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. @en

Studying mangroves in the field can be very difficult; therefore it is in some situations better to model in a flume. The question is, to what extend are the situation in the field and the situation in the flume comparable with each other? This study focusses on reflection in the wave flume. Wave energy in a flume can be damped by placing a wave absorber at the back of the flume, which absorbs part of the wave energy; part of the waves is reflected. In this study rock is used to build the wave absorber. Two aspects are studied: the influence of the lay-out of the wave absorber on the reflection in the flume, with as goal to find an optimal lay-out, fitting on a maximum horizontal length of 3.00 m, to minimise reflection; and the method of measuring the amount of reflection in a wave flume.

This thesis explores the nature of aeolian sediment availability and its influence on aeolian sediment transport. The aim is to improve large scale and long term aeolian sediment transport estimates in (nourished) coastal environments. The generally poor performance of aeolian sediment transport models with respect to measurements in coastal environments is often accredited to limitations in sediment availability. Sediment availability can be limited by particular properties of the bed surface. For example, if the beach is moist or covered with non-erodible elements, like shells. If sediment availability is limited, the aeolian sediment transport rate is governed by the sediment availability rather than the wind transport capacity. Aeolian sediment availability is rather intangible as sediment availability is not only affected by aeolian processes, but also by marine and meteorological processes that act on a variety of spatial and temporal scales. The Sand Motor 21 Mm3 mega nourishment is used to quantify the spatiotemporal variations in aeolian sediment availability and its effect on aeolian sediment transport. The Sand Motor was constructed in 2011 along the Dutch coast. Aeolian sediment accumulation in the Sand Motor region is low compared to the wind transport capacity, while the Sand Motor itself is virtually permanently exposed to wind and accommodates large fetches. Aeolian sediment availability is therefore likely to dominate aeolian sediment accumulation. Multi-annual bi-monthly measurements of the Sand Motor's topography are used for a large scale aeolian sediment budget analysis. The analysis revealed that aeolian sediment supply from the dry beach area, that is almost permanently exposed to wind, diminished a half year after construction of the Sand Motor. The reduction in aeolian sediment supply is likely due to the development of a beach armor layer. In the subsequent years, two-third of the aeolian sediment deposits originate from the low-lying beaches that are frequently flooded and therefore often moist. The importance of the low-lying beaches in the Sand Motor region is tested during a six-week field campaign. Gradients in aeolian sediment transport are measured during the field campaign as to localize aeolian sediment source and sink areas. A consistent supply from the intertidal beach area was measured that was temporarily deposited at the higher dry beach. The temporary deposits were transported further during high water, when sediment supply from the intertidal beach ceased, resulting in a continuous sediment supply to the dunes. The temporary deposition of sediment at the dry beach was likely promoted by the presence of a berm that affects the local wind shear. Moreover, the berm edge coincided with the onset of the beach armor layer that might have further promoted deposition of sediment. The measurements on spatiotemporal variations in aeolian sediment availability and supply inspired an attempt to capture the characteristics of aeolian sediment availability in coastal environments in a comprehensive model approach. The resulting model simulates spatiotemporal variations in bed surface properties and their combined influence on aeolian sediment availability and transport. The implementation of multi-fraction aeolian sediment transport in the model introduces the recurrence relation between aeolian sediment availability and transport through self-grading of sediment. The model was applied in a four-year hindcast of the Sand Motor mega nourishment as first field validation. The model reproduces the multi-annual aeolian sediment erosion and deposition volumes, and the relative importance of the intertidal beach area as source of aeolian sediment well. Seasonal variations in aeolian sediment transport are incidentally missed by the model. The model accuracy is reflected in a R2 value of 0.93 when comparing time series of measured and modeled total aeolian sediment transport volumes in the four years since construction of the Sand Motor. The results suggest that indeed significant limitations in sediment availability, due to soil moisture content and beach armoring, govern aeolian sediment transport in the Sand Motor region. A comparison with a simulation without limitation in sediment availability suggests that aeolian sediment availability in the Sand Motor region is limited to about 25% of the wind transport capacity. Moreover, both spatial and temporal variations in aeolian sediment availability as well as the recurrence relation between aeolian sediment availability and transport are essential to accurate long term and large scale aeolian sediment transport estimates. @en