EM 1110-2-1100 (Part II)
30 Apr 02
b. Inlet characteristics.
(1) Tidal inlets generally have a short, narrow channel passing between two sandy barrier islands
(Figure II-6-1) and connect the ocean (or sea) to a bay. Some bays are small enough (on the order of tens of
kilometers or less) for the water surface to rise and fall uniformly (co-oscillate) in response to the forcing
ocean tide. Larger estuaries sometimes have broader junctions with the sea and may be long enough
(hundreds of kilometers) to contain nearly an entire tidal wave length, thus having a variable water level at
a given instant of time throughout the bay. Most methods discussed in this chapter apply to inlets that are
closer to having a co-oscillating tide, but can be applied to most inlet systems as long as the tidal period is
long compared to the time required for a shallow-water wave to propagate from the inlet to the farthest point
in the bay, i.e.,
Lb
T >>
(II-6-1)
gdb
where Lb is the distance to the farthest point, db is average bay depth, and T is typically taken as 12.42 hrs
60 min/hr 60 sec/min or 44,712 sec (for locations with semidiurnal or twice daily tides) or 89,424 sec for
once-daily tides.
(2) The configuration of an individual inlet can vary significantly over time. Often the configuration is
highly influenced by geology or peculiarities of the site, rather than a simple equilibrium of sediment and
hydrodynamics. Convergence of flows from several directions at either side of the inlet can create strong
turbulence that scours the channel deeply through the narrowest part of the inlet, called the inlet gorge, and
silts in the channel on the bay and ocean sides. Maximum depths generally in the range of 4-15 m may occur
in such channels, whereas seaward channel depths may diminish to 1.5-3 m where the flow has diffused and
wave-driven sediment transport is important. Inside the inlet, water may diverge into one or more channels
among shoal areas created by the deposition of sand from the ocean beaches. The resulting bathymetry can
be a complex pattern of bars, shoals, and channels. Hayes (1980) shows the ocean-side ebb-tidal delta
morphology for an unstructured inlet (Figure II-6-2).
c. Inlet variables. Although inlet systems can be quite complex, for the purpose of simple hydraulic
analysis, the immediate inlet region can be approximated by key parameters which, although simplified,
permit an analytical treatment of its hydraulics and a useful analysis of inlet systems. Analysis by Vincent
and Corson (1980, 1981) shows the range of size of a number of inlet parameters based on 67 inlets that had
been subjected to little or no human intervention. Figure II-6-3 defines an oceanside channel length Lmw (used
for their particular study, and not to be confused with channel length defined later), depth at the crest of the
outer bar in the channel (DCC), and location of inlet minimum width Amw. Figure II-6-4 defines the area of
the ebb tidal delta, AED, bounded by the depth contour of DCC (until it parallels the shoreline), the line
joining this location and the shoreline and the line across the inlet minimum width. Cross-sectional area at
the minimum inlet width (Amw) is plotted against channel length (Lmw) in Figure II-6-5, against ebb tidal delta
area (AED) in Figure II-6-6, against maximum channel depth (DMX) measured at minimum width section
in Figure II-6-7, and against channel-controlling depth (DCC), the minimum depth across the outer bar, in
Figure II-6-8. All parameters vary in log-linear relation to Amw over several orders of magnitude. The
95-percent confidence bands are also plotted. O'Brien's (1931) observed that there is a direct relationship
between the inlet's minimum cross-sectional flow area A (this is the minimum area Ac, not necessarily the area
at the location of the minimum width), and the tidal prism (P) filling the bay (Figure II-6-9). This relationship
will aid in defining the stability of inlet channels. The tidal prism in this case is defined as the volume of
water entering through the inlet on a spring tide. Detailed methods to define inlet parameters will be
discussed later.
Hydrodynamics of Tidal Inlets
II-6-3
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