January 13, 2004
14:36
WSPC/101-CEJ
00094
Figure 2. Interpreted sediment-transport pathways at Shinnecock Inlet.
538
W. C. Seabergh & N. C. Kraus
Ebb Shoal
Qin
VEe
Bypassing Bar
(QB)in
Attachment Bar
VE
(QA)in
VBe
(QE)out
(Qbeach)in
VAe
VB
(QB)out
VA
(QA)out
Fig. 3. Concept sketch for reservoir inlet morphology model.
Figure 3. Concept sketch for reservoir inlet morphology model
The Reservoir Model has been applied to evaluate the interruption of the natural
bypassing rate by mining of the ebb shoal, bypassing bar, and flood shoal. In the
United States, ebb shoals are often mined for sediment bypassing (e.g. Cialone and
Stauble, 1998). Dredging of channels can also be represented, with placement pre-
scribed to downdrift morphologic features, such as to the adjacent beach. The model
shows the adaptation time of the inlet morphologic system toward re-establishment
of bypassing rates. Characteristic time scales of adaptation or recovery of the system
are calculated, and for smaller inlets, this might be 310 years, whereas for larger
inlets, the adaptation time can be 10 to 20 years for typical amounts of material
dredged for bypassing (50,000500,000 m3). The Reservoir Model was applied, for
example, to calculate dredging of the deposition basin at Shinnecock Inlet (Militello
26
and Kraus, 2001), which would function similarly to the deposition basin at a weir
jetty as discussed next.
3. Weir Jetty Systems
A weir jetty system is one of several methods for bypassing sediment at coastal inlets.
Others methods, as described above, include an offshore breakwater, a deposition
basin in the channel, a fixed pumping plant, or a jet pump suspended from a crane.
Figure 4 shows typical elements of a weir jetty system. The weir section, usually less
than about 300 m long, is a depressed region of the jetty that permits waves and the
longshore current generated by wind, waves, and tide to transport sediment moving
along the coast to enter a deposition basin located in the lee of the weir, thereby
reducing the amount of sediment entering the navigation channel. A weir also acts as
a breakwater and provides a semi-protected area for dredging the deposition basin.
Another benefit is that the weir allows flood currents to enter the inlet over the
weir and through the channel during flood flow with subsequent channeling of ebb
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