To what standard is the storm drainage system built?
Calgary’s underground storm-drainage pipes are designed only to handle low-intensity rainfalls, which are the majority of the rainfalls. In most communities in Calgary and other cities, the underground storm-drainage pipes are designed to meet the North American standard of draining one-in-five-year storms. To put this in perspective, the Calgary International Airport gets about 400 millimetres of precipitation annually. In Calgary, 2.54 cm (1 in.) of rainfall in one hour is a one-in-five-year storm and a storm-drainage system that is designed for one-in-five-year rainfalls can handle 80 per cent of the storms. A storm-drainage system that is designed for one-in-one-hundred-year rainfalls can handle 99 per cent of the storms. In most communities, the storm-drainage systems are designed to manage one-in-five-year rainfalls. In some older communities, the storm-drainage systems are designed to handle one-in-two-year rainfalls. All the communities built in the 1990s have a system of under
