112
R.H. Charlier and Chr. P. De Meyer
Chapter IV : Sediment Transport
1
Introduction
It is common practice to make a distinction between sediment transport parallel
to the shoreline and sediment transport tran~a,erse to the shoreline. In general the
longshore sediment transport, and in particular the gradients in this transport, are
responsible for the long-term changes in the coastline, whereas the cross-shore
transport is responsible for the short-term variations. In this connection it should
be observed that cross-shore transport may also cause long-term erosion or
accretion of the coast, but this is often difficult to detect because of the different
nature of transport phenomena.
Longshore transport occurs in a relatively narrow zone along the coast, and the
direction and magnitude is mainly determined by the height, period and direction
of the waves apart from the effect of possible tidal currents. The presence of this
longshore transport can be easily detected from the development of the coastline
and sea bed geometry near river mouths, headlands, groins, harbor moles, etc.
The short-term effects of cross-shore transport can also be easily detected, from
the changes in magnitude and position of breaker bars, erosion of the dunes
during storm surges, etc. As such the effect of seasonal variations in the wave
conditions on the coastline and seabed geometry are considered short-term
effects. Usually the net longshore transport per meter width is much larger than
the net cross-shore transport per meter width. However, the gross cross-shore
transport will be far higher. In principle under each wave crest sediment is
transported in the shoreward direction and trader each wave through in seaward
direction. During this motion the longshore current transports the sediment in
longshore direction. Therefore the gross cross-shore transport may" be of the same
order of magnitude as the gross longshore transport. Because of the nature of the
cross-shore transport the possible net long-term transport cannot be measured
directly. If a long-term change of the coastline cannot be explained by gradients
in the longshore transport it may be concluded that a net cross-shore transport
exists, but an indisputable proof of that is often hard to give.
R.H. Charlier and Chr. P. De Meyer
Chapter IV : Sediment Transport
1
Introduction
It is common practice to make a distinction between sediment transport parallel
to the shoreline and sediment transport tran~a,erse to the shoreline. In general the
longshore sediment transport, and in particular the gradients in this transport, are
responsible for the long-term changes in the coastline, whereas the cross-shore
transport is responsible for the short-term variations. In this connection it should
be observed that cross-shore transport may also cause long-term erosion or
accretion of the coast, but this is often difficult to detect because of the different
nature of transport phenomena.
Longshore transport occurs in a relatively narrow zone along the coast, and the
direction and magnitude is mainly determined by the height, period and direction
of the waves apart from the effect of possible tidal currents. The presence of this
longshore transport can be easily detected from the development of the coastline
and sea bed geometry near river mouths, headlands, groins, harbor moles, etc.
The short-term effects of cross-shore transport can also be easily detected, from
the changes in magnitude and position of breaker bars, erosion of the dunes
during storm surges, etc. As such the effect of seasonal variations in the wave
conditions on the coastline and seabed geometry are considered short-term
effects. Usually the net longshore transport per meter width is much larger than
the net cross-shore transport per meter width. However, the gross cross-shore
transport will be far higher. In principle under each wave crest sediment is
transported in the shoreward direction and trader each wave through in seaward
direction. During this motion the longshore current transports the sediment in
longshore direction. Therefore the gross cross-shore transport may" be of the same
order of magnitude as the gross longshore transport. Because of the nature of the
cross-shore transport the possible net long-term transport cannot be measured
directly. If a long-term change of the coastline cannot be explained by gradients
in the longshore transport it may be concluded that a net cross-shore transport
exists, but an indisputable proof of that is often hard to give.