12 December 2001
ECONOMIC FACTORS. The economic factors affecting the dredging of
Navy harbors are the following.
Amount of Material to be Dredged
. The mobilization and demobilization
costs will constitute a significant portion of the total project cost for small-volume
dredging projects. For large-volume dredging projects, the mobilization and
demobilization costs will only increase the cost per cubic yard by a relatively small
Distance From the Dredging Site to the Disposal Site. This distance
depends on the availability of disposal sites, the volume, and the environmental mental
quality of the dredged material. If the sediment is contaminated, regulatory agencies
may require dumping at a "contained" land disposal site. In many areas these sites are
limited. Ocean disposal sites are attractive alternatives because of their unlimited
capacity and general proximity to Navy harbors. In either case additional costs and time
delays may be incurred because the dredged material must be proven environmentally
clean prior to issuance of a dredging permit. Regardless of where the material is
dumped, cost is a function of distance to the disposal site and mode of transport.
Environmental Considerations. Some form of environmental
documentation is required for every dredging project, which can add substantially to
project costs. The minimum requirement is a Preliminary Environmental Assessment.
Additional chemical or biological testing may be required to supplement this
documentation. If ocean disposal is proposed, it is likely that bioassays will be required
at an additional cost. Most costly of all are environmental surveys of the dredge site
and the disposal site which may be required in environmentally sensitive areas or cases
of critical contamination. It is recommended to involve the local environmental office in
the planning discussions from the inception of the project.
New Work Versus Maintenance Dredging. Where an area has not
been dredged before, the bottom sediments may be consolidated and difficult to
dredge. The added time required to dredge new material may incur additional costs.
Other Factors. Other factors include the cost of fuel, competition
between private and public dredgers, and the configuration and use of the naval harbor
to be dredged. Refer to either Chapter 8 of The Handbook of Dredging Engineering
(Herbich, 1992) for an Economic Analysis for Disposal or Chapter 10 in Dredging: A
Handbook for Engineers (Bray et al., 1997) for a discussion of Dredging Costs and
GEOTECHNICAL FACTORS. Soil investigations and testing techniques
are generally similar to terrestrial procedures except that trial dredging may be resorted
to where conditions for a particular type of dredge are marginal. Soil classifications
generally refer to the PIANC adopted system.
Sediment Analysis. Sediment samples from the dredge area should be
obtained and analyzed.