EM 1110-2-1100 (Part II)
30 Apr 02
where
β = stability index (units of length5/time3)
Vmax = maximum velocity in the throat
VT = threshold velocity for sand transport
ac = critical cross-sectional area (as from Figure II-6-42)
(4) Czerniak (1977) applied this technique successfully at Moriches Inlet, New York. Bruun and
Gerritsen (1960) and Bruun, Mehta, and Jonsson (1978) developed an overall stability criteria for tidal inlets.
They based their relationships on a ratio P/M, where P is the tidal prism and M is the total annual littoral drift
(typical values for M and their determination are discussed in Part III). The stability of an inlet is rated good,
fair, or poor as given in Table II-6-4.
Table II-6-4
Inlet Stability Ratings
P/Mtot $ 150
Conditions are relatively good, little bar and good flushing
100 # P/Mtot # 150
Conditions become less satisfactory, and offshore bar formation becomes more
pronounced
50 # P/Mtot # 100
Entrance bar may be rather large, but there is usually a channel through the bar
20 # P/Mtot < 50
All inlets are typical "bar-bypassers"
P/Mtot # 20
Descriptive of cases where the entrances become unstable "overflow channels" rather
than permanent inlets
(5) Another aspect of inlet stability was discussed by Vincent, Corson, and Gingench (1991) in which
inlet channel geographical location and horizontal topology were studied from aerial photography. They
defined four stability indices, including minimum inlet width W, channel length L, change in geographical
position N, and orientation variability of inlet channel E. Figure II-6-44 shows types of geographic channel
instability. Figure II-6-45 shows the Little River Inlet, South Carolina, geographical variability. A stability
limit was chosen so as to classify an inlet as stable or unstable. These limits were based on rate of change
of width, length, position, and orientation. Records of 51 inlets over various time periods were examined and
a change of 100 ft/month was selected as a reasonable arbitrary limit for stability in length and width of W
and L. The 100-ft/month value was also selected as a divider between stable and unstable channel position.
Most values examined were below the 100 ft/month rate.
d. Scour hole problems.
(1) Scour hole problems can occur at a variety of inlet locations and much remains to be understood
about the mechanisms causing scour. Scour holes form as a result of the interaction of tidal currents, waves,
wave-generated currents, sediment and adjacent structures (usually jetties). Scour holes have been observed
at the tips of jetties, on the outside of the jetty trunk, and along the inner section of a jetty. Lillycrop and
Hughes (1993) review scour problems at Corps projects.
(2) Scour at jetty tips (Figure II-6-47) can be caused by flow separation during flood flow, when
turbulent eddies are generated. Ebb currents traversing through this region of scour also might contribute
II-6-52
Hydrodynamics of Tidal Inlets
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