Appendices 5-13 - Nautilus Cares - Nautilus Minerals

2.3 The SSFATE Model
Sediment dispersion modeling of unconsolidated surface sediments and hard rock competent
waste removal and discharge was carried out using the SSFATE model (see Swanson et al.
2007). SSFATE is a Lagrangian particle tracking model designed for determining the fate of
sediment in the water colum. Thousands of computer generated particle are assigned a mass
for the amount of material it represents and each is transported individually based on the
sediment type ot represents. After the transport calculation stage of the model, the results are
applied to a detailed bathymetric dataset and concentration grid using a Gaussian distribution
of the mass over area. This gives the effect that the particles move as a plume and not as a
clump of mass. Horizontal transport of material is due to advection by currents and diffusion.
Current velocity fields are imported into the model from a separate hydrodynamic model and/or
field data. Vertical transport is based on particle settling rates and turbulent mixing which the
model parameterises with vertical diffusion coefficients. Particle settling velocities are
calculated using Stokes’ law and through the complex processes of flocculation due to
cohesiveness.
Deposition is based on a probability which is a function of bottom stress and concentration.
Matter that is deposited can be resuspended if the critical bottom stress is exceeded. The
model employs two different resuspension algorithms. The first applies to material deposited in
the last tidal cycle (12 hours) and is from Lin et al. (2003). It accounts for the fact that newly
deposited material will not be consolidated and will therefore resuspend with less effort than
consolidated bottom material. The second algorithm is the Van Rijn method (Van Rijn, 1989)
and applies to all other material that has been deposited prior to the start of the last tidal cycle.
Swanson et al. (2007) summarise justifications and tests for these schemes.
The characterization of different dredge types is represented by the initial vertical distribution of
released material as well as the sediment grain size distribution. For example the majority of
sediment release from a trailer suction dredge is due to overflow of fine material. Therefore the
initial vertical distribution of material is set to release near the surface and the grain size
distribution is biased towards the finer material.
2.4 Sediment Data
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