Operational concerns

considered, a thorough analysis of the potential for resuspension and ero-

sion must be performed, to include frequency considerations.

Based on the detailed assessment, a value of Te should be added as the

erosion cap thickness component. The criteria used to calculate the thick-

ness to be added are equivalent to that used for the site screening dis-

cussed in Chapter 4. For projects in which no subsequent capping is

anticipated for a long time period (several decades or longer) or for which

materials for cap nourishment are not easily obtained, the recommended

cap thickness component to be added, Te, should be equivalent to the cal-

culated net cap erosion over the major portion of the mound over a period

of 20 years of normal current/wave energies or for a 100-year extreme

event. The 20-year ambient time interval and 100-year return interval for

storms are based on field experience gained to date. Twenty and one hun-

dred years as time periods are in the range of design periods for many en-

gineering structures. Note that calculated erosion at localized portions of

the mound or feature may be somewhat greater than the value of Te se-

lected. The corners of a mound would normally have an overlap of cap-

ping material, and the crest of a mound would normally have a greater cap

thickness; therefore, somewhat larger erosion could be tolerated over

these portions of a mound.

Selection of other values of ambient time periods, return intervals, etc.,

for calculating erosion thickness 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 acceptable level of risk. For pro-

jects in which subsequent capping is planned or for which materials for

cap nourishment can be easily obtained, higher erosion rates may be con-

sidered. In areas where available capping materials and current and wave

conditions are severe, a coarse-grained layer of material may be incorpo-

rated into the cap design to provide protection against erosive currents at

the site.

Selecting a cap thickness component for erosion is a function of the ac-

ceptable level of risk. Definitive guidance is difficult because the level of

risk acceptable will likely vary from project to project. Detailed guidance

on erosion thickness evaluation is found in Chapter 8, along with additional

discussion of the risk-related aspects associated with design cap thickness.

Operational concerns

At some locations, other considerations, termed operational, may have

to be considered when determining the final cap thickness. These include

ice gouging, anchoring, ability to place thin layers, unevenness of material

placement, etc. If these are serious considerations, then locations that

have significant potential for these types of operational considerations

would be poor choices for capping projects.

For most open-water disposal sites, the sites will be located sufficiently

far from shore and in sufficiently deep water that ice gouging should not

be a concern. Ice gouging is obviously only a problem in areas that re-

ceive significant amounts of ice in the winter (e.g., the Great Lakes). Ice

gouging occurs as ice thickness builds up, usually nearshore or adjacent to

Chapter 7 Dredged Material Cap Design