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.