Chemical isolation

structures, to such a thickness that the lower portion of the ice gouges and

displaces the bottom sediments. The thickness of the ice buildup de-

creases as distance from shore increases. Also as water depth increases,

ice gouging will be less of a concern. For those locations where ice goug-

ing may be problem, e.g., in situ capping sites nearshore, local experts

should be consulted as to the locations where ice gouging occurs and the

depth of the sediments disturbed.

Another operational concern is anchoring. Vessel anchors have the po-

tential to disturb bottom sediments (as do trawlers). While most any loca-

tion in shallow water (say 30 m or less) is subject to potential anchoring,

for most locations where open-water dredged material placement sites are

located, anchoring to such a degree that cap integrity is impacted will be

extremely rare. The anchors used by recreational vessels typically only

penetrate the bottom 1 to 2 ft. The relative area impacted by anchors com-

pared with the size of a cap is very small. Also, when the anchors are re-

moved, the area disturbed by the anchor is quickly filled. This is not true

for anchors from large ships, which can penetrate up to 5 to 10 ft. Thus

an area where ships routinely anchor would be a very poor choice for a

capping project.

Another operational concern is the ability to place a relatively thin cap

layer. Until recently, open-ocean capping operations made the controlled

placing of small thicknesses (less than 30 cm) difficult. For many of

those projects, the minimum cap thickness for most projects has been on

the order of 75 to 120 cm (2.5 to 4 ft). Recent experience from the Port

Newark/Elizabeth project at the Mud Dump (Randall, Clausner, and

Johnson 1994) and Puget Sound capping projects (Nelson, Vanderheiden,

and Schuldt 1994; Sumeri 1995) has shown that the sprinkling techniques

developed were successful and that layers about 15 to 20 cm (0.5 to 0.75 ft)

thick can be placed with reasonable assurance (though at increased cost

due to increased operational controls).

The placement process will likely result in some unevenness of the cap

thickness. This unevenness should be considered in calculation of the vol-

ume of capping material required.

If any of the above factors are significant for the site under considera-

tion, an additional cap thickness component for operational concerns, To,

should be added to the design cap thickness.

Chemical isolation

If a design function of the cap is to control contaminant flux, the poten-

tial for short-term and long-term flux through the cap should be evaluated.

The need for such an evaluation is dependent on the types of contaminants,

the potential for contaminant impacts, site and operational conditions, and

other factors. For example, if the reason for capping is to isolate a sediment

that is nontoxic to benthic organisms and exhibits bioaccumulation only

marginally above that for a reference sediment, the isolation provided by

the bioturbation thickness component will likely provide sufficient con-

trol, and there is little reason to conduct a detailed assessment. Conversely,

Chapter 7 Dredged Material Cap Design