Figure 2. Model computational grid for the Providence River and Upper Narrangansett Bay

ERDC TN-DOER-E12

July 2000

Using the flow fields provided by the 3-D hydrodynamic model, SSFATE was applied to simulate

the fate of dredged material released during dredging operations. The model uses a fine grid (20 m

(61 ft) square) that overlies the boundary-fitted hydrodynamic grid shown in Figure 2. This square

grid was used to provide finer resolution for the transport calculation while retaining the highly

efficient algorithms in SSFATE that advect Lagrangian particles through a square grid. Particle

movement is based on an advective velocity calculated from interpolated hydrodynamic model

results plus a diffusive velocity that vectorially adds a random component based on a typical

estuarine dispersion. A Chezy formulation is used to estimate the diffusion as a function of Manning

friction factor, local depth and local velocity (Rouse 1949).

Accurate specification of the sediment loads is crucial when applying SSFATE. For this application,

the dredged material loads were assumed to consist of continuous sources of material as opposed

to a series of individual events. This approximation was made since the bucket is drawn through

the water column with a new load of material approximately every 90 sec. Since the maximum

ambient velocity is 10-25 cm/sec (0.33-0.82 ft/sec), the maximum travel distance for a single plume

to move away from the dredge is only 9 to 22.5 m (30 to 74 ft). At this distance from the dredge,

there is not sufficient resolution to differentiate between consecutive bucket cycles. Thus, a

continuous load was used.