5% - eTheses Repository - University of Birmingham

2.1 Current Ripples
Current ripples (figure 3) are generally found with the finer
grades of quartz sand, typically less than 0.125 mm diameter.
However, they are also known to exist with 0.5 mm quartz sand
(RJS Whitehouse, private communication). These features are
transverse ridges with steep downstream (lee) slopes and
shallower upstream (stoss) slopes. Their height is invariably
less than 40 mm and their wavelength less than 600 mm ie, very
small compared to typical scales of the flow. Ripples migrate
downstream at velocities which are small compared with the stream
velocity (Yalin, 1972). Sediment is eroded from the stoss slopes
and deposited on the lee slopes (shown schematically in figure
4). Thus, each grain travels intermittently and is then buried
(Raudkivi, 1967).
2.2 Dunes/ sandwave s
Dunes (figure 5) (equivalent to sandwaves in our terminology)
also migrate downstream at speeds which are small compared with
the mean flow speed. They are much larger than current ripples -
reportedly reaching up to 20 metres in height and 1000 metres
in length with straight crests as much as 40 miles long
(Raudkivi, 1967). Such dunes have roughly triangular
longitudinal profiles, as depicted in figure 4. Their lee slopes
subtend angles less than 35 degrees to the horizontal,
approximately the angle of repose for quartz sand, with stoss
slope angles less than 5 degrees to the horizontal.
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