BI. Sediment Transport
.
65
2.4.2
High concentrations (c > 0.001)
In case of flow in the upper regime the sediment concentration profile should be
described by taking into account the hindered settling effect and the turbulence
damping effect (qb-factor)o Modelling of both effects requires the application of
numerical solution methods to solve eq. (5). The hindered settling effects tend to
increase the concentration because the fall velocity is reduced, while the
turbulence damping effect tends to decrease the concentration because the
sediment mixing coefficient is reduced.
0.9
~
0.8
o,
80
.... H-t
~'~ ~ °5
t
i
,
¢n
Fig. 7 : Concentration profiles (Vanoni, 1946)
According to Winterwerp et al. (1989), turbulence damping due to the presence
of large vertical concentration (density) gradients occurs for concentrations up to
c = 0.2. For increasing concentrations (c > 0.2) the concentration profile tends to
become uniform due to dominating hindered settling effects and turbulence
damping disappears because the concentration gradients disappear (Fig. 8).
Equation (16) in combination with eq. (A) representing hindered settling effects
shows reasonable agreement with measured values (ca ~- 0.35).