January 13, 2004
14:36
WSPC/101-CEJ
00094
535
Progress in Management of Sediment Bypassing at Coastal Inlets
down-coast beach. It can be difficult to fully intercept all the sand moving toward
an inlet. Natural bypassing of sand movement past the inlet may be facilitated by
structure orientation, e.g. overlapping jetties, but interception of the sand by the
channel may still occur.
Work conducted in the last 40 years has examined ways to direct the sediment to
a location where it may be handled by a small dredge and also minimize interruption
of natural bypassing. Examples include the dredging of a deposition basin in the
channel itself. This has been done at Carolina Beach, North Carolina (Vallianos,
1970), and at Sebastian Inlet, Florida (Bruun, Mehta, and Johnsson, 1978). Capture
of sediment in the lee of an offshore breakwater before reaching the inlet has been
implemented at Channel Island, California (Bruun, Mehta, and Jonsson, 1978) and
at Ventura Harbor, California (Hughes and Schwichtenberg, 1998).
A prominent example of sediment redirection is the weir jetty system. Several
weir-jetty systems have been constructed in the United States, but under limited
design guidance, a few have been considered successful. This paper examines se-
lected such inlets and systems and suggests, in some cases, what modification might
increase their performance. Also, in certain situations, the use of spurs on the sea-
side of jetties may prove beneficial in reducing the amount of littoral sediment from
entering the navigation channel and in controlling its location for rehandling. These
possible engineering actions are first placed in the context of the natural mechanisms
of sand bypassing at coastal inlets.
2. Natural Sand Bypassing at Inlets
Mechanisms of natural sand bypassing at inlets were first elucidated in a systematic
way by Bruun and Gerritsen (1959, 1960). Bruun (1991) gives an update on the
concepts and describes several case studies. These works identified two bypassing
modes as (1) bar bypassing, in which sand moves around the inlet along the bypass-
ing bars and ebb-tidal shoal, and (2) tidal bypassing, in which the sediment enters
the channel on flood current and exits on the other side of the inlet in the ebb cur-
rent. Bar bypassing inlets tend to have a large gross longshore sand transport rate
(the gross transport rate is equal to the sum of the rates from the left and right sides
of the inlet) as compared to tidal prism, whereas tidal bypassing inlets tend to have
a larger tidal prism as compared to the gross amount of material transported to
the inlet during the course of a year. Bruun and Gerritsen (1959, 1960) noted that
bypassing could take place at an inlet in both modes. Gaudiano and Kana (2001)
document a third type of natural sand bypassing mode called episodic bypassing,
by which sand is transferred downdrift by the collapse of a large portion of the ebb
shoal. Episodic bypassing is found at inlets with strong tidal flow as compared to
hydrodynamic influence by waves, and the periodicity of the South Carolina inlets
studied showed the process to occur about every 7 years, likely triggered by a storm.
Inlets at other locations may have longer or shorter cycles of episodic bypassing.
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