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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.

Coastal management in the Netherlands has the aim to defend coastal zones by preventing flooding and mitigating erosion. To that end, large-scale nourishments are placed in the nearshore, which are supposed to dynamically preserve the coastal zone over a timescale of years. To assess their effectiveness, these nourishments are monitored over large areas and long durations. As repetitive, in-situ measurements become too expensive, remote sensing offers an attractive alternative, mapping depth and near-surface current fields via depth inversion algorithms (DIA). However, the information that can be derived from remotely-sensed data is subject to improvement. In this study a 3D-FFT based DIA named XMFit (X-Band Matlab Fitting) is introduced, which is robust, accurate and fast enough for operational use. Focusing on depth estimates, the algorithm was validated for two case studies in the Netherlands: (1) the “Sand Engine”, a beach mega nourishment at a uniform open coast, and (2) the tidal inlet of the Dutch Wadden Sea island Ameland, characterizing a more complex coast. Considering both sites, the algorithm performance was characterized by a spatially averaged depth bias of −0.9 m at the Sand Engine and a time-varying bias of approximately -2 – 0 m at the Ameland Inlet. When compared to in-situ depth surveys the accuracy was lower, but the time resolution higher. Depth estimates from the Ameland tidal inlet were produced every 50 min by an operational system using a navigational X-Band radar to monitor the placement of a 5 million m3 ebb-tidal delta nourishment – a pilot measure for coastal management. Volumetric changes in the nourishment area over the year 2018, occurring at 7 km distance from the radar, were estimated with an error of 7%. Depth errors statistically correlated with the direction and magnitude of simultaneous near-surface current estimates. Additional experiments on Sand Engine data demonstrated that depth errors may be significantly reduced using an alternative spectral approach and/or by using a Kalman filter.

The recently developed Morphological Acceleration Factor (MORFAC) approach for morphodynamic upscaling enables numerical model simulations of coastal evolution at decadal to millennial time scales. Primarily due to the massive increase in modeling time scales it affords, the MORFAC approach is now standard in state-of-the-art commercially available coastal morphodynamic modeling suites. However, the general validity of the MORFAC concept for coastal applications has not yet been comprehensively investigated. Furthermore, a robust and objective method (as opposed to the subjective and inelegant trial and error method) for the a priori determination of the highest MORFAC that is suitable for a given simulation (i.e. critical MORFAC) does not currently exist. This communication presents some initial results of an ongoing, long-term study that attempts to rigorously and methodically investigate the limitations and strengths of the MORFAC approach. Based on the results of a strategically designed numerical modeling exercise using the morphodynamic model Delft3D, two main outcomes are presented. First, the main dependencies and sensitivities of the MORFAC approach to fundamental forcing conditions and model parameters are elucidated. Second, a criterion based on the Courant–Friedrichs–Levy (CFL) condition for bed form propagation that maybe used as a guide to determine the critical MORFAC a priori is proposed.

Responses due to coastal interventions may occur at multiple temporal and spatial scales. The morphological development in such cases may be a result of an interactive behaviour between the different scales. To illustrate such behaviour, the IJmuiden port was selected as a pilot case. The IJmuiden case showed that computing the morphological developments around such local coastal interventions is not trivial. The dominant force in the considered case is a coupled behavior covering multiple scales, which does not allow for a simple sub-division of scales. So, one single numerical model is not able to resolve all relevant scales. Hence, a hybrid (multi-scale) modeling approach is required, in which two or more models are combined. The results presented in this paper show that a coupling between a coastline model and area model (bypass, currents) already improves the predicted coastline development and longshore sediment transport. Further strategic research is currently being undertaken to further investigate the sensitivities of a hybrid modeling approach with the ultimate goal to develop an integrated modeling system that is able to assess the impact of anthropogenic activities on all relevant spatial and temporal scales.

In October and November 2009 a pilot project has been executed at the Delfland Coast in the Netherlands, constructing three small sandy headlands called Sand Groynes. Sand Groynes are nourished from the shore in seaward direction and anticipated to redistribute in the alongshore due to the impact of waves and currents to create the sediment buffer in the upper shoreface. The results presented in this paper intend to contribute to the assessment of Sand Groynes as a commonly applied nourishment method to maintain sandy coastlines. The morphological evolution of the Sand Groynes has been monitored by regularly conducting bathymetry surveys, resulting in a series of available bathymetry surveys. It is observed that the Sand Groynes have been redistributed in the alongshore, mainly in northward direction driven by dominant southwesterly wave conditions. Furthermore, data analysis suggests that Sand Groynes have a trapping capacity for alongshore supplied sand originating from upstream located Sand Groynes. A Delft3D numerical model has been set up to verify whether the morphological evolution of Sand Groynes can be properly hindcasted. Although the model has been set up in 2DH mode, hindcast results show good agreement with the morphological evolution of Sand Groynes based on field data. Trends of alongshore redistribution of Sand Groynes are well reproduced. Still the model performance could be improved, for instance by implementation of 3D velocity patterns and by a more accurate schematization of sediment characteristics.

The Seadarq software from Nortek is designed to derive wave, water depth and current information from ordinary navigation radar images. This is done by analysis of the wave propagation patterns using an inverse dispersion fitting technique. The remotely sensed data provides the unique opportunity to assess temporal and spatial propagation patterns of waves and currents over a significant stretch of water overlooked by a radar station. This exploratory study shows that radar data can provide very valuable insight in otherwise seldom disclosed current patterns. The spatial and temporal information of the radar data may allow us to calibrate our numerical model not only for water levels but now also for details in the currents patterns. Moreover, the radar data has improved the understanding of the hydrodynamics in a complex tidal inlet and around a mega-nourishment.

The Morphological Acceleration Factor (MORFAC) approach for morphodynamic upscaling enables the simulation of long term coastal evolution. However the general validity of the MORFAC concept for coastal applications has not yet been comprehensively investigated. Furthermore, a robust and objective method for the a priori determination of the highest MORFAC that is suitable for a given simulation (i.e. critical MORFAC) does not currently exist. This paper presents some initial results of an ongoing, long-term study that attempts to rigorously and methodically investigate the limitations and strengths of the MORFAC approach. Based on the results of a numerical modelling exercise using the morphodynamic model Delft3D, the main dependencies and sensitivities of the MORFAC approach are investigated. A criterion (based on the CFL condition for bed form migration) for the a priori determination of the critical MORFAC is also proposed.