Mound Geometry for Level-Bottom Capping (Cont.)

c. Predict contaminated mound geometry. An accurate prediction of

contaminated mound geometry is one of the most critical steps in

LBC project design. There are two primary methods to determine

mound geometry ranging from fairly simple to complex. The sim-

ple method is to assume a basic shape (e.g., a truncated cone or rec-

tangular prism with sloping sides), then estimate side slopes and

an apron width. A spreadsheet is an effective method to test a

range of expected heights and crest dimensions on footprint dimen-

sions and the corresponding cap volume required. A more rigorous

method is to use a numerical model such as the MDFATE model

(Moritz 1994; Moritz and Randall 1995) to predict mound geome-

try. Use of a numerical model allows the user to investigate the

impact of changing operations (disposal pattern, barge size, barge

velocity, etc.) on mound geometry.

d. Is the calculated contaminated mound geometry suitable? After

the contaminated mound footprint and elevation have been calcu-

lated, the project manager/designer must decide if the predicted

contaminated mound geometry meets project needs. The two basic

concerns are as follows: Will all the contaminated material (and

cap material) stay within any surface area constraints? Is the eleva-

tion of the capped mound sufficiently low so as not to interfere

with navigation and not experience excessive erosion? A reason-

able buffer distance between the edge of the contaminated mound

and the site boundary is 100 to 200 m. If the answer to both ques-

tions is yes, then the designer can proceed to the next step, comput-

ing cap volume required (described in more detail in Chapter 7 and

Appendix H). If the contaminated mound is predicted to spread

too near or over the site boundary or is too high, then the following

options should be investigated.

e. Calculated contaminated mound footprint is too large. If the

contaminated mound footprint extends beyond the site boundary or

is so large that the cost or volume of cap material required is a

problem, several options are possible. Once again the simplest so-

lution (but probably unattractive from the project perspective) is to

reduce the volume of material being placed. One option to reduce

spread is to make the mound taller by reducing the size of the area

over which disposal takes place. The mound shape can be changed

to make better use of available space; e.g., for the 1993 Port Newark/

Elizabeth project conducted in New York District, a triangular-

shaped mound was used. Figure 17 shows the rectangular mound

dimensions in the original design and Figure 18 shows the triangu-

lar mound design modification. Other options include dredging

pits and/or placing confining berms around the area (essentially

creating a CAD) or using a diffuser to reduce spread. A opera-

tional change such as reducing the barge velocity, changing ap-

proach direction of the disposal vessels, or disposing only when

the currents are in a favorable direction are other possible options.

To evaluate such options will require using a numerical model.

Long-term planning can help to create a de facto CAD site. Over a

period of several years, the New England Division made a series

of small mounds around a portion of their Central Long Island

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