EM 1110-2-1100 (Part V)
31 Jul 2003
with at least one long dimension that allows the hopper vessel to transit the site; i.e., on the order of
at least 1.5 km (5,000 ft). In some cases, the dredged material is taken to a rehandle site and
offloaded, then transferred to the beach by hydraulic pipeline or truck haul.
An alternate placement method is to dump material in a nearshore berm as close as possible to the
project beach, in water depths shallower than the depth of closure, where it will possibly be moved
ashore by wave action. Experiments in offshore dumping near New River Inlet, NC, in 1.82 to 3.65-
m- (6- to 12-ft) depth, resulted in a general onshore and lateral migration of fill material (Schwartz
and Musialowski 1980). Placing material in water this shallow requires special equipment such as
split hull barges, dredges, or other equipment to cast the material shoreward.
Offshore borrow sources have several favorable features. Suitable deposits can often be located close
to the project area. Offshore deposits, particularly linear and cape-associated shoals usually contain
large volumes of sediment with uniform characteristics and little or no silt or clay. Large dredges
with high production rates can be used. Environmental effects can be kept at acceptable levels with
proper planning.
An unfavorable aspect of offshore borrow operations is the necessity of operating under open sea
conditions. Restrictions on the placement of fill material on beaches during the sea turtle nesting
season often requires dredging during the winter, when wave energy is highest. In more protected
places, such as backbarrier or otherwise sheltered sources, less seaworthy dredging plant can be used.
Dredges capable of working in open sea conditions generally have higher rental and operating costs,
although this may be offset by greater production capacity.
Evaluation of offshore sources should also consider the possible effect of dredging a borrow area on
littoral processes along and adjacent to the project area. This analysis should include the use of a
numerical wave transformation model and the calculation of longshore sand transport rates and
transport rate gradients. Nearshore transformation of a project area's principal incident wave
conditions over the pre- and postdredging bathymetry should be simulated. The incipient breaking
wave conditions and littoral transport potential alongshore, leeward of the borrow area, should be
compared between conditions. The proposed limits or geometry of the borrow area may require
alteration to avoid unintended concentrations of wave energy, or alongshore transport gradients,
produced by the excavated topography. Additionally, borrow areas near the shoreline or inlet shoals
may result in accelerated transport of sediment from the beach to the dredged borrow area. In
general, where practicable, borrow areas should be sited in water depths greater than the estimated
depth of closure (a rough rule of thumb would be twice as deep).
In some cases, the original relief maybe restored by natural processes over time. This is more likely
to occur in active features such as inlet shoals than in relic features, or on ones that are active only
during intense storms. Because the depth of closure is well inshore of offshore relic sand borrow
sites, these borrow pits usually fill in with fine-grained material that is not suitable for beach fill.
V-4-14
Beach Fill Design
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