How cities can use rain gardens to help prevent flooding

A rain garden in Calgary, Alberta. A rain garden, or bioretention cell, is a shallow sloped area filled with plants and soil. (Maureen Flynn-Burhoe), CC BY How cities can use rain gardens to help prevent flooding Published: September 2, 2026 12.54pm EDT Share article Print article 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 A graphic explaining how rain gardens work. (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…