ERDC/CHL CHETN-IV-30
December 2000
(downdrift side of inlet throat). Spit platforms usually exhibit considerable relief having many
small shallow channels and numerous bars and bed forms. Spit accretion and downdrift
extension of the spit platform increase the length of the inlet channel, thereby decreasing flow
efficiency between the ocean and bay. Inlets of this type are susceptible to inlet channel
shortening particularly during storms when surges elevate water levels. At these times, early ebb
discharge from the backbarrier flows out the main channel as well as in a short cut route across
the spit platform.
At some inlets, seaward flow across the spit platform may become channelized in one of the
secondary shallow channels. Deepening of this channel may avulsively form a new, shorter
channel for water to exit the inlet. At other inlets the spit platform breaching process occurs
gradually over a period of 1 to 2 years. Depending upon the size of the inlet, this can result in
very large volumes of sand bypassing the inlet. Not only is a portion of the spit platform
transferred to the downdrift side of the inlet as the old channel fills, but also most of the former
ebb-tidal delta sediment is transported onshore to the downdrift beach as flow is diverted to the
new updrift inlet channel. This process of inlet sediment bypassing tends to be repeated every 4
to 8 years due to the continual enlargement and downdrift extension of spit platform, which
result from ongoing spit accretion and inlet migration and the oblique approach of the major
backbarrier creek at the inlet.
Model 6. Wave-Dominated Inlets. Wave-dominated inlets are defined as those in which
the distribution of sand bodies and general morphology of the ebb-tidal delta indicate the
dominance of wave-generated rather than tide-induced sediment transport. These inlets are
usually small (widths < 200 m) with shallow main ebb channels (depths < 6 m). They have sand
shoals that are pushed close to the mouth of the inlet, producing a slightly arcuate ebb delta
shape. The overall shallow nature of the distal portion of the ebb delta, much of which may be
exposed at low tide, coupled with its gently arcuate shape, allows waves to transport sand along
the periphery of the delta, especially at high tide (Figure 1b). The transport of sand along the
outer delta takes place in the same manner as that in the surf and breaker zones. Sediment
bypassing at these inlets occurs continuously, unlike the episodic landward bar migration
dominant in the other models. This mechanism of bypassing is similar to Bruun and Gerritsen's
(1959) "bar-bypassing model" and has been mathematically modeled by Kraus (2000).
Model 7. Jetty-Weir Inlet Bypassing. This mode of inlet sediment bypassing occurs at
certain jettied inlets containing one or two weirs with no settling basin (Figure 1c). Weirs are the
landward, submerged portions of jetties that allow sediment that is moving in the longshore
transport system to enter the jettied channel. This process is most active during storms when
energetic waves suspend large amounts of sand and strong longshore currents reach velocities of
a meter per second or more. During these conditions sand is easily transported over the weir and
into the adjacent channel. In the design inlet, sand entering the inlet via the weirs is transported
seaward by the dominant ebb-tidal currents. Ideally, sediment is eventually transported beyond
the end of the jetties where tidal and wave-generated currents move the sand onshore to the
downdrift beach. At some inlets, such as the entrance to Charleston Harbor, SC, a second weir
extending from the downdrift shoreline is present to reduce the amount of sand trapped by
downdrift jetties. This type of bypassing system works well only when the ebb currents clearly
dominate the flood currents and when the ebb discharge is capable of transporting the littoral
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