Average Water Depths

time periods are in the range of design periods for many engineering

structures. Note that erosion at localized portions of the mound or feature

greater than 1 ft would be allowed using these screening criteria. The cor-

ners of a mound would normally have an overlap of capping material, and

the crest of a mound would normally have a greater cap thickness; there-

fore, somewhat larger erosion could be tolerated over these portions of a

mound. Selection of other values of erosion thickness or time periods

should be based on site-specific factors (e.g., the degree of contamination,

distance to other resources), the level of confidence in the calculations,

and the level of risk acceptable to the parties involved.

For projects in which subsequent material placement and/or capping is

planned or for which materials for cap nourishment can be easily ob-

tained, higher erosion rates or shorter return periods for episodic events

may be considered as a criterion for purposes of site screening. In areas

where available capping materials are scarce and current and wave condi-

tions are severe, a coarse-grained layer of material (coarse sand, gravel, or

larger size materials) may be incorporated into the cap design to provide

protection against erosive currents at the site. Detailed guidance on evalu-

ation of erosion is found in Chapter 8 and Appendixes F and G.

Average Water Depths

Case studies have indicated that water depth is of particular interest in

evaluating the potential suitability of a site for capping operations (Palermo

1989). The deepest water depth for which a capping project has been exe-

cuted (as of 1995) is approximately 100 ft. However, definable dredged

material mounds have been created in water depths exceeding 400 ft

(Wiley 1995). Greater water depths generally provide more stable bottom

conditions with less potential for erosion. However, the greater the aver-

age water depth is at the site, the greater the potential is for water entrain-

ment and dispersion during placement. The expense and difficulty in

monitoring is also increased with a greater water depth.

As water depth increases, both the contaminated and clean material

must descend through a greater water column depth. More material is re-

leased to the water column during placement as compared with shallower

water placement, all other factors being equal. Therefore, the fraction of

the contaminated material that is not finally capped is greater.

Entrainment of ambient water causes the descending material to be-

come more buoyant; therefore, the effect of density stratification in the

water column needs to be evaluated. Although density stratification in the

water column may be encountered at some deep-water sites, stratification

is not likely to prevent the descent of the dredged material mass during

placement. The very cohesive fraction of mechanically dredged material

(clods or clumps) attains terminal speed quickly after release from a barge

and does not accelerate further with depth.

Chapter 4 Site Selection Considerations for Capping