lII. Sediment Transport
103
3.3.2
Particle Fall Velocity
Experiments of high sediment concentrations have shown a considerable
reduction of the particle fall velocity due to the presence of the surrounding
particles. This effect is known as the hindered settling effect. For flow conditions
with particles in the range of 50 to 500 ~tm the reduced particle fall velocity can
be described by a Richardson-Zaki (1954) type of equation :
w~, = (l-c) 5 w~
(47)
w .... = effective fall velocity in a fluid-sediment mixture
w~
= fall velocity in a clear fluid
Since wave-generated sediment concentrations may be rather large in the nearbed
region, the influence of the hindered settling effect has been investigated by
carrying out a few sensitivity computations. The sediment concentration profile
has been computed (numerically) for a reference concentration of 2,500, 5,000
and 10,000 rag/1 (0.0088, 0.177 and 0.353 oz/l). The results, in Fig. 17, show an
increase of the sediment concentrations due to the hindered settling effect.
Considering all uncertainties involved, the hindered settling effect seems to be of
minor importance for concentrations smaller than 10,000 mg/l (0.353 oz/1).
Measurements show that concentrations of this order of magnitude do not occur
outside the wave boundary layer.