Fig. 4. Simulation of flood current at Fire Island Inlet, New York (Kraus et al. 2003)
Burial of a portion of the bay bottom by the new flood shoal may be viewed as an
environmental enhancement if the bay bottom is not productive or as an environmental
degradation if the bottom is a resource. Often, the flood shoal will form near or at the
intra-coastal waterway passing parallel to the coast in the bay. In such a case, dredging
requirements for the intra-coastal waterway will increase.
7. Ebb Shoal Collapse
Ebb shoal collapse or deflation means migration of portion or the entire ebb shoal
onshore, alongshore, and offshore because of loss of the ebb-tidal current
(abandonment) over the shoal (Hansen and Knowles 1988; Pope 1991). The shoal can
collapse because of jetty construction or modification, or because an inlet is relocated.
Pope (1991) developed a conceptual framework (see Pope Fig. 5) of the morphologic
evolution of an ebb-dominated jettied inlet that moves from natural bypassing in its
original state to collapse of the ebb shoal and subsequent erosion of the ebb-shoal
platform with elapsed time. Byrnes et al. (2003) document collapse of the southern (up
drift) and northern (down drift) flanks of the ebb shoal at Grays Harbor, Washington,
and seaward translation of the central portion of the ebb shoal in response to
construction of long jetties at the turn of the 20th Century. Kana and McKee (2003)
discuss the twice-relocated Captain Sams Inlet in South Carolina, for which collapse of
the ebb shoal at the closed inlet was anticipated to nourish the beach (Kana and Mason
8. Cost of Construction and Maintenance Dredging
Jetty construction costs thousands of dollars per meter of length. As jetties extend into
deeper water, construction cost greatly increases because it is proportional to stone
volume, and larger physical plant is required for the construction. Long jetties relative
to the navigation channel depth or typical width of the surf zone will intercept greater
amounts of longshore sediment transport, requiring more planning and expense in
replacing natural bypassing with mechanical bypassing.
Maintenance dredging may offer the least-cost means of bypassing sediment, if the
waves and currents allow nearshore placement or pumping to the beach. Seabergh and
Kraus (2003) review bypassing techniques and engineering design considerations.
There is a tradeoff between the one-time construction cost to protect the channel from
sedimentation and constructing shorter jetties, but dredging more. Thus, jetty weirs,
jetty spurs, and channel deposition basins are strategies developed that combine
consideration of protecting the navigation channel from infilling while stockpiling or
directing sediment incident to the inlet to a convenient location for bypassing action.
Maintenance of coastal inlet navigation channels and the adjacent beaches brings
conflicting requirements. For example, jetties are built in part to confine and strengthen
the current, but the resultant seaward translation of the ebb shoal interrupts natural
sediment bypassing. In turn, interruption of the natural bypassing rates and pathways
compromise the integrity of the adjacent beaches, with potential feedback to destabilize