Effect of grain size sorting on the formation of shoreface‐connected sand ridges

Walgreen, M.

doi:10.1029/2002jc001435

Cited by 26 publications

(30 citation statements)

References 26 publications

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“…In case that wave-bedform feedbacks are important, both growth and migration are controlled by suspended load transport and, thus, reduce with an increase in the standard deviation of the sediment mixture. Note that the decrease in migration speed with increase in standard deviation of the mixture is in contrast with previous results (Walgreen et al 2003), which is due to the fact that, in their case, the migration speed was determined by bedload transport due to the neglect of wave-bedform interactions. Figure 9 reveals that growth rates and migration speeds become smaller if the fraction F 1 of fine sediment is increased.…”

Section: Sensitivity Results To Changes In Sediment Characteristicscontrasting

confidence: 54%

“…A considerable improvement with respect to Walgreen et al (2003) is that the modeled maximum variation in the fraction of fine and coarse sediment over sfcr has increased. The latter is contributed to the fact that interactions between waves and the topography are included.…”

Section: Comparison With Observationsmentioning

confidence: 99%

“…This is done by adopting and extending the sediment sorting module for a bimodal sediment mixture of Walgreen et al (2003). The first aim of this work is to investigate whether wave-bedform interactions play an important role in the initial growth of sfcr, as well as in the initial onset of grain sorting over sfcr.…”

Section: Introductionmentioning

confidence: 99%

“…Walgreen et al (2003) analyzed a model that considers the effect of sediment sorting on the formation of sfcr. Results of this model indicate that the phase shift between bed topography and mean grain size is due to the selective transport via suspended load of grains with different sizes.…”

Section: Introductionmentioning

confidence: 99%

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Effect of wave–bedform feedbacks on the formation of, and grain sorting over shoreface-connected sand ridges

2009

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The influence of wave-bedform feedbacks on both the initial formation of shoreface-connected sand ridges (sfcr) and on grain size sorting over these ridges on micro-tidal inner shelves is studied. Also, the effect of sediment sorting on the growth and the migration of sfcr is investigated. This is done by applying a linear stability analysis to an idealized process-based morphodynamic model, which simulates the initial growth of sfcr due to the positive coupling between waves, currents, and an erodible bed. The sediment consists of sand grains with two different sizes. New elements with respect to earlier studies on grain sorting over sfcr are that wave-topography interactions are explicitly accounted for, entrainment of sediment depends on bottom roughness, and transport of suspended sediment involves settling lag effects. The results of the model indicate that sediment sorting causes a reduction of the growth rate and migration speed of sfcr, whereas the wavelength is only slightly affected. In the case where the entrainment of suspended sediment depends on bottom roughness, the coarsest sediment is found in the troughs; otherwise, the finest sediment occurs in the troughs. Compared to previous work, modeled maximum variations in the mean grain size over the topography are in better agreement with field observations. Settling lag effects are important for the damping of high-wavenumber mode instabilities such that a preferred wavelength of the bedforms is obtained.

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Section: Sensitivity Results To Changes In Sediment Characteristicscontrasting

confidence: 54%

Section: Comparison With Observationsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Effect of wave–bedform feedbacks on the formation of, and grain sorting over shoreface-connected sand ridges

2009

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“…On the inner shelf, sediment sorting patterns along shoreface-connected ridges have been studied by Walgreen, de Swart & Calvete (2003), de Swart et al (2008 and Vis-Star, de Swart & Calvete (2009). These bed forms differ from sorted bed forms both in length and orientation.…”

Section: Introductionmentioning

confidence: 99%

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

2011

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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.

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Understanding coastal morphodynamic patterns from depth‐averaged sediment concentration

Ribas

Falqués

Swart

et al. 2015

Reviews of Geophysics

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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.

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Effect of grain size sorting on the formation of shoreface‐connected sand ridges

Cited by 26 publications

References 26 publications

Effect of wave–bedform feedbacks on the formation of, and grain sorting over shoreface-connected sand ridges

Effect of wave–bedform feedbacks on the formation of, and grain sorting over shoreface-connected sand ridges

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

Understanding coastal morphodynamic patterns from depth‐averaged sediment concentration

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