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

Citation
D. Calvete et al., Modelling the formation of shoreface-connected sand ridges on storm-dominated inner shelves, J FLUID MEC, 441, 2001, pp. 169-193
Citations number
34
Categorie Soggetti
Physics,"Mechanical Engineering
Journal title
JOURNAL OF FLUID MECHANICS
ISSN journal
00221120 → ACNP
Volume
441
Year of publication
2001
Pages
169 - 193
Database
ISI
SICI code
0022-1120(20010825)441:<169:MTFOSS>2.0.ZU;2-E
Abstract
A morphodynamic model is developed and analysed to gain fundamental underst anding of the basic physical mechanisms responsible for the characteristics of shoreface-connected sand ridges observed in some coastal seas. These al ongshore 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 equatio ns 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 rep resents a steady longshore current, driven by wind and a pressure gradient. The dynamics of small perturbations to this state are controlled by a phys ical mechanism which is related to the transverse bottom slope. This causes a seaward deflection of the current over the ridges and the loss of sedime nt carrying capacity of the flow into deeper water. The orientation, spacin g and shape of the modelled ridges agree well with field observations. Susp ended-load transport and spatially non-uniform wave stirring are necessary in order to obtain correct e-folding timescales and migration speeds. The r idge growth is only due to suspended-load transport whereas the migration i s controlled by bed-load transport.