Appendices 5-13 - Nautilus Cares - Nautilus Minerals

For these scenarios, the dual discharge pipes were constructed to face upwards. If the discharge
rate was at a maximum under these circumstances, then 4005 gpm (or 0.253 m 3 /s) of discharge
water would be directed upwards away from the seabed through two openings. The 5.8°C plume
would rise through the water column reaching a height of approximately 90 m above its starting
point, that is, 115 m above the seafloor (1,385 m below the surface). In contrast, the warmer 11.4°C
plume would rise through the water column reaching a height of approximately 120 m above its
starting point, that is, 145 m above the seafloor (1,355 m below the surface). At these heights above
the seafloor, the plume loses its jet momentum and buoyancy, since it has mixed sufficiently due to
the turbulence created by the jet, that is, its temperature and density are now the same as the
ambient water. The buoyancy differences are negligible after the jet phase and the plume centreline
dilution ratio (that is, minimum dilution ratio) is at least 1:180 at this point, that is, 1 part discharge to
180 parts ambient receiving water. Beyond this initial rapid dilution, the plume dynamics are now
influenced by the tidal currents and basin wide circulation of the Bismarck Sea. This phase of the
mixing dynamics is known as far field mixing and is the slowest of the mixing process of a discharge
plume.
Figure 3.1 shows a plan and cross-sectional view (between 1,300 m and 1,600 m depth) of the
plume in the far field at opposite phases of the tidal cycle. In the cross-sectional view in Figure 3.1
the bottom topography is shown as green. Note that the full plume is shown in Figure 3.1 and that
the concentrations of plume water within the ambient waters are coloured as follows:
• Plume concentrations are RED if dilutions are less than 1:300
• Plume concentrations are ORANGE if between 1:300 dilution and 1:5,000 dilution.
Figure 3.1 demonstrates that the plume is extremely patchy, since the discharge occurs in a tidally
varying current field for the 11.4°C scenario. That is, when currents are weak during the turning of
the tide, plume water builds up around the discharge point. In contrast, when currents are strong,
the plume is advected away from the discharge site quickly enough to prevent any buildup, thus
reducing plume concentrations within the ambient waters. The example plume extent in Figure 3.1
is about 0.96 km beyond the discharge site in the horizontal. The plume extent in the vertical is only
about 120 m in Figure 3.1 (between 1,350 m and 1,470 m below the surface) and remains within its
ambient temperature layer.
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