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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.
24
Chapter 4 Site Selection Considerations for Capping

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