Escoffier (1940) analyzed closure conditions for a tidal inlet channel by

comparing possible cross-sectional areas of the inlet with those predicted by a

stability criterion such as given in Equation 6-2. As noted by Seabergh (2003),

equilibrium cross-sectional area predicted through an Escoffier analysis implies

that the amplitude of the water-surface elevation in the bay is close or equal to

the amplitude in the tidal body connected to the inlet.

Kraus (1998) derived the form of Equation 6-2 through a process-based

model that accounts for the dynamic balance between ebb-tidal sediment

transport and the longshore sediment transport produced by waves. The power of

Table 6-2, and the coefficient *C *was determined to be of the form:

0.3

⎛ α π3nM WE4 / 3 ⎞

2

⎟

(6-4)

⎝

⎠

where

α

= nondimensional coefficient with value of order unity entering the inlet

sediment transport formula employed

2

= Manning's roughness coefficient squared, sec2/m2/3

cu m/sec)

Equation 6-4 does not explicitly account for a threshold of motion for

transport by the ebb-tidal current or by the longshore sediment transport rate. A

threshold could be significant for gravel and cobble beaches, both for transport

by the tidal current in the inlet and by waves at and adjacent to the inlet.

Equation 6-4 indicates that the value of *C *will increase if the gross longshore

transport rate decreases, all other factors being equal, giving a larger value of the

cross-sectional area *A*C in Equation 6-2 for the same tidal prism *P*.

Jarrett (1976) compiled information on the ratios of inlet width, *W *to depth,

(<100) tend to be hydraulically more efficient. This result is reasonable, because

small *W/D *values indicate relatively greater depth, hence weaker bottom friction.

A more hydraulically efficient channel implies a larger channel cross section for

the same tidal prism. The average *W/D *ratio for all the inlets studied by Jarrett

(1976) was 337, and the average *W/D *ratio for the 16 Atlantic coast dual-jettied

inlets was 67.

Byrne et al. (1980) compiled information on *W/D *for their 14 studied inlets

located in Chesapeake Bay and obtained an average *W/D *= 23. They conclude

that the cross section of smaller channels must, therefore, become more efficient

than that of larger channels to maintain stability. The observed departure in *W*/*D*

characteristics between large and small inlets occurs between approximately *A*C =

100 sq m to 500 sq m (1,076 sq ft to 5,082 sq ft).

269

Chapter 6 Inlet Morphology and Stability

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