As part of our 70th anniversary, we’re highlighting stories that connect our past with our present through a monthly series. In this article, we commemorate 70 years since Hurricane Hazel and the evolution of our flood warning and forecasting program to protect people and property from flooding.
Hurricane Hazel
In October 1954, Hurricane Hazel caused extensive damages and claimed 81 lives in Ontario. Following this devastation, the Province of Ontario and conservation authorities, including CVC, took action to help communities prepare for and reduce flood risks in the future.
Protecting People and Property through Flood Warning and Forecasting
In 1956, we initiated our flood forecasting and warning system as one of the ways to help protect people and property from flooding. We began monitoring watershed conditions, including rainfall, snow accumulation and stream flows. When flooding is possible or about to occur, we issue messages to municipal emergency management officials, the media and residents.
While the warning system helps communities prepare for flooding, improved stormwater infrastructure is important for flood prevention. At the time of Hurricane Hazel, there was no stormwater management. Excess runoff flowed through sewers and emptied into downstream creeks, rivers and lakes.
As the watershed became more urbanized with hard surfaces from pavement and buildings, less stormwater could absorb into the ground. More infrastructure was needed to support growing development. This included building larger conveyance systems to carry stormwater and stormwater ponds to hold rainfall and release it slowly back into local waterways.
Seventy years after Hurricane Hazel, there has been progress with the introduction of new stormwater management practices and better flood forecasting and warning system, but there is still more work as the frequency and intensity of storm events is anticipated to increase with changing climate conditions.
The Science of Flood Forecasting
Our Real-time Network
Since 2014, we have improved our real-time monitoring capabilities with a network of climate stations, rainfall and stream flow gauges. Today we have 59 gauges across the watershed that capture information in real time, alerting us to changes in weather conditions and water levels.
Using data from our gauge network and alerts from Environment Canada about inclement weather conditions, we run flood forecasting models to predict water levels based on weather forecasts and other factors like soil types and land use. This helps forecast flooding and predict impacts.
Differing Types of Flooding
Flooding can occur at any time of the year and can occur as the result of:
- Riverbanks along the Credit River and its tributaries overflowing. This is called riverine or fluvial flooding.
- When stormwater infrastructure in our communities is overwhelmed. This is called urban, overland or pluvial flooding.
For the Credit River, the largest floods usually happen in February through April due to river ice jams and winter rainfall, and snow melt, called the spring freshet.
Major flooding can also happen in fall from remnant hurricanes, but is less common.
For smaller creeks, especially in urban areas, the largest floods usually happen between June and September due to intense summer thunderstorms.
Flooding can also occur along the Lake Ontario shoreline due to high lake levels and wind and waves. This is uncommon, but usually happens between May and July.
2024 Storm Events
This summer we saw two storm events that led to flooding in the Credit River Watershed. The rainfall gauge at Pearson Airport, located just outside of the Credit River Watershed, experienced its #1 and #6 wettest days, wettest month, and wettest summer on record.
On July 16, over 100 milimetres of rain fell across parts of Mississauga, which caused severe flooding, both urban, overland and riverine. Moderate to major flooding also occurred in parts of Brampton and Halton Hills, which saw 60 to over 75 millimitres of rainfall.
On August 17, 90 to over 130 milimetres of rain fell again in Mississauga, but over a smaller area compared to the July 16 storm, causing major to severe flooding which was mainly urban overland flooding, with some riverine flooding. The heaviest rainfall occurred in east Mississauga and Etobicoke, just outside of CVC’s watershed.
Rainfall amounts varied significantly for both storms, depending on location and the data source.
Alertable Makes Receiving CVC Flood Warnings Simple
CVC issues flood warnings directly to watershed residents through our website, Facebook account, X (Twitter) account, and through Alertable.
Since 2020, we’ve used the Alertable App, a third-party emergency alert service, that offers residents near real-time CVC flood alerts. It’s free and you can choose to receive alerts through push notifications on your mobile device, SMS (text message), email or phone call.
Alertable helps residents living in flood-prone areas stay up-to-date on CVC flood warnings without having to actively search for that information.
Every second counts in a flood emergency. Sign up on the Alertable website today.
We also post data in real-time on our website. Go to the precipitation table to view recent water levels and rainfall information.
By: Jeff Wong, Senior Water Operations Engineer and Graeme MacDonald, Engineer, Hydrology and Hydraulics