DESCRIPTION OF SSFATE: SSFATE (Suspended Sediment FATE)

ERDC TN-DOER-E12

July 2000

11.6 km south of Fox Point Reach. The river then merges with upper Narragansett Bay, which

eventually splits into its east and west passages to Rhode Island Sound.

The navigation channel runs from upper Narragansett Bay through the Fox Point Reach. The channel

design depth is 12.2 m (40 ft) at mlw. It is typically 200-m- (650-ft-) wide below Fox Point Reach and

is flanked by wide shallow areas typically 1-2 m (3-7 ft) deep. In Fox Points Reach, the deep dredged

area widens to both banks of the river, 500 m (1,600 ft), to accommodate shipping traffic.

Hydrodynamic conditions at a dredging site play a crucial role in the modeling of suspended

sediment plumes. The tide range in the upper bay and Providence River averages 1.40 m (4.6 ft) in

the Providence Harbor, with the spring range being 1.71 m (5.6 ft). The tide is primarily a standing

wave with maximum tide-induced velocities approximately 90 out of phase with tide height.

Circulation in the Providence River and upper Narragansett Bay is dominated by the tides. Based

on an analysis of current meter data obtained at three locations in the river, Turner (1984) concluded

that approximately 70-80 percent of current variance occurs at tidal frequencies. More than 50 percent

of the variance is present at the semidiurnal M2 constituent (12.42 hr) period. Typical tidal velocities

in the river range from 10 to 25 cm/sec (0.32 to 0.82 ft/sec).

The tidal prism of the Providence River is approximately 32 106 m3 (1.1 109 ft3). Converted

to an equivalent flow of 1370 m3/sec (48,500 ft3/sec), it far exceeds the mean freshwater flow into

the system of 43.2 m3/sec (1,526 ft3/sec) (Ries 1990). This results in relatively small density-induced

flows compared to the tides, although two-layered estuarine circulation has been observed in the

Providence River (Turner 1984). This condition is most pronounced under stratified conditions and

when there is a relatively strong density gradient up the estuary as a result of low freshwater inflow.

DESCRIPTION OF SSFATE: SSFATE (Suspended Sediment FATE) computes suspended

sediment distributions resulting from dredging operations. SSFATE is a versatile computer

modeling system containing the following features:

Ambient currents can either be imported from a numerical hydrodynamic model or drawn

graphically using interpolation of limited field data.

A particle-based computational model computes the transport, dispersion, and settling of

suspended dredged material released to the water column.

Sediment source strength and vertical distribution from cutterhead, hopper, or clamshell type

dredges are either computed internally or prescribed as input data.

The fate of multiple sediment types or fractions can be simulated simultaneously.

Model output consists of concentration contours in both horizontal and vertical planes, time

series plots of suspended sediment concentrations, and the spatial distribution of sediment

deposited on the sea floor.

Sediment particle movement and concentration evolution can be animated over Geographic

Information System (GIS) layers depicting sensitive environmental resources and areas.

Depending on the resolution of the numerical grid employed, SSFATE can make predictions close

to dredging operations. However, the processes modeled are far field processes in which the mean