Evaluate stability, erosion, and consolidation (12,13)

Evaluate stability, erosion, and consolidation (12,13)

The deposit of contaminated dredged material must also be stable

against excessive erosion and resuspension of material before placement

of the cap. The cap material must be stable against long-term erosion for

the required cap thickness to be maintained. The potential for resuspen-

sion and erosion is dependent on bottom current velocity, potential for

wave-induced currents, sediment particle size, and sediment cohesion.

Site selection criteria as described above normally results in a site with

low bottom-current velocity and little potential for erosion. However, if

the material is hydraulically placed (as for a CAD site) or a site with

higher energy potential is considered, a thorough analysis of the potential

for resuspension and erosion must be performed, to include frequency con-

siderations. Conventional methods for analysis of sediment transport can

be used to evaluate erosion potential. These methods can range from sim-

ple analytical techniques to numerical modeling.

Consolidation of contaminated material needs to be examined for its ef-

fect on LBC mound slopes and volumes and on the volume occupied

within CAD sites. In general, consolidation of the contaminated dredged

material will result in more stable conditions. The same is true for con-

solidation of the cap material. However, consolidation of the cap results

in a reduced cap thickness. Therefore, the potential for cap consolidation

must be accounted for in the overall design of the cap thickness.

If the potential for erosion and consolidation of either the contaminated

material or cap is unacceptable, an alternative site (3), alternative capping

sediment (4), or alternative placement techniques (5,6) can be considered.

Guidance on evaluating long-term cap stability is found in Chapter 8 and

Appendixes F, G, and I.

Develop a monitoring program (14)

A monitoring program or site monitoring plan is required as a part of

any capping project design. The main objectives of monitoring normally

are to ensure that the contaminated sediment is placed as intended and with

acceptably low levels of contaminant release, the cap is placed as intended

and the required capping thickness is maintained, and the cap is effective

in isolating the contaminated material from the environment. Monitoring

plans for capping projects need to include a more intensive effort during

and shortly after placement operations and immediately after unusual

events (e.g., severe storms), with a declining level of effort in future years

if no adverse effects are detected. Physical, chemical, and biological ele-

ments may be included in a monitoring plan. In all cases, the objectives

of the monitoring effort and any remedial actions to be considered as a re-

sult of the monitoring must be clearly defined as a part of the overall pro-

ject design. Guidance on monitoring considerations for capping is found

in Chapter 9. Case studies of capping projects including conclusions

drawn from field monitoring efforts are described in Chapter 10.

Chapter 2 Design/Management Sequence for Capping