More often than not, basement flooding is a result of overland flooding, infiltration flooding or sewer backup, or a combination of two or all three of these types of flooding.
Canadian communities are designed to safely handle a certain amount of snow-melt and rainfall. When new communities are designed, municipal engineers and planners build underground sewer pipes and create overland flow routes to help convey stormwater and snowmelt away from urban areas. In many cases, streets, ditches, swales and even parks have been designed to carry water away from residential areas. All of these stormwater management structures and strategies can help reduce basement flooding.
Underground pipes are extremely expensive to build and install, and to offset this cost, municipal officials must build them to handle only a limited amount of rainfall and snowmelt. In most cases, underground stormwater pipes are designed to handle rainstorms that occur once every five years on average, or in other words, rainstorms that have a 20% chance of occurring in any given year. However, older parts of cities often have underground pipes that can only handle storms that occur once every two years on average, or in other words, rainstorms that have a 50% chance of occurring in any given year.
In newer subdivisions, including those built during and after the 1970s, overland flow routes were built to handle rainfall events that exceed the capacity of underground pipes. Overland flow routes may include streets with heightened curbs or ditches that run next to roadways. Generally, overland flow routes are designed to handle a rainstorm that has a 1% chance of occurring in any given year, or a rainstorm that might occur once every one hundred years. Again, overland stormwater management infrastructure must be designed to handle a limited capacity to help offset their considerable cost.
In the event that a rainfall or snowmelt event exceeds the capacity of underground pipes, or when a rainstorm exceeds the capacity of overland flow routes, overland flooding can occur. During the flood event, water will flow in areas that it usually would not, including through residential areas and over private properties. In older neighbourhoods, a lack of overland flow routes significantly increases the chances that overland flooding will occur. This flood water can then enter basements through windows, doors, vents and other openings. Overland flood water can also enter a home through reverse sloped driveways.
Although overland flooding will generally consist of relatively clean rainwater, it may wash soil and mud into your basement, or it may be tainted with chemicals, pet waste, salt, or other contaminates from city streets and local buildings.