EM 1110-2-1100 (Part II)
30 Apr 02
Figure II-6-4.
Ebb delta area measurement (Vincent and Corson 1980)
unimpeded by the continually narrowing opening. When it does begin to be so impeded and the
entrance acts as a barrier to the incoming tide, a delay in the advance of the tide will be caused, and
a difference of head will occur between the water outside the inlet and that inside. This will create
a hydraulic current into the bay, in addition to such movement as may be due solely to tidal wave
propagation.
It is clear that as long as the inlet is wide and deep in proportion to the area of the bay, hydraulic
currents will be small and tidal wave propagation will predominate. Tidal waves in the shallow waters near
the shore will be essentially waves of translation, that is, the whole body of water moves with practically the
same velocity horizontally. Now, if the size of the inlet becomes very small with respect to the bay area, tidal
wave propagation will be negligible, the flow through the inlet will be hydraulic, that is, the water surface
through the inlet will have a slope, causing a flow, and it can be considered mathematically in accordance
with known hydraulic laws.
(3) With this in mind, early development of inlet hydraulics achieved reasonable results by using
simplified approaches of steady-flow hydraulics to understand inlet currents and response of the bay (or
lagoon) tide (Brown 1928). Keulegan (1951, 1967) solved the one-dimensional, depth-averaged shallow
water wave equation for flow analytically. Others since have formulated a variety of analytical solutions for
inlets (including van deKreeke (1967), Mota Oliveira (1970), Shemdin and Forney (1970), King (1974),
Mehta and zsy (1978), Escoffier and Walton (1979), and DiLorenzo (1988)). Paralleling analytical
development, physical models were used for detailed studies of inlet design (see below). More recently,
numerical models have provided greater refinements and details using one-, two- and three-dimensional
longwave equations of motion (including developments by Harris and Bodine (1977), Butler (1980), and
Amein and Kraus (1991)). Some analytical models for inlets will be examined here to provide understanding
of the inlet system and because they actually produce usable information with minimal effort. Application
of the techniques of this chapter would include use of numerical models (e.g., Automated Coastal Engineering
System (ACES); Leenknecht et al. 1992). ACES includes a spatially integrated one-dimensional numerical
model. Additional information about ACES and other available models is provided in Part II-6-2, paragraph
m, "Other methods for inlet analysis."
II-6-6
Hydrodynamics of Tidal Inlets
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