INTEGRATING BIOLOGICAL AND DREDGING PROJECT DATA

ERDC TN-DOER-E9

May 2000

District1 can be used to estimate that a stationary point along a navigation channel would fall within

the boundaries of a 1,000-m suspended-sediment plume (although most dredging plumes are

typically less than 500 m) for approximately 2.8 to 4.3 days. The actual suspended-sediment

exposure experienced by sessile organisms may be highly variable depending on the hydrodynamic

conditions of the area near the dredging site. Tidal flushing may alleviate exposures to the plume

for periods of hours. Alternatively, a fluff zone may be established in which the settling of particles

is inhibited as suspended-sediment concentrations in the lower water column increase. This fluff

zone may continue to exist for a few weeks following the termination of the dredging project

(Wakeman, Peddicord, and Sustar 1975), but is less likely to persist in shallow waters adjacent to

the channel.

Hydraulic dredges mix large volumes of water with sediments to form a slurry that is either pumped

through a pipeline or into a hopper bin for discharge at a placement site or sidecast away from the

dredging site (Morton 1977). The rate of cutterhead rotation, vertical thickness of the dredge cut,

and swing rate of the dredge all affect turbidities generated by the dredging project (LaSalle 1990).

Based on total volume of dredged material produced annually, hydraulic cutterhead plants are the

most common dredging method used in the continental United States. Maximum concentrations

generally remain less than 500 mg/L and bottom suspended-sediment plumes are limited to within

500 m of the dredge (Havis 1988; LaSalle 1990). Advance rates for cutterhead dredges vary by

pipeline size and sediment type, but range from approximately 6 m/hr for 0.5- to 0.6-m (20- to

24-in.) pipeline dredges pumping sands to 18 m/hr for 0.7- to 0.8-m (27- to 30-in.) pipeline dredges

pumping silty material.1 Knowledge of dredge production and advance rates can be used to estimate

duration of plumes at a fixed point along a navigation channel. For example, hypothetical exposures

to suspended-sediment plumes generated by cutterhead dredging for sessile benthic organisms

immediately adjacent to the channel may last from approximately 1 to 3.5 days or longer (Figure 2)

depending on the project conditions and a variety of other factors.

Hopper dredges are self-contained vessels resembling barges that can maintain speeds of up to

3.6 m/sec (7 knots) while dredging (Morton 1977). Bottom turbidity associated with hopper dredges

is caused by the dragheads as they are pulled through bottom sediments. Surface turbidity may be

substantial if overflow occurs from the hoppers during loading, a practice that increases the sediment

content in the hopper bins (LaSalle 1990). Suspended-sediment plumes may range up to 1,200 m

on the bottom at concentrations up to 800 mg/L. Surface concentrations are dependent on whether

overflow is occurring.

INTEGRATING BIOLOGICAL AND DREDGING PROJECT DATA: It is a challenge to

integrate biological and engineering information to objectively evaluate potential impacts to a given

resource and to reach a consensus on such measures to protect that resource as environmental

windows. The basic problem, with respect to suspended sediments, is portrayed in its simplest form

in Figure 3. Given the project-specific conditions of a dredge plant, in situ sediment characteristics,

local hydrodynamics, and distributions of resources in space and time, exposures can be conceived

to fall within a matrix of concentration/duration combinations. Clearly those projects causing

minimal resuspension for short time intervals pose minimal risk of exposure above specific response

Personal Communication, C. Dyess, March 1999, U.S. Army Engineer District, Mobile.