Snow-clearing is a science!
SPREADING : The right amount
Nothing is better than sodium chloride, or unrefined table salt, to melt snow and ice. Contrary to popular belief, calcium is not used. As for those small stones familiar to Montrealers, they do not melt snow but are abrasives that increase contact with the pavement, making it less slippery. These stones, measuring a maximum of 6 mm in size, are heavier than sand and are not washed away by rain but remain in place.
The proportion of melting elements to abrasive elements used on the pavement depends essentially on the temperature. Down to -10˚C, only salt is used. Between -10˚ and -15˚C, a mixture of equal parts salt and stones is applied. Below -15˚C, a mixture of 90% stones and 10% salt is used. The colder it gets, the less effectively sodium chloride works. The mixture applied to sidewalks varies depending on weather conditions, as well as ground conditions.
LOADING: A matter of judgment
The decision to load snow onto trucks depends more on ground and weather conditions than on a mathematical formula. A dozen centimetres of fallen snow before an extreme cold wave will be a priority, whereas a 30 cm snowfall can be left to melt during spring thaw.
To plan operations, teams receive five weather bulletins every 24 hours: at 2 a.m., 10 a.m., 2 p.m. and 10 p.m. for short-term forecasts, and a daily bulletin for long-term forecasts.
DUMPING: High-level planning
Once the snow is loaded onto trucks, it is carted away to one of 12 surface dump sites or 16 sewer chutes that direct the snow to the wastewater treatment plant. Some 100 snow removal sectors are assigned to one of these 28 sites. This is known as the snow dumping strategy. To ensure that the maximum amount of snow is dumped into the sewer system, this strategy is often modified over the winter to take into account the hourly and seasonal capacity of the 28 sites. For example, when the flow rate in the sewer system is reduced, the trucks are directed to the surface dump sites. The truck schedules are predetermined by district.