150
Coastal Engineering: Theory and Practice
It may be “partial”, cutting off part of the drift, or perhaps “complété”,
that means ail the longshore drift is eut off. In most cases only part of the drift
is eut off, and a certain percentage of drift bypasses at the same time as accumulation takes place updrift. If the training walls lacks the required velocity
to flush the bar formation, narrowing the training walls at the entrance
enhances the velocity to flush the bar formation to attain equilibrium.
Sometimes improvements by dredging and by structures are combined
in that jetties or training walls are built to include bypassing arrangements
of permanent or intermittent nature.
Equilibrium by means of bypassing
Seen in larger perspective, it may be said that bypassing by sand pumps
aids two purposes: 1) protection of navigation channels against deposits by
longshore littoral drift materials; 2) protection of downdrift side beaches
against starvation caused by the littoral drift barrier. Various methods to
bypass the sédiments are discussed below.
• Sand pump on a Trestle updrift of the inlet
• Sand pump at the tip of the updrift Groin [on a short trestle]
• Sand pump on a Trestle downdrift of the inlet
Sand pumps are normally employed to re-establish the littoral drift
obstructed by a groin updrift of a tidal inlet, which is maintained artificially so as to avoid érosion at the downdrift of an inlet. For this purpose,
the sand pump is normally located on a jetty or mounted on a trestle
working over a sand trap updrift of the inlet Fig. 4.32(a). However, the disadvantages of the sand pumps are lack of adéquate mobility, their inability
to undertake dredging during high seas, when, a large quantity of drift is
expected to take place along the coast. In the event of a breakdown of the
pump, during a monsoon season, a shallowing of the updrift région takes
place resulting in a severe shoaling of the channel.
This could resuit in the shifting of the région of maximum drift away
from the location of the sand pump so that dredging during the subséquent
monsoon would be rendered rather inefficient.
In rare cases, the sand pumps can also be deployed at the downdrift of
the inlet over a trestle. In such a case, the approach channel itself acts as a
sand trap in addition to the trap at the updrift. Figure 4.32(b) depicts the
location of sand pump at the downdrift of inlet.
In order to avoid the drawbacks inhérent in fixed bypassing plants,
a floating dredger could be adopted for bypassing sand across the inlet
more conveniently. For this purpose, a sand trap is provided updrift of the
Coastal Engineering: Theory and Practice
It may be “partial”, cutting off part of the drift, or perhaps “complété”,
that means ail the longshore drift is eut off. In most cases only part of the drift
is eut off, and a certain percentage of drift bypasses at the same time as accumulation takes place updrift. If the training walls lacks the required velocity
to flush the bar formation, narrowing the training walls at the entrance
enhances the velocity to flush the bar formation to attain equilibrium.
Sometimes improvements by dredging and by structures are combined
in that jetties or training walls are built to include bypassing arrangements
of permanent or intermittent nature.
Equilibrium by means of bypassing
Seen in larger perspective, it may be said that bypassing by sand pumps
aids two purposes: 1) protection of navigation channels against deposits by
longshore littoral drift materials; 2) protection of downdrift side beaches
against starvation caused by the littoral drift barrier. Various methods to
bypass the sédiments are discussed below.
• Sand pump on a Trestle updrift of the inlet
• Sand pump at the tip of the updrift Groin [on a short trestle]
• Sand pump on a Trestle downdrift of the inlet
Sand pumps are normally employed to re-establish the littoral drift
obstructed by a groin updrift of a tidal inlet, which is maintained artificially so as to avoid érosion at the downdrift of an inlet. For this purpose,
the sand pump is normally located on a jetty or mounted on a trestle
working over a sand trap updrift of the inlet Fig. 4.32(a). However, the disadvantages of the sand pumps are lack of adéquate mobility, their inability
to undertake dredging during high seas, when, a large quantity of drift is
expected to take place along the coast. In the event of a breakdown of the
pump, during a monsoon season, a shallowing of the updrift région takes
place resulting in a severe shoaling of the channel.
This could resuit in the shifting of the région of maximum drift away
from the location of the sand pump so that dredging during the subséquent
monsoon would be rendered rather inefficient.
In rare cases, the sand pumps can also be deployed at the downdrift of
the inlet over a trestle. In such a case, the approach channel itself acts as a
sand trap in addition to the trap at the updrift. Figure 4.32(b) depicts the
location of sand pump at the downdrift of inlet.
In order to avoid the drawbacks inhérent in fixed bypassing plants,
a floating dredger could be adopted for bypassing sand across the inlet
more conveniently. For this purpose, a sand trap is provided updrift of the