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The nonlinear nature of bottom friction in shallow flow complicates its analysis, particularly in idealised models. For tidal flows, Lorentz’ linearisation has been widely applied, using an energy criterion to specify the friction coefficient. Here we propose an extension of this approach to storm surges, leading to a friction coefficient that may gradually vary over a storm event. The derivation is provided along with first results for a single channel.

In order to quantify the effects of mutual feedbacks between geomorphology and benthic activity, we present a twoway coupled idealised sand wave model. This model allows for an extensive parameter analysis and gives insight in the effects of different time scales. Our analysis shows that the behaviour of the coupled system is essentially different from that of an abiotic system. Where in the latter, the basic state (flat bed) is essentially static, in the former, biomass may show growth (or decay). In turn, this affects hydro- and sediment dynamics, such that the basic state can be considered dynamic.

Barrier coasts display a chain of islands, separated by tidal inlets that connect a back-barrier basin to a sea or ocean. Observations show that barrier island length generally decreases for increasing tidal range and increasing basin area. However, this has neither been reproduced in model studies nor explained from the underlying physics. This is the aim of our study. Here we simulate barrier coast dynamics by combining a widely used empirical relationship for inlet dynamics with a process-based model of the tidal hydrodynamics. Our model results show stable inlet systems with more than one inlet open that support the observed qualitative relationships and fit in existing barrier coast classifications. To explain this, we identify a competition between a destabilizing mechanism (bottom friction in inlets, tending to reduce the number of open inlets) and a stabilizing one (spatially varying pressure gradients over the inlets, tending to keep the inlets open).

River deltas and individual delta lobes frequently face reduction of sediment supply, either from the geologic process of river avulsion or, more recently, due to human activities such as river damming. Using a process-based shoreline evolution model, we investigate wave reworking of delta shorelines after fluvial input elimination. Model results suggest that littoral sediment transport can result in four characteristic modes of delta abandonment, ranging from diffusional smoothing of the delta (or delta lobe) to the development of recurved spits. A straightforward analysis of delta shape and wave characteristics provides a framework for predicting the mode of delta abandonment. The observed morphologies of historically abandoned delta lobes, including those of the Nile, Ebro, and Rhone rivers, fit within this framework. Our results provide quantitative insight into the potential evolution of active delta environments in light of future extreme reduction of fluvial sediment input.

This study focuses on the effects of changes in fluvial sediment supply on the plan-form shape of wave-dominated deltas. We apply a one-line numerical shoreline model to calculate shoreline evolution after (I) elimination and (II) time-periodic variation of fluvial input. Model results suggest four characteristic modes of wave-dominated delta development after abandonment. The abandonment mode is determined by the pre-abandonment downdrift shoreline characteristics and wave climate (which are, in turn, determined by previous delta evolution). For asymmetrical deltas experiencing shoreline instability on the downdrift flank, time-periodic variation in fluvial input influences the evolution of downdrift-migrating sandwaves. The frequency and magnitude of the riverine "forcing" can initiate a pattern that migrates away from the river mouth, interacting with the development of shoreline sandwaves. Model results suggest that long-period signals in fluvial delivery can be shredded by autogenic sand waves, whereas shorter-term riverine fluctuations can dominate the signal of the autogenic sandwaves. The insights provided by these exploratory numerical experiments provide a set of hypotheses that can be further tested using natural examples.

This study investigates the influence of basin geometry on the cross-sectional stability of double inlet systems. The inlet is in equilibrium when the amplitude of the inlet velocities equals the equilibrium velocity (~1 m s-1). This equilibrium is stable when after a perturbation the cross-sections of both inlets return to their original equilibrium value. The necessary amplitudes of the inlet velocities are obtained using an idealized 2DH hydrodynamic that calculates tidal elevation and flow in a geometry consisting of several adjacent rectangular compartments. Model results suggest that regardless of the inclusion or exclusion of bottom friction in the basin, stable equilibrium states exist. Qualitatively, the influence of basin geometry does not change the presence of stable equilibrium. Quantitatively, however, taking a basin surface area of 1200 km2, equilibrium values can differ up to a factor 2 depending on the geometry of the basin.

We investigate the effects of a mega-scale sand extraction trench (length ~200 km, width ~10 km, depth ~10 m, located in front of the Dutch coast) on tidal dynamics in the Southern North Sea. To this end, an idealized modeling approach is adopted, extending the classical problem of Kelvin wave reflection in rectangular semi-enclosed basins to account for lateral depth variations and the presence of a trench. The model results indicate changes in tidal range (zones of decrease and increase of the order of several cm), phase and currents (order cm/s) throughout the whole basin, which emphasizes the importance of a model domain on the scale of the basin. The changes in currents may structurally change sediment transport rates by several percents, which is relevant to coastal morphology. It is argued that this idealized model can be used to support a companion study carried out with a more detailed numerical engineering model. This is because of its ability to (1) provide insight into the physical mechanisms, (2) reveal the order of magnitude and area of influence of the effects and (3) allow for a quick sensitivity analysis with respect to trench geometry.

A new method is proposed to calculate the cross-sectional flow field in compound channels for 1D flow models. The proposed method involves a new parameterization of the interface stress between adjacent compartments, typically between the main channel and floodplain of a two-stage channel. This expression, proportional to the difference in squared velocities allows for a simple calculation of the average flow velocities in different compartments. For two-stage channels good agreement is found between the model results and data from literature. Also, it is shown that the description of compartment-averaged velocities may be applied to flows in channel cross-sections with arbitrary geometry, thus providing a potentially useful improvement to 1D river flow models.

Offshore sandwaves are rhythmic bed patterns widely present in shallow shelf seas. Observations from the North Sea indicate a trend of coarser sediment with a higher sorting at sandwave crests. In this study, we present an extended model for sandwave dynamics that accounts for the presence and transport of non-uniform sediment. The extension involves an active layer as well as a fractional calculation of sediment transport, which includes hiding-exposure effects. Simulations of a growing sandwave for a bimodal mixture indicate a coarsening trend towards the crest, whereas in the sandwave troughs the sediment remains mixed. The model results show qualitative agreement with observations from various sites in the North Sea.

[1] Tidal sandbanks are large-scale bed features present in many shallow shelf seas. Here we investigate the effect of nonuniform sediment on their dynamics, with a particular aim to explain observed surficial grain size variations over tidal sandbanks from a process-based modeling perspective. To this end, we use a linear stability analysis that describes the positive feedback mechanism between hydrodynamics, sediment, and the seabed responsible for sandbank formation on a horizontal shelf. In this model the sediment transport and bed evolution modules are extended by introducing an active layer and a bimodal sediment mixture. We include a dynamic hiding/exposure description of sediment transport, enhancing the transport of coarse grains and inhibiting the transport of finer grains. The model results show that for symmetrical tidal conditions, coarse grains tend to accumulate at the bank crests. Moreover, the growth rates of the perturbations increase compared to the case of uniform sediment, while the preferred wavelength and bank orientation remain unchanged. For asymmetrical tidal conditions we find a spatial phase shift between topography and the mean grain size fraction, indicating an accumulation of coarse grains on the lee side of the bank. The model results qualitatively agree with observations from banks on the Belgian continental shelf.