performed by McNair (1976), Mayor-Mora (1973, 1977), and Delmonte and
Johnson (1971).
Purpose of Model Investigation
The purpose of the investigation was to generate equilibrium inlet areas where
tide period, sediment size, and wave conditions were varied (including the situa-
tion of no waves present), creating differing hydraulic conditions. These data
were then to be used to examine an analytical solution to O'Brien's tidal
prism/minimum inlet cross-sectional area relationship:
πP
Ac =
(2)
T Um
where
T
= tidal period
Um = maximum velocity of water passing through the inlet
This relationship was analytically derived in O'Brien (1969) from the same
continuity equation considered by
Keulegan (1967) in
arriving at his
inlet hydrau-
lics relationships. The standard simplifying assumptions made by Keulegan
(1967) of constant bay surface area and inlet channel cross-section area during the
tidal cycle were invoked. Also assumed was uniform rise and fall of bay level in a
sinusoidal manner. Van de Kreeke (1992) discusses this development and also
mentions that the derivation is based on a sinusoidal current in the channel. If the
semidiurnal tide period of 44,712 sec and a rule-of-thumb current for large inlets
in equilibrium of 3.28 ft/sec (1.0 m/sec) (van de Kreeke 1992) is substituted for
Um in Equation 2, the resultant equation (in units of feet),
Ac = 2.14 10-5 P
(3)
is very close to O'Brien's (1969) relation,
Ac = 2.0 10-5 P
(4)
developed from data for inlets without jetties. The value of the equilibrium cur-
rent is dependent on the littoral climate (local wave and sediment characteristics)
affecting the inlet, i.e., the current needed to maintain the inlet, scouring the in-
coming littoral sand.
An idealized tidal inlet physical model (described in Chapter 2) molded in
concrete was constructed for the present study. It had a gorge or throat (region of
minimum area) of a cross-sectional area large enough that it could be filled in with
sand to create a movable-bed region, which could then respond to the action of
tidal currents and waves. The cross-sectional area could then adjust to its equilib-
rium area, dependent on the tidal period and ocean tide range, which controlled
the maximum velocity and the tidal prism.
2
Chapter 1 Introduction