Workshop on landslide hazards and risk management in Canada
Authors: Couture R 
Published: November 2010

The Cohasset A-52 drill site of Petro-Canada is situated in 34 M of water approximately 25 kM southwest of Sable Island (lat: 43-51-09N; long:60-37-44W; see fig.1 ). It is located south of West Bar in an area dominated by shoreface-connected ridges. The drill rig
Rowan Gorilla was situated on the south flank of such a shoreface ridge during drilling operations of January - March, 1986. The local bed dips at a slope of approximately 3 degrees to the south and is composed of medium - coarse size siliceous sand. Seismic data suggests that this sand ridge has
migrated to the south in modern times. Thus the Cohasset site appears to be a region of net=sand infilling. The site is intermittently rippled and Mega-rippled by waves and currents. Sand transport and scouring around the spud cane is considered by the diving observers, Wolf Sub-Ocean Ltd., to be
significant. This is thought to be due to winter storm-driven currents and wave Motion. The Geological Survey of Canada was invited by Petra-Canada Ltd. to carry out a joint _study on the transport of sand at the Cohasset site in order to assess seabed erosion and deposition and the conditions under
which sand transport takes place, Six (6) storm events were experienced during this study. The Most severe wave conditions of the study were experienced during our arrival at the rig. During the subsequent detection period the most severe conditions experienced were: MAXIMUM SIGNIFICANT WAVE HEIGHT
............ 4.5 M; MAXIMUM MEAN WAVE PERIOD................... 7.2 s; MAXIMUM NEAR BEO CURRENT ................... 0.8 m/s; MAXIMUM WIN ? SPEED ........................ 27 m/s; Sand was seen in suspension, the bed was covered with hummocky megaripples and scouring around the spud cans was taking
place at the time of our arrival; that is during the passage of storm 1. Despite the passage of several subsequent storms active suspended sediment transport of sand was detected only under storm 6. The prevailing direction of net sediment transport appears to be to the west. Transport took place
predominantly as bedload. Wave and current formed ripples were detected during each of the UFO dives. These appeared to change size and orientation from dive to dive, and illustrated bedload transport of sand under storms 2 - 6. The Majority (90%} of the suspended particulate Matter detected in
the sampling program was organic. Diatoms, organic detritus and pellets were suspended by wave motion during the middle four storms of the study: 30 January, 2 February, 5 February and 6 February, 1986. Minor amounts &amp;lt;&amp;lt; 107.) of fine-grain sand were sampled throughout the
study, but appeared to bear no relationship to wave or current strength. Sand was suspended during storms 1 and 6 which occurred on 29 January and 10 February, 1986. It is concluded that the natural seabed is stable under the storm conditions 2 - 5 observed during this study. It is also concluded
that active transport of bottom sediment (Mean diameter - 0.4 MM) took place during the storm 1 (of the 29 January, 1986). Threshold conditions for sand suspension were exceeded during this storm and were barely reached during storm 6. Megaripples are intermittently present. They form and are
destroyed within days of the passage of storms.