Roland Garnier

Modelling the formation and the long‐term behavior of rip channel systems from the deformation of a longshore bar

A nonlinear numerical model based on a wave- and depth-averaged shallow water equation solver with wave driver, sediment transport, and bed updating is used to investigate the long-term evolution of rip channel systems appearing from the deformation of a longshore bar. Linear and nonlinear regimes in the morphological evolution have been studied. In the linear regime, a crescentic bar system emerges as a free instability. In the nonlinear regime, merging/splitting in bars and saturation of the growth are obtained. In spite of excluding undertow and wave-asymmetry sediment transport, the initial crescentic bar system reorganizes to form a large-scale and shore-attached transverse or oblique bar system, which is found to be a dynamical equilibrium state of the beach system. Thus the basic morphological transitions “Longshore Bar and Trough” → “Rhythmic Bar and Beach” → “Transverse Bar and Rip” described by earlier conceptual models are here reproduced. The study of the physical mechanisms allows us to understand the role of the different transport modes: The advective part induces the formation of crescentic bars and megacusps, and the bedslope transport damps the instability. Both terms contribute to the attachment of the megacusps to the crescentic bars. Depending on the wave forcing, the bar wavelength ranges between 180 and 250 m (165 and 320 m) in the linear (nonlinear) regime.

Generation and nonlinear evolution of shore-oblique/transverse sand bars

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.

Shoreline instability under low‐angle wave incidence

The growth of megacusps as shoreline instabilities is investigated by examining the coupling between wave transformation in the shoaling zone, long-shore transport in the surf zone, cross-shore transport, and morphological evolution. This coupling is known to drive a potential positive feedback in case of very oblique wave incidence, leading to an unstable shoreline and the consequent formation of shoreline sandwaves. Here, using a linear stability model based on the one-line concept, we demonstrate that such instabilities can also develop in case of low-angle or shore-normal incidence, under certain conditions (small enough wave height and/or large enough beach slope). The wavelength and growth time scales are much smaller than those of high-angle wave instabilities and are nearly in the range of those of surf zone rhythmic bars, O(102 − 103 m) and O(1 − 10 days), respectively. The feedback mechanism is based on: (1) wave refraction by a shoal (defined as a cross-shore extension of the shoreline perturbation) leading to wave convergence 26 shoreward of it, (2) longshore sediment flux convergence between the shoal and the shoreline, resulting in megacusp formation, and (3) cross-shore sediment flux from the surf to the shoaling zone, feeding the shoal. Even though the present model is based on a crude representation of nearshore dynamics, a comparison of model results with existing 2DH model output and laboratory experiments suggests that the instability mechanism is plausible. Additional work is required to fully assess whether and under which conditions this mechanism exists in nature.

Understanding coastal morphodynamic patterns from depth‐averaged sediment concentration

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.

Modeling the response of shoreface-connected sand ridges to sand extraction on an inner shelf

Shoreface-connected sand ridges are rhythmic bedforms that occur on many storm-dominated inner shelves. The ridges span several kilometers, are a few meters high, and they evolve on a timescale of centuries. A process-based model is used to gain a fundamental insight into the response of these ridges to extraction of sand. Different scenarios of sand extraction (depth, location, and geometry of the extraction area; multiple sand extractions) are imposed. For each scenario, the response timescale as well as the characteristics of the new equilibrium state are determined. Results show that ridges partially restore after extraction, i.e., the disturbed bathymetry recovers on decadal timescales. However, in the end, the ridge original sand volume is not recovered. Initially, most sand that accomplishes the infill of the pit originates from the area upstream of the extraction, as well as from the areas surrounding the pit. The contribution of the latter strongly decreases in the subsequent time period. Depending on the location of the pit, additional sand sources contribute: First, if the pit is located close to the downstream trough, the pit gains sand by reduction of sand transport from the ridge to this trough. Second, if the pit is located close to the adjacent outer shelf, the ridge recovery is stronger due to an import of sand from that area. Furthermore, pits that are located close to the nearshore zone have a weak recovery, deeper pits have longer recovery timescales, wide and shallow pits recover most sand, while multiple sand pits slow down the recovery process.

Observation and Modeling of Crescentic Bars in Barcelona Embayed Beaches

Two events of crescentic bar formation in La Barceloneta beach are analyzed (Mediterranean coast of Spain). They occurred on October 2003 and on December 2005, during Eastern storms with offshore root-mean-square heights between 1.5 m and 2 m, peak periods of about 9 s and angles of incidence relative to shore-normal between 15 and 30. The final alongshore spacings are about 300 m in the first event and about 100-200 m in the second. A nonlinear morphodynamic model is then applied to La Barceloneta conditions to reproduce these two events. The model is able to simulate the final shape of the crescentic bars, reproducing the observed spacings.

Stabilizing effect of random waves on rip currents

[1] The instability leading to the formation of rip currents in the nearshore for normal waves on a nonbarred, nonerodible beach is examined with a comprehensive linear stability numerical model. In contrast to previous studies, the hypothesis of regular waves has been relaxed. The results obtained here point to the existence of a purely hydrodynamical positive feedback mechanism that can drive rip cells, which is consistent with previous studies. This mechanism is physically interpreted and is due to refraction and shoaling. However, this mechanism does not exist when the surf zone is not saturated because negative feedback provided by increased (decreased) breaking for positive (negative) wave energy perturbations overwhelms the shoaling/refraction mechanism. Moreover, turbulent Reynolds stress and bottom friction also cause damping of the rip current growth. All the nonregular wave dissipations examined give rise to these hydrodynamical instabilities when feedback onto dissipation is neglected. When this feedback is included, the dominant effect that destroys these hydrodynamical instabilities is the feedback of the wave energy onto the dissipation. It turns out that this effect is strong and does not allow hydrodynamical instabilities on a planar beach to grow for random seas.

Short‐crested waves in the surf zone

This study investigates short-crested waves in the surf zone by using the mesh-free Smoothed Particle Hydrodynamics model, GPUSPH. The short-crested waves are created by generating intersecting wave trains in a numerical wave basin with a beach. We first validate the numerical model for short-crested waves by comparison with large-scale laboratory measurements. Then short-crested wave breaking over a planar beach is studied comprehensively. We observe rip currents as discussed in Dalrymple (1975) and undertow created by synchronous intersecting waves. The wave breaking of the short-crested wavefield created by the nonlinear superposition of intersecting waves and wave-current interaction result in the formation of isolated breakers at the ends of breaking wave crests. Wave amplitude diffraction at these isolated breakers gives rise to an increase in the alongshore wave number in the inner surf zone. Moreover, 3-D vortices and multiple circulation cells with a rotation frequency much lower than the incident wave frequency are observed across the outer surf zone to the beach. Finally, we investigate vertical vorticity generation under short-crested wave breaking and find that breaking of short-crested waves generates vorticity as pointed out by Peregrine (1998). Vorticity generation is not only observed under short-crested waves with a limited number of wave components but also under directional wave spectra.

A mechanism inhibiting rip channel formation for oblique waves

Previous numerical modelling studies based on 2DH morphodynamical model show that oblique waves tend to inhibit the formation of rip channel systems, but the mechanisms were not investigated. Field observations do not always agree with this model result, thus, understanding the mechanisms seems essential. To this end, the global analysis technique, originally developed to describe the long term behavior of bars (saturation of the bar growth), is also applied here to the initial stage of the bar evolution (formation of the bars). As a result, rip channels grow slower for larger wave angle because of the weakening of the instability mechanism -that only depends on the cross-shore current- rather than the increase of the damping due to the diffusive bedslope transport.

Nearshore Sand Bars

This review summarizes the morphological characteristics and dynamics of nearshore sand bars observed in the surf zone of sandy beaches worldwide, with length scales ranging from tens to hundreds of meters and time scales ranging from hours to weeks. They include shore-parallel bars (straight and crescentic) and transverse bars of different types. Furthermore, the present knowledge on the physical processes behind their formation and development is discussed.