Appendices 5-13 - Nautilus Cares - Nautilus Minerals

3 RESULTS AND DISCUSSION
The calculation of the complete pre-mining removal and placement operation (comprising the
surface unconsolidated sediments and some hard rock competent waste) using the ROV and
the Seafloor Mining Tool (SMT) was undertaken with the SSFATE model described herein. This
operation will be carried out, intermittently, as required over a 20 month period.
The SSFATE simulations demonstrated that due to the fast falling velocity of the coarse sands
and heavier materials, these larger particles would fall down-slope, but near the tip head sites.
In contrast, the finer particles, the clays and silts would spread more widely since their lower fall
velocities enabled them to be carried by the tidal currents before settling.
Figure 3.1 shows the depositional thicknesses of the placement from the ROV and SMT
excavated pre-mining material surrounding the Solwara 1 mining lease site after this operation is
completed in full. Bottom thicknesses, shown in Figure 3.1, varied from 0.18 mm to small areas
adjacent to the tip heads site slightly exceeding 500 mm or 0.5m. Some deposition was noted to
fall outside the areas shown in Figure 3.1, but at a thickness that was less than the measured
natural sedimentation rate for the site (being 0.18 mm over a 20 month period).
The model results also detailed the composition of the benthic loading in the area as shown in
Figure 3.2 for the mining sites and Figure 3.3 for the area in general. Figure 3.2 shows the
particle size distributions for sediment that returned to the mining zone, while Figures 3.3a to
Figure 3.3e show the extent of the individual sediment size classes. The coarse sands were
found to settle very close to the tip head sites, given the larger settling velocities of these
particles. These sands are predicted to mound near the tip head site by SSFATE. This is to be
expected, although the SSFATE model does not account for post settlement slumping or any
instability associated with these higher sediment peaks. It is possible that these mounds may
collapse and spread the coarser sands down the slope of the Solwara-1 mound. In contrast, the
significant quantities of clays and silts within the unconsolidated sediments have very low
settling velocities and hence are subjected to the ambient currents. While the SSFATE model
accounts for the bathymetric profile of the site as shown in Figure 3.1 and Figure 3.3, a small
proportion of these clays, silts and some fine sands are predicted to settle back on the mining
site (maximum thickness on mining area < 376mm) due to the ambient currents and vertical
turbulence. Indeed, the benthic loading predicted within the mining boundaries, shown in Figure
3.2 and Figure 3.3, is comprised of these clays, silts and fine sands from the unconsolidated
sediments.
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