5% - eTheses Repository - University of Birmingham

there is no evidence to suggest that their formation mechanisms
are different. Accordingly, we choose not to distinguish between
these different morphologies in the present classification.
However, we note that dunes are distinguished in two categories,
as discussed by Alien (1984) . The first category relates to more
three dimensional dunes with steeper lee faces, termed
megaripples, simple dunes, dunes or sandwaves. The second
category relates to flatter dunes, straighter crested with longer
wavelengths and traditionally termed sandwaves, rippled
sandwaves, diminished dunes, scaloid sandwaves and transverse
bars or bars.
3 THE FLOW OVER SANDWAVES
The main features of flow over a sandwave comprise boundary layer
separation at the crest, shear layer growth, reattachment in the
trough and establishment of a new boundary layer on the stoss
slope, all as included schematically in the back step flow of
figure 2. Here we discuss the shear layer and the interaction
of its embedded vortices with sediment particles viewed as a
discrete second phase in the style of literature on the dynamics
of multiphase flows. Discussion of boundary layer aspects
appears in appendix 1.
3.1 The Shear Layer
The stoss boundary layer separates from the bed at the crest,
where the vorticity generated on the upstream boundary becomes
1-7