Underflow Classification

ERDC TN-DOER-N7

August 2000

to 1,026 to 1,270 wet-g/L density). However, concentrations at the upper end of this range may not

be navigable, and the term `fluid mud' used here is not meant to imply navigability. Solids in the

pipeline are generally about 15 percent by weight or 150-200 g/L (Schubel et al. 1978).

Underflow spreading controls the configuration of the final deposit. Limited observations indicate

that the final deposit is a series of strata laid down as the underflow shifts and grows larger in

response to bottom topography. Maximum deposit thickness was about 0.3 m (1 ft) for a typical

2-day disposal operation in Mobile Bay and about 1.8 m (6 ft) for a 10-day disposal in the James

River (Nichols and Thompson 1978).

Underflow Classification. A fluid mud underflow can be characterized by its density, and speed,

and can behave as a turbulent turbidity current, or laminar slowly-spreading underflow, as shown

in Figure 2. The turbulent or laminar condition

is critical to underflow characteristics and con-

centrations. Laminar flows have smooth parti-

cle trajectories while turbulent flows have

chaotic motions, in addition to the mean flow,

which change flow properties. Turbulent un-

derflows or turbidity currents have lower fric-

tion and a characteristic billowing head just

behind their leading edge. Turbulent under-

flows have velocity profiles that, unlike open-

channel flows, have highest velocities near the

bed. Since turbulent underflows generally en- Figure 2. An underflow classification scheme using

local flow and concentration conditions

train ambient water, they grow vertically and

tend to have lower concentrations than laminar

underflows. Since one goal is to avoid turbidity generation, low-density turbulent underflows are

to be avoided in most cases as they are readily entrained into the overlying flow. Entrainment of

water into the underflow, and growth in underflow thickness, can occur under turbulent underflow

conditions. As noted earlier, density and viscosity differences between the fluid mud and the

overlying water column inhibit entrainment.

Pipeline discharges originate as turbulent flows, but empirical evidence suggest that they transition

to laminar underflow after spreading a short distance along a relatively flat bottom (Thevenot et al.

1992). Deceleration brings a low- or high-concentration turbulent underflow into the laminar

regime. Slow-spreading laminar undeflows have a velocity profile with 0 velocity at the bed, a

concentrated zone of relatively high fluid shear, and often a plug-flow or 0-shear zone near the

surface. Further description of laminar underflow properties will be provided in the next section.

Laminar underflows do not entrain ambient water. In fact, evidence suggests that they increase in

concentration as they spread, apparently as the result of settling. Underflows of both turbulent and

laminar types can be entrained into the overlying water column under certain conditions. Static beds

are generally formed by deposition from any of the underflow types as shown in Figure 2.