Source: The Conversation – Canada
The Canadian capital of Ottawa was bombarded with 167 millimetres of rain in five hours earlier this summer in what the city called a one-in-200-year event. Close to 5,800 basement-flooding reports followed, and the worst of it hit the city’s west end, including the Crystal Beach neighbourhood.
Climate change is expected to make rainfall heavier. Short, intense downpours overwhelm drainage systems. Our research focuses on how rain gardens can help prevent flooding, and where cities should put them.
A rain garden, or bioretention cell, is a shallow sloped area filled with plants and soil. Water from a roof or road runs into it instead of a catch basin. Some soaks away, some drains out over a few hours and the rest reaches the sewer.
However, while rain gardens are useful, how and where they’re planted can impact how beneficial they are. Before the July 1 storm in Ottawa, we had already spent three years building a computer model of Crystal Beach.
Instead of testing a hypothetical storm, we ran the actual July 1 rainfall record through it four times: the neighbourhood as it stands today, plus rain gardens on private lots, along streetside municipal right-of-way areas (roads/walking or cycling paths) and in both places at once.
Our results showed that placement matters far more than how much land the gardens cover.
Read more:
New stormwater infrastructure is needed for Canadian cities to handle increased urban flooding
(Township of Langley)
What our model showed
At the storm’s height, without rain gardens, standing water covered 8.4 hectares of Crystal Beach, or about 12 per cent of the neighbourhood. We counted ground as flooded once the water’s height reached 15 centimetres above the ground, a threshold widely used in urban flood modelling because it’s roughly the height of a doorstep, the point at which water starts getting into buildings.
Our model showed that rain gardens on private lots alone cut that to 6.2 hectares, a 27 per cent reduction. Street-side right-of-way gardens alone cut it far more: to two hectares, a 76 per cent reduction, despite occupying less ground. With gardens in both places, the flooded area fell to 1.2 hectares, an 86 per cent reduction, and the deepest water dropped from about 1.5 metres to just over one metre.
Right-of-way gardens did more despite covering less ground because they sit on the routes that water already takes when going downhill. The timing shows the effect. Without gardens, the flooded area more than tripled between 2 p.m. and 2:45 p.m. as the drainage system was overwhelmed. With street gardens, it levelled off around 2 p.m. and receded from there.
The starkest impact was what happened to hazardous flooding: water fast enough to knock someone off their feet or float a car, moving faster than 0.6 metres per second. Without rain gardens, 1,095 square metres met that threshold at the peak. Private-lot gardens cut that by one fifth. Street-side gardens alone brought it to zero.
Read more:
Ontario floods: How nature-based solutions can promote effective flood management
Pipes and plants do different work
Gardens in both private lots and right-of-way areas also cut total runoff volume by 80 per cent and nearly halved peak flow through the pipe network, easing pressure downstream too.
Rain gardens demonstrate the complementary role green and grey infrastructure can play in water management. Pipes can move water away faster once it has already pooled; but they do nothing to change how much water gets there in the first place.
A rain garden works earlier in that chain, intercepting rainfall before it reaches the sewer system, so there is simply less water for the pipe to carry.
Studies of combined green and grey systems elsewhere find the same pattern: the two are complementary rather than interchangeable, and a network built with both tends to outperform one built around either alone.
Grey upgrades treat the symptom, moving water that has already accumulated. Green infrastructure reduces the runoff before it forms.
(City of Calgary)
What municipalities can take from this
Right-of-way rain gardens are the most defensible place to begin. The city owns the land, controls maintenance and doesn’t need to persuade a single homeowner. At least in this location, rain gardens also did more per square metre than gardens on private property.
What one storm shows won’t hold for every storm, though. That is an argument for testing drainage plans against real historical rainfall, not only the standardized storms codes are built around.
These are still modelled results, and models are kinder to stormwater controls than field monitoring is. Over time, installed rain gardens compact and silt up, so flooding hazards can return if gardens are not properly maintained.
Even with gardens in both places, the flooding did not disappear entirely: an area larger than two football fields was still under water at the peak, in the same low-lying pocket that flooded before. What changed was how much of the neighbourhood flooded, and how dangerous the water there was.
Jennifer Drake receives funding from the National Research Council of Canada.
Ali Zoghi does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
Original source: https://analysis1.mil-osi.com/2026/09/02/how-cities-can-use-rain-gardens-to-help-prevent-flooding/