LONG-TERM COASTAL INLET CHANNEL AREA STABILITY

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Abstract: The equilibrium-area concept for tidal inlets has been a useful approach to understand the

adjustment of an entrance channel's minimum cross-sectional area to the basic hydraulic and

sedimentation characteristics of the inlet and bay it serves. This paper examines the concept in terms

of inlets that apparently are not in equilibrium, maintaining a smaller area than the equilibrium area

that is indicated by the Escoffier diagram. Is the Escoffier approach too simplified or is the response

sometimes a very long-term process? Other methods and concepts imply equilibrium area values

smaller than predicted by the Escoffier approach.

INTRODUCTION

The equilibrium area concept for tidal inlets was 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 form of this equation is:

Ac = CPn

(1)

where Ac is the minimum inlet cross-sectional area in the equilibrium condition, C is an empirically

determined coefficient, P is the tidal prism (typically during the spring tide), and n is an exponent

usually slightly less than unity. The empirical coefficients C and n are usually determined by the best

fit to data. Recent work by Kraus (1998) derived the form of Eq. 1 by a process-based model that

accounted for the dynamic balance between inlet ebb-tidal transport and longshore sand transport at

the inlet entrance. Kraus obtained an explicit expression for C in Eq. 1. Hughes (2002) derived an

equilibrium cross-sectional area relationship that not only matched field inlets, but also laboratory-

scaled inlets, which were not reconciled by previous expressions.

Using the above equation for equilibrium area and coupling it with Escoffier's (1940, 1977)

concept of simultaneously solving the analytic equilibrium area equation and the inlet's hydraulics

for various channel areas of a particular inlet, one can determine stable and unstable channel areas

(Fig. 1) for sandy inlets. Also, this analysis is used as a preliminary design tool to understand the

inlet's response. Typically one-dimensional numerical or analytical models have been used to

determine the inlet hydraulics in the initial approach. The interpretation of this curve (known as the

"closure curve") has had two approaches, but Van de Kreeke (1992) clarified the interpretation that

is shown in Fig. 1. Others had interpreted the area value at the peak velocity as being the location of

the equilibrium area.

This concept implies that equilibrium area is achieved once the inlet's bay fills completely, i.e., the

bay tide range is equal to the ocean tide range (assuming a resonant condition does not exist due to

bay geometry). This conclusion is based on application of this concept to many inlets with initial bay

1 U.S. Army Engineer Waterways Experiment Station, Coastal and Hydraulics Laboratory, 3909 Halls

Ferry Road, Vicksburg, MS 39180-6199 USA. William.C.Seabergh@erdc.usace.army.mil

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