Submerged Discharge - tech-frame-rev040040

Framework for Dredged Material Management

May 2004

4.4.2 Submerged Discharge

Submerged discharge is a control measure which may be considered to reduce

water-column impacts. The use of a submerged point of discharge reduces the area of

exposure in the water-column and the amount of material suspended in the water column

susceptible to dispersion. The use of submerged diffusers also reduces the exit velocities

for hydraulic placement, allowing more precise placement and reducing both

resuspension and spread of the discharged material. Considerations in evaluating

feasibility of a submerged discharge and/or use of a diffuser include water depth, bottom

topography, currents, type of dredge, and site capacity. Design specifications for

submerged diffusers are available, and the diffusers have been successfully used for

disposal operations (Neal, Henry, and Green 1978, Palermo 1994).

4.4.3 Lateral Containment

Lateral containment is a control measure which may be considered to reduce

benthic impacts. The use of subaqueous depressions or borrow pits or the construction of

subaqueous dikes can provide containment of material reaching the bottom during open-

water disposal, resulting in a reduced bottom area being affected by the placement. Such

techniques reduce the areal extent of a given disposal operation, thereby reducing both

physical benthic effects and the potential for release of contaminants. Considerations in

evaluating feasibility of lateral containment include type of dredge, water depth, bottom

topography, bottom sediment type, and site capacity.

Simply selecting a site amenable to lateral containment such as an existing bottom

depression or valley can be effective. Placement of material in constructed depressions

such as abandoned borrow pits has also been proposed. Submerged dikes or berms for

purposes of lateral containment have been constructed or proposed at several sites. Such a

proposal would not necessarily involve added expense to the project if the material used

for the berm comes from the same or another dredging project.

4.4.4 Thin-Layer Placement

The intentional spreading of hydraulically pumped dredged material over broad

areas to achieve overburdens less than 12 inches thick has been termed "thin-layer"

placement. The objective of thin-layer placement is to minimize impacts on benthic

fauna and to speed their recovery, particularly in estuarine environments. This strategy is

based upon knowledge that a portion of the benthos can migrate upward through the

dredged material overburden, usually present as a fluid mud layer. This concept has been

developed and demonstrated in Mississippi Sound by the Mobile District. Results of

monitoring studies indicated that recovery was enhanced in shallow, turbid Gulf coast

estuaries. A distinction should be made between thin-layer placement in open-water

applications and high-pressure spray disposal on marsh surfaces. Although sometimes

referred to as thin-layer placement, the latter case involves different equipment

requirements and generally is suitable for relatively small volumes of dredged material,

whereas open-water thin-layer placement uses conventional hydraulic equipment (with