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Freshwater pulses (during which river discharge is much higher than average) occur in many estuaries and strongly impact estuarine functioning. To gain insight into the estuarine salinity response to freshwater pulses, an idealized model is presented. With respect to earlier models on the spatiotemporal behavior of salinity in estuaries, it includes additional processes that provide a more detailed vertical structure of salinity. Simulation of an observed salinity response to a freshwater pulse in the Guadalquivir Estuary (Spain) shows that this is important to adequately simulate the salinity structure. The model is used to determine the dependency of the estuarine salinity response to freshwater pulses for different background discharge, tides, and different intensities and durations of the pulses. Results indicate that the change in salt intrusion length due to a freshwater pulse is proportional to the ratio between peak and background river discharge and depends linearly on the duration of the pulse if there is no equilibration during the pulse. The adjustment time, which is the time it takes for the estuary to reach equilibrium after an increase in river discharge, scales with the ratio of the change in salt intrusion length and the peak river discharge. The recovery time, that is, the time it takes for the estuary to reach equilibrium after a decrease in river discharge, does not depend on the amount of decrease in salt intrusion length caused by the pulse. The strength of the tides is of minor importance to the salt dynamics during and after the pulse.

Tidal sand ridges are large-scale bedforms with horizontal dimensions of several kilometers and heights in the order of tens of meters, which occur on outer shelves of coastal seas. In order to study the long-term evolution of these ridges, an idealized nonlinear numerical model was developed. With this tool, the sensitivity of the characteristics of these finite-height ridges, in particular, their shape and growth time, to 1D/2D configuration (topography varies in one/two horizontal dimensions), tidal ellipticity and critical bed shear stress for sand erosion was investigated. In the case of a 1D configuration, the root mean square height hrms of the bedforms first grows exponentially and hereafter saturates. In the end, ridges in static equilibrium are obtained, i.e., hrms remains constant. In contrast, when the configuration is 2D, ridges are found with spatially meandering crests that oscillate in time. Initially the bedforms are composed of a finite number of bottom modes. The meanders occur if bottom modes with crests normal to those of the initially preferred bedform exist, and their topographic wavenumbers are in the order of that of the preferred bedform or smaller. In addition, the vertical distance between the crest and trough levels should be larger than around 80% of the maximum water depth. Generally, the global growth time, i.e., the time at which hrms stops increasing after the exponential growth stage of the bedforms, is slightly larger for a 2D than for a 1D configuration. The ridge shapes are sensitive to the tidal ellipticity, while they are hardly sensitive to the critical bed shear stress. The global growth time varies non-monotonically with the tidal ellipticity, and it increases if the critical bed shear stress is included. Comparison between the model results and field observations suggests that the model is able to simulate the gross characteristics of the Dutch Banks and the Flemish Banks in the southern North Sea and that these ridges may still be growing.

The initial- and long-term evolution of shoreface-connected sand ridges (sfcr) is investigated with a new nonlinear model (MORFO56) that employs finite difference methods, rather than spectral methods. MORFO56 uses depth-averaged shallow water equations, including sediment transport and bed updating. Moreover, it includes full wave-topography feedbacks, wave shoaling and refraction, and wave radiation stresses. First, effects of relaxing the rigid-lid assumption and quasi-steadiness on the initial growth and migration of sfcr are quantified, by conducting a series of short-term runs. It turns out that assuming a free lid and unsteady flow results in larger wavelengths and larger migration speeds of sfcr. Furthermore, the new model is able to simulate the finite amplitude behaviour of sfcr for more realistic bottom slopes than earlier spectral models. Finally, the role of wave-topography feedbacks in the initial formation of sfcr is examined. Model simulations show that sfcr in the presence of these feedbacks are more trapped to the shoreface, with an offshore extent of approximately 1 km. Moreover, growth of sfcr is enhanced considerably by wave-topography feedbacks. The specific inclusion of radiation stresses does not affect this result.

A morphodynamic model has been applied to explain the formation of transverse sand bars at Duck beach, USA. The model describes the feedback between waves, rollers, depth-averaged currents and bed evolution, so that self-organized processes can develop. The wave and bathymetric conditions measured at Duck are used to perform the simulations. Subsequently, modelled bar characteristics are compared with those observed there. Realistic positive feedback leading to formation of the observed bars only occurs if the resuspension of sediment due to bore turbulence is included in the model. Also, the offshore root mean square wave height must be larger than 0.5 m and the offshore wave incidence angle larger than 15o (offshore boundary is at 8 m depth), conditions that occur at Duck only 25% of the time. The modelled shape (wavelength, cross-shore extent and crest orientation) and growth rate agree with data, but the model overestimates the migration rates.