Acceptability of design

volumes for capping. The requirement is to cap the deposit of contami-

nated material with the required thickness of capping material. The areal

extent of the contaminated material deposit and required cap thickness are

the key factors in calculating the volume of cap material. For example, if a

large volume of contaminated material were placed in a subaqueous de-

pression or pit (a CAD project), the deposit could be satisfactorily capped

with a relatively small volume of capping material. Additional considera-

tions on cap areas and volumes are provided in Appendix H.

Acceptability of design

Once the total cap thickness is determined, the calculations used to ar-

rive at each of the components should be reexamined and the acceptability

of the design evaluated. Some recalculations using an iterative process

may be necessary because total cap thickness influences the water depth

above the cap, which influences erosion potential, and total cap thickness

as placed influences the magnitude of consolidation of the cap. However,

in most cases, the calculations will not be overly sensitive to the overall

cap thickness, and recalculation of specific thickness components should

not be required.

The overall design of the cap should also be examined with respect to

acceptability from the operational, logistical, and economic perspectives.

If the total cap thickness is too large for effective placement, or the

needed volume of cap material is not available, or the anticipated cost of

capping too great, alternate sites or other disposal alternatives should be

considered.

Considerations for Intermediate Caps

Some capping projects could be designed in the context of anticipated

multiuse or multiuser applications. In such a case, one site (e.g., a

subaqueous borrow pit) could be selected for placement of contaminated

sediments from several projects. If several placements of contaminated

sediments are to be placed with such frequency that the site could not ef-

fectively recolonize, there would be no pathway for bioaccumulation or

benthic toxicity. Also, if the site is located in a sheltered area, or the en-

ergy from low-frequency events would not cause significant erosion, no

placement of cap material or placement of a intermediate cap with a lesser

thickness. That is, one that has a shorter return period level of erosion pro-

tection or less capabilities for chemical or biological isolation than the

full design cap could be considered. Determining an appropriate thick-

ness for an intermediate cap would require an evaluation of the same proc-

esses as described above, but the design parameters (especially those for

long-term flux, return periods for storms, etc.) should be selected to repre-

sent the time periods anticipated between dredged material and intermedi-

ate cap placement and final cap placement.

Chapter 7 Dredged Material Cap Design