COMPUTATIONAL MODEL

ERDC TN-DOER-E10

April 2000

suspended material is assumed to occur very near the bottom. For dredging operations using a

hopper dredge, both near-bottom and near-surface sources are modeled. Near-surface sources are

needed if overflow operations are performed. Clamshell dredges release material continuously as

the clamshell is pulled through the water column. Thus, the vertical distribution of suspended

sediment released by a clamshell dredge extends over the entire water column. In addition, since

overflow operations can occur with the placement of material into a barge using a clamshell dredge,

a near-surface source is also implemented for clamshell dredges. A detailed discussion of the

sediment sources in SSFATE is provided in Johnson and Parchure (1999).

Simulation durations with SSFATE are not anticipated to be greater than a day or so. Thus, although

the sources for cutterhead and clamshell dredges can move during the day, the greatest movement

of the sediment source will occur with a hopper dredge. To account for this movement, the user

specifies a line along which dredging takes place at a specified rate. When the hoppers are full, the

simulated dredge moves to the placement site and releases the material. When the dredge returns

to the dredging site, a new dredging line is specified. This procedure continues until the simulation

is completed.

COMPUTATIONAL MODEL: Depending on the resolution of the numerical grid employed,

SSFATE can make predictions very near dredging operations; however, the processes modeled are

primarily far field processes in which the mean transport and turbulence associated with ambient

currents dominate. Transport and dispersion of suspended material from a sediment source are

predicted by a particle-based model using a random walk procedure.

The following basic equations determine the location of each particle at the next time-step in the

simulation:

X n+1 = X n + DX

(1)

Y n+1 = Y n + ∆Y

(2)

Z n + 1 = Z n + ∆Z

(3)

where

∆X = U∆T + Lx

(4)

∆Y = V∆T + Ly

(5)

∆Z = Wsi ∆T + Lz

(6)

and

X,Y,Z

= location of particle in the x-, y-, and vertical directions, respectively

U,V

= mean ambient velocity in the x-, and y-directions, respectively