The present study aimed to include the turbulence effects derived from a road located over the crest of a dike, in its probabilistic safety assessment. This was done by building two different computation fluid dynamics models (RANS K-); one of a dike with a road on top and one without it. Both models were validated with experimental data collected from the Wave overtopping simulator experiments performed in the Netherlands. These models were used to produce training data sets which were later used for constructing emulators (computationally cheaper models) which allowed to reduce the computational burden from the required stochastic modelling. These new emulators allowed to calculate bottom shear stress time series in different locations along the dike profile. With these time series, it is possible to estimate the potential scouring depth per wave volume routed. These emulators allowed to model different probabilistic overtopping scenarios without running the CFD models again. The results showed that when assessed under extreme climate scenarios, the presence of a road may reduce the dike safety by almost 50% with respect to the case where no road is present. In addition, it is also concluded that the spatial grass quality distribution is a more important factor for determining dike safety than the spatial grass cover thickness. ... Read more

The land-sea interface is a very attractive location for humans to settle. In the case of low lying, sedimentary coastlines this can be a risky location, as these shorelines are inherently dynamic in nature. Accelerating rates of relative sea level rise will increase coastal erosion, creating world-wide growing demands for coastal protection along urbanized shores. Starting point of this project is that the key to sustainably adapt to this situation is to be found in smart, pro-active sediment management using ‘building-with-nature’ (BwN) approaches, rather than in traditional reactive approaches involving expansion of static, hard coastal defense structures. ... Read more

Because wind is one of the main forcings in storm surge, we present an idealised process-based model to study the influence of topographic variations on the frequency response of large-scale coastal basins subject to time-periodic wind forcing. Coastal basins are represented by a semi-enclosed rectangular inner region forced by wind. It is connected to an outer region (represented as an infinitely long channel) without wind forcing, which allows waves to freely propagate outward. The model solves the three-dimensional linearised shallow water equations on the f plane, forced by a spatially uniform wind field that has an arbitrary angle with respect to the along-basin direction. Turbulence is represented using a spatially uniform vertical eddy viscosity, combined with a partial slip condition at the bed. The surface elevation amplitudes, and hence the vertical profiles of the velocity, are obtained using the finite element method (FEM), extended to account for the connection to the outer region. The results are then evaluated in terms of the elevation amplitude averaged over the basin’s landward end, as a function of the wind forcing frequency. In general, the results point out that adding topographic elements in the inner region (such as a topographic step, a linearly sloping bed or a parabolic cross-basin profile), causes the resonance peaks to shift in the frequency domain, through their effect on local wave speed. The Coriolis effect causes the resonance peaks associated with cross-basin modes (which without rotation only appear in the response to cross-basin wind) to emerge also in the response to along-basin wind and vice versa. ... Read more

We present a quick assessment method using an idealized one-dimensional linear model to explore the influence of multiple retention basins, whose construction is proposed as a measure to reduce tidal amplitudes in estuaries/tidal channels. To this end, we have developed a process-based network model for the cross-sectionally averaged water motion, including width and depth convergence (thus extending earlier studies), bottom friction, radiation damping and allowing for multiple basins at arbitrary locations. For frictionally dominated tidal channels, model results show that construction of a retention basin generally leads to a reduction of the tidal amplitude at the channel head. This reduction is stronger for larger basins and for basins closer to the landward side. Strikingly, for weak to moderate friction, a basin situated sufficiently close to the seaward side may trigger the opposite and undesired effect (tidal amplification) and nonlinear interaction among basins may occur. The model is then applied to the Ems estuary (Germany), where the construction of nine designated retention basins is currently under consideration. For parameter values representing the present-day Ems situation, constructing all nine proposed basins is estimated to result in a tidal amplitude reduction of 0.87 m (from 1.51 m to 0.64 m). A systematic model analysis of all 29 = 512 combinations shows that 86% of this reduction can be achieved by selecting only four of these basins. Importantly, shifts in the frictional regime, as experienced by the Ems in the past, may drastically change the effect of retention basins. Even though the efficiency and flexibility of this exploratory model allow for extensive sensitivity studies, we recommend to combine it with a detailed model, for the purpose of both efficiency and accuracy ... Read more

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