This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Canada's capital, Ottawa, was bombarded with 167 millimeters (6.6 inches) of rain in five hours earlier this summer in what the city called a 1-in-200-year event. Close to 5,800 reports of basement flooding followed, and the worst of it hit the city's west end, including the Crystal Beach neighborhood.
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 affect 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 neighborhood as it stands today, plus rain gardens on private lots, along streetside municipal rights-of-way (roads, walking paths or bike paths), and in both places at once.
Our results showed that placement matters far more than how much land the gardens cover. At the storm's height, without rain gardens, standing water covered 8.4 hectares (21 acres) of Crystal Beach, or about 12% of the neighborhood. We counted ground as flooded once the water level reached 15 centimeters (6 inches) above ground, a threshold widely used in urban flood modeling 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 (15 acres), a 27% reduction. Streetside right-of-way gardens alone cut it far more, to 2 hectares (5 acres), a 76% reduction, despite occupying less ground. With gardens in both places, the flooded area fell to 1.2 hectares (3 acres), an 86% reduction, and the deepest water dropped from about 1.5 meters (5 feet) to just over 1 meter (just over 3 feet).
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