Spreading over large areas (Cont.)

selected for the split-hull hopper dredge Dodge Island, which started the

capping operation with the goal of quickly covering the contaminated

mound with 15 cm (6 in.) of sand cap. Variations in the vessel's track line

down the lane were expected to spread the material evenly over the area.

Sediment profile image (SPI) profiles (see Chapter 9) at a spacing of a

5-m run perpendicular to the lanes conducted after a few passes had been

made showed no area without sand and most areas to have a 15-cm (6-in.)-

thick cover, apparently confirming the model predictions. Lanes 75 m

(250 ft) wide were selected for the hopper barge Long Island. This value

is about equal to the width at 0.5 of the maximum thickness. The majority

of the cap was placed with the Long Island. See Chapter 10 for additional

details on this project.

Several factors have to be considered when using disposal lanes for cap

placement. Hopper dredges have superior seakeeping abilities compared

with towed barges and thus will be better suited to open-ocean placement.

Towed barges for lane disposal probably should be restricted to protected

areas. When the cracked-hull technique is used, once the hull is cracked it

cannot be closed until the vessel is empty. Thus, when the vessel reaches

the end of a line, it continues to discharge cap material while turning. So,

to reduce the spread of cap material beyond the contaminated footprint,

the vessel should turn before reaching the edge of the contaminated mate-

rial. It is likely more effective to cap the outer edge of a contaminated

mound using a series of straight segments around the perimeter of the foot-

print. Also, while a vessel that is using direct pump-out to discharge mate-

rial can stop the pump during turns, the dredge operators would much

prefer to keeping pumping. Thus, similar considerations will have to be

made regarding where the turn is conducted.

Turning radius is another factor that needs to be considered for cap

placement using disposal lanes. Modern hopper dredges have bow thrus-

ters and can turn in less than their own length; therefore, they can often

proceed down adjacent disposal lanes. Older hopper barges and less ma-

neuverable hopper dredges have larger turning radii and therefore may

only be able to cap every 2nd or 3rd disposal lane. This is not a problem,

but requires more accurate record keeping to confirm no lanes are missed.

The decision on how the dredge or barge is operated, i.e., adjacent lanes,

or every 2nd, 3rd, 4th lane, etc., should be made in consultation with the

operator. Keeping a record of track plots is highly recommended. In pro-

tected waters, a 1,000-m3 towed hopper barge needs about 120 m to turn

while maintaining speed and control (Parry 1994). Because of individual

variations between vessels, it is prudent to consult with the vessel opera-

tors early on in the process to obtain the best estimates of sea-keeping

abilities turning radii, etc.

How long it takes to discharge the capped material is another factor to

be considered for cap "sprinkling." When the Dodge Island cracked its

hull 0.3 m (1 ft) during the Port Newark/Elizabeth project, the 2,000-m3

(2,600-yd3) load of sand exited in 20 to 30 min, translating to a rate of

65 to 100 m3/min. During direct pump-out, the Long Island emptied its

roughly 9,600-m3 load in 2 to 3 hr, translating to a discharge rate of 53 to

89 m3/min. Hopper dredges can use their water cannons to produce rea-

sonably continuous discharge rates. In fact, they can turn off their water

cannons to reduce the discharge rate during turns.

Chapter 5 Equipment and Placement Techniques