SYSTEM OVERVIEW

ERDC TN-DOER-E10

April 2000

detrimental impacts. An ability to display the dispersion of suspended sediments from a dredging

site in a format that can be merged with known distributions of biological resources is a requirement

that powerfully enhances impact assessments. Also, a "hands-on" tool that would enable the

dredging project manager or resource agency representatives to specify a range of simulated

scenarios and have model solutions quickly and readily available for interpretation would be a

significant improvement over existing technologies.

Given these considerations, SSFATE is being developed to fulfill an obvious need for a modeling

tool that can be easily customized to simulate a broad spectrum of dredging scenarios, accommo-

dating essentially any hydrodynamic setting and most typical dredge plants. SSFATE is not

intended to be an analytical tool per se, but rather a screening tool. Its utility is particularly suited

for assessing the likelihood that resuspended sediments generated by a specific project would pose

substantial risk to resources or habitats of concern, thereby allowing environmental windows to be

appropriately applied or modified. Obviously, if SSFATE output showed negligible overlap of

suspended/deposited sediments and resource distributions, the need for a stringent window to avoid

conflicts would be questionable. Conversely, where output from SSFATE indicated a high

probability of impact, an individual window could be accepted with a higher degree of confidence

in its technical justification, and lead to consideration of other means to minimize impacts.

SYSTEM OVERVIEW: SSFATE is a versatile computer modeling system containing many

features. For example, ambient currents, which are required for operation of the basic computational

model, can either be imported from a numerical hydrodynamic model or drawn graphically using

interpolation of limited field data. 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. In addition, particle movement can be

animated over Geographic Information System (GIS) layers depicting sensitive environmental

areas.

SSFATE employs a shell-based approach consisting of a color graphics based, menu-driven user

interface, GIS, environmental data management tools, gridding software, and interfaces to supply

input and display output data from the model. SSFATE runs on a personal computer and makes

extensive use of the mouse (point/click) and pulldown menus. Data input/output is interactive and

mainly graphics based. The system supports a full set of tools to allow the user to import data from

standard databases, a wide variety of GISs, and other specialized plotting/analysis programs.

SSFATE can be set up to operate at any dredging operation site and includes a series of map-

ping/analysis tools to facilitate applications. Initial setup for new locations of dredging operations

can normally be accomplished in a few hours, unless numerical hydrodynamic models are run to

provide flow fields. At the heart of the system is a computational model that predicts the transport,

dispersion, and settling of suspended dredged material released to the water column as a result of

dredging operations. An integral component of the modeling system is the specification of the

sediment source strength and vertical distribution.

SSFATE SEDIMENT SOURCES: At the present time, sediment sources in SSFATE represent

the introduction of sediment into the water column only as the result of a cutterhead dredge, a hopper

dredge, or a clamshell dredge. The strength of each source is based on the Turbidity Generation

Unit concept proposed by Nakai (1978). For the cutterhead dredge source, introduction of