III. Sediment Transport
Chapter II : Bed Material Suspension and
Transport in Uniform Waves
1
Introduction
85
Wave motion over a movable fine sand bed (50 to 500 ~tm) can generate a
sediment suspension with relatively large sediment concentrations in the nearbed region as shown by Nakato et al. (1977) and by Bosman (1982) for the ripple
regime, and by Horikawa et aL (1982) and by Staub et aL (1984) for the plane
bed (sheet flow) regime.
The variation of the instantaneous sediment concentrations in space and time
which is relevant for the cross-shore transport processes in non-breaking waves,
appears to be a complicated physical process particularly in the ripple regime.
This process seems hardly predictable due to the random character of the
phenomena involved. Cross-shore transport processes in non-breaking waves are
caused by various mechanisms :
• the asymmetry of the wave motion yielding larger onshore peak velocities
under the wave crests than offshore peak velocities under the wave troughs
which may result in a net offshore-directed transport in case of a ripplecovered bed or in a net onshore directed transport in case of a plane bed (sheet
flow),
• the generation of net mean 0~ulerian) onshore-directed velocities in the nearbed region which may result in a net onshore-directed transport (Fig. 12),
• the generation of forced long waves due to a mean water surface decrease
("trough") under large-amplitude wave groups resulting in secondary
offshore-directed orbital velocities under the "trough" and a net offshoredirected transport because the sediment concentrations are largest under
large-amplitude waves (Fig. 12).
7~me-averaged sediment concentrations which are relevant for cross-shore and
longshore transport processes due to breaking waves are less complicated to
describe and have been studied by many researchers. Transport processes in
breaking waves are caused by the following mechanisms :
• the generation of net mean offshore-directed velocities in the lower layers
(undertow, Fig. 12),
• the generation of large-scale horizontal circulation cells with longshore
currents and with offshore-directed rip currents (Fig. 12).
Chapter II : Bed Material Suspension and
Transport in Uniform Waves
1
Introduction
85
Wave motion over a movable fine sand bed (50 to 500 ~tm) can generate a
sediment suspension with relatively large sediment concentrations in the nearbed region as shown by Nakato et al. (1977) and by Bosman (1982) for the ripple
regime, and by Horikawa et aL (1982) and by Staub et aL (1984) for the plane
bed (sheet flow) regime.
The variation of the instantaneous sediment concentrations in space and time
which is relevant for the cross-shore transport processes in non-breaking waves,
appears to be a complicated physical process particularly in the ripple regime.
This process seems hardly predictable due to the random character of the
phenomena involved. Cross-shore transport processes in non-breaking waves are
caused by various mechanisms :
• the asymmetry of the wave motion yielding larger onshore peak velocities
under the wave crests than offshore peak velocities under the wave troughs
which may result in a net offshore-directed transport in case of a ripplecovered bed or in a net onshore directed transport in case of a plane bed (sheet
flow),
• the generation of net mean 0~ulerian) onshore-directed velocities in the nearbed region which may result in a net onshore-directed transport (Fig. 12),
• the generation of forced long waves due to a mean water surface decrease
("trough") under large-amplitude wave groups resulting in secondary
offshore-directed orbital velocities under the "trough" and a net offshoredirected transport because the sediment concentrations are largest under
large-amplitude waves (Fig. 12).
7~me-averaged sediment concentrations which are relevant for cross-shore and
longshore transport processes due to breaking waves are less complicated to
describe and have been studied by many researchers. Transport processes in
breaking waves are caused by the following mechanisms :
• the generation of net mean offshore-directed velocities in the lower layers
(undertow, Fig. 12),
• the generation of large-scale horizontal circulation cells with longshore
currents and with offshore-directed rip currents (Fig. 12).