3. Sédiment Transport
55
The orbital paths of water particles which are nearly circular till the
depth is half the wave length become elliptical shorewards of this point.
Shoreward, there will be water particle movements at the bottom and the
wave is said to feel the bottom. The velocity and accélération of particle
movement increases with decreasing depths and at a depth depending on
the sédiment size and wave characteristics, the bottom sédiments are put
into motion. At this point, the material movement will be relatively small;
but with decreasing depth, the movement increases and near the breaker,
a considérable quantity of sédiments are thrown into suspension due to the
high turbulence associated with the breaking of waves. These are then easily
moved by even slow moving currents.
This movement of material both in suspension and as bed load alters
the existing profile. The change goes on till equilibrium is reached between
the wave System which is being altered by the temporally changing depths
and the bed profile which is similarly changed by the waves. However,
equilibrium is seldom reached because the waves are continuously changed
by meteorological factors. Sédiment transport mechanism in plan view is
depicted in Fig. 3.12.
Considering the action of waves on any beach in plan, the wave crests
reaching the shore are seldom parallel to the shoreline or the underwater
contours. The effect of this oblique attack of the waves on the shore is
to generate two components of the fluid velocity, of which, one along the
direction parallel to the shore is called longshore currents responsible for
the transport of sédiment along the shore. This is referred to as “longshore
sédiment transport”.
The second component of the velocity in the direction normal to the
shore, transport the sédiment in the direction perpendicular to the shoreline. This mode of sédiment transport is referred to as “onshore-offshore
sédiment transport” or cross-shore transport. The longshore sédiment transport, however, is more dominant and mainly responsible for the shoreline
instabilities.
The sédiments especially the material thrown into suspension at the
breaker zone is easily transported by the longshore currents. Apart from
this, the zig-zag path described by the water mass and the sédiments in
the foreshore due to the uprush and backrush from the breakers also cause
transport of material along the shore. The transport of material in the
longshore direction by waves and currents near the shore is known as littoral
drift, sometimes the material so transported is also called by the same
name.
55
The orbital paths of water particles which are nearly circular till the
depth is half the wave length become elliptical shorewards of this point.
Shoreward, there will be water particle movements at the bottom and the
wave is said to feel the bottom. The velocity and accélération of particle
movement increases with decreasing depths and at a depth depending on
the sédiment size and wave characteristics, the bottom sédiments are put
into motion. At this point, the material movement will be relatively small;
but with decreasing depth, the movement increases and near the breaker,
a considérable quantity of sédiments are thrown into suspension due to the
high turbulence associated with the breaking of waves. These are then easily
moved by even slow moving currents.
This movement of material both in suspension and as bed load alters
the existing profile. The change goes on till equilibrium is reached between
the wave System which is being altered by the temporally changing depths
and the bed profile which is similarly changed by the waves. However,
equilibrium is seldom reached because the waves are continuously changed
by meteorological factors. Sédiment transport mechanism in plan view is
depicted in Fig. 3.12.
Considering the action of waves on any beach in plan, the wave crests
reaching the shore are seldom parallel to the shoreline or the underwater
contours. The effect of this oblique attack of the waves on the shore is
to generate two components of the fluid velocity, of which, one along the
direction parallel to the shore is called longshore currents responsible for
the transport of sédiment along the shore. This is referred to as “longshore
sédiment transport”.
The second component of the velocity in the direction normal to the
shore, transport the sédiment in the direction perpendicular to the shoreline. This mode of sédiment transport is referred to as “onshore-offshore
sédiment transport” or cross-shore transport. The longshore sédiment transport, however, is more dominant and mainly responsible for the shoreline
instabilities.
The sédiments especially the material thrown into suspension at the
breaker zone is easily transported by the longshore currents. Apart from
this, the zig-zag path described by the water mass and the sédiments in
the foreshore due to the uprush and backrush from the breakers also cause
transport of material along the shore. The transport of material in the
longshore direction by waves and currents near the shore is known as littoral
drift, sometimes the material so transported is also called by the same
name.