Nearshore sandbars

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It is recommended to read this article in conjunction with the article Shoreface profile.


Introduction

Fig. 1. Wave transformation in the nearshore zone.



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[1]. Their presence promotes the breaking of waves further away from the shoreline; they thus reduce the wave forces exerted directly on the shore[2]. The cartoon of Fig. 1 shows a typical example of the transformation and breaking of incident waves in the nearshore zone.

Bar formation

The process of bar formation is still a topic of 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[3]. 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[4][5]. 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[6][7] (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)[8][9][10]. See Shoreface profile for more explanations.

Laboratory experiments[11][12] and process-based morphodynamic modeling[13][14] 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[15][16]. Wave breaking will thus initiate a bar by creating a trough at the breakpoint and a hump seaward of the breakpoint (assuming that the shoreface slope seaward of the breakpoint is in equilibrium). The breakpoint will then move seawards 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[17][18].


Fig. 2. Onshore-offshore asymmetry of the wave orbital velocity and acceleration in the shoaling zone. The maximum onshore orbital velocity in the wave crest phase is substantially larger than the maximum offshore orbital velocity in the wave trough phase (sometimes called positive skewness). The acceleration of offshore to onshore wave orbital velocities is also substantially larger than the acceleration in the opposite direction (sometimes called positive asymmetry). In most cases this will induce net onshore sand transport, although in some cases the opposite may also happen (see Sediment transport formulas for the coastal environment).
Fig. 3. Profiles (green and blue) schematically representing two successive stages of bar formation by wave breaking on an initially non-barred shoreface (red).

Bar migration

Fig. 4. Cross-shore depth profiles of the surf zone at Skallingen [19] (Denmark) and Egmond [20] (Netherlands) showing systems of multiple nearshore bars at different years. For both, 0 m depth corresponds approximately to mean sea level. At Skallingen the bar crests move in onshore direction, whereas at Egmond the bar crests move in offshore direction. At Egmond, the outer bar decays at the seaward limit of the surf zone at 8 m depth. Both coasts are storm-dominated. Symbols: median grain size [math]d_{50}[/math][mm], mean significant wave height [math]H_s[/math][m], mean wave period [math]T[/math][s], tidal range [math]TR[/math][m].

Nearshore bars are not static features but move in onshore or offshore direction depending on the wave climate. Ruessink and Terwindt, 2000[21]) 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[22]. Most observations indicate a long-term net offshore migration[23], but on some other coasts the bar location migrates in a landward direction[19] (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[24]. 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[25]. 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[26][27]. 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[28][19].


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 is not fully understood, although some qualitative features are reproduced by semi-empirical models[29]. 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[30]. 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[31]); 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[24].

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[23]. 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[32] and by the tide range[33].

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[34].


Related articles

Shoreface profile
Parametric equilibrium models
Closure depth
Rhythmic shoreline features


References

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The main author of this article is Job Dronkers
Please note that others may also have edited the contents of this article.

Citation: Job Dronkers (2021): Nearshore sandbars. Available from http://www.coastalwiki.org/wiki/Nearshore_sandbars [accessed on 27-09-2021]