Formation of rhythmic sorted bed forms on the continental shelf: an idealised model

Abstract

An idealised model is presented to study the formation of sorted bed forms generated by a wind-driven along-shore current. The study employs a linear stability analysis to describe the time development of perturbations of both bottom composition and bed elevation, superimposed on a flat bed composed of a sediment mixture homogeneously distributed in space. The model considers both bed and suspended loads and takes into account the averaged influence of waves on the flow field and the transport of sediment. The results show that the positive coupling between waves, along-shore current and the erodible heterogeneous bed leads to the amplification of two modes, which exhibit distinct characteristics. A first mode is found to be dominant when moderate hydrodynamic conditions are considered and is primarily amplified by the convergence of sediment transport induced by the changes in the bed elevation. This mode has wavelengths of the order of hundred metres and has coarse (fine) sediments in its troughs (crests). By increasing the height of the waves and/or the strength of the steady current, the second mode can become dominant. This mode is characterised by shorter wavelengths and results from the interaction between the convergence of sediment transport related to changes in the bottom composition and that induced by perturbations of the bed elevation. These bed features can have an up-current or a down-current shift between the centre of the coarse-grained bands and the trough of the bottom wave. Typical growth times of the amplified features are of the order of hundreds of days and the migration rates, in the direction of the along-shore current, range between 0.1 and 10 m per day. A qualitative comparison of the model results with field observations indicates that the generation of two distinct modes provides a possible explanation for the broad range of characteristics of the natural bed features.

Rippled scour depressions characterization through interferometric data at Barra da Lagoa — Moçambique beaches, Southern Brazil

“…It means that, where the intenser acoustic answer were observed in the SSS images, the bathymetry shows a lower elevation, that varies from 0.5 to 1 meter and is composed of rippled coarse sediment, which have a steeper and deeper face downdrift, reflecting higher velocities south to north bottom current [15]. This also, reinforces the hypothesis of 'selforganization' [2], but we also believe in some interaction between flow and bathymetry, as [9]. The SB presents at Barra da Lagoa -Moçambique beaches seems to be stable as surveys was conducted in 2013 and 2014, and no positional changes were found…”

“…The generation of such features has been associated with cross-shore currents [1]; along shore currents [2]; combined current and waves [7] storm generated downwelling [8]; tidal scour [3]; and also wind driven along-shore current [9] Reference [2], started with the hypothesis that those features, present worldwide, is related to sediment sorting feedback ('self-organization') rather than interactions between flow and bathymetry. They referred to the features as SB, being part of a major feature, and not a simple depression, followed by [10] [11].…”

“…The features are a low bathymetric relief on the order of centimeters to meter, composed by rippled coarse sand, at the scale of meters to hundred meters wide and meters to kilometers long, mostly perpendicular to the shoreline, adjacent to a well sorted fine sediment [2][7][5] [9]. It acts as a sink or source of coarse and fine sand [10][11], as well as a possible nursery area for some species of fish, increasing the habitat heterogeneity in the area [12].…”

Modelling the two-way coupling of tidal sand waves and benthic organisms: a linear stability approach

“…In a study into current-generated sorted bed forms, van Oyen et al [50] found that the resulting eigenmodes could be assigned to either roughness or topography. In our model, we observe that the eigenmodes can be related to either biomass or topography, but only to a certain extent.…”

“…Whereas the majority of the idealised model studies into morphological features only consider the morphological evolution of the bed, some studies included a coupling between bed topography and multiple grain size fractions in order to explain observed phase differences between them [21,41,50,51]. Moreover, for riverine environments a similar approach has been used, here focussing on the coupling between vegetation and morphodynamics [3,16].…”