Description of the SSFATE Numerical Modeling System

ERDC TN-DOER-E10

April 2000

Description of the SSFATE

Numerical Modeling System

PURPOSE: This technical note describes the numerical modeling system SSFATE (Suspended

Sediment FATE), which is being developed to compute suspended sediment fields resulting from

dredging operations. Both theoretical aspects of the computations made within SSFATE and

application aspects of the shell-based personal computer program are discussed.

BACKGROUND: SSFATE was developed in response to a need for tools to assist dredging project

managers confronted by requests for environmental windows. Environmental windows, intended

to protect biological resources or their habitats, are requested during the interagency coordination

process for dredging projects (Reine, Dickerson, and Clarke 1998). In many cases, decisions

regarding environmental windows must be based on limited technical information because potential

impacts are linked to a host of site- and project-specific factors. For example, navigation dredging

operations in different reaches of the same waterway may pose risks to different resources, or

potential impacts may vary dependent on the type of dredge plant involved. Few tools exist to

evaluate such concerns early in the environmental window negotiation process. Consequently, a

general inability to address "What if" questions associated with given dredging project scenarios

tends to ensure that recommended environmental windows are conservative, and perhaps over-

restrictive (Reine, Dickerson, and Clarke 1998).

Some of the most frequently asked "What if" questions during dredging project coordination are

related to resuspension and dispersion of sediments at the dredging site. Suspended sediments are

a primary concern of resource agencies, as exposure of aquatic organisms to elevated suspended

sediment concentrations is perceived to be a major source of detrimental impact. Likewise,

redeposition of suspended sediments can be a significant concern if sensitive bottom-dwelling

organisms (e.g., oysters or sea grasses) are present in the vicinity of a dredging project. Accurate

information on the spatial dynamics of dredge-induced suspended sediments is therefore a critical

necessity in establishing the overall need for protective windows.

Environmental windows are associated with a majority of dredging projects in many U.S. Army

Corps of Engineers Districts (Reine, Dickerson, and Clarke 1998). However, presently available

modeling tools for predicting suspended sediment behavior were not designed with environmental

windows negotiation in mind. For logistical reasons, models that require complicated, extensive

hydrodynamic databases, grid building, or high-end computer support are not suitable. These models

are more appropriate for large, controversial projects. Clearly, funding constraints alone would

hinder application of expensive numerical models to the evaluation of numerous environmental

windows.

To be truly effective as a dredging project management tool with respect to windows, models should

be capable of running multiple simulations in a relatively short span of time so that a number of

alternative dredging scenarios can be evaluated to determine those with the least probabilities of