Use of multiple disposal points or lanes

impact velocity on the bottom, which may displace previously placed mate-

rial (Parry 1994). Additional discussion of this phenomena is provided

later in this section. Bridging of sand over the hull opening is typically

much less of a problem in modern hopper dredges that have water cannons

in the hoppers to help fluidize the sand.

Barge towing and positioning are generally a factor of weather condi-

tions. In good weather, barges may be transported and positioned with a

tug directly alongside. This allows for more precise dump positioning.

Also, if the barge is under tow, the line length may be as short as 30 or

45 m with lateral offsets on the order of one barge width. In poor weather,

the tow length may be increased to 175 to 300 m where lateral offsets may

be several barge widths.

For even placement of material around a point, vessel approach head-

ings should be varied. Vessel operators generally prefer to approach the

disposal site from the direction of travel to the site because that direction

affords the shortest time to travel and dispose. However, continuous

dumping along one transect may concentrate material in a manner or loca-

tion that is less than ideal for the capping project. When weather permits,

approach direction should be specified so that the most even coverage of

dredged material can be accomplished. But, for poorer weather conditions,

operators should be afforded the flexibility to approach the placement area

from the safest direction based on the prevailing winds and waves at that

time.

Use of multiple disposal points or lanes

For large projects (say 100,000 to 200,000 m3 or more) in shallow

water (say 20 m and less), point dumping of contaminated material at a

single location may create a mound unacceptably tall. To avoid this,

placement can be divided among multiple buoy locations to create a larger

(footprint) but less thick mound. This was done for the 1993 New Haven

Harbor Project (Fredette 1994). The other option is to place material

along a line or in lanes. For example, the 1993 Port Newark/Elizabeth

project had an EPA Region II restriction not to have the capped mound ex-

tend above the 23-m (75-ft) depth contour. Because the existing depth aver-

aged about 25 m (83 ft), point dumping the 448,000 m3 (586,000 yd3) of

contaminated dredged material would have created a mound extending

well above the 23-m depth restriction. To keep the mound elevation be-

low the limit, a triangular mound was designed, with three lanes with a

width of 150 m (500 ft) wide by 350 to 450 m (1,150 to 1,480 ft) long (see

additional discussion in Chapters 6 and 10). To assist the contractor in sit-

ing the placements, each apex of the triangle had taut-moored buoys. To

reduce the chance of placing material outside the lanes, the contractor was

directed to dispose of all material within 60 m (200 ft) of an imaginary

line connecting the apex buoys. Additional details on this project can be

found in Chapter 10.

For capping projects, both point dumping and spreading material over

specific lanes have been used, sometimes both on the same project. For

small projects (say 25,000 m 3 or less) where the contaminated sediment

Chapter 5 Equipment and Placement Techniques