Excess inflow and infiltration is a common cause of sanitary sewer backups and basement flooding. Infiltration describes water that comes from the surrounding soils and enters sewer via cracks in the pipes and leaky connections. Inflow is when clean rainwater enters the sanitary sewer via direct connections. One major source of inflow is when weeping tiles are connected directly to the sanitary sewer system. This kind of connection is rarely, if ever, allowed now but used to be quite common prior to about the 1980’s. In many cities in Canada, residential developments built prior to the 1980’s are a major contributor of inflow and, therefore, are factors contributing to sewer backup flooding. One particular neighbourhood in London, Sherwood Forest, was identified as being a major contributor to the series of basement flooding events that struck the city over many years. This particular neighbourhood was built in the 1970’s and 1980’s, prior to the weeping tile disconnection bylaws of 1985. The problem is further exacerbated by the soil conditions, which a highly compacted clay that do not infiltrate water easily, leading to heavier reliance on weeping tiles and other constructed water conveyance mechanisms, and a relatively high degree of impermeable surface, which also prevent the absorption of water into the ground.
After a series of widespread basement flooding events, in 2013 the City of London began a program to disconnect weeping tiles from the sanitary sewer system. Weeping tiles are some form of porous pipe that is used for water conveyance and dispersal, usually set within a trench filled with aggregate material. When this weeping tile is connected to the sanitary sewer lateral, it drains rainwater directly into the sewer system and can lead to basement flooding as the added rainwater fills the sewer system beyond capacity, causing it to back up into homes. For decades it has been standard practice all across Canada to ban the connection of weeping tiles to the sanitary sewer lines. However, in subdivisions built prior to the 1980’s it is common for many homes to do just that. London’s pilot program targeted one specific neighbourhood in which many older homes had such connections.