EM 1110-2-1100 (Part II)
30 Apr 02
shows that variable entrance area increases the rate of rise of flood tide and decreases the rate fall, causing
higher maximum flood velocities than ebb, creating a flood-dominant inlet. These conditions are important
to help interpret the movement of sediment through inlets and the locations where shoals form.
i.
Multiple inlets.
(1) Often a lagoon is connected to the ocean by more than one inlet. The basic simplified hydraulics can
be determined by methods introduced previously or may be numerically modeled for more detailed effects
such as residual currents and net circulation (van de Kreeke 1976, 1978). Multiple inlets with a common bay
can impact inlet stability if an inlet changes or if an inlet is added (Ward 1982). Changes to a multiple inlet
system may be gradual (for example, one inlet gradually lengthens as littoral transport creates shoal regions
in the inlet), or change may be rapid (e.g., a hurricane cuts a channel through a barrier beach and begins to
rapidly expand in size, capturing a significant portion of the tidal prism from a previously existing inlet
connected to the same bay).
(2) For an existing system of multiple inlets, the K values may be added together
Kinlet 1 % Kinlet 2 % ......... Kinlet n ' Kall inlets
(II-6-23)
to determine the total K for the system, from which the bay tide amplitude and phase lag may be determined
(Figures II-6-18 and II-6-20). A total maximum discharge (for the total of all inlets) from the Vmax determined
from Figure II-6-19 is
2π
Qmax '
ao Ab Vmax
(II-6-24)
T
(3) This total maximum discharge then may be apportioned between all inlets
Qmax ' Q1max % Q2max % Q3max%.....
(II-6-25)
with the above equation knowing that, for example, for three inlets
Qmax
Q1max '
(II-6-26)
K2
K3
1%
%
K1
K1
Q2max and Q3max may be determined by placement of the appropriate K values in the denominator in the same
relative positions as in the above equation.
(4) For the case of an inlet connected to a bay that is segmented into separate internal bays connected
by narrow openings, reference is made to Dean (1971) and Mota Oliveira (1970).
j. Tidal jets. Some analytical approaches to flow patterns at inlets treat parameters affecting the
"plume" of water exiting the inlet (i.e., ebb flow). Joshi (1982) examined the effect of bottom friction,
bathymetric changes, and lateral entrainment of water due to turbulent mixing. Generally, as bottom friction
increases, the plume becomes wider and water velocity along the center line decreases. As bottom slope
increases, the plume contracts. zsy (1977) and zsy and nlata (1982) developed relationships to
determine plume expansion and center-line velocity, as shown in Figures II-6-32 and II-6-33. Example
Hydrodynamics of Tidal Inlets
II-6-37
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