Board Adjusting Outflows To Assist Salvage Efforts
August 3, 2015
The International St. Lawrence River Board of Control has agreed to allow temporary flow fluctuations at the Moses-Saunders Hydropower Dam in order to assist in efforts to salvage a sunken tug boat that remains located in the North Channel of the St. Lawrence River at Cornwall, Ontario.
Two tugs sank here on Monday, 22 June 2015, in separate incidents. These tugs were involved in an attempt to move a third vessel (a work barge) into place to facilitate the demolition of the old international bridge. This reach of the St. Lawrence River is characterized by very strong currents, which were a factor in the sinkings and which have created challenges with respect to the salvage operations. Salvage efforts have been ongoing since the sinkings, and the Board and its associates have been in communication with the salvage experts during this time. One of the sunken tugs was successfully removed on Saturday, 1 August, 2015.
The remaining tug is expected to be removed this coming week, and the Board will allow outflows to be decreased during daylight working hours to reduce current velocities and the risk of hazardous conditions in the vicinity of the remaining sunken vessel. The Board specifies the outflows from Lake Ontario, generally according to Regulation Plan 1958-D, as required in the Orders from the International Joint Commission. The current Plan-prescribed outflow is 8520 m3/s (300,900 cfs). The Board has agreed to allow 14-hour long daytime outflow reductions for up to seven days to facilitate the remaining salvage operations. Starting on Tuesday, 4 August, outflows will be decreased to as low as 6810 m3/s (240,500 cfs) each day by 6 am, and ramped back up to Plan flow each night starting at 8 pm.
Water level impacts of the flow fluctuations will be most evident on the St. Lawrence River. The decreased outflows expected during the salvage operations will result in water levels on Lake St. Lawrence rising during the day, but the gates at Iroquois Dam will be lowered partially into the water to help maintain levels below the upper alert level of 73.87 m (242.35 ft) upstream of Moses-Saunders Dam. However, strong winds, especially those with a westerly component, may result in temporary high levels in excess of this threshold. When outflows are ramped back up each night, levels on Lake St. Lawrence will temporarily decrease by as much as 80 cm (31.5 in).
Water levels downstream of Moses-Saunders will also fluctuate with the daily varying outflows, with levels declining during the day and rising during the night. However, the Beauharnois Dam will help attenuate these fluctuations on Lake St. Francis, and also on Lake St. Louis, near Montreal, where inflows from the Ottawa River will also contribute. The impact of the outflow deviations on water levels is estimated to be at most 15 cm (5.9 in) on Lake St. Francis, and 20 cm (7.9 in) near Montreal.
The net effect of the flow variations will also cause as much as 3 cm (1.2 in) of water to be temporarily stored on Lake Ontario (relative to Plan 1958-D). This water will then be removed from Lake Ontario as quickly as possible following the removal of the remaining tug. The Board currently anticipates being able to complete this process within a period of between five to eight weeks.
Lake Ontario recently peaked at 75.23 m (246.82 ft) on 8 and 10 July, 14 cm (5.5 in) below the monthly mean upper criterion (h) limit of 75.37 m (247.3 ft). Lake Ontario was 22 cm (8.7 in) above long-term average as of 29 July 2015.