Navigation and Positioning Controls

Navigation and Positioning Controls

Once the dredging equipment and placement techniques and potential

capping site have been selected, the needs for navigation and positioning

equipment and controls can be addressed. The objective here is to place

both the contaminated and capping materials (whether by the bargeload,

hopperload, or by pipeline) at the desired location in a consistently accu-

rate manner so that adequate coverage by the cap is attained.

Navigation (the science of getting vessels from place to place) and

positioning (accurately locating an object) are two of the most important

factors in designing and implementing a successful capping project. Accu-

rate positioning is necessary for any dredged material disposal operation

in open water to ensure the material is located within the appropriate dis-

posal site boundaries. For a capping project, contaminated material place-

ment requirements are similar, but may be more restrictive in that

placement of material within a specified radius, along a given linear tran-

sect, or similar location may be required. For the capping phase, materi-

als must be adequately placed to cover the previously placed

contaminated material. Therefore, knowing the precise navigation and po-

sitioning is of principal importance to allow proper capping.

For pipeline placement in shallow water, the desired positioning of the

pipeline discharge can be maintained with little difficulty. Accurate navi-

gation to the placement site and precise positioning during material place-

ment by bottom-dump barge or hopper dredge is more difficult, especially

for sites well offshore.

There exist a number of methods to position barges and hopper dredges

for placement of dredged and cap material. One of the most common is

placement near a taut-moored buoy. The other common methods are elec-

tronic positioning systems (EPS) including range-azimuth, LORAN-C

(low-frequency), microwave (high-frequency), and differential global posi-

tioning system (DGPS). Detailed guidance on all aspects of hydraulic sur-

veying to include these positioning methods is found in USACE Engineer

Manual 1110-2-1003, Hydrographic Surveying (USACE 1991). Estimated

positional accuracy for each of the electronic positioning systems is

shown in Table 4.

Taut-moored buoys

Taut-moored buoy positioning requires locating and placing a buoy an-

chored and moored in such a way as to minimize buoy movement during

placement operations. At USACE New England Division1 disposal sites

in 20- to 25-m depths, the taut-moored buoy has a watch circle diameter

of about 20 m. Positioning of dredged material placement equipment is

specified to occur within some distance of the buoy during disposal. Elec-

tronic placement errors are minimized with this method (except for initial

The New England Division has been changed to the New England District.

Chapter 5 Equipment and Placement Techniques