Divers cleared ‘frazil’ ice at city’s Pont-Viau drinking water intake
The City of Laval’s public affairs department released a series of photos on World Water Day last month, detailing a challenging scuba diving operation undertaken in January when a water intake on the Rivière des Prairies had to be cleared of crystalized “frazil” ice to keep Laval’s drinking water supply flowing.
It’s worth noting that the temperature on the day of the operation at the Pont-Viau water treatment station was – 30 degrees Celsius – making it all the more dramatic.
The frazil ice menace
According to several sources, including the Canadian Journal of Civil Engineering and the Cold Region Technical Digest, frazil ice is notorious for blocking water intakes, as ice crystals can accumulate and build up on a piece of protective equipment known as the intake trash rack.
January and February were exceptionally cold and led to conditions conducive to the formation of frazil
The trash rack prevents water-borne debris (such as logs, boats, animals or waterweed) from entering the water intake at the pumping station. However, at a certain point the frazil ice begins to adhere to objects in the water, and can actually damage trash racks.
A winter-time first
While the City of Laval’s water management department undertakes an inspection of all water intakes at least once a year, this was the first time an operation of this sort was conducted in mid-winter beneath freezing cold river waters and flowing ice.
The diving company hired for the job, SMM Service, sent a team that included an assigned diver, a backup diver if needed, and two support personnel.
Hours of submerged work
The equipment that was used to deal with the blockage problem included an ice axe and a pneumatic hammer, the latter being specially-adapted for underwater use. The operation lasted up to six hours, including preparation time.
Although the formation of frazil ice is considered to be rare in the Rivière des Prairies and the Rivière des Mille Îles – both of which are used by the city for drinking water intakes – the months of January and February this past winter were exceptionally cold and led to conditions conducive to the formation of frazil.