EM 1110-2-1100 (Part V)
31 Jul 2003
(b) Backbarrier sources. Sediment deposits in the backbarrier marsh, tidal creek, bay, estuary, and
lagoon environments behind barrier islands and spits have been used in the past for beach fill. They are an
attractive source because they are protected from ocean waves and are often close enough to the project beach
to allow direct transfer of the material by pipeline. This eliminates the need for separate transport and transfer
operations. However, most backbarrier sediments are too fine-grained to use as beach fill. In addition, many
backbarrier areas are highly important elements in the coastal ecosystem and are sensitive to disturbance and
alteration by dredging. Material in backbarrier sediments coarse enough for beach fill is generally confined
to overwash deposits and flood tidal shoals associated with active or relic inlets. Overwash deposits occur
on the landward margin of the barrier where storm waves have carried beach and dune sediments across the
island or spit. Flood tidal shoals occur inshore of tidal inlets and consist of sediment transported by tidal
currents flowing in and out of the inlet. These sediments are usually derived from littoral drift from adjacent
beaches. Overwash deposits and relict flood tidal shoals may be ecologically important because they may
provide suitable substrate for marsh growth. In addition, on retreating barriers, they may comprise a reserve
of sand that will be recycled into the active beach deposits as retreat progresses. Flood tidal shoals at an
active inlet may be suitable for borrow sites because the material removed is likely to be replaced by ongoing
inlet processes. However, dredging material from active flood tidal shoals can adversely alter both the
hydraulic conditions in the inlet and wave action on adjacent shores. A study of the hydraulic effects should
be made prior to dredging flood-tidal shoals.
(c) Harbors, navigation channels, and waterways. Creation of harbors, navigation channels, and
waterways, and deepening or maintenance dredging of existing navigation projects often requires the
excavation and disposal of large volumes of sediment. In some cases, where the dredged sediment is of
suitable quality, it can be used as fill on nearby beaches rather than placing it in offshore, upland, or contained
disposal sites. Operations of this type are economically attractive because dual benefits are realized at
considerably less cost than if both operations were carried out separately.
Maintenance dredging of projects in low energy environments such as estuaries or protected bays is
least likely to produce suitable beach-fill material. In such areas, the dredged material often consists
of clay, silt, and very fine sand. However, when dredging new harbors, channels or waterways, or
deepening existing channels in low energy areas, the dredge may cut into previously undisturbed
material of suitable characteristics.
Dredged material from higher energy areas, such as rivers above tidewater and open coast inlet
shoals, is often more acceptable for beach fill. On barrier coasts, inlets trap beach sediment that has
been carried to the inlet by littoral drift. Therefore, material dredged from inlets is typically similar
to the native material on the project beach. However, sediment compatibility tests should be
performed to determine its suitability for use as beach fill.
(d) Offshore sources. Investigations of potential offshore sources of beach-fill material under the Coastal
Engineering Research Center's (CERC's), Inner Continental Shelf Sediments Study (ICONS), by the USACE
Districts and others (i.e., Bodge and Rosen (1988)), indicate that large deposits of suitable material often
occur in offshore deposits. Data from the Atlantic coast show that the most common suitable sources are in
ebb tidal shoals off inlets, and in linear and cape-associated shoals on the inner continental shelf, such as
those shown in Figure V-4-4. Potential sources on the inner shelf have also been identified in submerged
glaciofluvial features, relic stream channels, and featureless sheet-type deposits.
V-4-12
Beach Fill Design
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