1 Introduction
Coastal inlets are narrow waterways that connect a bay, lagoon, or similar water body
with a larger water body that generates motion between them, such as forced by the tide
in the oceans and by seiching in the Great Lakes. Inlets serve commercial, military, and
recreational functions. They are also central to the health of marine and coastal terrestrial
organisms by allowing water exchange and by being a conduit for movement of
organisms and nutrients between the sea and estuaries or bays. Consequently, inlets have
been the subject of considerable research in coastal engineering and science.
In the present study, motivation for improved understanding of inlet processes lies in
reducing the cost of maintaining navigable channels and more reliably predicting the
functioning of proposed inlet modifications. Related concerns are the influence of an
inlet or planned inlet modification on the adjacent beaches and the associated ecosystem.
The overall aim of an inlet maintenance program is to establish and sustain a dynamic
state of equilibrium for the inlet morphology with minimal adjustment required of the
adjacent beaches and minimal change to the ebb- and flood-tidal shoals.
Background
The physical environment of a tidal inlet is determined by the forcing of the
periodically reversing tidal current, and by waves, wave-induced currents, wind-induced
currents, and storms. The interaction includes the type of sediment (grain size) and
sediment transport, and the morphologic response of the inlet to the hydrodynamic
forcing, which then feed back to the hydrodynamics. Two types of inlet stability are
commonly recognized, one referring to locational stability and orientation of the inlet
channel or gorge, and the other to stability of the inlet channel cross-sectional area. The
stability of an inlet depends on the balance between two physical processes, the wave-
induced longshore current that transports sediment toward the inlet, creating a tendency
towards closure and migration, and the tidal flow within the inlet that tends to scour the
bottom and banks of the channel, removing sediment from the inlet throat (Johnson 1919;
Bruun and Gerritsen 1959).
Most field and theoretical studies of inlet stability have concerned large tidal inlets
with sand-sized sediment, as commonly found on all coasts of the United States. Such
inlets are prevalent and have great economic and environmental functions. However,
coastal inlets are also found on rocky coasts, and small inlets that tend to close are of
environmental concern (Goodwin 1996). Although inlet channel cross-sectional area has
been investigated (e.g., Le Conte 1905; O'Brien 1931, 1969; Jarrett 1976; Byrne et al.
1980; Moody 1988; Hume and Merdendorf (1990); Kraus 1998; Hughes 2002), the role
of sediment size in the stability of inlets has received almost no investigation (Simpson
1
Chapter 1 Introduction
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