January 13, 2004
14:36
WSPC/101-CEJ
00094
550
W. C. Seabergh & N. C. Kraus
weir should be less transmissive than if the longshore transport were unidirectional
(right-to-left) and in this case it is desirable to capture all the sediment in the
basin. To accomplish this, the difference between the right-to-left sediment and net
sediment volumes must be temporarily stored where it is available to be moved
back during left-to-right movement. In other words, a portion of the right-to-left
movement must be prevented from entering the deposition basin. Offsetting the weir
seaward of the original shoreline may be beneficial with regard to the creation of a
temporary fillet region that fills when the longshore sediment transport is directed
from right to left in Fig. 11. After the fillet has grown to the shoreward edge of the
weir, sediment will begin entering the deposition basin. Capturing the net transport
minimizes sediment-handling costs. If right-side jetty orientation will permit wave
approach to move sediment out of the fillet region back up-coast, this is possible. If
the right-side jetty creates a large shadow zone so that sediment does not move left
to right out of the fillet, a possible up-coast groin may hold sediment, and so it is
available for transport to the right. Figure 12 illustrates this approach.
As mentioned in the previous paragraph, designing weir sections to capture and
bypass only the net transport is reasonable guidance if there is a relatively greater
volume of sediment approaching from the predominant direction. In the case of
balanced left-to-right and right-to-left transport, it may not be desirable to construct
a weir jetty, unless sediment is moving seaward along the jetty to the tips of the
jetty. Then, to reduce the strength of sediment pathways to the jetty, a weir might
be constructed. However, other methods may be employed to prevent sediment from
entering the channel, such as jetty spurs on the seaward side of the jetty, or groin
Fig. 12. Placement of a groin updrift of weir jetty to aid in capturing only net sediment volume in
deposition basin.
Figure 12. Placement of a groin updrift of weir jetty to aid in capturing only net sediment
volume in deposition basin.
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