Erosion

monitoring data to differentiate between changes in surface elevation due

to consolidation as opposed to those potentially due to erosion. It is im-

portant to note that the total mound height for an LBC project or fill

height for a CAD project can decrease (due to consolidation of the con-

taminated layer or underlying native sediment) without the need to nour-

ish the cap.

The consolidation analysis also holds importance for any required as-

sessment of potential long-term flux of contaminants through the cap.

The magnitude of consolidation of underlying sediments will determine

the amount of water potentially moving (advecting) upward into the cap.

Changes in the void ratio of the cap must also be considered in determin-

ing the distance to which this water is expressed upward into the cap.

If the selected material for the cap is fine-grained material (defined as

material with more than 50 percent by weight passing a #200 sieve), the

change in thickness of the material due to its own self-weight or due to

other cap components should be considered in the overall design of the

cap thickness. An evaluation of cap consolidation should be made in this

case, and an additional cap thickness component for consolidation, Tc,

should be added so that the appropriate cap thickness is maintained. Such

consolidation occurs over a period of time following cap placement, but

does not occur more than once.

If the cap material is not a fine-grained material, no consolidation of

the cap may be assumed, and no additional increase in the isolation thick-

ness is necessary. However, consolidation of the underlying contaminated

sediments may occur, and a consolidation analysis may be necessary to

properly interpret monitoring data. Procedures for evaluation of consolida-

tion are given in Chapter 8 and Appendix I.

Erosion

If there is potential for erosion, the total cap thickness should include a

thickness component for erosion, Te, which may occur primarily due to

long-term continuous processes (i.e., tidal currents and normal wave activ-

ity) or episodic events such as storms. This portion of the total thickness

can be lost after many years of normal levels of wave and current activity,

after an abnormally severe storm season, or in a few days during extreme

events. Monitoring activities should result in detecting the loss of cap fol-

lowed by a management decision to place additional material to bring the

cap back to its design thickness.

A screening level assessment of erosion potential should first be con-

ducted. This assessment may be conducted as a part of the site screening

process described in Chapter 4. This assessment can be based on simple

analytical or empirical methods. If the screening assessment indicates lit-

tle or no potential for erosion, no detailed assessment need be conducted,

and no erosion cap thickness component is needed. If the screening as-

sessment indicates a potential for erosion, a more detailed assessment

should be conducted. If the contaminated material is to be hydraulically

placed (as for a CAD site) or a site with higher energy potential is being

Chapter 7 Dredged Material Cap Design