EM 1110-2-1100 (Part II)
30 Apr 02
Problem II-6-2 shows the effects of a flat bottom with friction. zsy and nlata provide additional
information for sloping bottoms and the effect of a longshore current approaching perpendicular to the ebb
jet, deflecting it downcoast. Joshi and Taylor (1983) examined the alongshore circulation created by tidal
jets and predicted that currents toward the inlet along the shoreline (induced by the ebb tidal jet or plume) can
range from 0.01 to 0.1 m/s within 10 half-widths (of the minimum inlet width) of the natural inlet entrance.
With a jetty in place, the range of induced currents along the shoreline is reduced, finally becoming zero at
the junction of the shoreline and jetty. The above work is based on a simplified bathymetry and does not
include wave-induced longshore currents, which can be an order of magnitude larger than jet-induced
longshore currents. Ismail and Wiegel (1983) found that waves increased the rate of ebb jet spread.
k. Tidal dispersion and mixing.
(1) Flow through inlets can play an important role in the exchange processes and flushing of the bay,
both of which impact water quality and bay ecology. Tidal currents, along with wind-driven currents and in
some cases density-driven circulation, can drive dispersion processes in the bay where particles of water (or
other substances, e.g., pollutants) are scattered or diluted. Tidal dispersion and its role in flushing of bays
has been studied by many investigators including Geyer and Signell (1992), Zimmerman (1988), van der
Kreeke (1983), Dyer and Taylor (1973), and Sanford, Boicourt, and Rives (1992). There is typically a broad
variance in the dispersion coefficient values that can be determined for the processes causing dispersion over
a large area. Therefore, analytic techniques to determine flushing of a bay are limited to small basins, and
assume complete internal mixing, with detailed numerical simulations required for larger bays. Tidal
dispersion is important in regions where flow separations occur; e.g., in regions of abrupt geometric change,
with the inlet entrance channel a prime example. The extent of dispersive influence is limited to the tidal
excursion in the bay (the distance covered by a particle entering the bay at the start of flood tide until the end
of flood tide). For tidal dispersion to influence large spatial scales, spacing between major topographic
features must be less than the tidal excursion distance (Geyer and Signell 1992).
(2) For small basins, where it is assumed that waters in the basin are completely mixed at all times,
van de Kreeke (1983) defined residence time as the average time for a particle to enter, then leave the basin,
with this average based on many particles. For small basins, and with the following relationship met
V
> 1.5
10 >
(II-6-27)
εP
where <V> is the average volume of the bay over the tidal cycle, ε is the fraction of new water entering the
bay from the sea each tidal cycle, and P is the tidal prism. A value of ε = 0.5 is often used when there is no
other basis for estimation of this factor. Then residence time can be calculated by
V
τr
(II-6-28)
'
T
εP
with τr residence time and T tidal period. The time to reduce the mass of a substance's concentration by a
factor of "e" (2.3) is equal to τr and to reduce it a factor of 10 is 2.3τr.
l. Wave-current interaction. The region where the navigation channel intersects and/or passes through
the outer ebb shoal can be a region of intense wave-current interaction. Wave-current interaction can produce
difficult navigation and dredge operation conditions. Also, interaction of waves, currents, and local
bathymetry/channel typically produces the most severe channel shoaling problems at inlets. These
interactions will be discussed in various combinations.
(1) Wave-current interaction.
Hydrodynamics of Tidal Inlets
II-6-39
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