Demonstration of the SSFATE Numerical Modeling System

ERDC TN-DOER-E12

July 2000

Demonstration of the SSFATE

Numerical Modeling System

PURPOSE: This technical note presents a demonstration of the SSFATE (Suspended Sediment

FATE) model. SSFATE fulfills a need for a suspended sediment plume modeling tool that enables

the user to quickly and cost-effectively simulate multiple dredging project scenarios. The flexibility

inherent in SSFATE, coupled with an ability to simulate most commonly used dredge plants (e.g.,

bucket, hydraulic pipeline, or hopper) and dredging practices (e.g., barge or hopper overflow,

multiple plants operating simultaneously), is intended to assist in the negotiation process of

reasonable, objectively defined environmental windows. This note exemplifies an application of

SSFATE for an actual dredging project involving consideration of environmental windows.

BACKGROUND: The U.S. Army Engineer District, New England (NED), prepared a Draft

Environmental Impact Statement (DEIS) for the proposed Providence River and Harbor Mainte-

nance Dredging Project (U.S. Army Engineer District (USAED), New England, 1998). Comments

received on the DEIS recommended that dredging be allowed only at certain times (environmental

windows) to minimize potential impact to marine resources. Because the proposed project is

estimated to last 18 months with 24-hr/day dredging, constraints on periods when dredging is

allowed would extend the schedule even longer. The NED is conducting additional studies to

determine whether the use of environmental windows is justified both environmentally and

economically. Modeling of the plumes generated from the dredging process to determine the extent

and duration of such plumes is one component of these additional studies. This component provided

an ideal opportunity for an initial demonstration of the SSFATE model.

Under contract with the U.S. Army Engineer Research and Development Center (ERDC),

Vicksburg, MS, Applied Science Associates, Inc. (ASA) was responsible for the development of

the SSFATE model. Because of this development and ASA's experience in modeling various

processes in Narragansett Bay and the Providence River, ASA was retained by NED to conduct a

modeling study to aid in addressing environmental issues identified during interagency coordination

at the DEIS. The modeling study involved an application of SSFATE to estimate total suspended

solids (TSS) concentrations and the spatial extent of suspended sediment plumes generated by the

dredging operation, as well as an application of ASA's WQMAP modeling system (ASA 1997) to

compute the concentration of copper in the water column (Swanson, Isaji, and Ward 2000). A series

of sites were selected along the channel for generation of suspended sediment plumes. Different

bucket types and barge overflow protocols were chosen to simulate the range of expected actual

conditions. This technical note describes only the SSFATE application component of the modeling

study.

DESCRIPTION OF THE STUDY AREA: The Providence River and upper Narragansett Bay

(Figure 1) are connected to Rhode Island Sound through the east and west passages of Narragansett

Bay. The Providence River in the Harbor area down to the upper end of Fuller Rock Reach is

approximately 500-m- (1,700-ft-) wide. It continues to widen to its mouth at Conimicut Point located