Underflow Spreading from an Open-Water Pipeline Disposal

ERDC TN-DOER-N7

August 2000

Underflow Spreading from

an Open-Water Pipeline Disposal

PURPOSE: This technical note provides a conceptual review of the mixing and dispersion

processes associated with open-water pipeline discharges from hydraulic dredging operations. This

is the first step in developing improved predictive tools or models for water column suspended-

sediments and turbidity impacts, and bottom spread of disposed dredged material under DOER.

This note emphasizes underflow plume spreading of disposed dredged material.

BACKGROUND: "Good dredging procedures, already known but not always practiced, will

reduce dredge-induced turbidity but also will result in a more economic operation" (Saucier et al.

1978). Questions have long been raised about the fate of material at a disposal site, dispersion of

sediment into nearby bottoms, and/or spread of material along the bottom (Johnson et al. 1999).

These issues closely connect to pipeline disposal, an economical method to place hydraulically-

dredged material into a nearby open-water disposal area. Pipeline disposal, once more common, is

now often restricted by environmental considerations. Turbidity has been the most common issue

of concern related to pipeline discharge, and an underflow of fluid mud is created as material

descends to the vicinity of the bed. Fluid-mud underflow account for the spreading of most disposed

material. To justify the pipeline disposal method and to meet the goals of the Clean Water Act,

information on the post-disposal behavior of the material is needed to properly select and size

disposal sites, and to specify conditions of discharge.

Wherever practicable, adverse turbidity levels around a pipeline discharge should be minimized,

and the spread of the material should be contained within designated disposal areas. Pipeline

placement of dredged material requires engineering and operational controls on discharge condi-

tions to accomplish some specified mound configuration. This technical note describes processes

that interact with a simple pipeline disposal case as material first enters the environment. The

approach is conceptual rather than practical.

PIPELINE DISCHARGE TURBIDITY: As it exits a pipeline or diffuser, dredged material

generally has both high momentum and high density, and the behavior of the material will depend

on many factors: slurry properties, initial trajectory, whether or not it exits into air or water, currents

and shear stresses in the water column, and topography of the bottom. The geometry of discharges

varies widely, and can include a variety of baffle or deflector plates, cylindrical or conical diffusers.

Alternatives for Reducing Pipeline Discharge Turbidity. Several options are available to

minimize surface and near-surface turbidity around a pipeline discharge. The most viable are

simply to direct the discharge vertically downward while submerging the discharge end of the pipe.

This concept has been advanced with the development and application of submerged diffusers

which reduce discharge velocity and entrainment.