On the redistribution and sorting of sand at nourishments: Field evidence and modelling of transport processes and bed composition change

Increasingly large sand nourishments are used for the maintenance of sandy coasts around the world. There is, however, still very limited understanding of their behaviour (i.e. redistribution) and effects on the marine environment. This PhD thesis explores the morphological reshaping of shoreface nourishments (i.e. long bunds of sand placed at the sub-tidal bar with volumes of 1 to 5 million m3) based on data at 19 field sites as well as the reshaping of mega nourishments (i.e. temporary land reclamations) using data of the 'Sand Motor' at the Dutch coast (with a volume of 21 million m3). Considerable cross-shore profile change takes place at shoreface nourishments, consisting of a landward movement of the nourishment crest and erosion of the seaward edge of the nourishment, erosion directly landward of the shoreface nourishment (in the first 100 to 150 m) and some accretion in the inner surfzone (at MSL -2m). Especially the water-level gradient driven currents and onshore transport due to wave skewness are responsible for the morphological change, which could be modelled with Xbeach using a lookup table with initial sedimentation-erosion rates for possible climate conditions. Mega nourishments, on the other hand, reshape predominantly in alongshore direction as a result of the alongshore wave-driven current. Design graphs showing the erosion rates, life span and maintenance volumes were made for the mega nourishments, which can be used for the planning phase of projects. Making a differentiation between the non-rotating foreshore and active surfzone proved to be essential for an accurate representation of the wave-driven alongshore transport in 1D coastline models. Furthermore, the lifetime of the nourishment is related to the sensitivity of the alongshore wave-driven transport to a shoreline rotation, which is affected especially by the wave energy. The recent upscaling of the sand nourishment volume in the last decades (i.e. to Sand Motor scale) and increasing anthropogenic pressure also comes with questions regarding the impact that is made on the natural environment. This thesis investigated the development of the bed sediment composition at the 'Sand Motor' using field measurements and numerical modelling, since bed composition is relevant for marine ecology. Considerable alongshore heterogeneity of the bed composition (D50) was observed as the Sand Motor evolved over time with (1) a coarsening of the lower shoreface of the exposed part of the Sand Motor (+90 to +150 µm) and (2) a deposition area with relatively fine material (50 µm finer) just North and South of the Sand Motor. The alongshore heterogeneity of the D50 is most evident outside the surfzone (i.e. seaward of MSL -4 m), while alongshore variation in D50 was relatively small in the surfzone itself (i.e. landward of MSL -4 m). Preferential erosion of the finer sand fractions takes place during mild to moderate wave conditions, while a reduction of the local armouring of the bed takes place during storms which mobilize all sediment fractions and mix the the top-layer of the bed with the relatively finer substrate. A 3D multi-fraction morphological model gave a good hindcast of 2.5 year of observed spatial and temporal changes in D50 at the Sand Motor, which showed that the coarsening of the bed after construction of the Sand Motor is mainly due to the tidal contraction at the Sand Motor. The currents transport especially the fine grains of the sand mixture, which are more easily suspended than the coarse grains. This difference in suspension behaviour is the main cause of the observed bed composition changes at the lower shoreface. Within the surfzone the difference in suspension behaviour of the size fractions will be smaller, as the energetic conditions can suspend all size fractions. The current findings imply that large-scale bed composition changes can take place at any coastal structure which has a considerable impact on the tidal currents.