228
..........
R.H. Charlier and Chr. P. De Meyer
In the first three options, placement is effected by pumping or truck dumping,
allied with some profiling where necessary. The fourth option calls for a decision
as to where to place the material on the seabed and how. On a high wave energy
coast the work can usually only be done by a floating plant ; i.e. dredges and
hopper barges, or vessels that combine both these capabilities. Dredged sand can
be pumped from them into shallow water and onto the beach, but for placements
further offshore in deeper water, dumping from a hopper vessel is usually the
most economic procedure. An offshore placement is thus determined by the draft
of the vessels available and the thickness of the dumped material on the seabed.
A dredge hopper vessel grounded on its own load is the last thing required. (Pig.
26).
The sand source for offshore nourishment placements is usually from the deeper
seabed offshore of the normal active zone, where the sediment mobility is low, its
backfilling rate is very slow and its response is largely dominated by only the
larger storm wells. The final option that requires consideration (last choice) is
whether to spread the nourishment evenly over the offshore shallower seabed
(Profile Placement) or concentrate it as heaps along a line nearly parallel to the
beach and form a seabed structure that imitates a natural offshore storm bar (Bar
Placement). The optimum combinations of options for an offshore placement
becomes quite complex for it must consider public relations, politics,
nourishment volume, mechanical plant and vessel size and delivery rates, the
probably effective 'life" of the placement and combine all these factors to attain
the maximum economy and public endorsement. The worst enemy of economy is
double handling : double the work, double the cost. (Smith, A.W. and Jackson,
L.A., 1990, The siting of beach nourishment placements : Shore and Beach 58,
1, 17-24). (fig. 27).
Usually, the actual field location is determined before the work is even
provisionally designed. Sometimes the reverse applies : the placement location is
dominated by the field conditions, so the other design factors have to be modified
to suit. Sometimes, both the type of plant and the placement location are
determined by the anticipated weather conditions most likely to exist when the
work is to be done ; but in the end, the finally selected placement location holds
by far the greatest impact upon the behavior of the nourishment material, both
during, and after the work is complete.
The number of choices for a nourishment placement is quite small in the dune
zone ; on the beach ; in the swash zone and through the fine weather breaking
zone ; distributed over the near beach seabed ; on the seabed in the storm bar
location.
..........
R.H. Charlier and Chr. P. De Meyer
In the first three options, placement is effected by pumping or truck dumping,
allied with some profiling where necessary. The fourth option calls for a decision
as to where to place the material on the seabed and how. On a high wave energy
coast the work can usually only be done by a floating plant ; i.e. dredges and
hopper barges, or vessels that combine both these capabilities. Dredged sand can
be pumped from them into shallow water and onto the beach, but for placements
further offshore in deeper water, dumping from a hopper vessel is usually the
most economic procedure. An offshore placement is thus determined by the draft
of the vessels available and the thickness of the dumped material on the seabed.
A dredge hopper vessel grounded on its own load is the last thing required. (Pig.
26).
The sand source for offshore nourishment placements is usually from the deeper
seabed offshore of the normal active zone, where the sediment mobility is low, its
backfilling rate is very slow and its response is largely dominated by only the
larger storm wells. The final option that requires consideration (last choice) is
whether to spread the nourishment evenly over the offshore shallower seabed
(Profile Placement) or concentrate it as heaps along a line nearly parallel to the
beach and form a seabed structure that imitates a natural offshore storm bar (Bar
Placement). The optimum combinations of options for an offshore placement
becomes quite complex for it must consider public relations, politics,
nourishment volume, mechanical plant and vessel size and delivery rates, the
probably effective 'life" of the placement and combine all these factors to attain
the maximum economy and public endorsement. The worst enemy of economy is
double handling : double the work, double the cost. (Smith, A.W. and Jackson,
L.A., 1990, The siting of beach nourishment placements : Shore and Beach 58,
1, 17-24). (fig. 27).
Usually, the actual field location is determined before the work is even
provisionally designed. Sometimes the reverse applies : the placement location is
dominated by the field conditions, so the other design factors have to be modified
to suit. Sometimes, both the type of plant and the placement location are
determined by the anticipated weather conditions most likely to exist when the
work is to be done ; but in the end, the finally selected placement location holds
by far the greatest impact upon the behavior of the nourishment material, both
during, and after the work is complete.
The number of choices for a nourishment placement is quite small in the dune
zone ; on the beach ; in the swash zone and through the fine weather breaking
zone ; distributed over the near beach seabed ; on the seabed in the storm bar
location.