Figure 3. Location of simulated dredging sites in the Providence River and Upper Narrangansett Bay from north to south

ERDC TN-DOER-E12

July 2000

ambient TSS concentrations typically vary from 1.63 to 14.8 mg/L (USAED, New England, 1998).

These can be added to the model predicted concentrations to obtain total concentrations.

Figure 3. Location of simulated dredging sites in the Providence River and Upper Narrangansett Bay from

north to south: Fox Point Reach North (FPRN), Fox Point Reach Central (FPRC), Bullock

Point Reach South (BPRS), and Rumstick Neck Reach Central (RMRC)

The choice of the TSS concentration levels selected for the presentation of model results corresponds

to the ranges presented by Newcombe and Jenson (1996) in a description of their fisheries impacts

model. They analyzed a series of studies of fish response to suspended sediment and developed a

set of empirical equations that relate biological response to duration of exposure and TSS concen-

tration.

The plan view portion of Figure 4 shows the near surface (0-2 m [0-6 ft]) extent of the sediment

plume for the Fox Point Reach Central (FPRC) dredging site with a release rate of 1.5percent,

whereas, the cross-section view presents the TSS concentration over the entire water column. The

lower panel shows the down channel extent of the plume reached during low slack water while the

upper panel shows the up channel extent reached during high slack water. At low slack water, the

plume (at a concentration greater than 20 mg/L) extends approximately 1,000 m (3,300 ft) down

channel of the dredging site. There is some residual material up channel of the site that was

generated during the previous flood tide. At high slack water, the plume extends approximately

780 m (2,550 ft) up channel, with a small amount of material remaining in the water column down

channel of the site. The tidal circulation moves the plume up and down the channel away from the

release site. Thus, any location away from the immediate dredging site experiences elevated

concentrations for only a portion of the tidal cycle.