Modelling the formation of shoreface-connected sand ridges on storm-dominated inner shelves

Abstract

A morphodynamic model is developed and analysed to gain fundamental understanding of the basic physical mechanisms responsible for the characteristics of shorefaceconnected sand ridges observed in some coastal seas. These alongshore rhythmic bed forms have a horizontal lengthscale of order 5 km and are related to the mean current along the coast: the seaward ends of their crests are shifted upstream with respect to where they are attached to the shoreface. The model is based on the two-dimensional shallow water equations and assumes that the sediment transport only takes place during storms. The flux consists of a suspended-load part and a bed-load part and accounts for the effects of spatially non-uniform wave stirring as well as for the preferred downslope movement of sediment. The basic state of this model represents a steady longshore current, driven by wind and a pressure gradient. The dynamics of small perturbations to this state are controlled by a physical mechanism which is related to the transverse bottom slope. This causes a seaward deflection of the current over the ridges and the loss of sediment carrying capacity of the flow into deeper water. The orientation, spacing and shape of the modelled ridges agree well with field observations. Suspended-load transport and spatially non-uniform wave stirring are necessary in order to obtain correct e-folding timescales and migration speeds. The ridge growth is only due to suspended-load transport whereas the migration is controlled by bed-load transport.

Modification of the Quaternary stratigraphic framework of the inner-continental shelf by Holocene marine transgression: An example offshore of Fire Island, New York

“…In addition, the swales in the exposed Holocene transgressive surface would also be expected to migrate as the ravinement surface is exposed at the seafloor and subjected to erosion, albeit at a lower rate than the sand ridge migration; the mobile modern sediment (sand ridges) is migrating over a lessmobile substrate. Nonetheless, interpretation of southwest migration of these ridges is consistent with models of shoreface-connected ridge migration rates of 1 to 10 m/yr in energy environments similar to southern Long Island (Trowbridge, 1995;Calvette et al, 2001;and Vis-Star et al, 2007) and similar to rates of sand ridge movement recognized offshore of the Outer Banks, NC (Thieler et al, 2013).…”

“…As the ridges grow in amplitude, they undergo alongshore, down-current migration, while at the same time the eroding shoreface retreats out from under them. A widely cited theory of origin and maintenance of shorefaceattached sand ridges is that of Trowbridge (1995), modified by Calvette et al (2001) and Vis-Star et al (2007), who used stability analysis to show that sand ridges can be created, maintained, and enhanced during storm-driven alongshore-directed flow. Fig.…”

Self-organization mechanisms for the formation of nearshore crescentic and transverse sand bars

“…Another approach commonly adopted to study the initial growth of a pattern is linear stability analysis which has already been successfully applied to describe the formation of various features at different scales ranging from ripples (Blondeaux 1990) to offshore patterns like sand waves (Hulscher, De Swart & De Vriend 1993) and shoreface-connected ridges (Calvete et al 2001).…”

Data-driven components in a model of inner-shelf sorted bedforms: a new hybrid model

“…The calibration parameter in this framework is γ c and is adjusted to maintain an order of magnitude difference between the two terms on the right-hand side of Eq. (4), similar to the methodology of Calvete et al (2001). For all experiments in this contribution, γ c = 0.07.…”

“…We suggest that this mismatch, coupled with the strong forcing condition that is required to move sediment in the model (i.e., large U w ), has lead to a smaller permissible parameter space where the morphodynamic term and the new GP derived predictor are interoperable. We define the permissible parameter space by the scaling argument made previously by Calvete et al (2001): γ c should be set to a value that maintains the ratio between the two terms on the right side of Eq. (4) to ∼ 1 order of magnitude.…”