APPLICATION OF THE PARTICLE TRACKING MODEL TO PREDICT FAR-FIELD FATE OF SEDIMENT SUSPENDED BY NEARSHORE DREDGING AND PLACEMENT , BRUNSWICK GA

The Engineering Research and Development Center (ERDC) is supporting the USACE Savannah District in conducting a multi-year study to evaluate and validate numerical models for predicting dredged material transport at nearshore and open-water sites (Smith et al,. 2007). Accurate predictive models are necessary for selecting and managing nearshore placement sites. An example of this procedure is dredging performed at Brunswick, GA. Dredged material from the Brunswick Harbor Entrance Channel has recently been placed in a series of channel-adjacent, open-water dredged material placement sites approximately 1200m south of the navigation channel. Dredged material removed from the entrance channel is composed of approximately 80 percent sand and 20 percent silt and clay. The dredged material does not meet guidelines for direct beach placement. Therefore, nearshore placement is considered a promising alternative to direct beach placement under which winnowing by wave action will naturally separate sand and silt fractions. Nearshore transport predictions are required to address natural resource, beneficial use, and site capacity issues. The Particle Tracking Model (PTM) is applied at the Brunswick site as a diagnostic tool to judge the combined capabilities of other models, including hydrodynamic and wave models and as a predictive tool to determine the far-field fate of dredged materials. PTM is a Lagrangian Particle Tracking Model (Demirbilek et al 2005). In general terms, a Lagrangian modeling framework is one which moves with the flow. In PTM, the constituent being modeled is discretized into a finite number of particles that are followed as they are transported by the flow. Sufficient particles are modeled such that transport patterns are representative of all particle movement from the specified sources. One benefit to this technique is that particle pathways (including time histories) are readily identified. In addition, Lagrangian models generally use a fraction of the CPU time required by Eulerian models. This makes them well suited for simulating multiple scenarios. The PTM interface is in the Surface-water Modeling System (SMS) graphical user interface (Zundel 2005). Due to this basic yet versatile formulation, PTM is a Lagrangian particle tracking model with the capacity for a wide range of applications. One of PTM’s primary purposes is the determination of the far-field fate of dredged material. PTM applies hydrodynamics and waves from external models to transport suspended sediments to provide both qualitative and quantitative assessment of the consequences of dredging activities. The purpose of this current work is to demonstrate the capabilities and limitations of PTM for evaluating nearshore placement of dredged material at Brunswick. These PTM simulations are part of a larger study to develop a Dredged Material Management Plan that includes optimization of nearshore dredged material placement for littoral zone nourishment. The PTM simulations performed in this work pertain to the transport of sand and silt from two existing dredged material placement locations. The objective is to demonstrate the PTM application for nearshore placement of dredged material and to compare model results to the fluorescent tracer study observations.