The approximate 18 sq m geotextile mat had a pillow structure filled with concrete on the
landward edge to anchor the mat to the bed. The rolled up geotextile mat was deployed
by a barge-mounted crane. Once on the bottom, the mat was extended seaward. Three
interlocking Beachsaver Reef units were then placed on the seaward side of each
geotextile mat with the use of the crane. Divers were used to align the individual units
and insure that the interlocking was accomplished.
An initial survey in March 2002 indicated that the 2.7 m contour was nearly
parallel with the shoreline and the end of the groins. Just before placement a new survey
in June 2002 indicated that the 2.7 m contour had moved seaward on the eastern one-
third of the groin cell and landward on the western end. In order to maintain the desired
depth of placement, excavation with a barge-mounted backhoe was required along the
eastern portion of the line due to deposition of sand in the spring of 2002. The
orientation of the line of Beachsaver Reef units was angled slightly seaward to parallel
the 2.7 m contour. As the reef line reached the center of the cell, sand fill was placed on
the bed before the filter cloth was placed to bring the bed elevation up to the required
depth, due to scour of the bottom as the units were being placed. The barge-mounted
backhoe excavated the sand from the nearshore on the landward side of the placement
line. This cut and fill requirement increased the installation time. A new alignment was
selected in the middle of the cell to bring the line more landward to alleviate the
requirement for fill. The final alignment of the placement of the 72 3-m-long units had a
seaward bow in the line due to the change in bed elevation at the time of placement from
the pre-installation survey five months before. This change in bed elevation is indicative
of the strong flood currents just seaward of the groins that have caused the nearshore to
change elevations frequently over time. Placement was originally scheduled for mid-
June but was delayed to mid-August due to environmental concerns during the mating
season of horseshoe crabs (Limulus polyphemus) in Delaware Bay. The Beachsaver
Reef installation was completed over a five-week period from 16 August to 25
September 2002, with a total of 25 working days.
Double-T Sill Installation
In cell 6, a prefabricated concrete Double-T structure was placed and is expected
to act as a sill across the seaward end of that cell to create a perched beach. The name
comes from the shape of the precast concrete units with two vertical legs on a flat base
when viewed on its end looks like two TT's. This Double-T design is typically used for
parking garage supports. The units for this particular application for coastal erosion
prevention are made of marine grade concrete and reinforced with coated rebar. The 9.14
m-long and 3.66 m-wide units are placed end to end in an inverted position with the flat
surface on the sand bed at 2.7 m depth (Figure 5). The vertical legs extend 0.8 m in the
water column and a single unit has a weight of 17.3 metric tons. For this application, an
extension of the legs on one end of each unit allowed an interlocking effect between the
units, designed to maintaining a linear orientation of the sill. The sill was located near
the seaward end of the groins and was attached to the groins at both ends by rock,
creating a closed structure.
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