EM 1110-2-1100 (Part II)
30 Apr 02
Figure II-6-7. Amw versus maximum channel depth at minimum width
section DMX (Vincent and Corson 1980)
(6) Keulegan's assumptions of prismatic channel cross section and vertical bay walls greatly simplify
prototype conditions, because natural inlets generally have a complex morphology, making accurate
determination of effective hydraulic radius, channel length, cross-sectional area, and bay area difficult.
Considerable subjectivity is required to determine these values from bathymetric charts. Some aid is provided
later in determining this information. Figure II-6-16 shows the variation of bay tide amplitude (ab) to that
of the ocean (ao) and the phase lag (ε) for various values of the coefficient of filling K. Figure II-6-17 defines
phase lag and provides a sample of output that can be determined from simple analytical models.
Approximate values for ken, kex, and f are given in Example Problem II-6-1. For flow entering an inlet
channel, Ken is usually taken between 0.005 and 0.25. For natural inlets, which are rounded at the entrance,
Ken 0.05 or less. For inlets with jetties and flow bending sharply as it enters the inlet channel, Ken = 0.25
may be appropriate. The exit flow is usually taken near unity, meaning kinetic head is fully lost. If there is
significant flow inertia, as in a very channelized bag, Kex may be less than 1.0.
(7) King (1974) solved the same equations but included the effect of inertia (first term of Equation II-6-
2). If inertia effect is important, then at times when the tide curves of ocean and bay intersect, there
still would be a flow into the bay, e.g., on flood flow there would still be movement of the water mass into
the bay, even as the bay elevation dropped below that of the ocean. This would be likely to occur when L
is large (channel is long, and therefore a large mass of water moving through the inlet has significant inertia
to move against an opposing head difference). Also the possibility exists that the inlet system could have a
Helmholtz frequency (or pumping mode, where the basin oscillates uniformly) that is tuned to the forcing
II-6-10
Hydrodynamics of Tidal Inlets
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