It is recommended to read this article in conjunction with the article Shoreface profile.
Nearshore sandbars, also called breaker bars, are a common feature of the surf zone of sandy coasts worldwide. Their occurrence is related to the shoreface slope, which should be smaller than about 1/30. They are the result of the dynamic interaction between the shape of the coastal profile and the transformation of waves as they propagate onshore; at the same time they are an important agent in this interaction. Their presence promotes the breaking of waves further away from the shoreline; they thus reduce the wave forces exerted directly on the shore. The cartoon of Fig. 1 shows a typical example of the transformation and breaking of incident waves in the nearshore zone.
Under high-energy wave conditions, nearshore sandbanks tend to be straight, with a parallel orientation to the coast, or at a small angle to the shoreline. Sandbars close to the shore can break up in smaller sandbanks with crescentic shapes under less energetic wave conditions. This usually occurs in conjunction with the development of a rip cell system, as described in the articles Rip current and Rhythmic shoreline features.
Understanding sandbar dynamics has greatly progressed thanks to the development of the Argus video monitoring system, which allows continuous observation of the presence and migration of sandbanks under a wide variety of conditions. Nevertheless, the behavior of sandbanks is not yet fully understood and is the subject of ongoing research. Observations suggest that bar formation is related to the prevalence of onshore sand transport by incident waves before breaking and the prevalence of offshore transport after breaking. Because there is no prevalence in an equilibrium situation, this must be understood as follows. Under intensifying incident waves, a small positive perturbation of the equilibrium seabed profile (small hump) in the shoaling zone (i.e. before breaking) will migrate in onshore direction, whereas a small positive perturbation in the surf zone (i.e. after breaking) will migrate in offshore direction. Onshore transport prior to breaking is mainly due to the interaction of the shoaling wave with the seabed that generates higher velocities and stronger acceleration of onshore wave orbital motion compared to offshore wave orbital motion (Fig. 2). Offshore transport is mainly due to wave breaking that produces strong turbulence and uplift of sand from the seabed that is transported seaward by so-called undertow (the return flow in the lower part of the water column compensating for the onshore mass transport in the upper part of the water column between wave trough and crest). See Shoreface profile for more explanations.
Laboratory experiments and process-based morphodynamic modeling show that nearshore sandbars can develop as a result of wave breaking on the shoreface. Waves breaking on a non-barred shoreface induce a net seaward sand transport caused by the undertow current in a zone landward of the breakpoint. Sediment mobilization in this zone (which can be enhanced by seabed stirring due to longshore currents induced by obliquely incident breaking waves) is largely responsible for the strength of this seaward sand transport. Wave breaking will thus initiate a bar by creating a trough at the breakpoint and a hump just seaward of the breakpoint (assuming that the shoreface slope seaward of the breakpoint is in equilibrium). The breakpoint will then shift towards the hump and the initial bar will follow until reaching a position further down the shoreface slope where the breaker-induced offshore sand transport is weakened and in equilibrium with wave-induced onshore transport (Fig. 3). This mechanism illustrates that shorefaces subject to vigorous wave breaking will usually exhibit a barred profile. In situations of less intense wave breaking the bar will not grow high, but take the form of a terrace.
Nearshore bars are not static features but move in onshore or offshore direction depending on the wave climate. Ruessink and Terwindt, 2000) found that on the Dutch coast bars migrate offshore under energetic waves (storm periods), while under mild waves (long-period waves, swell, non-breaking onshore propagating surf bores) the migration direction is onshore. This study also showed that offshore migration dominates when [math]H_s/d[/math] (ratio of significant wave height [math]H_s[/math] to water-depth-above-crest [math]d[/math]) is larger than 0.6 and onshore migration when this ratio is smaller than 0.3.
During storm periods, large offshore bar displacements can occur in a short time. Landward bar migration is much slower; long periods of onshore motion are required to offset the seaward migration of a single high-wave period. Most observations indicate a long-term net offshore migration, but on some other coasts the bar location migrates in a landward direction (Fig. 4). This seems to be a feature of highly dissipative coasts, i.e. coasts with a fine-sandy gently sloping shoreface, where waves are breaking gradually without developing strong undertow currents. Onshore bar migration generally occurs during long periods of swell-dominated conditions. In some cases the bar eventually welds to the shoreline, leaving a non-barred shoreface. The process of onshore bar migration is often associated with the formation of bar-rip systems with a longshore rhythmic variation. This is described more in detail in the article Rhythmic shoreline features, where it is shown that longshore sand transport processes also play an important role in the generation and evolution of nearshore sandbars.
In situations where the net bar migration is directed offshore, the bar eventually decays when the water depth above the crest becomes too large to induce frequent wave breaking and associated convergence of sand transport. When this outer bar decays, energetic incident waves reach more easily the intertidal beach and are capable to remove sand for generating a new bar that subsequently starts moving offshore. However, at some coasts, observations show the generation of an outer bar offshore at the location where energetic incident waves start breaking on the shoreface.
Offshore sandbar migration implies accretion at the outer (seaward) bar flank and erosion of the inner side. This does not necessarily proceed through sand transfer from the inner to the outer bar flank; the outer bar flank can accrete with sand imported from offshore. Offshore sandbar migration is thus not synonymous with sand loss to deeper water.
Multiple bar system
Dissipative coasts with a wide surf zone usually have several (often 3 or 4) more or less parallel bars. The shoreface bathymetry of the Dutch coast at Katwijk is shown as an example in Fig. 5. Rozynski and Lin (2015) described the nearshore bar system at the micro-tidal Baltic coast of Poland as follows: (1) when waves are mild, the surf zone is narrow, and they break only over the innermost bar; (2) higher waves begin to break over the second bar, the surf zone now includes two bars, and the breakers can include spilling or plunging modes or both; (3) during heavy storms, the outer bars contribute to wave-energy dissipation as well – the surf zone now includes four or more bars and is several hundred meters wide; various combinations of spilling and plunging modes are then possible, resulting in very complicated alongshore and cross-shore driven sediment patterns; (4) variations in wave set-up and wind-driven storm surges (in a range of 1 m) further modify the breaking regimes during the build-up, peak and recession of storms.
The dynamics of multiple bar systems is not fully understood, although some qualitative features are reproduced by semi-empirical models. It has been suggested that Bragg scattering – the resonant reflection of low-frequent waves (infragravity waves) in a multiple bar system – plays a role in their formation. Multiple bars can arise from various other mechanisms, such as sand bar splitting (which has been observed during low-energy conditions, several weeks after the incidence of high-energy waves); a secondary breaking event when waves have reformed after breaking on the outer bar; distinct wave breakpoints at high water and low water on the shoreface of tidal coasts. There is only limited sand exchange between multiple bars because of small sand transport capacity in the troughs between the bars.
An often observed particularity of many nearshore bar systems is the orientation with respect to the shoreline. In many cases the bars make a small angle of 2-4 degrees, their distal end at the outer edge of the nearshore region pointing in the direction of the littoral drift. When the distal part decays offshore, the most inner part separates from the shoreline and starts moving offshore. More generally, the behaviour and alongshore variability of inner bars and the shoreline is influenced by wave breaking patterns on the outer bars and by the tide range.
While multiple bar systems are typical for wide dissipative high-energy beaches, there are situations where nearshore bars are absent from such beaches. For example, no bars were present in the nearshore zone of the coastal stretch between The Hague and Rotterdam (Netherlands). The groyne field along this coastal stretch was suggested as a possible reason, because a nearshore bar developed spontaneously after the groyne field was covered by a beach nourishment.
- Shoreface profile
- Parametric equilibrium models
- Closure depth
- Rhythmic shoreline features
- Rip current
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