Characterize contaminated sediment (2)

characterization and placement of the contaminated material, aspects re-

lated to the characterization and placement of the capping material, and as-

pects related to the capping site under consideration. Each of these

aspects must be initially examined in a parallel fashion (see Blocks 2, 3,

and 4 of Figure 2). Further, the interrelationship and compatibility of

these three aspects of the design are critical.

Characterize contaminated sediment (2)

The contaminated sediment must be characterized from physical, chemi-

cal, and biological standpoints. Physical characteristics are of importance

in determining the behavior of the material during and following place-

ment at a capping site. In situ volume (to be dredged), in situ density (or

water content), shear strength, compressibility, and grain-size distribution

are needed for evaluations of dispersion and spread during placement,

mounding characteristics, consolidation, and long-term stability and resis-

tance to erosion. These data should be developed using standard techniques.

Some chemical and biological characterization of the contaminated

sediment is normally performed as a part of the overall evaluation for suit-

ability for open-water placement. Guidance on characterization of con-

taminated sediments is found in Chapter 3.

Select a potential capping site (3)

The selection of a potential site for capping is subject to the same con-

straints and tradeoffs as any other open-water placement site. The major

considerations in site selection include bathymetry, bottom slopes, cur-

rents, water depths, water column density stratification, erosion/accretion

trends, proximity to navigation channels and anchorages, bottom-sediment

characteristics, and operational requirements such as distance to the site

and wave climate. However, in addition to normal considerations, the cap-

ping site should ideally be in a relatively low-energy environment with

little potential for erosion or disturbance of the cap. While capping at a

low-energy site is desirable, such sites are not always available. Higher

energy sites can be considered for dredged material capping, but a de-

tailed study of erosion potential is required; increases in cap thickness to

account for potential erosion or use of a coarser grain-size material may

be required.

Consideration should be given to the following factors during selection

of a potential capping site. Bathymetry forming a natural depression will

tend to confine the material, resulting in a CAD project. Placement of ma-

terial on steep bottom slopes should generally be avoided for a capping

project. Water column currents affect the degree of dispersion during

placement and the location of the mound with respect to the point of dis-

charge. Of more importance are the bottom currents, which could poten-

tially cause resuspension and erosion of the mound and cap. The effects

of storm-induced waves on bottom-current velocities must be considered.

For some sites, other processes such as prop wash may need to be consid-

ered. The deeper the water is at the site, the greater the potential is for

Chapter 2 Design/Management Sequence for Capping