The dynamics of small‐amplitude perturbations of an otherwise rectilinear coastline due to the wave‐driven alongshore sediment transport is examined at large time and length scales (years and kilometers). A linear stability analysis is performed by using an extended one‐line shoreline model with two main improvements: (1) the curvature of the coastline features is accounted for and (2) the coastline features are assumed to extend offshore as a bathymetric perturbation up to a finite distance. For high incidence angles, instability is found in accordance with Ashton et al. (2001). However, it is seen that instability is inhibited by high waves with long periods and gently sloping shorefaces so that in this case the coastline may be stable for any angle. Similarly, there is no instability if the bathymetric perturbation is confined very close to the coast. It is found that the traditional linearized one‐line model (Larson et al., 1987) tends to overpredict the coastline diffusivity. The overprediction is small for the conditions leading to a stable coastline and for moderate incidence angles but can be very dramatic for the conditions favoring instability. An interesting finding is that high‐angle waves instability has a dominant wavelength at the linear regime, which is in the order of 4–15 km, one to two orders of magnitude larger than the length scale of surf zone rhythmic features. Intriguingly, this is roughly the same range of the wavelength of some observed shoreline sand waves and, in particular, those observed along the Dutch coast. A model application to this coast is presented.

The coupling between topography, waves and currents in the surf zone may self-organize to produce the formation of shore-transverse or shore-oblique sand bars on an otherwise alongshore uniform beach. In the absence of shore-parallel bars, this has been shown by previous studies of linear stability analysis, but is now extended to the finite-amplitude regime. To this end, a nonlinear model coupling wave transformation and breaking, a shallow-water equations solver, sediment transport and bed updating is developed. The sediment flux consists of a stirring factor multiplied by the depth-averaged current plus a downslope correction. It is found that the cross-shore profile of the ratio of stirring factor to water depth together with the wave incidence angle primarily determine the shape and the type of bars, either transverse or oblique to the shore. In the latter case, they can open an acute angle against the current (up-current oriented) or with the current (down-current oriented). At the initial stages of development, both the intensity of the instability which is responsible for the formation of the bars and the damping due to downslope transport grow at a similar rate with bar amplitude, the former being somewhat stronger. As bars keep on growing, their finite-amplitude shape either enhances downslope transport or weakens the instability mechanism so that an equilibrium between both opposing tendencies occurs, leading to a final saturated amplitude. The overall shape of the saturated bars in plan view is similar to that of the small-amplitude ones. However, the final spacings may be up to a factor of 2 larger and final celerities can also be about a factor of 2 smaller or larger. In the case of alongshore migrating bars, the asymmetry of the longshore sections, the lee being steeper than the stoss, is well reproduced. Complex dynamics with merging and splitting of individual bars sometimes occur. Finally, in the case of shore-normal incidence the rip currents in the troughs between the bars are jet-like while the onshore return flow is wider and weaker as is observed in nature.

We present a novel process‐based morphodynamic model, which includes transport processes due to both velocity and acceleration skewness and a new formulation for intrawave motions, that successfully simulates observations of onshore sandbar migration. Results confirm findings of previous studies, in which each process was considered separately and in which sediment transport was computed from the observed water motion. However, our results indicate that accounting for the joint action of both velocity and acceleration skewnesses causes major improvement of the modeled onshore bar migration and is essential to accurately model the evolution of the entire cross‐shore bottom profile, when compared with observations. We also demonstrate that the morphodynamics in the shoaling zone are dominated by velocity skewness (bed shear stresses), while sediment transport induced by acceleration skewness (pressure gradients) controls the morphodynamics in the inner surf zone.

The process of formation of a rip channel/crescentic bar system on a straight, sandy coast is examined. A short review of earlier studies is presented. A morphodynamic stability model is then formulated. The resulting model includes a comprehensive treatment of shoaling and surf zone hydrodynamics, including wave refraction on depth and currents and waves. The sediment transport is modeled using a total load formula. This model is used to study the formation of rip currents and channels on a straight single‐barred coast. It is found that this more comprehensive treatment of the dynamics reveals the basic rip cells predicted in earlier studies for normal incidence. Also as before, cell spacings (λ) scale with shore‐to‐bar crest distance (X_b), while growth rates decrease. The λ increases with offshore wave height (H) up to a saturation value; increasing H also increases instability. Experiments at off‐normal wave incidence (θ > 0) introduce obliquity into the evolving bed forms, as expected, and λ increases approximately linearly. the e‐folding times also increase with θ. At normal incidence, λ increases weakly with wave period, but at oblique angles, λ decreases. Tests also reveal the presence of forced circulation cells nearer to the shoreline, which carve out bed forms there. The dynamics of these forced cells is illustrated and discussed along with the associated shoreline perturbation. Transverse bars are also discovered. Their dynamics are discussed. Model predictions are also compared with field observations. The relevance of the present approach to predictions of fully developed beach states is also discussed.

A morphodynamic model is developed and analysed to gain fundamental understanding of the basic physical mechanisms responsible for the characteristics of shoreface-connected sand ridges observed in some coastal seas. These alongshore rhythmic bed forms have a horizontal lengthscale of order 5 km and are related to the mean current along the coast: the seaward ends of their crests are shifted upstream with respect to where they are attached to the shoreface. The model is based on the two-dimensional shallow water equations and assumes that the sediment transport only takes place during storms. The flux consists of a suspended-load part and a bed-load part and accounts for the effects of spatially non-uniform wave stirring as well as for the preferred downslope movement of sediment. The basic state of this model represents a steady longshore current, driven by wind and a pressure gradient. The dynamics of small perturbations to this state are controlled by a physical mechanism which is related to the transverse bottom slope. This causes a seaward deflection of the current over the ridges and the loss of sediment carrying capacity of the flow into deeper water. The orientation, spacing and shape of the modelled ridges agree well with field observations. Suspended-load transport and spatially non-uniform wave stirring are necessary in order to obtain correct e-folding timescales and migration speeds. The ridge growth is only due to suspended-load transport whereas the migration is controlled by bed-load transport.

This review highlights the important role of the depth‐averaged sediment concentration (DASC) to understand the formation of a number of coastal morphodynamic features that have an alongshore rhythmic pattern: beach cusps, surf zone transverse and crescentic bars, and shoreface‐connected sand ridges. We present a formulation and methodology, based on the knowledge of the DASC (which equals the sediment load divided by the water depth), that has been successfully used to understand the characteristics of these features. These sand bodies, relevant for coastal engineering and other disciplines, are located in different parts of the coastal zone and are characterized by different spatial and temporal scales, but the same technique can be used to understand them. Since the sand bodies occur in the presence of depth‐averaged currents, the sediment transport approximately equals a sediment load times the current. Moreover, it is assumed that waves essentially mobilize the sediment, and the current increases this mobilization and advects the sediment. In such conditions, knowing the spatial distribution of the DASC and the depth‐averaged currents induced by the forcing (waves, wind, and pressure gradients) over the patterns allows inferring the convergence/divergence of sediment transport. Deposition (erosion) occurs where the current flows from areas of high to low (low to high) values of DASC. The formulation and methodology are especially useful to understand the positive feedback mechanisms between flow and morphology leading to the formation of those morphological features, but the physical mechanisms for their migration, their finite‐amplitude behavior and their decay can also be explored.