Bathymetry

capping, but a detailed study of erosion potential is required; increases in

cap thickness to account for potential erosion may be required. Monitor-

ing and maintenance costs may also be higher for higher energy sites.

Special consideration of site bathymetry, currents, water depths, bottom-

sediment characteristics, and operational requirements such as distance,

sea state, etc., are required in screening or selecting sites for capping

(Truitt 1987a; Truitt, Clausner, and McLellan 1989).

Bathymetry

Site bathymetry influences the degree of spread during placement of

both contaminated and capping material. The flatter the bottom slope, the

more desirable it is for LBC projects, especially if material is to be placed

by hopper dredge. If the bottom in a disposal area is not horizontal, a

component of the gravity force influences the energy balance of the bot-

tom surge (the lateral movement of the disposed material as it impacts sea

bottom) and density flows due to slope following impact of the discharge

with the bottom. It is difficult to estimate the effects of slope alone, since

bottom roughness plays an equally important role in the mechanics of the

spreading process. To date, LBC projects in which the material was me-

chanically dredged and released from a barge have been executed at sites

with slopes up to 1:60 (Science Applications International Corporation

(SAIC) 1995a) and in which material was placed by hopper dredge at sites

with slopes up to 1:225 (i.e., New York Mud Dump site). Placement of

material on steep bottom slopes (steeper than one degree 1:60) should gen-

erally be avoided for a capping project (Truitt 1987a). Bathymetry form-

ing a natural depression tends to confine the material, resulting in a CAD

project. This is the most desirable type of site bathymetry for a capping

project.

Currents

Water column currents affect the degree of dispersion during placement

and mound location with respect to the point of discharge. Of more impor-

tance are bottom currents, which could potentially cause resuspension and

erosion of the mound and cap. The effects of storm-induced waves on

bottom-current velocities must also be considered. Capping sites need

to have current and wave climate characteristics that result in long-term

stability of the capped mound or deposit.

Collection of basic current information is necessary at prospective dis-

posal sites to identify site-specific conditions. The principal influence of

currents in the receiving water during placement is to displace or offset

the point of impact of the descending jet of material with the bottom with

respect to the point of release (by a calculable amount). Water column cur-

rents need not be a serious impediment to accurate placement, nor do they

result in significantly greater dispersion during placement (though the offset

Chapter 4 Site Selection Considerations for Capping