January 13, 2004
14:36
WSPC/101-CEJ
00094
548
W. C. Seabergh & N. C. Kraus
3.2. Weir jetty design based on prototype and modeling experience
Many lessons can be extracted from the examination of weir jetty systems that
have been built in the latter part of the 20th century. Problems are varied and
include scour along the structure, tidal currents migrating through the deposition
basin, and sediment bypassing the basin near the shoreline, as examples. In most
cases, the problems can be solved by some modification or adjustment to the project
("project tuning").
3.2.1. Deposition basin location and size
At many weir jetty sites, the shoreward edge of the deposition basin was placed
at about the location of the low-water line of the adjacent beach. Murrells Inlet
(Fig. 9) is an example. Under normal conditions, sediment transport is along the
inner surf zone at the landward end of the weir, and sediment moves along the shore
landward of the basin, creating a spit that elongates toward the interior channel
or into the main navigation channel. Flood flow, together with waves and wave-
generated longshore currents over the weir, develop the spit feature and curve it
around the inlet shoulder. The ebb current shears off the sediment from the wrapping
spit, and sediment moves into the channel-side of the basin (early ebb flow was
aligned with the basin while currents exited over the weir, which shut off once the
mid-tide level of the weir is reached); then, some sediment is carried seaward along
the navigation channel to the ebb shoal (Fig. 9). The interior spit was formed during
construction of the jetties before the south jetty was built, and greater wave energy
wrapped the spit tighter into the interior channel.
The ebb current directed toward the deposition basin is expected to decrease as
the tide level falls. Because the weir elevation at Murrells Inlet is at mean tide level,
ebb flows are concentrated in the navigation channel during the later stages of ebb
flow. To prevent sediment from passing over the weir and bypassing the deposition
basin on the shoreward side once inside the weir, a possible solution would be to
move the deposition basin landward relative to the weir, or in the case of an existing
weir, cover a portion of the landward edge of the weir with stone, creating an offset
with respect to the basin. Figure 11 illustrates this setback of the basin. Also shown
in Fig. 11 is that, for more energetic wave conditions, suspended sediment could pass
over the more seaward portion of the weir and settle in the deposition basin. The
location of maximum sediment transport over the weir is at the location of wave
breaking on the seaward side of the weir. This figure was derived from laboratory
studies (Seabergh, 1983). For small or typical waves, sediment would wrap landward
and into the basin. Because of greater basin depth, the sediment will be constrained
to the basin. As the deposition basin fills and becomes shallower, there will be a
tendency to develop a sediment pathway that will extend to the rear of the basin and
out toward the bay and interior channels. Regular basin dredging can alleviate this
potential problem. If the dredging schedule is irregular or the transport episodic, a
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