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Dispersal of recent sediments and mine tailing in a shallow-silled fjord, Rupert Inlet, British Columbia Johnson, Ronald Dwight

Abstract

Rupert Inlet (fjord) is the site of large-scale submarine disposal of mine tailing and waste rock. This thesis considers the pre-operational condition and the initial two years of operation. Repetitive seismic surveys, grain size analysis from cores and samples, bottom current measurements together with geologic, topographic and oceanographic considerations are used to establish the natural erosional-depositional pattern and its effect on tailing distribution. Connection of Quatsino Sound to the Holberg-Rupert basin via Quatsino Narrows is a post-glacial feature which initiated the present sedimentation regime. Prior deposits, flat-lying clayey silt, were eroded by bottom currents originating at the Narrows, moved up inlet then re-deposited, generally in Holberg Inlet above Coal Harbour and in Rupert Inlet off the present Mine site. Bottom currents result from the incursion of flood tide water through Quatsino Narrows into the basin. Density dictates the intrusion level into the indigenous water column, while tide height and range contribute to the velocity of resulting currents. Water above the intrusion level moves headward, while water below that level forms counter-currents in Rupert and Holberg inlets which are convergent below the Narrows. Release of pressures built up during flood tide causes strong up-inlet bottom currents on early ebb tide. Maximum bottom currents are theorized as occurring when the flood tide water is denser than all the basin water. Currents are more persistent up inlet, but both up and down inlet commonly exceed 50 cm/sec and occasionally 100 cm/sec The theoretical maximum up-inlet current may approach 300 cm/sec Sediment grain size and sorting indicate that maximum currents below the Quatsino Narrows extend more strongly up Rupert Inlet. While tailing deposition is widespread throughout the basin and occasionally observed being entrained out Quatsino Narrows on ebb tide, greater than 90% is restricted to Rupert Inlet. Below the outfall pipe tailing have caused slides damming the bottom of the inlet and usefully ponding tailing. Tailing are at uniform grade from the fan to Quatsino Narrows, with maximum winnowing by currents in the lower reach. Natural sedimentation attempts to confine the tailing to Rupert Inlet. This desired effect is thwarted by too much fine tailing being allowed to enter the water column. Further study of current structure of the water column and of density relationship on either side of the sill is necessary. Modification of engineering design may accomplish lower turbidity levels The monitoring program has provided the type of multi-disciplinary data required for environmental evaluation of inlets.

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