ERDC/CHL CHETN-IV-30
December 2000
sands out the jettied channel. In addition, tidal and/or wave-generated currents must be
sufficiently strong to rework the sand onshore that is deposited at the mouth of the jettied
channel.
Model 8. Jettied Inlet Bypassing. The amount of sand that bypasses jettied inlets depends
on a number of factors such as jetty length, inlet size, channel depth, tidal current strength, and
ebb-tidal delta morphology. There are few jetties that bypass sand at a rate equal to the net
longshore transport rate. In most cases, the excess sand accumulates along the updrift beach,
causing progradation or is deposited in the jettied channel and has to be dredged. Sand that
bypasses the inlet is either moved offshore by rip currents or enters the jettied channel through
wave overtopping and/or movement through porous riprap (Figure 1c). Storm waves and large
wave swell produce strong longshore currents, which are directed toward the jetty along the
updrift shoreline. This obstruction commonly produces intense rip currents that transport littoral
sands offshore to the ebb-tidal delta. At some inlets, such as Merrimack River Inlet, MA, storm
waves occasionally transport sand over the updrift jetty and into the adjacent channel (Hubbard
1975).
At sites where jetty construction consists of porous riprap, sand can move through gaps between
the stones during periods of high wave energy and/or high water. Sand deposited in the jettied
channel by various mechanisms is transported seaward by dominant ebb currents to the ebb-tidal
delta. The presence of jetties serves to funnel the ebb discharge thereby displacing the ebb delta
offshore into deeper water. The deeper overall depth of the ebb delta reduces the effect of waves
and retards the formation of bar complexes. Sediment bypassing is accomplished through
transport along the outer bar by wave action primarily during storms.
Model 9. Outer Channel Shifting at Jettied Inlets. At some jettied inlets, such as
Moriches Inlet on Long Island, NY, the ebb-tidal delta is dynamic, and channel migration is an
active process. Sand bypassing at inlets of this type occurs in a manner similar to Model 4 where
the outer channel is deflected downdrift due to the preferential buildup of sand on the updrift side
of the main ebb channel (Figure 1c). During periods of high tidal discharge a new pathway is cut
through the deflecting sand shoal producing a straighter channel that is hydraulically more
efficient. The breaching produces a packet of sand that has been transferred from the updrift to
the downdrift side of the delta. Some of this sand is then transported onshore by wave action,
completing the bypassing process.
FINAL OBSERVATIONS: Sediment bypassing at tidal inlets occurs through a range of
processes that have been identified in six models of natural inlets and three models of jettied
inlets. A commonality of unstructured inlets is that sand bypassing ultimately results in the
formation, landward migration, and attachment of large bars to the downdrift shoreline. The
amount of sediment contained in these bars varies between 50,000 to more than 200,000 m3 of
sand. The actual volume is dependent on inlet size inlet, ebb-tidal delta morphology, rate at
which sand is delivered to the inlet, and the type of bypassing mechanism. Bar welding at these
inlets is a repetitive process with a frequency of 4 to 10 years. FitzGerald (1988) and Gaudiano
and Kana (2000) have documented episodic welding of bar complexes onto the downdrift beach
at inlets in South Carolina. An exception to this trend are the wave-dominated inlets where
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