Typical Contaminated Mound Geometry

For LBC sites, the geometry of the contaminated material mound de-

pends on the physical characteristics of the material (grain size and cohe-

sion) and the placement technique used (hydraulic placement will result in

greater spread than mechanical placement). Assuming that the material

from multiple barge loads or pipeline can be accurately placed at a single

point, the angle of repose taken by the material and the total volume

placed will dictate the mound spread.

However, few data are available on the volume changes resulting from

entrainment of water during open-water placement or the shear strengths

of dredged material initially deposited in open-water sites. For these rea-

sons, a priori estimates of mound spread made to date have been made

based on the observed characteristics of previous mounds created with

similar placement techniques and similar sediments (Palermo et al. 1989).

Models have been developed that will account for the development of

mounds due to a number of barge or hopper discharges (Moritz and Ran-

dall 1995; SAIC 1994). The Corps' mound building model that models

Multiple Disposals from barges and hopper dredges and their FATE

(MDFATE) is a modification of the STFATE model. In the MDFATE

model, a streamlined version of the STFATE model is run for each barge

disposal. Thus, the input requirements for MDFATE are similar to those

for STFATE. In MDFATE, the program keeps track of the mound thickness

in each grid cell, then algebraically adds the thickness from subsequent

disposals with avalanching when mound steepness exceeds critical values.

MDFATE allows a number of typical disposal patterns to be automated; it

allows moving barges and can import actual site bathymetry in real-world

coordinates. MDFATE also allows interaction with the LTFATE model

(Scheffner et al. 1995). This allows the mound created in MDFATE to be

eroded by waves and currents during mound creations that may last months.

A more detailed description of MDFATE can be found in Appendix E, and

a more detailed description of LTFATE can be found in Appendix F.

Similar to the output from STFATE, output from the MDFATE model

includes the volume of material on the bottom and contour and cross-

section plots of mound bathymetry. Figures 14 and 15 show typical

MDFATE output. One limitation of MDFATE is that it has been verified

on only one actual project to date (Moritz and Randall 1995).

A model developed for the New England Division Disposal Area Moni-

toring System, the DAMOS capping model (Wiley 1994), is also based on

the STFATE model. While it does not consider moving vessels or erosion

by waves and currents, it has the advantage of having been verified for a

number of mounds constructed by the New England Division in Long Is-

land Sound.

Typical Contaminated Mound Geometry

As noted in the previous chapter, for LBC projects, virtually all of the

mounds created have been constructed using mechanical dredging with

transportation and placement by bottom-dump barges. The resulting

Chapter 6 Sediment Dispersion and Mound Development and Site Geometry During Placement