Overflows and bypasses occur when sanitary sewers become overloaded during heavy rain events when too much rainwater enters the sanitary system, as shown below. The primary cause of this is the direct connection of weeping tiles to the sanitary system. It is estimated that there are approximately 50,000 homes within the City of London that have weeping tiles directly connected to the sanitary sewer. These flows can overwhelm the sewer system during heavy rainfall events which in turn can cause basement flooding through floor drains and basement plumbing fixtures, as well as sewer system overflows and bypasses.
The City of London is committed to system improvements for its sanitary and combined sewer infrastructure. Since 2008, over $40 million has been spent on separation of combined sewers and the mitigation of sewer system overflows. In total, 49 km of sanitary sewer has been replaced since 2008, some of which had previously been combined sewer. Construction of separate storm and sanitary sewers effectively reduce the volume of storm drainage diverted to the sanitary sewer system and prevent/ reduce sewer system overflows to the Thames River. The following map also highlights the areas within the City that remain to be separated as well as sections of combined sewer that have been replaced since 2013.
The Pollution Prevention and Control Plan was initiated in 2012 to identify, investigate and reduce these sewer system overflows through the design of a prioritized infrastructure replacement program roadmap.
The Basement Flooding Grant Program provides subsidies to homeowners to disconnect weeping tiles and protect themselves from basement flooding. The following figure highlights the amount of grant program money invested by the City each year and the total number of participants. Participation in the grant program increased following the intense rainfall events in 2015 and 2016, when a significant number of basement floodings occurred. Since 2008, there have been approximately 340 home owners participate and receive funding for the grant program. Recent changes to the program now cover 90 % of the costs of the program, to a maximum upset limit.