I~. Sediment Transport
...............
93
The experimental results indicate a simultaneous decrease of the near-bed
concentrations and the ripple height when the wave height increases from H~ =
0.69 m (2.21 ft.) to H~ = 1.1 m (3.52 ft.). This can be observed more clearly in
Fig. 14 showing the concentrations at three elevations (z = 0.025 h, 0.05 h and
^2
0. lh) as a function of the mobility parameter q' = U~/((s- 1)gdso ) in which l]~
is the amplitude of near-bed orbital velocity. The ripple height is also plotted as a
function of W. Fig. 14 clearly shows that the concentrations are largest for ~F =
150 in the presence of ripples with a height of 0.02 m. For ~I" > 150 there is a
gradual transition from the ripple regime (height ~ 0.02 m) with relative large
concentrations to the smooth bed regime (smooth fiat ripples of 0.001 m) with
relatively small concentrations. It is most likely that the ripple-generated eddies,
which are most effective in the entrainment of particles from the bed, are
gradually disappearing for ~F = 150 resulting in a gradual decrease of the
concentrations, as shown in Fig. 14.
Bosman (1982)
phenomena '
and Nap-Van Kampen (1988) observed the following
increasing concentrations for increasing wave heights,
slightly flatter concentration profiles for increasing wave heights,
constant concentration gradients at each wave height (for H~ < 0.18 m or
0.58 ft.),
two layer concentration profiles for H~ = 0.2 m (0.64 ft.) and 0.23 m (0.74 ft.)
with constant concentration gradient in near-bed region z/h < 0.15 and
increasing concentration gradients for z/h > 0.15,
decreasing ripple height for increasing wave height (upto H~ = 0.18 m or
0.58 ft.).
These results show slightly flatter concentration profiles for a decreasing ripple
heights, which is related to a somewhat less intensive mixing in the near-bed
layer. The concentrations in the region z/h > 0.15 are relatively large for wave
heights ofH~ = 0.2 (8 in.) and 0.23 m (9 in.) in the experiments of Bosman. This
may be caused by the effect of occasional breaking of the waves.
2.2.2
Concentrations by Breaking Waves over a Flat Bed
Bosman (1982, 1986) observed two phenomena :
,, the near-bed concentrations are approximately constant (~ i0 kg/m 3 or
0.6 lb./ft?) for increasing wave heights,
• the concentrations at higher levels show a large increase for wave heights
increasing from H~ = 0.12 m (0.38 ft.) (non-breakingwaves) to H~ = 0.19 m
(0.61 ft.) (plunging breaking waves).
...............
93
The experimental results indicate a simultaneous decrease of the near-bed
concentrations and the ripple height when the wave height increases from H~ =
0.69 m (2.21 ft.) to H~ = 1.1 m (3.52 ft.). This can be observed more clearly in
Fig. 14 showing the concentrations at three elevations (z = 0.025 h, 0.05 h and
^2
0. lh) as a function of the mobility parameter q' = U~/((s- 1)gdso ) in which l]~
is the amplitude of near-bed orbital velocity. The ripple height is also plotted as a
function of W. Fig. 14 clearly shows that the concentrations are largest for ~F =
150 in the presence of ripples with a height of 0.02 m. For ~I" > 150 there is a
gradual transition from the ripple regime (height ~ 0.02 m) with relative large
concentrations to the smooth bed regime (smooth fiat ripples of 0.001 m) with
relatively small concentrations. It is most likely that the ripple-generated eddies,
which are most effective in the entrainment of particles from the bed, are
gradually disappearing for ~F = 150 resulting in a gradual decrease of the
concentrations, as shown in Fig. 14.
Bosman (1982)
phenomena '
and Nap-Van Kampen (1988) observed the following
increasing concentrations for increasing wave heights,
slightly flatter concentration profiles for increasing wave heights,
constant concentration gradients at each wave height (for H~ < 0.18 m or
0.58 ft.),
two layer concentration profiles for H~ = 0.2 m (0.64 ft.) and 0.23 m (0.74 ft.)
with constant concentration gradient in near-bed region z/h < 0.15 and
increasing concentration gradients for z/h > 0.15,
decreasing ripple height for increasing wave height (upto H~ = 0.18 m or
0.58 ft.).
These results show slightly flatter concentration profiles for a decreasing ripple
heights, which is related to a somewhat less intensive mixing in the near-bed
layer. The concentrations in the region z/h > 0.15 are relatively large for wave
heights ofH~ = 0.2 (8 in.) and 0.23 m (9 in.) in the experiments of Bosman. This
may be caused by the effect of occasional breaking of the waves.
2.2.2
Concentrations by Breaking Waves over a Flat Bed
Bosman (1982, 1986) observed two phenomena :
,, the near-bed concentrations are approximately constant (~ i0 kg/m 3 or
0.6 lb./ft?) for increasing wave heights,
• the concentrations at higher levels show a large increase for wave heights
increasing from H~ = 0.12 m (0.38 ft.) (non-breakingwaves) to H~ = 0.19 m
(0.61 ft.) (plunging breaking waves).