SUMMARY - doern30005

Technical Note DOER-N3

March 1998

relatively higher mob/demob costs because those dredges require tugs to move, support vessels to

transport pipe or barges, and in some cases, land-based equipment for reworking of the placed

dredged material. Hopper dredges placing dredged material in open water typically have lower

mob/demob costs because a hopper dredge is an oceangoing vessel requiring less support.

However, if a hopper dredge is placing material via a direct pump-out buoy, mob/demob costs could

be comparable with mechanical and pipeline dredges.

Hopper dredges placing material nearshore by bottom discharge can help minimize overall dredging

project costs in two ways: (a) the lower mob/demob cost becomes more attractive (relative to other

dredge types); and (b) shorter haul distance to the nearshore placement site saves time and fuel

costs. Pipeline dredges can also reduce overall dredging project costs by placing material nearshore.

A pipeline dredge discharging into the nearshore reduces its own mob/demob costs by eliminating

those factors associated with land-based equipment, and the shorter pumping distance reduces

operating time and fuel consumption.

Limitations that must be considered are that some hopper dredges may be unable to access desired

shallow-water placement sites because of draft limitations. Alternatives include "light-loading,"

which reduces the vessel draft, and speciality shallow-draft hopper dredges. Pipeline discharging

into the nearshore may also be limited because of the more energetic hydrodynamic environment

closer to shore, which can create large forces on the pipeline.

Ultimately, projects considering nearshore placement of dredged material must optimize costs by

closely examining the dredging requirement, cost of available equipment specific to the project of

interest, type of nearshore feature desired, and volume of material to be dredged. In some cases,

the benefitting parties may have to contribute larger portions of the dredging and placement costs

to ensure that the desired feature is constructed with appropriate equipment if the selected equipment

or method is more expensive than the least-cost method.

SUMMARY: Planning and designing a dredging project with nearshore placement alternatives

first require knowledge of the physical and ecological environment. Engineering decisions can

then be made to determine potential placement alternatives that, in turn, provide a basis for decisions

driven by economic factors. The results of such decisions may provide a beneficial, economically

feasible nearshore placement alternative.

Under the DOER Program, investigations will be conducted in the Evaluation and Design of

Nearshore Placement of Mixed Sediments Work Unit to enhance opportunities for nearshore

placement of mixed sediments. Additionally, other work units in the Nearshore and Offshore

Placement Focus Area will include physical model investigations of mixed sediment movement,

improvement of existing numerical models determining the fate of dredged material, and environ-

mental effects of material placed in the nearshore. Benefits from this research will include improved

use of maintenance dredged material for shoreline stabilization, increased acceptance by regulatory

agencies of nearshore placement as a viable long-term disposal alternative, and lower costs for

overall maintenance dredging operations.

POINTS OF CONTACT: For additional information contact the authors, Mr. Gregory L. Wil-

liams (601-634-2089, williag3@ex1.wes.army.mil), and Ms. Terri L. Prickett (601-634-2337,