TAble 1. Size Fraction Distribution of Dredged Material

ERDC TN-DOER-E12

July 2000

The sediment fraction distribution of the dredged material is

Table 1

shown in Table 1, and is based on measurements made in the

Size Fraction Distribution

Providence River (USAED, New England, 1998). This distri-

of Dredged Material

bution contains equal fractions by weight of clay, fine silt and

Percent

medium silt, with an additional one-half fraction of fine sand.

Material

Composition

Clay

5.6

The specification of the sediment source strength for input

Fine Silt

5.8

to the model is based on an estimated production rate of

Medium Silt

5.8

7,700 yd3/day (0.068 m3/sec). Assuming a solid fraction of

Fine Sand

2.8

0.2 (20 percent solids) by weight and a sediment density of

2,600 kg/m3, the mean rate of sedi-

Fluid

80.0

Table 2

ment dredged is 35.4 kg/sec. The

Model Source

source strength is then defined as the percent of this amount that is

Strengths of TSS

released to the environment during the dredging operation. The as-

TSS

sumed release rates correspond to different bucket types and operational

Release

Loading

protocols. These rates are1.5 percent for an environmental bucket that

Rate,

seals the bucket load, 2 percent for a conventional bucket, and 4 percent

percent

kg/sec

for a conventional bucket with allowance for barge overflow during 1.5

0.53

loading operations (USAED, New England, 1998). The actual model

2.0

0.71

loading rate or source strength for TSS is shown in Table 2.

4.0

1.42

Table 3 shows the percent of material released as a

Table 3

percent of local depth. The load is distributed over

Vertical Distribution of Material

the water column with decreasing release amounts

Released into the Water Column

from bottom to surface to better simulate release

during Dredging Operations

distributions (USAED, New England, 1998).

Percent of Local

Percent of Material

Depth

Released

The assumed locations of the dredging-generated

80 100 (surface)

sediment plumes in various channel reaches are

60 80

shown in Figure 3. The northernmost reach, Fox

40 60

Point, has two sites in its northern (FPRN) and

20 40

central (FPRC) areas. The next reach to the south,

0 20 (bottom)

Fuller Rock, has one site situated at its midpoint

(FRRC). The Bullock Point Reach has a site lo-

cated at its southern end (BPRS) and the Rumstick Neck Reach has a site at its midpoint (RNRC).

The FPRC, BPRS and RNRC sites were each evaluated individually for the three release rates. Three

additional scenarios that combined FPRN and FPRC; FPRC and FRRC; and FPRN, FPRC and

FRRC were also simulated with a 2 percent release rate.

MODEL RESULTS: The modeling results are presented below in two forms: (a) color contours

of concentrations in plan and vertical section views that show the maximum horizontal and vertical

extent of the resulting sediment plumes; and (b) time-series plots of concentrations at the release

site and at specified distances of 150 m (500 ft) and 300 m (1,000 ft) up and down the channel from

the release point. These locations correspond to the monitoring requirements for the Boston Harbor

Navigation Improvement Project (USAED, New England, 1998). The concentrations presented are

excess concentrations since no ambient levels are included. The range of historically observed