The City of Mississauga is crediting its underground stormwater infrastructure for protecting residents and area infrastructure against the impacts of a major storm earlier this month.

On July 16, Mississauga experienced a 100-year storm with approximately 106 mm of rainfall in only a few hours – much more than the average 75 mm the city typically receives for the entire month of July. Heavy rains falling in a short period of time caused high water levels and localized flooding on roads and along creeks, rivers and streams.

While some parks flooded, trails washed out, embankments eroded, debris flowed into neighbourhoods, and some residents experienced property damage like basement flooding or water pooling in their backyards, the city says the impacts could have been far greater without stormwater infrastructure actively managing the situation.

Most of the unusual quantity of rain was managed through a network of underground pipes, designated stormwater ponds, culverts and overland flow routes along roadways.

Separate from the sanitary wastewater system, the city’s stormwater system collects stormwater from properties and carries it to a receiving waterbody. Maintaining this system lessens the risk of flooding, reduces creek erosion and protects water quality.

Projects funded by the city’s stormwater charge, such as the construction of Lake Saigon—a stormwater management pond at Matheson Blvd E. and Hurontario Street—protected flood-susceptible lands downstream within the Cooksville Creek watershed. These infrastructure improvements were key in mitigating the impact of flooding within the city, highlighting the importance of the city’s stormwater charge as a critical investment.

Last June, the city approved its first stormwater master plan. The plan will refine the city’s stormwater program, management systems and infrastructure.

The city’s program to build, expand, and maintain its stormwater system is fully funded over the next 10 years because of the self-funded stormwater charge. However, there is still a need to appropriately contribute to the Pipe Reserve Fund to allow for the future replacement of the city’s aging storm water pipes.

The city is seeking a commitment from all levels of government and is advocating for additional funding to help bridge the infrastructure gap.

This latest storm isn’t the first time Mississauga has experienced heavy rains.

On July 8, 2013, the city experienced a storm where more than 126 mm of rain was recorded at Toronto Pearson airport over a stretch of 10 hours. Another recorded storm was in 1954 – known as Hurricane Hazel – and dropped more than 200 mm of rain over Mississauga in 24 hours.

Stormwater infrastructure is sized to convey flows based on peak intensity. For example, 50 mm of rain falling intensely over two hours is more likely to cause issues in the urban environment than if 50 mm of rain fell slowly over two days. In addition to peak intensity, the total amount of rain can overwhelm storm drainage systems. For example, 50 mm of rain requires greater system capacity than 5 mm.

These two factors (intensity and total rainfall) help define storms by assigning a probability of occurrence or specific return period. More intense and higher-volume storms occur theoretically less often, whereas less intense, lower-volume storms can occur several times every year. A so-called ‘100-year storm’ has a probability of occurring of 1 per cent on any given year.

Currently, Mississauga has more than 1,900 kilometres of storm sewer pipes in its drainage system. If laid out end-to-end, these pipes would connect Mississauga to Winnipeg. The stormwater drainage system also includes more than 51,000 catchbasins, 270 kilometres of ditches, 150 kilometres of creeks, and 81 stormwater management facilities that help to collect, drain, and clean the city’s rainwater runoff before it enters Lake Ontario.