dredged during inlet cutting that was pumped down drift (white strip of dry sand in
bottom of photograph), in front of a seawall and condominium.
Empirical predictive formulas are available to calculate the volume of the ebb shoal
complex (Walton and Adams 1976) and, with less reliability, the volume of the flood
shoal (Carr de Betts 2000). These formulas are based on the tidal prism P, which is the
amount of water entering through an inlet on ebb or exiting on flood, exclusive of river
flow and wind-generated currents. These predictive expressions have the form:
VF = BPk
(1)
flood shoal), and B and k = empirical coefficients. The values of the empirical
coefficients for prediction ebb shoal volume depend weakly on a wave-energy
parameter as determined for the three ocean coasts of the United States and in
recognition that greater wave energy will tend to transport material off the shoal. The
formulas of Carr de Betts (2000) were derived only for Florida inlets and do not have a
wave dependence.
The time scale for growth of inlet morphologic features is long and can require centuries
because (a) the supply of sediment is limited by the longshore transport rate at the site,
(b) the ebb shoal will bypass sediment down drift and to other inlet morphologic
features while it is growing, and (c) storms can reduce the size of shoals. The Inlet
Reservoir Model (Kraus 2000) can estimate the time rate of volume growth and natural
bypassing of inlet morphologic features. The characteristic time scale τ of this model is:
Ve
τ=
(2)
Qg
of transport arriving at the inlet. This ratio expresses the capacity of the ebb shoal to
hold sediment to its rate of supply.
3. Interruption of Natural Bypassing
The modes of natural sediment bypassing at natural and stabilized inlets have been
discussed by FitzGerald et al. (2001). Sediment pathways for natural bypassing depend
on the wave conditions, particularly between typical seas and storms (Militello and
Kraus 2003). If the jetties are long relative to the surf zone, thereby pushing the ebb
shoal outside the active littoral zone, or if the navigation channel is dredged very deep,
total interruption of natural bypassing is possible, as reported by Olsen (1977) for St.
Marys Entrance, Florida. Sediment must then be bypassed mechanically, as part of
maintenance dredging. The time scale for natural bypassing is long, perhaps extending
to hundreds of years, and it depends on τ. Applications of the Inlet Reservoir Model for
decadal and century-long calculation of natural bypassing have been made by Kraus
(2000), Kraus et al. (2003), Zarillo et al. (2003), and Dabees and Kraus (2005).
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