[II. Sediment Transport '
91
2.2
Time-Averaged Concentrations
Time-ax, eraged concentrations (averaged over many waves) are relevant for the
current-related sediment transport. Time-averaging is necessary to eliminate the
large random scatter of the instantaneous concentrations. Time-averaged
concentrations have been measured by various researchers. Some of the
experimental results are analyzed here focusing on the following cases :
1. concentrations by non-breaking waves over a rippled bed,
2.
concentrations by breaking waves,
3.
concentrations in the sheet flow layer.
2.2.1
Concentrations by Non-Breaking Waves over a Rippled-Bed
In case of oscillatory flow over a rippled bed the instantaneous sediment
concentrations are varying in time and space, as shown in Fig. 13. To eliminate
these variations, it is necessary to average the concentrations over time (many
waves) and over space (many bedforms). In laboratory conditions this can be
done quite simply by moving the measuring instruments forwards and backwards
over a certain distance during the sampling period.
Nieuwjaar-Van Der Kaaij (1987) observed the following phenomena :
• concentrations increase for increasing wave height H~
• negligible small concentrations for z/h > 0.3
° constant concentration gradient at each wave height Hs
° steeper concentration profiles for increasing wave heights
° ripple height increases slightly for increasing wave height
Van Rijn (1987) observed the following phenomena :
• increasing concentrations for increasing wave height upto Hs = 0.69 m
(2.21 fl.) and decreasing concentrations for I-L = 1.1 m (3.52 ft.) in the nearbed layer,
• two layer concentration profiles with large gradients in the near-bed layer of
z/h < 0.05 (3 ripple heights) and small gradients for z/h > 0.05,
• decrease of ripple height from A r = 0.02 m (0.8 in.) to 0.001 m (0.04 in.) for
wave height increasing from H~ = 0.69 m (2.21 ft.) to H~ = 1.1 m (3.52 ft.).
91
2.2
Time-Averaged Concentrations
Time-ax, eraged concentrations (averaged over many waves) are relevant for the
current-related sediment transport. Time-averaging is necessary to eliminate the
large random scatter of the instantaneous concentrations. Time-averaged
concentrations have been measured by various researchers. Some of the
experimental results are analyzed here focusing on the following cases :
1. concentrations by non-breaking waves over a rippled bed,
2.
concentrations by breaking waves,
3.
concentrations in the sheet flow layer.
2.2.1
Concentrations by Non-Breaking Waves over a Rippled-Bed
In case of oscillatory flow over a rippled bed the instantaneous sediment
concentrations are varying in time and space, as shown in Fig. 13. To eliminate
these variations, it is necessary to average the concentrations over time (many
waves) and over space (many bedforms). In laboratory conditions this can be
done quite simply by moving the measuring instruments forwards and backwards
over a certain distance during the sampling period.
Nieuwjaar-Van Der Kaaij (1987) observed the following phenomena :
• concentrations increase for increasing wave height H~
• negligible small concentrations for z/h > 0.3
° constant concentration gradient at each wave height Hs
° steeper concentration profiles for increasing wave heights
° ripple height increases slightly for increasing wave height
Van Rijn (1987) observed the following phenomena :
• increasing concentrations for increasing wave height upto Hs = 0.69 m
(2.21 fl.) and decreasing concentrations for I-L = 1.1 m (3.52 ft.) in the nearbed layer,
• two layer concentration profiles with large gradients in the near-bed layer of
z/h < 0.05 (3 ripple heights) and small gradients for z/h > 0.05,
• decrease of ripple height from A r = 0.02 m (0.8 in.) to 0.001 m (0.04 in.) for
wave height increasing from H~ = 0.69 m (2.21 ft.) to H~ = 1.1 m (3.52 ft.).