January 13, 2004
14:36
WSPC/101-CEJ
00094
551
Progress in Management of Sediment Bypassing at Coastal Inlets
systems to keep sediment reaching the shadow of the jetty, so that the sediment will
be moved away from the jetty when wave direction changes.
Consideration of bypassing of sediment to the down-drift beaches should be part
of a weir system. Regular dredging of the deposition basin can provide sediment
for this bypassing. Such bypassing maintains continuity in sediment movement and
integrity of the down-drift beach. Some natural bypassing around the jetty tip,
especially during storms, is also expected. Planning of weirs, spurs, and groins is
done as part of a sand-sharing system approach that involves means and costs for
bypassing the appropriate volume of material, accounting for natural bypassing of
sediment.
3.2.3. Jetty weir construction material
Jetty weirs have been constructed from concrete sheet piles, wooden piles, and rubble
rock. Each of these materials can be placed to any required elevation after examina-
tion of the necessary considerations to determine height. The typical crest elevation
range is between low water up to about mid-tide. Concrete and wooden piles can
be set for a more precise elevation than rubble rock weirs. However, Murrells Inlet,
with a rubble rock weir, functions well. The elevation of rubble rock weirs can be
more easily adjusted later on, if necessary, than concrete or woodpile weirs. Also,
wave reflection is less for rubble rock than for the sheet pile. Wave reflection may
increase undesirable wave energy in the channel and may alter sediment pathways
on the seaside of the weir as they approach the structure. Seabergh (1983), based
on monochromatic wave model studies (which would tend to emphasize the effect),
noted offshore movement of sediment before it reached the weir section due to the
interaction of incident and reflected waves.
One other approach that would function as a weir, but offer greater wave pro-
tection is a highly permeable jetty. Over time, adding additional stone or removing
stone might be placed to adjust permeability.
3.2.4. Weir elevation
Weir elevation is determined from a combination of leading parameters including
tide range, wave height, inlet bay response (which will define timing of maximum
ebb and flood flows with respect to tide elevation) and magnitude of left- and right-
directed longshore sediment movement in the vicinity of weir.
The weir elevation must be such to allow sediment to pass over it and into the
deposition basin. A low crest elevation would maximize sediment transmission by
overtopping; however, wave energy would be relatively greater than if the weir were
higher. Significant wave transmission would introduce wave energy into what is typ-
ically protected water for a regular jettied inlet. Multiple wave directions (incident
and reflected) inside the entrance could be a hazard to navigation. Also the weir
provides protection for dredging activity in the deposition basin and minimization
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