1
Introduction
Background
The equilibrium area concept for a tidal inlet has been a useful approach to
understand the adjustment of the minimum cross-sectional area of an entrance
channel to the basic hydraulic and sedimentation characteristics of the inlet it
serves. Based on a concept originated by LeConte (1905), O'Brien (1931, 1969)
examined field data from tidal inlets through sandy barriers on the west coast of
the United States and determined a relationship between the minimum cross-
sectional flow area of the entrance channel and the tidal prism (the volume of wa-
ter flowing into the bay during the flood tidal cycle or conversely, the volume of
water flowing out of the bay during the ebb portion of the tidal cycle usually at
spring tide conditions). This relationship defines the equilibrium area.
The form of this equation is
Ac = CPn
(1)
where
Ac =
the minimum inlet cross-sectional area in the equilibrium condition
C =
an empirically determined coefficient
P =
the tidal prism (typically during the spring tide)
n =
an exponent usually slightly less than unity
C and n are usually determined by best fits to data sets. Many investigators have
found regional differences in the values of C and n.
Jarrett (1976) refined Equation 1 and developed expressions for the east, west,
and Gulf coasts of the United States based on additional field data. Others have
examined this relationship with regard to reduced wave exposure of more pro-
tected shorelines (Riedel and Gourlay 1980) and relative to inlet channel area
magnitude (Byrne, Gammisch, and Thomas 1980). Kraus (1998) developed a
process-based model to calculate equilibrium area, and Hughes (1999) introduced
a simple expression relating maximum discharge to depth of scour in a coastal
inlet. Laboratory inlet work for equilibrium areas with movable beds has been
1
Chapter 1 Introduction
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