Overview of MDFATE (Cont.)

accounted for as well as sloping or depression disposal areas. Differences in

material composition can be considered, and layering of different materials in

the hopper can be modeled also. Based on material properties, currents, etc.,

stripping of fines is accounted for, and an estimate of how the material accumu-

lates on the seafloor is provided. STFATE output consists of plots of mound

footprint coverage and thickness of bottom accumulation. MDFATE modifies

the existing bathymetric grid according to the STFATE-predicted mound foot-

print and bottom thickness. Subsequent STFATE outputs are appended to the

grid, thus creating a composite mound.

For the week-long simulations, LTFATE models the long-term processes

affecting the created composite mound. The processes modeled include morpho-

logical changes resulting from cohesive and noncohesive sediment erosion,

noncohesive sediment avalanching, and cohesive sediment consolidation. For

the sediment erosion processes, LTFATE requires input from hydrodynamic

databases for tides and waves. The tidal current time-series is generated from

user-specified tidal constituents for the site of interest by the program TIDE.

Wave statistics from the Wave Information Study (WIS) are used (provided by

the user for the site of interest) by the program HPDSIM to generate a wave

time-series and ultimately wave-induced currents. The net resulting tidal and

wave currents are then used to drive the sediment transport portion of the model.

These two routines are also used by the STFATE model within MDFATE to

generate the water column currents that affect material settling for the short-term

processes.

A summary of the noncohesive and cohesive sediment transport algorithms

used by MDFATE can be mound in the description of LTFATE (Appendix F).

The avalanching routine applied in LTFATE is based on a routine developed

by Larson and Kraus (1989), who adapted the work of Allen (1970) on slope

failure. Allen's (1970) experiments showed that two limiting slopes occurred,

angle of initial yield and the residual angle after shearing, which were influenced

by the particle deposition-rate gradient, particle concentration at the time of

deposition, and particle size and density. Allen (1970) examined the effect of a

larger deposition rate at the top of a slope versus the toe of a slope, which in

effect produced a steepening by rotating the slope around the toe. When the

slope becomes unstable, it avalanches, and a new more stable slope is formed.

To account for consolidation of cohesive sediment, the procedure developed

by Poindexter-Rollings (1990) for predicting the behavior of a subaqueous

sediment mound was followed. The consolidation calculations used by

Poindexter-Rollings (1990) and used in LTFATE were based on finite strain

theory introduced by Gibson, England, and Hussey (1967). Numerical solutions

were developed by Cargill (1982, 1985). Finite strain theory is well-suited for

the prediction of consolidation in cases of thick deposits of fine-grained sedi-

ments because it provides for the effect of self-weight, permeability that varies

with void ratio, nonlinear void ratio-effective stress relationship, and large

strains (Scheffner et al. 1995).

Appendix E MDFATE Model