Rupert Inlet Research Articles

Hydrodynamics of Turbid Underflows. II: Aggradation, Avulsion, and Channelization

A two-dimensional, depth-averaged finite-volume model is used to study the hydrodynamics of turbid underflows. A sensitivity analysis is performed with regard to model input parameters and the initial bed properties. Simulations are then performed to illustrate aggradation and avulsion in an existing submarine channel. The conditions under which channelization of deposits occur are also examined. The effect of numerical dissipation on flow ignition is examined and the high resolution properties of the model are found to produce satisfactory results. The model is applied to historical field-scale events, including the Reserve Fan in Lake Superior and Rupert Inlet in British Columbia.

Integrated geophysics for mineral exploration in drift‐covered volcanic terrains: examples from northern Vancouver Island, Canada

Known mineral occurrences in northern Vancouver Island are typically hosted in volcanic units of the Bonanza Group. At a local scale, the mineralization is associated with advanced argillic bedrock alteration and is often intimately related to porphyry intrusions. On a larger scale, faults are thought to exert the most significant control on the distribution of mineralized host rocks. Poor exposures and a complex glacial history limit the use of traditional methods of geological mapping and mineral exploration in this region and to date geophysical methods have been under‐utilized. Here we present findings from four standard geophysical (gravity, magnetics, electromagnetics and seismic refraction) methods, recently deployed here to elucidate the subsurface geology, as well as to identify new targets for base metal exploration.Results at two different sites show that the integrated interpretation of geophysical data, constrained by physical rock property measurements, yields detailed images of the subsurface at a fraction of the cost of drilling. At one site, east of Rupert Inlet, the final subsurface model shows that the Bonanza Group is not nearly as extensive as previously presumed. An extension of the Holberg Fault is identified some 50 km east of the visibly mapped outcrop and an extensive zone of alteration around the fault is recognized. Furthermore, a number of the methods provide support for the existence of a porphyry dike at this site. At the second site, north of Rupert Inlet, magnetic and electromagnetic data prove effective at mapping alteration and locating shear zones beneath a relatively thin drift cover. Together, these results help outline a strategy for exploration in drift‐covered terrains and show that a redirection of exploration effort is warranted in the case of northern Vancouver Island.

Marine Tailings Disposal Simulation

The disposal of mine tailings in deep water body requires comprehensive assessment of the potential environmental impacts, and estimates of the effective storage capacity of the bottom bathymetry for accommodating the anticipated volume of disposed tailings over the life of a mine. A mathematical simulation model was developed for this purpose to predict the distribution and long-term deposition patterns of tailings in a fjord. The model describes the mechanics of the tailings slurry flow along the bottom of the receiving water body and the settling of suspended particles, as well as the redistribution of the deposited sediments due to slumping of unstable slopes. Specific processes simulated in the model also include entrainment at the top of the density current, entrainment associated with internal hydraulic jumps in areas of changing bottom slopes, hindered settling and coagulation of suspended particles, and sediment deposition and bathymetric changes resulting from the deposition of large quantities of tailings. Long-term tailings deposition patterns are predicted by analyzing the flow of the tailings slurry in a series of time increments, from a few months to a year. The bathymetry is assumed to remain unchanged within each time increment, and the model first simulates the steady-state flow field for the tailings slurry along the bottom and the resulting deposition rates for several sizes of suspended particles. Then the estimated tailings deposition rates are combined with slope stability considerations to predict the new bathymetry of the receiving water body. Data from several operating marine tailings disposal systems were reviewed to assess the applicability of specific relationships used in the model. The model was calibrated using data from one of the most extensively studied marine tailings disposal systems, the Island Copper mine in Rupert Inlet of Vancouver Island, British Columbia. Comparison of simulated tailings deposition patterns in Rupert Inlet with data from field observations and measurements suggests that calibration of the model is satisfactory and provides confidence in the capability of the model to predict deposition patterns and suspended sediment concentrations for proposed mines.

Arsenic and copper residues in waterbirds and their food down inlet from the Island Copper Mill

Since 1971, Island Copper Mine has been operating an open-pit mine, mainly for copper on the north shore of Rupert Inlet. Rupert Inlet is the most easterly inlet of Quatsino Sound on northern Vancouver Island, British Columbia (Fig. i). The ore body was estimated to contain 250 million tons of reserves. Mining is proposed to be continued until 1996, at which time the reserves will have been depleted (Island Copper Mine, pers. comm.). The mine processes daily 41,000 tons of ore, containing on average 0.52% copper (Waldichuk and Buchanan 1980). Mine tailings are discharged into Rupert Inlet at a depth of 50 m. The tailings now cover the bottom of Rupert Inlet (originally i00 m deep), as well as much of that of Holberg Inlet and part of eastern Quatsino Sound, past Quatsino Narrows (Fig. i). The tailings contain an average of about 700 mg/kg -I copper and 5 mg/kg -I arsenic (Thompson and Paton 1975). The concentrations of the same metals in natural sediments of Rupert Inlet are 44 mg/kg -I copper and 5 mg/kg -I arsenic (Waldichuk and Buchanan 1980). As chemical data were lacking, a pilot study was initiated in March 1976 to determine arsenic and copper residues in tissues of birds in Quatsino Sound and connecting inlets. After finding elevated arsenic residues in bird livers in 1976, waterbirds and their food chain were further investigated for arsenic and copper from October 1981 through March 1982. The results of the 1976 and 1981-82 analyses are presented here.

Turbidity currents and submarine channel formation in Rupert Inlet, British Columbia: 2. The roles of continuous and surge‐type flow

Acoustic backscatter images are presented of the continuous flow turbidity current resulting from mine tailing discharge into Rupert Inlet. The current flows partially within a submarine channel, which both hooks to the left and meanders. The images show the turbidity current spilling over the outer levees at channel bends. A model for continuous turbidity flow incorporating overspill is developed and applied to the left‐hooking section of the submarine channel. It is very likely that the observed overspill is primarily an inertial effect resulting from the outer bank being more sharply curved than the channel axis. The model results are combined with a sediment accumulation budget based on successive seismic profiling surveys and with results for surge‐type flows (taken from the companion paper (Hay, this issue)). It shows that surges are primarily responsible for the transport of tailing to large distances from the point of discharge and should recur at 1‐ to 2‐day intervals on average. This is consistent with the characteristics of coarse‐grained turbidites observed in cores from the levees. Relative to the intervening fine‐grained mud deposits the turbidites increase both in thickness and frequency in the downchannel direction. Furthermore, the surge recurrence interval obtained from the number of turbidites per core and the local accumulation rate is 2–5 days, essentially the same as that obtained from the model. Suspended tailing transport within the channel appears to be dominated by surge‐type turbidity currents except close to the outfall. It is concluded that the surge‐type flows are most responsible for the formation and morphology of much of the channel. It is not clear, however, that the meanders are formed principally by surges. Although the surges occur frequently and transport most of the sediment in the meander reach, the volume transport is dominated by continuous flow.

Turbidity currents and submarine channel formation in Rupert Inlet, British Columbia: 1. Surge observations

Acoustic backscatter images obtained at 107 and 200 kHz of surge‐type turbidity currents resulting from mine tailing discharge are presented. Excess densities in the head of the order of 100 kg m−3 are estimated from differences in acoustic backscatter intensity and in the amplitude of the bottom‐reflected signal. These excess densities are comparable to estimates based on surge height and speed, the speed being determined by assuming a universal head shape. Unlike gravity current surges from an impulsively started steady source, the surge‐type turbidity flows consist of a high concentration slug of short duration followed by a longer‐lived wake in which sediment concentrations are lower by 2–3 orders of magnitude. The profile of the high concentration slug resembles those for density slugs on small bottom slopes obtained in laboratory experiments. Discrete scatterers, probably fish, are observed to avoid the surges.

Early diagenesis of copper and molybdenum in mine tailings and natural sediments in Rupert and Holberg inlets, British Columbia

The distribution and behaviour of Cu and Mo in interstitial waters collected from Cu- and Mo-rich mine tailings and natural sediments in Rupert and Holberg inlets (Vancouver Island, British Columbia) are discussed. Dissolved Fe, Mn, [Formula: see text], and Σ H2S distributions and solid-phase Cu, Mo, Mn, and organic carbon data are used to constrain the interpretation of Cu and Mo diagenesis in both tailings and natural sediment facies. Cu is released to solution at or near the sediment–water interface in both facies either by decomposing labile organic material or as a probable result of oxidation of sulphide minerals within the tailings. Mo is significantly enriched (up to 0.6 μmol L−1) in tailings pore water, probably as a result of dissolution of soluble Mo oxides produced initially by oxidation of MoS2 during milling of the ore.In natural sediments, both Cu and Mo are removed from solution at depth, probably by precipitation as their sulphides or in solid solution with an iron sulphide phase. Similar reactions appear to be occurring at depth in the tailings. The benthic flux of copper from the tailings into overlying seawater is similar in magnitude to that observed in natural sediments; there is no indication from the present data that remobilization of copper from the tailings is perturbing the pre-mine Cu distribution in Rupert Inlet. Unlike that of Cu, the benthic flux of Mo is unique to the tailings but is insufficient to produce a measureable anomaly in inlet waters because of the short water residence time and the relatively high Mo concentration characteristic of seawater.

A diatom chronology for sediments in a high accumulation rate environment: Rupert Inlet, British Columbia1

Concentration profiles of large diatoms in gravity cores from the rapidly accumulating mine tailing deposit in Rupert Inlet exhibit variations with sediment depth which are ascribed to seasonal effects. Time‐series of near‐surface chlorophyll a measurements and phytoplankton cell counts, together with observations of an unusually massive bloom of Coscinodiscus gigas, are used to attribute peak concentrations of frustules of C. gigas, Coscinodiscus centralis, and Coscinodiscus radiatus in the sediments to late spring‐midsummer and early fall blooms of these species. Sediment accumulation rates estimated from the frustule concentration profiles range from 57 to 120 cm yr−1 and are similar to rates obtained from bathymetric and continuous seismic profiling surveys. From these rates it is inferred that very fine laminae (about 0.1 mm thick) may be tidally induced, and a rough estimate of 5 m d−1 is made for the in situ settling velocity of C. gigas.

Interstitial water metabolite chemistry in a marine mine tailings deposit, Rupert Inlet, B.C.

The distributions of [Formula: see text], [Formula: see text],[Formula: see text], titration alkalinity, Ca2+, and pH in interstitial waters collected from mine tailings and natural sediments in Rupert and Holberg inlets (Vancouver Island, British Columbia) are discussed. In natural sediments, dissolved ammonia, phosphate, and titration alkalinity increase with depth, and sulphate decreases, following established patterns for coastal sediments. In rapidly accumulating tailings, however, no dissolved phosphate enrichment is observed despite significant sulphate reduction. This phosphate depletion is accompanied by a calcium enrichment and a relatively high pH, which is probably due to the addition of lime to the tailings during milling. Solubility considerations suggest that the phosphate distributions in both tailings and natural deposits are controlled by equilibrium with respect to carbonate fluorapatite. Lime addition appears to increase the supersaturation of the pore water with respect to carbonate fluorapatite, resulting in phosphate consumption by the tailings. The tailings thus inhibit benthic phosphate regeneration, which is now known to be an important factor in the phosphorus budget of some coastal waters. It is demonstrated that such inhibition is not important in Rupert Inlet due to a short water residence time; however, it could be of significance in poorly mixed or stratified water bodies including lakes and some fjords.

Submarine channels in rupert inlet, British Columbia: II. Sediments and sedimentary structures

In a previous paper the morphological evolution of submarine channels in the Rupert Inlet mine tailing deposit was traced through a succession of three distinct periods or phases over a five-year period: a meandering channel phase, an apron phase and a rechannelized phase. The distributions of sediment properties including grain size, specific gravity, and Cu and Fe concentrations for these phases are presented here. Copper-rich, coarse-grained deposits were found along channel axes and down the apron flanks. Sandy, fining-upward turbidites in cores from the levees during the meandering channel phase were characterized by higher Cu (and lower Fe) concentrations than the adjacent mud, and had sharp basal contacts sometimes exhibiting flame and load-pocket structures. The average thickness of, and fraction of the sediment column occupied by, turbidites was found to increase with distance downchannel, indicating a downchannel decrease in the importance of continuous (as opposed to surge-type) flow to the channel-building process. Deposition rates, determined from the local time rates of change of tailing deposit thickness, ranged from 0.1 to 4 m yr −1 at the core sites and were used together with the number of coarse-grained turbidites per core to yield a minimum turbidity current surge recurrence interval of 2–5 days during the meandering channel phase.

Submarine Channels In Rupert Inlet, British Columbia: I. Morphology

The evolution of submarine channel systems in a mine-tailing deposit is presented. Three successive phases of channel development were observed in time-series of seismic reflection, side-scan sonar and bathymetric surveys: (1) a meandering channel phase; (2) an “apron” phase, in which the channel was obliterated; and (3) a rechannelized phase. The meander phase consisted of a leveed channel with three morphologically distinct reaches: a left-hooking upper reach (axial slope decreasing from 9 to 2°); a middle meander reach (1° slope, 500–300 m and 100 m meander wavelength and average amplitude); and a straight lower reach (0.5° slope). The meanders are geometrically similar to those in rivers. Channel width and depth decreased down-channel suggesting continuous loss of solids by deposition and overspill. The morphology of the channel is compared to that of deep-sea fan valleys, particularly the Redondo, which exhibits a meandering-braided transition at an increase in slope from 1 to 2°. Braided-meandering and meandering-straight transitions in subaqueous channels are discussed in the context of the slope—discharge dependence of such transitions in subaerial rivers. Probable trajectories of the Rupert Inlet and Redondo systems in slope—discharge parameter space are proposed, and illustrate the importance to the occurrence of meanders in subaqueous channels of the entrainment of ambient fluid and of suspended sediment loss through deposition and channel overspill.

On the remote acoustic detection of suspended sediment at long wavelengths

Experimental results are presented which indicate a linear relation between the time‐averaged amplitude of the envelope of the backscattered acoustic pulse at 192 kHz and the square root of suspended sediment concentration in the 10 to 103 mg 1−1 range. Particle sizes ranged from 2 to 140 μm. The measurements were made in a negatively buoyant, mine‐tailing discharge plume in a submarine channel at depths of 60 to 90 m in Rupert Inlet, British Columbia. From the theory of acoustic backscatter from a solid elastic sphere in the Rayleigh region it is shown that if the pressure amplitude of the backscattered wave is Rayleigh distributed, then such a linear relation is to be expected. Expressions for the optimum acoustic frequency for the detection of dilute suspensions at a given range and for the minimum detectable concentration are obtained assuming a thermal noise background. The possibility that bubbles contribute to the backscatter is considered and found to be unlikely on the basis of probable bubble lifetimes.

Remote Acoustic Detection of a Turbidity Current Surge

A turbidity current surge has been detected in a leveed submarine channel in Rupert Inlet, British Columbia, with the use of acoustic sounders operating at 42.5, 107, and 200 kilohertz.