!I/. Sediment Transport
99
The diffusion concept is valid when the slope of measured concentration profiles
represented in a plot of log c versus z are inversely proportional to the fall
velocity in case of constant wave conditions. To investigate this, some
experimental results presented by Roelvink (1985) are analyzed. Roelvink
measured concentration profiles of different size fractions above a rigid bottom
consisting of artificial ripples. A limited amount of sediment particles was fed to
the (oscillatory) flow. Fig. 15 shows measured concentration profiles for various
size fractions in case of a constant wave height. Since the wave height is
constant, the local mixing coefficient will be approximately constant assuming
that this parameter is not or only weakly correlated to the sediment particle size.
As can be observed, there is a tendency for the slopes of the concentration
profiles to be inversely proportional to the particle fall velocity, which supports
the applicability of the diffusion concept for wave induced mixing. A similar
conclusion is given by Van Der Graaff (1988). Nielsen (1984) questions the
applicability of the diffusion equation for wave-induced mixing. He presents
some data showing that the eddy trapping mechanism is the dominant suspension
mechanism rather than the diffusion process. All sand particle sizes, that are
small enough to be trapped in an eddy will travel with the eddy until it dissolves
and their time-averaged concentrations will become distributed in very much the
same way, irrespective of settling velocity.
99
The diffusion concept is valid when the slope of measured concentration profiles
represented in a plot of log c versus z are inversely proportional to the fall
velocity in case of constant wave conditions. To investigate this, some
experimental results presented by Roelvink (1985) are analyzed. Roelvink
measured concentration profiles of different size fractions above a rigid bottom
consisting of artificial ripples. A limited amount of sediment particles was fed to
the (oscillatory) flow. Fig. 15 shows measured concentration profiles for various
size fractions in case of a constant wave height. Since the wave height is
constant, the local mixing coefficient will be approximately constant assuming
that this parameter is not or only weakly correlated to the sediment particle size.
As can be observed, there is a tendency for the slopes of the concentration
profiles to be inversely proportional to the particle fall velocity, which supports
the applicability of the diffusion concept for wave induced mixing. A similar
conclusion is given by Van Der Graaff (1988). Nielsen (1984) questions the
applicability of the diffusion equation for wave-induced mixing. He presents
some data showing that the eddy trapping mechanism is the dominant suspension
mechanism rather than the diffusion process. All sand particle sizes, that are
small enough to be trapped in an eddy will travel with the eddy until it dissolves
and their time-averaged concentrations will become distributed in very much the
same way, irrespective of settling velocity.