Inner flan

Inner flank

At the edge of the main mound, the inner flank of the mounds slope

downward at a slope of approximately 1:35 to 1:70 with most of the

mound slopes between 1:35 and 1:50. For the Port Newark/Elizabeth

mound, the inner flank extended from the mound crest down to an elevation

of about 1.0 m above the preplacement bottom.

Outer flank

For the Port Newark project, a break in slope generally occurred at the

1.0-m elevation; the outerflank then sloped down to an elevation of about

0.30 to 0.15 m at a slope of about 1:115. Data from the New England Di-

vision projects have not been examined in sufficient detail to determine if

a similar feature exists for those mounds.

Apron

During the dynamic collapse phase (when the energy of the vertically

descending jet of material disposed from a barge or hopper dredge is con-

verted to horizontal velocity), some portion of the low shear strength, fine-

grained material with high water contents may be transported a considerable

distance from the disposal point. At the completion of the contaminated

material placement, an apron of fine-grained material, typically 1 to 15 cm

in thickness but extending up to several hundreds of meters beyond the

main mound flanks, has occurred on almost all LBC projects. The apron

has been defined as that portion of the material less than about 15 to 30 cm

in thickness, because 20 to 30 cm is the resolution limit for high-quality

bathymetry in water depths of 25 m or less.

A sediment profiling camera (SPC) can reliably measure apron thick-

ness from 1 to 2 cm up to 20 cm. Thus, the outer limit of the apron should

be defined as the point at which the apron can no longer be conclusively

distinguished by the SPC, a thickness of 1 to 2 cm. Some contaminated

material extends beyond the apron edge as defined by the 1- to 2-cm SPC

limit; however, the percentage of the total volume is likely extremely

small.

The apron typically exhibits an overall slope of 1v:1000+h at the Port

Newark/Elizabeth project, and overall apron slope of about 1:2,000 was

observed on downward sloping bottoms. If the inner edge of the apron is

assumed to be 15 cm in thickness, the width of the apron for the Port

Newark/Elizabeth project was about 300 m. The STFATE model and

MDFATE model and the DAMOS capping model can be used to predict

the apron dimensions.

Recent experience with a New York District 1997 capping project

placed in the Mud Dump site illustrated the potential for slope adjust-

ments when fine-grained mounds are created with heights exceeding about

10 ft. In one case, a portion of a contaminated mound with a height of

12 ft had a slope adjustment resulting in an after adjustment height of 6 to

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