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Storm surge barriers (SSBs) protect the hinterland of estuaries against flooding, while in open state the tidal dynamics are maintained to some extent. Even when tidal dynamics are maintained, tidal conditions are inevitably affected by the confinement of the flow. As a result, intertidal flats – providing important ecosystem services – face losses through erosion. In this work, we integrated research on (1) morphological consequences of the Eastern Scheldt SSB (The Netherlands, 1987) and (2) intertidal flat nourishments mitigating these negative eco-morphological consequences. Through decades of data, we show that the SSB induced persistent erosion. We have demonstrated that sand nourishments on tidal flats can effectively mitigate SSB-induced erosion and ecological consequences. We have found that net lowering of tidal flats in systems as the Eastern Scheldt will proceed, also given accelerated sea level rise. Even though tidal flat nourishments are an effective mitigation strategy, we urge to steer first on minimizing irreversible hydromorphological changes caused by future SSB projects (e.g., considered for Galveston Bay). Through monitoring data on intertidal flat sand nourishments in this system, we identify the potential of these mitigation measures and provide recommendations for future SSB deployments and mitigation.

Numerical studies of sediment transport and morphodynamics in the swash zone generally use numerical models based on the depth-averaged shallow water equations. However, such models still have difficulty predicting the sediment transport (and subsequently morphodynamics) in the swash (see e.g. Ruffini et al., 2020). Depth-resolving models, such as models solving the ReynoldsAveraged-Navier-Stokes (RANS) equations have previously shown to accurately simulate swash hydrodynamics and also sediment transport in the surf zone (Jacobsen et al., 2014). However, applications of a RANS model capable of simulating sediment transport in the swash are limited. In this study, we present preliminary results from our RANS model on sediment transport in the swash.

To contribute to solving scientific and practical questions, numerical morphodynamic models like Delft3D are often used to predict the hydrodynamics, sediment transport processes and morphological development of coastal systems. In such models, many of the processes are parameterized based on a variety of assumptions. One of the parameterized variables is the bedform-related hydraulic roughness ks, which is often assumed to be related to the ripple height. This roughness affects the magnitude and vertical structure of the flow and, consequently, the magnitude of the sediment transport. Yet, their sensitivity to ks is not well understood.

This report aims to understand how mega-scale and meso-scale morphological changes affect tidal dynamics and sand transport within the Scheldt estuary. With mega-scale we refer to the whole Scheldt estuary; with meso-scale we refer to tidal channels within a macro-cell. To that end we have applied a validated two-dimensional horizontal (depth-averaged) Delft3D model of the whole Scheldt estuary including flow and sand transport processes with both tidal and wind forcing. The model simulations were morphostatic, i.e. the bed levels were fixed. First, we have ran simulations using historical bed levels (1973, 1983, 2006, and 2011) to further validate the model and study the impact of the large-scale morphological changers. Then we have systematically investigated how a possible further deepening of the Gat van Ossenisse and undeepening of the Middelgat affects tidal dynamics and sand transport in the Scheldt estuary.

Most knowledge on the morphodynamic behaviour of shoreface and beach nourishments originates from data analysis studies. Numerical modeling tools have been used successfully in hindcasting behaviour of nourishments, but do not yet have the predictive power for reliably forecasting.

A 2D RANS-VOF model is used to simulate the flow and sand transport for two different full-scale laboratory experiments: i) non-breaking waves over a horizontal sand bed (Schretlen et al., 2011) and ii) plunging breaking waves over a barred mobile bed profile (Van der Zanden et al., 2016). For the first time, the model is not only tested and validated in terms of water surface and outer flow hydrodynamics, but also in terms of wave boundary layer processes and sediment concentration patterns. It is shown that the model is capable of reproducing the outer flow (mean currents and turbulence patterns) as well as the spatial and temporal development of the wave boundary layer. The simulations of sediment concentration distributions across the breaking zone show the relevance of accounting for turbulence effects on computing suspended sediment pick-up from the bed.