From a physical perspective, the Sand Motor is a large, protruding body of sand along the Delfland coast. Modifying the coast on a kilometer-scale inherently has an effect on coastal hydrodynamics, which involve currents and surface waves. These will behave differently due to the presence of the Sand Motor. In turn, the modified nearshore hydrodynamics will influence the morphological development of the Sand Motor.@en

The behaviour of subtidal sandbars can be strongly influenced by the introduction of sand nourishments in the coastal system. This study focuses on the impact of nourishments on subtidal bar behaviour at spatio-temporal scales beyond a single nourishment project. It aims to determine the long-term behaviour of subtidal sandbars along an entire coastal cell, taking into account both the unnourished and nourished regime, and covering various types of nourishments. The analysis is based on over 50 years of sandbar evolution along the Delfland coast, a 17-km long coastal cell at the Dutch North Sea coastline protected by groynes and maintained with frequent sand nourishments. Observations reveal clearly different sandbar behaviour during the unnourished (first 20 years) and nourished periods of the dataset. Introduction of the first beach nourishments (nourished sand primarily placed at the subaerial beach) was found to stimulate sandbar development along previously unbarred sections of the coast. Shoreface nourishments (nourished sand placed at the seaward face of the pre-existing subtidal sandbar) tended to migrate shoreward rapidly at a rate of 20 to 60 m/year at this coast, thereby forcing the pre-existing sandbar to weld to the dry beach. An abrupt transition of sandbar dynamics was observed following a major nourishment operation (∼ 37.5 Mm3 of nourished sand) that covered the entire coastal cell. A new, shallow sandbar formed with a degree of alongshore variability that was unprecedented at the Delfland coast over the full study period. These results imply that individual nourishments can influence the formation and migration of individual sandbars, while continued nourishments can fundamentally change long-term sandbar dynamics along an entire coastal cell.@en

Artificial sand nourishments are a common measure to mitigate coastal erosion problems. Such nourishments can have an impact on currents and waves near the beach, especially when the nourishment has a large size. As nourished beaches often have a recreational function, these altered wave and current patterns may pose a threat to swimmers. This study has investigated the impact of nourishments on currents, waves and swimmer safety. De Sand Motor, an experimental large-scale nourishment at the Dutch coastline south of The Hague, served as a central case study. Using a combination of current measurements at sea and computer models, this study has revealed several interesting flow patterns around the Sand Motor, amongst others the presence of large eddies in the tidal flow. To determine the impact of such flow patterns on swimmer safety, the presence and spatial spreading of beach users at the Sand Motor was monitored with a set of cameras. Although the tidal eddies have a clear influence on currents and sand transport around the Sand Motor, their impact on swimmer safety remains limited. At the part of the Sand Motor where hazardous currents due to tidal eddies may occur, hardly any beach users are present due to the large distance from beach entrances, parking lots and restaurants. The most significant hazard is formed by tidal currents in the artificial lagoon, which has been incorporated in the initial design of the Sand Motor. Especially in the first years after construction of the nourishment, currents in the channel connecting the lagoon to the North Sea were quite strong, while that part of the Sand Motor can be crowded on nice summer days. The findings of this study enable engineers to incorporate swimmer safety considerations in the design of future nourishments. Furthermore, more fundamental insights into waves and currents around the Sand Motor contribute to the understanding of sediment transport, coastal erosion and eventually prevention of coastal flooding.@en

Operational nearshore current forecasts based on numerical model simulations are gaining popularity as a measure to increase the safety of swimmers. Applying remotely-sensed bathymetry in these model simulations is often proposed in order to cope with rapidly changing nearshore bathymetry. Errors in the remotely-sensed bathymetry may negatively affect performance of the hydrodynamic model. Hence, this study aims to determine the sensitivity of modelled nearshore currents (with a strong focus on rip currents) to errors in remotely-sensed bathymetries. The errors in the remotely-sensed bathymetries (depth inversion algorithm applied to video stream) were quantified with a length scale-aware validation technique, providing useful insights in the contribution of pattern and amplitude errors to the total error throughout the analysis domain and over a range of bathymetric length scales. Subsequently, simulations with a nearshore hydrodynamic model were performed, using both in-situ and remotely-sensed bathymetries as an input. A comparison of predicted rip currents on either bathymetry yielded performance statistics for operational current forecasts on remotely-sensed bathymetries, taking the model with in-situ bathymetry as a reference. Linking these performance statistics back to the quantified errors in the remotely-sensed bathymetry finally revealed the relation between errors in flow and bathymetry. Of all rip currents generated on an in-situ bathymetry, 55% were reproduced on the remotely-sensed bathymetry, showing that models predicting nearshore currents on remotely-sensed bathymetry have predictive value. Positive rip current predictions were promoted significantly by accurate reproduction of the pattern and amplitude of nearshore bars at length scales between 200 and 400 m. In contrast to the length-scale aware validation technique applied here, commonly used domain-wide bulk error metrics lack important information about spatial variations in the quality of remotely-sensed bathymetry.

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For a large beach nourishment called the Sand Engine – constructed in 2011 at the Dutch coast – we have examined the impact of coastal forcing (i.e. natural processes that drive coastal hydro- and morphodynamics) and groundwater
recharge on the growth of a fresh groundwater lens between 2011 and 2016. Measurements of the morphological change and the tidal dynamics at the study site were incorporated in a calibrated three-dimensional and variabledensity
groundwater model of the study area. Simulations with this model showed that the detailed incorporation of both the local hydro- and morphodynamics and the actual recharge rate can result in a reliable reconstruction of the growth in fresh groundwater resources. In contrast, the neglect of tidal dynamics, land-surface inundations, and morphological changes in model simulations can result in considerable overestimations of the volume of fresh groundwater. In particular, wave runup and coinciding coastal erosion during storm surges limit the growth in fresh groundwater resources in dynamic coastal environments, and should be considered at potential nourishment sites to delineate the area that is vulnerable to salinization.@en

Impact of spatial variability in the nearshore bathymetry on net sediment transport rates has been investigated for a selection of observed beach states at the Dutch coast. The beach states comprise a longshore bar trough and two transverse bar rip situations, which were present at the large scale Sand Motor nourishment at the Holland coast. These observed bathymetric features were then applied multiple times next to each other along a longer stretch of coast to obtain a repeating pattern of the considered beach state. The wave transformation towards the shore, alongshore wave-driven and water-level setup driven currents and sediment transport were computed with the Delft3D model, which has been applied successfully for many other studies at the Sand Motor. It was found that net sediment transport is considerably influenced for the most pronounced transverse bar rip configuration, which was most prominent for conditions with small wave angles (i.e. less than 10° from shore-normal). Furthermore, a decrease in transport rates is found for conditions from larger angles of wave incidence (i.e. 30 to 45° from shore-normal). Impacts of the bathymetries of longshore bar trough and the less pronounced transverse bar rip system on net sediment transport rates were much smaller. The actual cause for the enhancement (or decrease) of the net transport for the transverse bar rip configuration is expected to be related to 1) the oblique orientation of the rip-channel for the considered configuration as well as to 2) a more diffusive pattern of the wave breaking as a result of the refraction on the spatially variable bathymetry.@en

The presence of complex nearshore sand bar patterns (i.e. alongshore bathymetric variability) has an impact on local currents, affecting recreational safety and nearshore mixing processes. This study assesses the evolution of alongshore bathymetric variability along the Delfland coast in The Netherlands, over the first 5 years after construction of a mega-scale beach nourishment (the Sand Motor) in the central part of the coastal cell. A total of 38 bathymetric surveys was conducted over this period. Alongshore variability was quantified by subtracting an alongshore averaged bathymetry from the actual surveyed bed levels for both the intertidal and subtidal zone. From 2 years after construction onwards, the subtidal nearshore bathymetry at the Sand Motor is considerably more alongshore variable than the adjacent parts of the Delfland coast. Intertidal variability tends to be high in areas where beach groynes are present.@en

In recent years, the application of large-scale beach nourishments has been discussed, with the Sand Motor in the Netherlands as the first real-world example. Such protruding beach nourishments have an impact on tidal currents, potentially leading to tidal flow separation and the generation of tidal eddies of
length scales larger than the nourishment itself. The present study examines the characteristics of the tidal flow field around protruding beach nourishments under varying nourishment geometry and tidal conditions, based on extensive field observations and numerical flow simulations. Observations of the flow field around the Sand Motor, obtained with a ship-mounted current profiler and a set of fixed current profilers, show that a tidal eddy develops along the northern edge of the mega-nourishment every flood period. The eddy is generated around peak tidal flow and gradually gains size and strength, growing much
larger than the cross-shore dimension of the coastline perturbation. Based on a 3 week measurement period, it is shown that the intensity of the eddy modulates with the spring-neap tidal cycle. Depth-averaged tidal currents around coastline perturbations are simulated and compared to the field observations. The occurrence and behavior of tidal eddies is derived for a large set of simulations with varying nourishment size and shape. Results show that several different types of behavior exist, characterized by different combinations of the nourishment aspect ratio, the size of the nourishment relative to the tidal excursion length, and the influence of bed friction.@en

Mega-scale, concentrated nourishments are nowadays proposed as mitigation for coastal erosion or to widen beaches for recreation. The intention of such concentrated nourishments is to spread their sediment in both alongshore and cross-shore direction. The mixing process of sediment and other suspended matter by hydrodynamic processes is of key importance for an evaluation and prediction of these coastal projects. Surf zone velocity oscillations and eddies on timescales of up to 30 minutes are contributing factors to this mixing. One of the questions herein is how the artificial altering of the natural profile affects the velocity fluctuations and spatial patterns. This paper will discuss the characteristics of low frequency velocity motions in a new man-made setting; the large Sand Engine nourishment protruding 1 km into the sea. Measurements will be presented from an alongshore array of 6 ADCPs which were deployed during a 6-week field campaign with the intention to investigate amongst others the effect of the wind sea climate and the strong (>0.5 m/s) alongshore tidal currents at the site. The observed magnitudes of both very low frequency (f<0.004Hz) and infragravity motions (0.004