Mota Oliveira (1970) determined from numerical experiments that for an inlet coefficient of

repletion, *K, *from about 0.6 to 0.8, that the bed load capacity of the tidal currents reaches a

maximum. The Keulegan (1967) "*K*" repletion coefficient, is defined as

2 *ga*o

with *F *= ∑ ki +

(2)

2π ao Ab

4*R*

where *T = *tidal period; *a*o = tidal amplitude; *A*c = the channel cross-section area; *A*b = the surface

area of the bay; *F= *the impedance of the inlet; *k*i = entrance, exit and other energy loss coefficients;

to the average depth of the channel. The repletion coefficient is roughly equal to the decimal fraction

that the bay fills, e.g., for *K *= 0.6, the bay fills approximately 60 %. Therefore, Mota Oliveira's

analysis supports the concept that equilibrium inlets can have bays that do not fill completely. This

phenomenon of bed load capacity efficiency can be explained by the relation of water level to the

time of maximum currents. For inlets that do not fill their bays completely, greater current

magnitudes exist for maximum ebb flow due to their occurrence at lower water levels. Flood flow is

at a higher water level, and maximum flood currents will be weaker, due to a larger channel cross-

section at the higher water level. Therefore, the seaward flushing of sediments through the inlet is

most efficient in the range of *K *values of about 0.6 to 0.8. This hydrodynamic process might be

expected to lead to some inlets tending to be in "equilibrium" with bays that do not fill completely.

It should be noted that other factors could contribute to whether tidal inlet channel currents are ebb

or flood dominant. For example, Boon and Byrne (1981) showed that inlets with large open bays

tend toward flood dominant currents, and inlets with bays that have highly variable areas, e.g.,

containing marsh and small channels, contribute to ebb dominant currents.

Skou (1990) examined the Escoffier curve and defined the response ability as "the most optimum

situation for an inlet to remain stable." The response ability is determined by calculating the gradient

of the Escoffier curve and plotting this slope versus the cross-section area of the inlet. The location

along the curve where the gradient was a maximum defined the area that would be able to respond to

change the fastest. Though Skou's interpretation did not define this as the equilibrium area, it was

always larger than the "critical area," i.e., the area associated with the peak of the Escoffier curve.

These two criteria will be examined for the case of a "low-*K" *inlet in the next section.

Many inlets have low Keulegan *K *values. This fact indicates the inlet is not in equilibrium if the

value is below 0.6 to 0.8 (by Mota Olivera's work) and below about 2.0 (the value of *K *when the bay

fills completely), by Escoffier's analysis. A listing of some inlets that have or have had low *K *values

is shown in Table 1. When an Escoffier analysis is performed, typically the equilibrium area is

associated with the bay filling completely. An example of the Escoffier method is shown for

Barnegat Inlet, New Jersey (see Fig. 2). This inlet connects the Atlantic Ocean to a very large bay

(surface area of 123 million m2) with the entrance channel passing between two parallel jetties

spaced 305 m apart (see Figure 3). The inlet has sands ranging from 0.25 mm to 0.60 mm in

diameter. In 1968, the bay filled only 9 % of its capacity and had a Keulegan *K *of 0.09. Fig. 2

shows the location of the 1968 area along the stability curve. Note that it was very close to unstable

Seabergh

3

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