EM 1110-2-1100 (Part II)
30 Apr 02
(8) Hoerls special function distribution. Tests of several regression equations and related curves revealed
that the best fit of the data from the four inlets was attained by a "Hoerls" special function distribution, given
in general form by:
b
VR ' a FI e cF I
(II-6-41)
where a, b, and c are the best-fit coefficients. The regression equation was solved for filling index FI values
corresponding to various channel depths, which allows for the proper "C" value to be used in the "daily shoal
volume" equation developed in Phase II of this method.
f. Inlet weir jetty hydraulic and sediment interaction. The subject of weir jetty design is dealt with in
detail by Seabergh (1983) and Weggel (1981), who discuss several hydraulic and sedimentary aspects
including flow over the weir and its elevation, location, length, jetty alignment and orientation, as well as the
shoreward end of the weir. The function of the weir jetty is to ideally collect the net sediment movement
behind the weir section, where it can then be handled in protected waters and bypassed. A sedimentation
benefit may occur in that sediment-laden longshore currents are kept away from the channel. Also, the ebb
currents may be hydraulically enhanced, as seen in Figure II-6-48, creating ebb dominance of channel
currents, which can aid in flushing sediment seaward.
(1) Weir location. With respect to tidal currents within the jetty system, the farther the weir is from the
navigation channel, the less likely it is to capture channel ebb currents that are directed seaward. This can
depend on the location of the predominant ebb channels and their orientation. Care must be taken in
evaluation of ebb flow direction because once the jetty system is constructed, the channel orientation may
change due to removal of wave effects and consequent sediment movement, which, for example, may have
deflected the ebb channel downcoast.
(2) Weir length. Primary transport over the weir exists at its intersection with the shoreline. If the wave
climate is mild, the weir should only be as long as necessary to prevent a chance of closure, something that
has not yet been noted to occur in existing weir jetty systems. The length to prevent closure would need to
be evaluated based on wave conditions, beach slope, orientation of the structure, etc. (Weggel 1981). If the
wave climate is highly variable, the weir should probably extend further oceanward so as to include a large
percentage of the breaker zone since heavy transport over the weir will occur at the breaker location for larger
wave conditions; otherwise, the sediment will move offshore along the jetty. Another factor influencing weir
length will be consideration of the amount of flow that is desired in the system. If a design objective is to
obtain high ebb dominance of flow in the navigation channel, then the weir should be longer, if this would
not interfere with other constraints, such as placement of a portion of the weir too close to the channel. The
complete hydraulic flow situation must be considered to determine whether the additional flow provided by
a wider weir will substantially augment ebb flow predominance and provide additional scouring ability in the
entrance channel (see Seabergh (1983)).
II-6-60
Hydrodynamics of Tidal Inlets
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