January 13, 2004
14:36
WSPC/101-CEJ
00094
552
W. C. Seabergh & N. C. Kraus
of downtime. Weggel (1981) describes wave transmission formulas to estimate wave
height in the deposition basin.
Weir elevation with respect to inlet hydrodynamics is a subtle, but key parameter
for the timing of maximum flows and flow volume over the weir. Inlets with a small
bay tide range with respect to the ocean tide range have maximum currents in
the inlet at high and low water. Therefore, the flood current would be strongest at
high water, when wave energy would likely to be the greatest. A weir at an inlet
such as this should be relatively higher than for an inlet that has a bay that nearly
completely fills. If the inlet bay tide range is nearly as great as the ocean range, then
maximum flood and ebb currents occur at mid-tide level. Probably the greatest care
with respect to currents would be in regard to the direction of ebb flow. Presence of
a strong ebb current directed towards the weir may cause a gradual cutting of the
region between the deep deposition basin and the shallower water that separates the
basin and the channel. Eventually, the channel might be pulled through the basin,
causing a dispersal of sediments coming over the weir.
3.2.5. Weir length
Early weir designs called for long weir sections. This was thought necessary to pre-
vent a storm from bringing in so much sand as to impound the weir and isolate
it from the littoral system. To date, complications associated with storms have not
been documented. As noted in the discussion on basin location, most of the sediment
crosses over the weir at its intersection with the swash zone. For larger waves, some
sediment is transported over the weir at the breaker line with the aid of flood tidal
currents and longshore currents generated by breaking waves. If the breaker zone is
located beyond the seaward limit of the weir, the sediment will likely be diverted
along the outer portion of the jetty and enter the navigation channel as a tip shoal.
Weir length is a tradeoff between wave protection in the channel and deposition
basin area and the possibility of diverting significant amounts of sediment seaward.
Local beach slope is also a factor in determining how far seaward the weir should
extend. Flatter nearshore slopes require longer weirs to prevent too much sediment
from bypassing the weir with a potential to enter the channel.
4. Spur Jetties
A jetty spur may be defined as a relatively short structure added to a jetty that
flanks a navigation channel through an inlet. The spur will typically be nearly per-
pendicular to the jetty, but may be oriented at some angle with respect to the jetty
in the range of 0 to 45 degrees. The spur may be added on the beachside of a jetty
to prevent sediment from entering the inlet or may be placed on the channel side
to divert the tidal current away from the jetty to reduce scour and possible jetty
instability. This section discusses spurs placed on the beach or seaward side of a
jetty.
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