62
R.H. Charlier and Chr. P. De Meyer
Van Rijn (1984) presumed a concentration-dependent alp-factor and found by
calibration using measured concentration profiles, the following expression (Fig.
5):
(C~ 0"8 2(C10"4
~= 1+~0)
- \C~-)
(13)
in which :
= local concentration (volume)
Co
= maximum concentration = 0.65
Fig. 5 shows one of the calibration results (experiment of Einstein and Ning
Chien, op cir.). As can be observed, the ~P-factor according to eq. (13) yields
good agreement in the near-bed region, but a lesser agreement in the upper layer.
With qb = 1, computed concentrations are an order of magnitude too large
compared with measured values (Fig. 5).
Finally, it is noted that the qb-factor is only important for concentrations larger
than about 0.001 (- 2.5 kg/m 3 or 0.16 lb/ft?), (Van Rijn, 1984).
2.4
Concentration Profiles
Expressions for the sediment concentration profiles can be obtained by
integration of the convection-diffusion equation (6) using relevant expressions for
the mixing coefficient (see eqs. 7 - 11 with • = 1 ). As bed-boundary condition a
reference concentration (c,) at a small distance (a) above the mean bed is applied
(known as the integration constant).