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. 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 shore. The cartoon of Fig. 1 shows a typical example of the transformation and breaking of incident waves in the nearshore zone.
The process of bar formation is still a topic of research. It has been suggested 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. 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 for sufficiently large waves. 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 compensating for the onshore Stokes' mass transport). See Shoreface profile for more explanations.
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 is on average stable or migrates in a landward direction (Fig. 3). During long periods of swell-dominated conditions the bar eventually welds to the shoreline, leaving a non-barred shoreface. 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 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. The onshore bar migration is often complex due to the formation of bar-rip systems with a longshore rhythmic variation, described more in detail in the article Rhythmic shoreline features. Longshore sand transport processes also play an important role in the generation and evolution of sandbars.
Multiple bar system
Dissipative coasts with a wide surf zone usually have several (often 3 or 4) more or less parallel bars (Fig. 3). The dynamics of multiple bar systems are 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 also arise from sand bank splitting, which has been observed during low-energy conditions, several weeks after the incidence of high-energy waves. Another 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 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.
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