A simple morphological model is considered which describes the interac
tion between a tidal flow and an erodible bed in a shallow sea. The ba
sic state of this model describes a spatially uniform tide over a flat
bottom where the flow vector is represented as a tidal ellipse. The l
inear stability of this solution is analysed with respect to bed form
perturbations. Results are presented for both a uni-directional and ci
rcular tide. In the former case the wave-length and the orientation of
the fastest growing bed mode agree well with those of tidal sand bank
s. However, this model only predicts the growth of large-scale sand ri
dges. With a simplified numerical model we tentatively show that the e
ffects of secondary currents on the sediment transport trigger the for
mation of instabilities at an essentially smaller scale, viz, sand wav
es. Another limitation of a model with uni-directional tides is that n
o selective modes found are the first to become unstable if the model
parameters are varied. In the case of a circular tide, critical model
parameters are found below which the basic state is stable. We conclud
e that this provides a starting point for the development of a weakly
non-linear analysis, which will yield information on the amplitude beh
aviour of marginally growing bed forms.