12 December 2001
Dredge. (1) To excavate or move soil or rock underwater. (2) A vessel, or item of
floating plant, equipped with means to move or excavate soil or rock underwater.
Hopper. A funnel-shaped chamber in which materials are stored temporarily and later
discharged through the bottom.
CURRENT DREDGING DESIGN PRACTICE. Dredging practice and
plant equipment is determined based on whether one is dealing with river dredging,
estuary or coastal dredging, ocean dredging, beach replenishment, etc. Preliminary
investigations must first be conducted to provide data to confirm that dredging will be
necessary and that environmental regulations can be satisfied. A preliminary study will
provide estimates of the sedimentation regimes, disposal areas, and the type of
dredging plant that will be employed.
Navy Harbors. The dredging of Navy harbors may involve the dredging
of clay and silt from estuarine harbors or the dredging of sand from harbors on open
coasts. By 1980, 87 percent of the Navy's total annual maintenance-dredging volume
consisted of cohesive sediments, while 13 percent consisted of sand. A large part of
the total dredging in naval harbors consists of removing shoaled material from under
berthing piers. Other dredging activities include dredging of navigation channels and
turning basins, as well as channel-entrance bypassing.
Dredging Research Program. The USACE Dredging Research Program
(DRP) conducts studies to develop up-to-date information on dredging technology. The
results of these studies are compiled in technical reports that may be obtained from
WES. Additional information can be found on the USACE web site.
. Extensive discussion
of current practices, including agitation
dredging, barrier curtains, water injection dredging, sediment bypassing, and jet scour
arrays, can be found in Chapter 7 of Dredging: A Handbook for Engineers (Bray et al.,
1997) and the Handbook of Dredging Engineering (Herbich, 1992).
PROJECT DEPTH. According to the Permanent International Association
of Navigational Congresses (PIANC), the required dredge depth for ship channels is
determined by summing up the following components:
Reference Water level minus lowest low water level
Admissible ship draft, including trim and density change allowances
Vertical ship motions, including squat, pitch and roll
Net required under-keel clearance
Depth sounding precision
Sediment deposition thickness between scheduled dredgings