90
R.H. Charlier and Chr. P. De Meyer
The phenomena above the trough are :
• the (random) scatter is also quite large (not shown),
• two larger concentration peaks after flow reversal and probably generated by
leeside eddy-velocities (time lag is larger compared with concentration
measurements at the crest),
• two smaller concentration peaks after maximum flow and probably generated
by stoss-side velocities,
• asymmetrical concentration distribution,
• the peaks above the trough are smaller than those above the crest due to
dispersion and settling of sediment particles.
According to Bosman, the scatter is mainly caused by (slight) local ripple
changes resulting in small differences in the local velocities and hence
concentrations. Based on this, it seems very difficult to relate the local
instantaneous sediment concentration to a local instantaneous fluid velocity. The
measurements of Nakato et al. (1977), with periods in the range of 1 to 3 seconds
show similar results as those of Bosman. Mathematical models to compute the
instantaneous sediment concentrations in the ripple regime are not yet available.
2.1.2
Sheet flow regime
Instantaneous concentrations generated by non-breaking waves in the sheet flow
regime have been measured by Horikawa et al. (1982) in oscillatory flow over a
sand bed (also = 200 ~m) in a wave tunnel using an electro-resistance
concentration meter. The maximum concentrations are generated at the moment
of maximum velocities (® = 90°). The concentration is minimum at the moment
of minimum velocities (O = 0°).
The thickness of the concentration layer is about 0.02 m (0.064 ft.). At this level
the time-averaged concentration was about 0.5 kg/m 3 (0.03 lb/ft?). The velocities
have been determined by applying a camera technique focusing on the interior
particles to eliminate the wall effect. Analysis of the instantaneous sand transport
rates show that most of the transport occurs below the initial bed level (® = 30 °,
60 °, 90°).
R.H. Charlier and Chr. P. De Meyer
The phenomena above the trough are :
• the (random) scatter is also quite large (not shown),
• two larger concentration peaks after flow reversal and probably generated by
leeside eddy-velocities (time lag is larger compared with concentration
measurements at the crest),
• two smaller concentration peaks after maximum flow and probably generated
by stoss-side velocities,
• asymmetrical concentration distribution,
• the peaks above the trough are smaller than those above the crest due to
dispersion and settling of sediment particles.
According to Bosman, the scatter is mainly caused by (slight) local ripple
changes resulting in small differences in the local velocities and hence
concentrations. Based on this, it seems very difficult to relate the local
instantaneous sediment concentration to a local instantaneous fluid velocity. The
measurements of Nakato et al. (1977), with periods in the range of 1 to 3 seconds
show similar results as those of Bosman. Mathematical models to compute the
instantaneous sediment concentrations in the ripple regime are not yet available.
2.1.2
Sheet flow regime
Instantaneous concentrations generated by non-breaking waves in the sheet flow
regime have been measured by Horikawa et al. (1982) in oscillatory flow over a
sand bed (also = 200 ~m) in a wave tunnel using an electro-resistance
concentration meter. The maximum concentrations are generated at the moment
of maximum velocities (® = 90°). The concentration is minimum at the moment
of minimum velocities (O = 0°).
The thickness of the concentration layer is about 0.02 m (0.064 ft.). At this level
the time-averaged concentration was about 0.5 kg/m 3 (0.03 lb/ft?). The velocities
have been determined by applying a camera technique focusing on the interior
particles to eliminate the wall effect. Analysis of the instantaneous sand transport
rates show that most of the transport occurs below the initial bed level (® = 30 °,
60 °, 90°).