Are we prepared for an overwhelming storm?

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12 July 2013

The Greater Toronto Area (GTA) has recently experienced an extraordinary storm which inundated the region with water in a 24-hour period. The area’s infrastructure was strained under the heavy rainfall, and widespread flooding resulted in commuter delays and impacted power supply significantly. Ken Chow, an experienced water resources engineer based in engineering, architecture and environmental consulting company GHD’s Mississauga office, weighs in on the overwhelming demands placed on the region’s storm water systems.

These recent weather events in the GTA far exceed the rainfall typically seen in the area. The most recent major storm event, Hurricane Hazel (1954) produced less rainfall over a one day period.

“Storm water systems, which are comprised of the flow of rain over city surfaces such as roads, sidewalks and medians and the storm drains that collect this rain and move it underground, are typically designed with significantly large, unlikely storm events in mind,” explains Ken.

The upper boundary of rainfall that is considered during the design of storm water systems is termed a ’100 year storm’ which means that there is a one-in-one hundred chance that a storm of a certain magnitude will occur in the local area.

Ken continues, “100 year storm values are estimated by reviewing historic local rainfall data; the GTA’s 100 year storm estimate is 100 – 110 mm of rain over a 24-hour period. The recent storm dropped 127 mm of rain in 24-hours in the hardest hit locations.”

Concern over the function of regional infrastructure has grown as a result of the wide-spread flooding, however Ken explains that this isn’t unexpected: “As heavy rain enters the storm water system and runs downhill towards Lake Ontario, the system can become overwhelmed. With rainfall at the scale we’ve seen recently I would expect there to be some flooding, especially in urban areas with older infrastructure.”

The majority of the GTA’s urban infrastructure is 50 – 80 years old, which puts it at greater risk of becoming overwhelmed during an unexpectedly large storm.

“Surface flooding in Mississauga, for example, was generally less severe than in other areas. The newer infrastructure in Mississauga, which is approximately 20 – 40 years old, combines an underground system with overland flow routes and was able to handle the influx of storm water better; though flooding was still common and widespread.”

In some cases, older infrastructure may not be able to cope well with serious rain events in growing areas, as the larger urban footprint channels more storm water from surrounding areas into the system. Inefficiencies may also arise over time, which reduce the capacity of storm water the system can handle during an intense storm as seen recently. 

Ken explains, “There is no way to look into the future and know what magnitude of storm may hit. In this case, the 100 year storm threshold was surpassed, and the region’s infrastructure was overwhelmed. Our infrastructure couldn’t carry all of the storm water; it backed up, and this resulted in widespread flooding.

“In the future, the guidelines for storm water management will take this recent storm into consideration; however there is no way to know how these storms will impact the 100 year storm guideline. There has to be a balance between planning for the estimated worst case, and day-to-day operability and impacts on the community.”

“The use of the 100 year storm guideline, which is incorporated into infrastructure planning in the GTA, is the industry standard – storm water systems can’t realistically be designed with biblical proportions in mind – and it is extremely rare that the intensity of rainfall seen recently wouldn’t cause localised flooding in any system.”

The potential for flooding is known, and local governments in the GTA have already been proactively replacing pipe systems with larger capacity pipes in the last few years to reduce the likelihood of flooding. Designing systems to a higher standard (beyond the 100 year storm criteria, for example) would result in increased costs up front and higher long term maintenance costs.

Following the recent extreme weather, homeowners have been particularly impacted by surface flooding on their properties and in many cases into their basement.

“Basement flooding was more severe in areas with combined sewer system, which handles storm water and sanitary sewer water together. In the case of a major storm, a backup of the combined sewer system can result in a backup of storm water into the basement via basement flood drains

“This is why many basements were flooded – both the storm water and sanitary sewer systems were affected - although it can be expensive, considering a sewer backflow prevention device is one way homeowners can avoid the problem in the future,” concludes Ken.

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