Operational Requirements

The increased water entrainment with deep-water placement may also

result in a greater spread of the more fluid material on the bottom, but en-

trainment reduces the overall potential energy at bottom impact. Field

studies indicate that the bottom surge does not spread at a faster rate than

that occurring in shallower depths, although because of additional entrain-

ment, the initial thickness of the surge increases as depth increases

(Bokuniewicz et al. 1978). Greater care in control of placement may there-

fore be required as water depth increases to develop a discrete mound of

contaminated material and adequate coverage of the mound with capping

material.

Comparison of predictive models for fate of placed material and field

monitoring of Puget Sound Dredge Disposal Analysis (PSDDA) sites in

Seattle's Elliott Bay and Everett's Port Gardner Bay show the high degree

of reliability of these models for prediction of mound footprint extent in

water depths of 300 to 400 ft (Wiley 1995). Also, the accuracy of available

electronic positioning equipment used during disposal is validated.

The use of a deep-water site for capping generally holds an advantage

over a shallower site from the standpoint of cap stability from erosive

forces. Deep water acts as a buffer to wave action, and the resulting wave-

induced currents from storm events are smaller than in shallow water.

Therefore, deep-water sites are usually quiescent, near bottom low-energy

environments that are better suited to capping from the standpoint of cap

stability, but this must be balanced against potential material loss during

placement. Generally, a greater water depth at a site has more favorable

influence on long-term cap stability than unfavorable influence on disper-

sion during the placement process (Truitt 1986b).

Operational Requirements

Among the operational criteria that need to be considered in evaluating

potential capping sites are site volumetric capacity, nearby obstructions or

structures, haul distances, bottom shear due to ship traffic (in addition to

natural currents), location of available cap material, potential use of bot-

tom drag fishing equipment, and ice influences. The effects of shipping

are especially important since bottom stresses due to anchoring, propeller

wash, and direct hull contact at shallow sites are typically of a greater

magnitude than the combined effects of waves and other currents (Truitt

1987a). Methods for calculating prop-wash velocities are available

(Palermo et al. 1996).

Chapter 4 Site Selection Considerations for Capping