A horrible rain stroke Toronto this week on June 16th, 2024. The press says three thunderstorms poured down at the same time in three hours.

According to the precipitation measurement, it’s a 100-year rainfall event, but the press also says that this is the third 100-year rainfall in the past 11 years, which rings an alarm to me. 

Many online sources including some government websites explain the "100-year rainfall" as that the chance to have such a rainfall is 1% in every year. This is correct theoretically, but it doesn’t reveal the real meaning about this term. It’s like telling the truth by a riddle.

The fundamental meaning of the term “100-year” is about the return period of an event. It means that based on the historical data on average such a rainfall happens every 100 years. So, when you heard that this is the third 100-year rainfall in the past 11 years, something just clicks.

When scientists categorize a rainfall by return period, historical data are used to define what is a typical 100-year rainfall. Based on the current trend, this benchmark 100-year rainfall could be out of date. The old 100-year rainfall could be, for example, the new 50-year rainfall or even 10-year rainfall.

When looking at the historical data of Toronto, I see the increase of temperature is astonishing, like an evidence of climate change slap on my face, but the change of precipitation is not that obvious. However, this recent “third” 100-year rainfall in the past 11 year is alarming. If the trend stands, it adds another inconvenient proof for that climate change is real.

Consequently, the government guideline on the assessment of new constructions should be updated.

When the authority approves a new construction, an assessment must be conducted to assess what impact the new construction can bring to the regional flooding under storms.

Most of urban flooding is caused by insufficient drainage system and sewer overflow. Here are a few possible examples:

• Some new constructions turn a green land to a grey concrete, reducing the land capacity to absorb rain water and routing a large amount of rainwater to underground sewers.

• Some new constructions are built on old cities, whose aging drainage system already has a lot of difficulty to deal with storms. With more habitants moving in to the region and therefore, an increase of household wastewater discharge, the old sewer is surcharged more easily and frequently in raining days. In such cases, old pipes require retirement, and new pipes should be installed.

Some constructions have more impacts than the other. The flood assessment must tell what impact a new construction can bring to regional flooding and what measures should be taken to mitigate the impact.

Required by government guideline, such assessments are usually conducted using benchmark rainfall events with return periods like 10 years, 50 years, 100 years, etc., and vulnerable areas often requires an assessment under 100-year rainfall.

However, if the 100-year benchmark rainfall is out of date, we are underestimating the impact and downgrading prevention and mitigation criteria, for it’s not a 100-year benchmark rainfall but instead, a 50-year rainfall or even 10-year rainfall.