EM 1110-2-1100 (Part II)
30 Apr 02
in which all terms have been previously defined except for "C," which is termed the "potential shoaling
factor." As stated previously, the quantity of material entering and depositing in the channel increases with
the depth of the cut. However, until sufficient channel depth is attained to intercept all materials moving into
the inlet, some material continues to bypass the inlet via the sloping face of the ocean bar below the bottom
of the channel cut. Accordingly, C is defined as the portion of the total alongshore transport to the inlet
entering the domain of the channel. In order to determine C, a regression analysis was conducted to correlate
the inlet bar siltation with those factors judged to be most influential in the filling and flushing of such
channels.
(5) Phase III: Regression analysis. The following three factors were selected as dominant in influencing
the magnitude of shoaling at a particular inlet site having a dredge-maintained ocean bar channel. These
factors are described as follows:
(a) Ebb tide flow is the primary factor acting to flush intrusive littoral materials from the inlet
environment. Its influence in the analysis is represented by the symbol E∆T , which is the difference between
the mean ebb tide flow energy flux across the ocean bar at its natural elevation and the mean ebb tide flow
energy flux through the cross section of the excavated ocean bar navigation channel. It is assumed that the
tidal discharge is not significantly altered from one condition to the other. A basic concept in the tidal energy
flux difference is that the tidal flow velocities directed seaward over the ocean bar at its natural elevation, in
combination with wave agitation, are rapid enough to prevent accumulation of sediments above the natural
bar depth. If a section of the ocean bar is deepened by a navigation channel, the related average local flow
velocity is diminished and sediment deposition is initiated.
(b) Wave energy reaching the littoral zones adjacent to the inlet is the primary factor controlling the
quantity of littoral material moving toward the inlet and, as such, determines the shoaling characteristics of
an ocean bar navigation channel. In the analysis, the unrefracted wave energy flux per unit width of wave
crest offshore of the area of interest, designated Ew , is the basic measure of sediment transport toward the
inlet.
(c) The depth of an ocean bar channel determines the degree to which a channel will trap the littoral
sediments entering the inlet environment. In the analysis, the amount of channel entrenchment, and hence
the measure of sediment entrapment potential, is taken as the ratio DR of the depth of the channel to the depth
at which the seaward slope of the ocean bar meets the sea bottom. Each of these depths is measured from the
natural elevation of the ocean bar; therefore, the ratio DR represents the extent to which the ocean bar's
seaward slope has been incised by the channel.
(6) Normalized, independent filling index. The channel sedimentation potential increases or decreases
as each of the three factors E∆T, Ew, and DR increases or decreases, respectively. The factors were combined
as follows to establish a normalized, independent variable FI (filling index) for the regression analysis.
(E∆T @ EW @ DR)
ML 2
FI '
(II-6-37)
1014
T2
The normalized, dependent variable selected for the analysis was the ratio of the volume of channel infill to
the computed volume of the total alongshore sediment influx to the inlet multiplied by 100. This percentage
value VR is referred to as the "volume ratio." The regression analysis was based on data from four dredged-
maintained inlets within the boundaries of the Wilmington District: Oregon, Beaufort, Masonboro, and
Lockwoods Folly Inlets. Information available consisted of: (a) measured tidal discharges or inlet throat
cross-sectional areas, which permitted tidal discharge computations by means of tidal prism-inlet area
relations, (b) site wave statistics representing one or more years of wave gauge records, (c) detailed
II-6-58
Hydrodynamics of Tidal Inlets
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