EM 1110-2-1100 (Part II)
30 Apr 02
Table II-6-3
Tidal Prism-Minimum Channel Cross-sectional Area Relationships
Location
Metric Units
English Units
Ac = 3.039 x 10-5 P1.05
Ac = 7.75 x 10-6 P1.05
Atlantic Coast
Ac = 9.311 x 10-4 P0.84
Ac = 5.02 x 10-4 P0.84
Gulf Coast
Ac = 2.833 x 10-4 P0.91
Ac = 1.19 x 10-4 P0.91
Pacific Coast
Ac = 7.489 x 10-4 P0.86
Ac = 3.76 x 10-4 P0.86
Dual-Jettied Inlets (O'Brien)
Ac is the minimum cross-sectional area in square meters (square feet) and P is the tidal prism in cubic meters (cubic feet).
(2) Inlet tidal prisms versus minimum cross-sectional area for Jarrett's data is plotted in Figure II-6-9.
(3) Work by Byrne et al. (1980) indicated that for inlets with minimum cross sections less than 100 m2
(1,076 ft2), there was a departure from the relationships developed above. They studied small inlets on the
Atlantic Coast in Chesapeake Bay. Their relationship for area and tidal prism is
Ac ' 9.902 10&3 p 0.61 (metric units)
(II-6-32)
(4) Jarrett's (1976) work pertains to equilibrium minimum cross-sectional areas at tidal channels from
one survey at a given date. Byrne, DeAlteres, and Bullock (1974) have shown that inlet cross section can
change on the order of 10 percent in very short time periods (see Figure II-6-40). This, of course, is due
to the variability in tidal currents and wave energy. A storm may bring a large amount of sediment to an inlet
region, since the inlet tends to act as a sink for sediment and the tidal current may require a certain time period
to return the minimum cross-sectional area to some quasi-equilibrium state. Brown (1928) notes that for
Absecon Inlet, New Jersey, "a single northeaster has been observed to push as much as 100,000 cubic yards
of sand in a single day into the channel on the outer bar, by the elongation of the northeast shoal, resulting
in a decrease in depth on the centerline of the channel by 6 to 7 feet." Sediment changes such as this will
affect the hydraulics of the inlet system, which in turn will remodify channel depth and location of sediment
shoaling.
(5) Adding jetty structures to natural inlets modifies the inlet's morphology. Careful engineering design
can reduce the amount of induced change. For example, jetties are placed so that the minimum cross-
sectional area of the inlet is maintained, leaving the volume of water exchanged over a tidal cycle (i.e., the
tidal prism) unchanged from the natural state. During design of inlet training structures, the question of
appropriate spacing must be answered so that excessive scour does not occur and cause settling or
displacement of the structures. Using O'Brien's formula for jettied inlets, the minimum distance between
jetties can be calculated. Figure II-6-41 shows lines of average depth expected for given jetty spacing and
tidal prism. The data points plotted on Figure II-6-41 describe actual field conditions for 44 inlets. No
attempt was made to analyze or judge whether problems existed at a particular project. For example, a very
large tidal prism with very narrow jetty spacing may develop problems of erosion along one of the jetties or
very high velocities might exist, producing navigational difficulties. Also, if spacing of the jetties is too wide,
the channel thalweg may meander in a sinuous manner, making navigation more difficult and reducing
efficiency, so as to increase shoaling and lead to possible closure. If the same minimum area is maintained
between the entrance channel's jetties that existed for the natural inlet, the tidal prism will be the same, and
tides will flush out the bay behind the inlet as well as they did in the natural state. Actually, a more
hydraulically efficient channel usually will exist at a jettied inlet because sediment influx is reduced and there
are fewer shoals.
II-6-48
Hydrodynamics of Tidal Inlets
Previous Page
