EM 1110-2-1100 (Part V)
31 Jul 2003
(e) Environmental factors. In general, environmental effects of borrow operations can be made
acceptable by careful site selection, and choice of equipment, technique and scheduling of operations.
Restoration of flora and fauna often takes place in a short time after operations (Stauble and Nelson 1984).
Alterations in physical features (the pit left behind after excavation) may, in some circumstances, be restored
by natural processes.
One effect of borrow operations is direct mortality of organisms due to the operation itself, and
destruction or modification of the natural habitat. Direct mortality of motile fauna, such as fish, is
usually not great because they move to other areas during the borrow operation. Sessile flora and
fauna cannot vacate the area; mortality of these organisms is therefore higher. However, they usually
are replaced by the reproduction of survivors or stocks in unaffected peripheral areas (Nelson 1985;
Johnson and Nelson 1985).
Another consideration is the destruction or modification of the habitat needed for survival of native
species. A common alteration is the exposure of a substrate that differs from the natural substrate
as a result of excavating overlying material. Many marine benthic, and some pelagic, organisms are
adapted to specific substrate conditions. Even though larvae of the native species reach the affected
area, they may not survive.
In comparing borrow sites, it is necessary to consider whether or not natural substrate will be
modified by the planned operation. This depends on the thickness of the surficial layer and the depth
of excavation needed to produce sufficient fill. In many instances, where the layer of suitable fill
material is thin, an increase in the areal extent of the borrow area will allow excavation of sufficient
material without altering substrate conditions. While this alternative increases direct mortality, it will
preserve favorable conditions for repopulation of native organisms.
In subaqueous areas, detrimental effects on native organisms, both within and near the borrow site,
may occur due to suspending silt and clay size material in the water column as a result of the
dredging operation. Deposits containing more than a small amount (generally taken as less than 10
percent by volume) of silt and clay are thus less desirable sources of fill from an environmental
standpoint. In addition, the fine fraction will be unstable in the beach environment.
(f) Utilization of bypassing/backpassing material. Consideration should be given to bypassing sand
across tidal inlets from accreted areas at updrift jetties and from ebb and flood deltas at inlets. Likewise,
back-passing of sand from a terminal downdrift jetty to an updrift beach-fill project should be evaluated as
a possible cost-effective sand recycling measure. Different types of sand transfer systems are discussed in
Richardson (1977). The effect of these measures on adjacent beaches must be evaluated.
(2) Borrow site exploration. A field exploration program to locate and characterize potential borrow
sources is usually necessary for offshore and backbarrier environments. For a detailed discussion of
procedures, see Prins (1980) and Meisburger (1990). In terrestrial areas, information on deposits is usually
available from state geological surveys. There may be existing commercial sand and gravel mining
operations. For existing navigation projects, or planned improvements, information on the dimensions and
characteristics of material to be dredged is usually available. Field exploration programs involve four phases:
a preliminary office study, general field exploration, detailed survey of the site, and characterization of
potential sites. The geographical area covered by these investigations is limited by the distance from the
project site that is within an economically feasible range for transportation of fill material. Borrow sources
within a few miles of the site should be considered initially. Sources farther away should be considered only
if no suitable sources are within this range.
Beach Fill Design
V-4-15
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