the effects of daily sea surface level variability have been assessed, although MSLR acts on the coastline more significantly. Particularly, the effects of MSLR on the dry beach area have been assessed yielding a loss of between 25% to 44% of initial area, increasing as MSLR scenarios are more pessimistic. These effects are also visible on the mean lifetime (time necessary for the amplitude to decrease by a factor e) of the ZM resulting in a period from 30 to 26 years. Hence, the most important recommendations of this study are that a new mega-nourishment should be constructed in this area and, in the case the same shape is chosen, it should be placed at the south-west from the original position of the ZM. Finally, in future studies about the effect of MSLR in this area it is important to account for the erosion of the coast, as done in the present study, given that it accounts for 50% of the dry beach area loss (the other 50% being due to inundation)