January 13, 2004
14:36
WSPC/101-CEJ
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W. C. Seabergh & N. C. Kraus
keeping it away from the navigation channel and possibly aiding its return to the
beach system through re-diversion of the longshore current from going around the
ends of the jetties to a 180◦ rotation of the current vector. Examples of jetty spurs
in the United States are shown in Fig. 13. Table 2 summarizes the characteristics
of these spurs with regard to length, angle with the jetty, and location along the
length of the jetty. Typically, spurs are located about 75 % of the jetty length from
the local shoreline. The Bakers Haulover jetty spur was placed at the end of the
jetty.
Spurs also act as a breakwater and provide wave height reduction for the beach
in its lee. If added to a weir-jetty system, it may provide wave reduction for dredging
operations in the deposition basin. The spur may also reduce wave activity along
the shore inside the weir near the basin. This would reduce spit migration towards
the navigation channel. Another possible benefit for a new jetty system with spurs
is that the outer tips of the jetties may not need to extend seaward as far as a
system without a weir jetty, because seaward transport along the jetty is minimized
(Bottin, 1981).
A spur jetty may also be included as part of a beach nourishment plan to aid
in maintaining the sediments in the beach area (Walther and Dombrowski, 1999).
A spur could be placed on a down-drift jetty as well as on the more typical up
drift jetty if it was thought that the nourished down-drift beach might have a
tendency to locally feed back along the down-coast jetty towards the navigation
channel.
4.2. Spur jetty design considerations
Governing factors for spur design are location along the jetty, spur elevation, spur
length, distance from the shore, beach slope, water depth, length, angle with struc-
ture, crest elevation, whether submerged or emergent, width of crest if submerged
and wave climate.
4.2.1. Spur location
As noted from Table 2, existing spurs have been placed from about 60% of the jetty
length from the shoreline to 100%, with 75% typical. This location will depend on
local conditions near the jetty, such as bottom slope, wave climate, and proximity
of the shoreline. These will determine where waves are breaking and where sedi-
ment transport will be greatest. For relatively short jetties or a flat bottom slope,
wave breaking can occur seaward of a jetty system and sediment transport will be
strongest in many cases at the location of the breaker. A spur may not function sat-
isfactorily if this situation occurs frequently, as there is a small potential to intercept
and divert sediment pathways.
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