The 1968 jetty modification was very effective in preventing the
movement of sediment through the structure into the navigation
channel.
The jetty, however, is more reflective and while it
appears that it might be contributing to erosion of the adjacent
shoreline north of the structure, the extent of its contribution
to erosion has not been 'quantified. A study of the erosion
problem at Saco River is currently underway, and historical
trends indicate recession of the shoreline after raising and
widening the jetty in 1898.
SUMMARY:
Sand-tightening of coastal structures using conventional methods
of rehabilitation by removing armor stone and placing stone cores
of low permeability, steel sheet-piling, and/or concrete cores is
very effective. The low permeability, stone core was appropriate
at Manasguan Inlet since the existing jetties had deteriorated
and the armor stone had to be moved prior to reconstruction.
The
cost of this procedure is very high, however, for permeable
structures that are not in a deteriorated state, particularly
those constructed in relatively deep water.
Sand-tightening
structures by placing sealing stone and a relatively smooth
surfaced armor over the existing structure has also been
effective, but as illustrated at Saco River, may induce
undesirable wave reflection characteristics and result in erosion
of adjacent shorelines.
Sealing coastal structures with chemical gels and/or cementitious
materials may be significant in returning high economic benefits,
as opposed to the more conventional sealing methods.
The
development of sealant mixtures and injection methods has
significantly advanced in recent years, and many coastal projects
could benefit from the application of these technologies.
In
addition to site specific performance of chemical gels and
concrete sealants, large-scale model tests have been conducted
along with long-term field exposure tests (Simpson, et al 1990).
The data provide insight into the effectiveness and durability of
sand tightening materials and techniques. Based on field
experience, cement-sand sealants injected into structure voids
appear to be very effective. The Mission Bay Jetty cement-sand
sealant curtain was effective for some 30 years before
deterioratation.
It was never repaired even though the structure
was battered with intense storms that caused extensive armor
stone displacement. Other West Coast structures that have been
sealed over the years with cementitious sealants appear to have
remained effective. Preliminary field experience with chemical
gels (sodium silicates) injected into structure voids in Florida
appear to show promise, however, the gels have not been in place
long enough to evaluate their durability/longevity.
Large-scale physical model tests have been conducted to evaluate
materials and methods of sealing voids in rubblemound structures.
Sealants that had shown potential for success in field
applications were selected for evaluation.
Test results revealed
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