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Fraser Delta

Submarine Slope Failure and Sediment Stability
by Gwyn Lintern, Phil Hill, Kim Conway

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    The Project

    Geotechnical analysis has shown that parts of the Fraser Delta might be susceptible to slope failure. Coastal communities and infrastructure surrounding the southern Strait of Georgia would be at risk in the case of a large failure. Though there is no evidence of a catastrophic failure, there is ample data showing small scale failures. These small failures could rupture power transmission cables that supply electricity to Vancouver Island, while large failures could damage or even destroy important infrastructure. Furthermore, the tsunami generated by such a hypothetical failure would propagate across the Strait of Georgia and impact the shorelines of the Gulf and San Juan Islands as well as mainland British Columbia.

    An array of instruments has been developed to measure liquefaction and failure events. In the past, it has been difficult to measure and capture these very short failure events, with long time intervals between, using using short term moored instruments. Deployment and networking of the present instrument packages will be facilitated by the Victoria Experimental Network Under the Sea (VENUS) Project. The project will install a fibre optic cable from the central Strait of Georgia allowing real time observations so that the parameters described below can be studied in great detail (milliseconds to years). It is hoped that eventually the instruments developed here may be used to detect conditions leading to slope weakness here and at other locations in Canada.


    Possible Causes of Failure

    forces_fullsize

    Oversteepening, natural and dredge disposal

    The Fraser river discharges many tonnes of sediment per year. This is confined to a relatively small area of the mouth ever since the confinment of the navigation channel by the construction of the Steveston Jetty in the 1930′s. Depositional rates are greater at low tide, when the salt wedge is located at the river mouth (Kostachuck et al., 1989). The quickly deposited sediment is loosely packed (Chillarige et al, 1997a, b). This simple oversteeping of the delta front is probably the main cause of failures in the area. Up to five hundred thousand cubic meters of dredged sediment is deposited per year in the designated disposal site at Sand Heads, which may be further adding to the buildup of loosely packed and unstable sediment.

    Repeat multibeam (sea floor bathymetry) surveys below show that there was generally a net accumulation of sediment during the year 2001 to 2002. However, in the year 2002 to 2003 large sections began to fail, including a 10 m thick deposit in the upper canyon area (Hill, 2006).

    Tide, gas, earthquakes, storms, groundwater, erosion

    It is known that the bed at shallow depth fails periodically due to tidal drawdown resulting in excess pore pressures (Christian et al., 1997). The seabed in this area contains a significant volume of gas (1%), which leads up to an 80% attenuation in pore pressure within the top 5 m, and which also likely affects the shear strength of the soil (Chillarige et al., 1997b; Christian, 1998; Christian et al., 1997, 1998). We further hypothesize that storm surges and seismic activity lead to pore pressure increases and corresponding decreases in sediment strength. Pore pressures are also affected by seasonal groundwater flows. Finally, strong tidal currents have been measured at the toe of the delta slope (Kostaschuk et al., 1995; Barrie and Currie, 2000). These currents are strong enough to erode and undercut the base of the delta area.


    SLIP instruments

    The variety of factors involved in generating slope failures at the Fraser Delta has made investigations difficult. A number of researchers with different expertise have made use of many different classes of instruments over the past two decades. The SLIP Instruments include piezometers to measure pressures, in the water column and within the bed. These penetrate to a depth of 5 m beneath the seafloor. To measure the variation in gas release, acoustic instruments send their beams up through the water column to acoustically visualize gas bubbles. Hydrophones are placed at strategic locations to allow acoustic detection of earthquakes and large scale sediment movements. Accelerometers measure seismic activity, and along with inclinometers, measure associated sediment movements (strain). Seasonal groundwater flows are measured by the combination of piezometers and thermistors.

    SLIP_fullsize Details of the overall Seismic Piezometer (see diagram)

    1. Trawl-proof housing.
    2. Deployment post. This will fit inside a sleeve containing weights for push-in deployment at a controlled drop (1 m/s). The weight sleeve will be removed after deployment.
    3. Electronics package. To contain power supply, data acquisition device, data processing device and communications via RS-485 to external network.
    4. Optical Backscatter sensor, or other measure of suspended sediment.
    5. Absolute pressure gauge/transducer. This will measure hydrostatic pressure at depths between 15 m and 150 m.
    6. Differential transducer. To measure the pore pressure at the bottom of the instrument relative to hydrostatic pressure.
    7. Zero volume change valve. This will allow the connection to pressure transducers to be closed, necessary for instalment of the instrument, as well as for maintaining a constant hydrostatic pressure for measurement of surface waves at the seismic pressure temperature tip.
    8. Mudplate. To hold the electronics package (3) and instruments (4,5,6) above, and the extension to instrument package below.
    9. Rigid extension. Must have internal conduits to allow cabling and/or tubing from lower instrument package to the differential pressure transducer and electronics package.
    10. Seismic Pressure Temperature Tip. See following description

    Additional specifications:

    1. All connections between instruments and electronics package must be water tight.
    2. All structural components must be corrosion resistant under salt water conditions.
    3. All instruments must communicate via RS-485, RS-232 or analog (0 to 10V), and must operate on 5 to 24 (preferred) volts.

    SLIP_tip_fullsizeSLIP tip (see diagram)

    i. Porous (filter) entrance and conduit to sensing face of the pore pressure transducer.
    ii. Pressure transducer conduit. A transducer will be located above the mudplate, and will measure pore pressure at the cone tip.
    iii. Inclinometer- to be used as redundancy to accelerometer tilt.
    iv. Thermistor for measurement of pore water temperature.
    v. Triaxial accelerometers and tilt sensors. Will be used to measure natural seismic events and imposed conventional seismic signals.
    vi. Triaxial geophones (seismic velocity transducers). Will be used to measure natural seismic events and imposed conventional seismic signals.
    vii. Electronics and amplification.
    viii. Casing with Pipe extension joint. This joint will allow wiring and a pressure conduit to pass through from the SPT tip to the extension shaft.
    ix. Pigtail to carry power to the devices and return amplified signals to the surface (6m).





    Schedule

    The work completed to date includes review of existing data, site survey, equipment design, purchase of instruments, contracting for the build of prototype instruments, and testing of these instruments. A borehole piezometer installed 93 m below Roberts Bank is currently sending live pore pressure data to VENUS. Several scientific cruises have been conducted to examine the oceanographic, sediment dynamics and geotechnical characteristics of the piezometer locations. The DDL has been deployed for a short time in 2008 (see Lintern and Hill, 2010) and is scheduled for redeployment by June 2011. The SLIP array will be deployed in late 2011 or early 2012.


    Acknowledgements

    Instrumentation for this portion of VENUS was largely funded through the original VENUS grant to the University of Victoria from the Canadian Foundation for Innovation. Instrumentation and expertise has also been provided by Prof. Gilliane Sills, Department of Engineering Science, Oxford University. Background survey and planning work, including shiptime, is funded by the Earth Science Sector of Natural Resources Canada through the Public Safety Geoscience Program. Additional planning for dredged sediment disposal experiments in the study area have been supported by Environment Canada’s Disposal at Sea Program.

    For more information contact:

    Dr. Gwyn Lintern, Project Manager
    Natural Resources Canada
    Institute of Ocean Science
    9860 West Saanich Road
    P.O Box 6000 / B.P. 6000
    Sidney, BC, V8L 4B2
    Tel. (250) 363-6416
    Fax: (250) 363-6565
    E-mail


    References

    • Lintern and Hill, 2010. An Underwater Laboratory at the Fraser River Delta. Eos Trans. AGU, 91, 38, doi:10.1029/2010EO380001.
    • Barrie, J.V. and Currie, R.G. 2000. Human impact on the sedimentary regime of the Fraser River Delta, Canada. J. Coastal Res., 16, 747-755.
    • Chillarige, A.V., Morgenstern, N.R., Robertson, P.K., and Christian, H.A. 1997. Liquefaction and seabed instability in the Fraser River delta. Canadian Geotechnical Journal, 34: 520-533.
    • Chillarige, A.V., P.K. Robertson, N.R. Morgenstern, and H.A. Christian, 1997. Evaluation of the in situ state of Fraser River sand. Can. Geotech. J. 34: 510-519.
    • Christian, H.A., Mosher, D.C., Barrie, J.V., Hunter, J.A. and Luternauer, J.L. 1998. Seabed slope instability on the Fraser River delta. . In: Geology and Natural Hazards of the Fraser River Delta, British Columbia, (Eds. J.J. Clague, J.L. Luternauer, and D.C. Mosher), Geological Survey of Canada Bulletin 525, p. 217-230.
    • Christian, H.A., Mosher, D.C., Mulder, T., Barrie, J.V. and Courtney, R.C. 1997a. Geomorphology and potential slope instability on the Fraser River delta foreslope, Vancouver, British Columbia. Canadian Geotechnical Journal, 34, 432-446.
    • Christian, H.A., Woeller, D.J., Robertson, P.K., and Courtney, R.C. 1997b. Site investigations to evaluate flow liquefaction slides at Sand Heads, Fraser River delta. Canadian Geotechnical Journal, 34, 384-397.
    • Hart, B.S., Prior, D.B., Barrie, J.V., , Currie, R.G. and Luternauer, J.L. 1992. A river mouth submarine failure complex, Fraser Delta, Canada. Sedimentary Geology, 81, 73-87.
    • Hill, P.R. (2006). Evaluation Of The Sand Heads Disposal Site And Feasibility Of Beneficial Use Of Dredge Spoil On Roberts Bank. Report to Environment Canada, Disposal at Sea Program, June 2006, 26 pp + 23 figures.
    • Kostaschuk RA, Luternauer JL (1989) The role of the salt-wedge in sediment resuspension and deposition: Fraser River estuary, Canada. Journal of Coastal Research 5:93-101
    • Kostaschuk, R.A., Luternauer, J.L., Barrie, J.V., LeBlond, P.H. and Werth von Deichmann, L. 1995. Sediment transport by tidal currents and implications for slope stability: Fraser River Delta, British Columbia. Can. J. Earth Sci., 32, 852-859.
    • McKenna, G.T., Luternauer, J.L., and Kostaschuk, R.A., 1992. Large-scale mass-wasting events on the Fraser River delta front near Sand Heads, British Columbia. Canadian Geotechnical Journal, 29, 151-156.
    • Mosher, D.C. and Hamilton, T.S. 1998. Morphology, structure and stratigraphy of the offshore Fraser delta and adjacent Strait of Georgia. . In: Geology and Natural Hazards of the Fraser River Delta, British Columbia, (Eds. J.J. Clague, J.L. Luternauer, and D.C. Mosher), Geological Survey of Canada Bulletin 525, p. 147-160.
    • Ricketts, B.D. 1998. Groundwater flow beneath the Fraser River delta, British Columbia: a preliminary model. In: Geology and Natural Hazards of the Fraser River Delta, British Columbia, (Eds. J.J. Clague, J.L. Luternauer, and D.C. Mosher), Geological Survey of Canada Bulletin 525, p. 241-255.
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