With two torrential summer rainfalls happening in less than a month in Niagara-on-the-Lake, and similar weather patterns happening on a consistent basis in the Greater Toronto Area since June, the 100-year and 500-year rainfall events have become curious distinctions.
As a result, many are questioning the validity of these statements.
Using rainfall data and climate patterns from the Niagara Region, infrastructure engineer Steve Miller, from the Niagara Peninsula Conservation Authority, explains how to understand the 100-year rainfall phenomenon.
“You can think of it as a probability,” he said.
“The 100-year rainfall has a one per cent chance of happening every year. And much the same with a 500-year storm, which has a 0.2 per cent chance of occurring every year.”
So, using his explanation, these types of storms can happen more often than described, albeit the chances are slim.
In Niagara-on-the-Lake, the June 18 storm that flooded homes and property and washed out ditches and roadways, “brought up to 45 (millimetres) (1.75 inches) of rain in the eastern regions of the Niagara Peninsula watershed,” the conservation authority stated in a news release at the time.
However, during the same storm, Queenston residents Win and Kal Laar recorded 101 mm in just an hour at their Sheppard Crescent home.
For a storm to be classified as a 100-year storm in Niagara, Miller said four inches (101.6 mm) of rain must fall in 12 hours, while a 500-year storm is 11 inches (279.4 mm) over the same period of time, which would put the June 18 storm somewhere in the middle of the 100-year and 500-year classifications.
Geographical location is also important to determining the type of storm.
“But in other locations, like the west coast of B.C., that is a rainforest — their 100-year storm looks different,” he added.
In an interview with The Lake Report in July, Lord Mayor Gary Zalepa — incorrectly, based on the Laars’ calculations — used the 500-year rainfall calculation to defend the community’s drainage infrastructure.
“In the St. Davids area on that day, they experienced three-and-a-half inches of rain in less than an hour and that’s almost a 500-year rain event,” he said in noting the infrastructure is not built to manage that volume of water in that short period of time.
In Niagara, average rainfall amounts vary depending on where you are.
“For instance, our southern coast, the coast along Lake Erie or Port Colborne and Fort Erie. They get about 1,000 (millimetres) of rain a year,” said Miller.
“The north part, you know, St. Catharines, Niagara-on-the-Lake, gets a little bit less, about 890 mm. So the magnitude of the storm generally depends on where you are.”
