January 13, 2004
14:36
WSPC/101-CEJ
00094
534
W. C. Seabergh & N. C. Kraus
sink and accumulate sediment moving along the coast and from shallow shoals that
flank the inlet. A major engineering effort is often required to remove this sediment
from the inlet entrance by dredging the channel and placing the dredged material
at an economically feasible and physically appropriate location along the coast, for
example, such that it does not readily return to the channel during times of longshore
transport reversal.
A central element of coastal inlet maintenance is bypassing of sediment from the
inlet region back to the adjacent beaches. Artificial bypassing mimics or preserves
the pathways of sediment in the littoral zone and harmonizes the requirement for
deepened navigation channels within the context of the natural sediment-sharing
system of inlets and beaches. Many coastal inlets have jetty structures that stabi-
lize the location of the navigation channel, protect vessels from waves breaking in
shallow water, and reduce channel infilling by sediment. Sediment eventually accu-
mulates on the seaside of these structures and moves around the seaward jetty tips
into the navigation channel. Sediment may also pass over a jetty at the shoreward
end by either being blown by wind or washed over in a combination of sediment
accumulation, elevated water level, and wave run-up during storms. If there is a net
movement of sediment along the coast toward the inlet, one side may tend to accu-
mulate sediment (until it then passes by the tip of the jetty into the channel), while
the beach down drift is eroded due to sediment blockage. The above description is
a simplified representation of the complex movement of coastal sediment, but it is
a characteristic pattern at many stabilized inlets.
Several methods and actions have been developed to maintain a deep channel and
accommodate the bypassing and handling of sediment at inlets (Bruun, 1978, 1981;
Richardson, 1991). Hereafter, such sediment will be assumed to consist primarily
of sand (and perhaps gravel), because fine-grained material is not usually placed
on beaches. A typical sand bypassing operation involves a mobile dredge that can
move to necessary locations within the inlet to clear the shoaled channel and pump
or transport the sand to the beach or the nearshore. A problem with the on-demand
dredging procedure is that sand that has reached the channel prior to scheduled
maintenance dredging may reduce navigation reliability. Weather and environmental
windows can limit when the channel is dredged to only a few months a year in some
locations. Similarly, the down-drift beach may require sand when a sea-going dredge
cannot be scheduled.
As opposed to dredging, "fixed systems" have been designed to intercept the
sand before it reaches the channel. South Lake Worth Inlet and Lake Worth Inlet,
Florida, have stationary slurry pumping plants operating from platforms on jetties.
These systems are designed to intercept sand before it reaches the channel, and then
to pump it past the inlet and to the down-drift beach. At Indian River, Delaware,
a jet pump has been deployed from a crawler crane to permit some mobility to
this "fixed" type bypassing system (Clausner, et al., 1991). Sediment volumes of
more than 75,000 cubic meters/year are bypassed from the south jetty fillet to the
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