IV. Retreating Shorelines
281
River mouths but also entrances to lagoons or embayments act as a brake for
littoral transport. If subjected to tides, the tidal current may not be strong enough
to carry away sediments that accumulate on the downcoast side of the opening. It
can be dredged, pumped or one can use the available wave energy to expedite
passage ; groin construction is usually necessary. This method has been called bypassing.
However, the term is also used for a mechanical approach whereby dredging
units of barge on pipeline disposal are put to work ; or by fixed loading devices
feeding a pipeline passing under or around the entrance of the inlet, or of the
structure (e.g. groin). By-passing is thus removal updrift before the material
accumulates at a site deemed unfavorable. The material can be used for beach
nourishment provided it is placed along the beach where wave action can spread
it, outside of an effect zone from a downdrift marine structure, yet within the
limit of an offshore bar built during the winter.
A low section (weir) in an updrifl jetty can provide a sheltered pre-dredged
deposition basin acting as a reserve for beach nourishment purposes.
Floating plants can only be used where the impounding zone is subject to periods
of light wave action, or by breaking into the landward section of the
impoundment and dredging behind the beach berm, leaving a protective barrier
for the dredge. Dredging a sand trap in the water of an inlet protected by jetties is
a by-passing technique, particularly when tidal currents are strong ; the trap can
be periodically emptied. A set-up completed with a shore-connected breakwater is
the most elficient and the costliest.
As said elsewhere the three methods of by-passing are land-based dredging,
floating dredges and land-based transport (trucks).
Compensation-dredging can be considered as related to sand by-passing in that
dredging of material is carried out in a given site and the material used to nourish
the beach system elsewhere. The use of dredge spoils, by the US Army Corps of
Engineers, mentioned earlier, is in fact "involuntary" compensation-dredging.
3,9
Conclusion
Man's actions at rather considerable distance from a specific site often are
additional causes for rapid erosion. Beaches along the Gulf of Brnin have been
depleted because of an important barrage built on the Niger River. Indeed river
damming and construction of hard structures - piers, harbor jetties, etc - along
riverbanks, lead to sediment starvation and as a corollary to an impoverishment
in coastal sand supplies.
281
River mouths but also entrances to lagoons or embayments act as a brake for
littoral transport. If subjected to tides, the tidal current may not be strong enough
to carry away sediments that accumulate on the downcoast side of the opening. It
can be dredged, pumped or one can use the available wave energy to expedite
passage ; groin construction is usually necessary. This method has been called bypassing.
However, the term is also used for a mechanical approach whereby dredging
units of barge on pipeline disposal are put to work ; or by fixed loading devices
feeding a pipeline passing under or around the entrance of the inlet, or of the
structure (e.g. groin). By-passing is thus removal updrift before the material
accumulates at a site deemed unfavorable. The material can be used for beach
nourishment provided it is placed along the beach where wave action can spread
it, outside of an effect zone from a downdrift marine structure, yet within the
limit of an offshore bar built during the winter.
A low section (weir) in an updrifl jetty can provide a sheltered pre-dredged
deposition basin acting as a reserve for beach nourishment purposes.
Floating plants can only be used where the impounding zone is subject to periods
of light wave action, or by breaking into the landward section of the
impoundment and dredging behind the beach berm, leaving a protective barrier
for the dredge. Dredging a sand trap in the water of an inlet protected by jetties is
a by-passing technique, particularly when tidal currents are strong ; the trap can
be periodically emptied. A set-up completed with a shore-connected breakwater is
the most elficient and the costliest.
As said elsewhere the three methods of by-passing are land-based dredging,
floating dredges and land-based transport (trucks).
Compensation-dredging can be considered as related to sand by-passing in that
dredging of material is carried out in a given site and the material used to nourish
the beach system elsewhere. The use of dredge spoils, by the US Army Corps of
Engineers, mentioned earlier, is in fact "involuntary" compensation-dredging.
3,9
Conclusion
Man's actions at rather considerable distance from a specific site often are
additional causes for rapid erosion. Beaches along the Gulf of Brnin have been
depleted because of an important barrage built on the Niger River. Indeed river
damming and construction of hard structures - piers, harbor jetties, etc - along
riverbanks, lead to sediment starvation and as a corollary to an impoverishment
in coastal sand supplies.