Select and characterize capping sediment (4)

water entrainment and dispersion during placement. However, deeper

water depths also generally provide more stable conditions on the bottom

with less potential for erosion. Numerical models for prediction of water

column behavior, mound development, and long-term stability against ero-

sion may be used in evaluating site conditions. Guidance on site selection

for capping is found in Chapter 4.

Select and characterize capping sediment (4)

The cap sediment used in a project should be carefully selected. How-

ever, for economic reasons, a capping sediment is usually taken from an

area that also requires dredging or is considered advanced maintenance

dredging. If this is the case, there may be a choice between projects.

Scheduling of the dredging is also an important consideration. In other

cases, removal of bottom sediments from areas adjacent to the capping

site may be considered.

The capping sediment is characterized as described above for the con-

taminated sediment. However, the capping sediment must be one that is

suitable for open-water placement (i.e., a clean sediment). The evaluation

of a potential capping sediment for open-water placement acceptability

must be accomplished using appropriate techniques under either CWA or

MPRSA. Physical characteristics of the capping sediment are also of par-

ticular interest in capping design. Density (or water content), grain-size

distribution, and cohesiveness of the capping sediment must be evaluated.

Selection of the capping sediment should be carefully considered because

the capping material must be compatible with the contaminated sediment

and this compatibility is related to dredging and placement equipment and

techniques. Previous studies have shown that both fine-grained materials

and sandy materials can be effective capping materials. Guidance on se-

lecting and characterizing capping sediment is found in Chapter 3.

Select equipment and placement technique for contaminated

sediment (5)

A variety of equipment types and placement techniques have been

used for capping projects. The important factors in the placement of con-

taminated material are reducing water column dispersion and bottom

spread to the greatest possible extent. This minimizes the release of con-

taminants during placement and provides for easier capping. For LBC the

dredging equipment and placement technique for contaminated sediment

must provide a tight, compact mound. This is most easily accomplished

with mechanical dredging and barge release (point dumping). If CAD is

under consideration, hydraulic placement of the contaminated material

may be acceptable.

Specialized equipment and placement techniques can also be consid-

ered to increase control during placement and reduce potential dispersion

and spread of contaminated material. These might include use of sub-

merged diffusers or submerged discharge points for hydraulic pipeline

placement, hopper dredge pump-down with diffuser, or gravity-fed tremie

Chapter 2 Design/Management Sequence for Capping