Factors Contributing to Turbidity Generation

ERDC TN-DOER-N7

August 2000

Factors Contributing to Turbidity Generation. Factors contributing to near-surface turbidity

generation could include:

a. Spreading and or stripping of material at the water surface.

b. Gas entrained in the dredged material and released during disposal.

c. Stripping of material by the water column during descent.

d. Entrainment of material by the water column during underflow spreading.

The fluid mud underflow contains the greatest portion of disposed material and the greatest potential

for turbidity generation by the fourth factor. Field observations indicate that at times of high bed

shear-stress, entrainment of underflow material can generate a turbid plume extending some

distance from the discharge and not necessarily downstream from the discharge. Thus, the area of

concern with respect to water column impacts of a pipeline discharge is not confined to the vicinity

of discharge, but also includes the area of the underflow that might extend hundreds or thousands

of feet from the discharge.

The Entrainment Process. The entrainment of sediment from a fluid-mud layer into the water

column is described, and an entrainment algorithm is proposed by Teeter (1994). A detailed

theoretical and laboratory investigation of fluid mud entrainment is presented by Kranenburg and

Winterwerp (1997). Density and viscosity differences between the fluid mud and the overlying

water column inhibit entrainment. Thus, both fluid mud density and viscosity reduce entrainment

into the overlying water column and maintain a fluid mud underflow as a distinct feature (Teeter

1994). The calculation of total entrainment depends on a reasonable estimate of the total underflow

area and underflow properties.

The next section reviews underflow characteristics and some methods of analysis.

UNDERFLOW SPREADING: Whatever the configuration of the discharge port and its orienta-

tion, most discharged material reaches the bed in shallow water (less than about 4.5 m (15 ft)) shortly

after disposal. Once near the bed, sediments form fluid mud layers which flow away from the point

of discharge, depending on bottom slope, ambient currents, and their initial discharge trajectory.

As the bottom layer quickly thickens at the point of discharge, it behaves as a density flow and

spreads under the influence of gravity (Neal, Henry, and Greene 1978). The higher the thickness

and solids content of the layer, the greater the density effect. It has been estimated that 95-99 percent

of discharged sediment mass descents to the bottom layers with 30 m (100 ft) or so of the point of

a pipeline discharge (Schubel et al. 1978; Neal, Henry, and Greene 1978). In Mobile Bay, for

example, 99 percent was found to be dispersed along the bottom in the form of fluid mud (Nichols

and Thompson 1978).

Fluid mud is also a term used in conjunction with channel navigability as those mud concentrations

which do not inhibit navigation. The interaction of mud and a vessel comes about primarily through

viscosity, though density can also have an effect (Teeter 1992a). The range of concentrations is

similar for the fluid mud definition used here, roughly 10 to 400 dry-g/L (corresponding roughly