Disconnecting Weeping Tiles

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.

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Understanding and Assessing Impacts

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.

Identifying Actions

Recognizing that it had a major problem on its hands, the City of London commissioned a study looking into the means of solving the issue. The subsequent report indicated that the most cost-effective method of reducing sanitary sewer overflows and basement flooding would be to address the issue at its source: the point at which rainwater is entering the sanitary sewer system. The report indicated that a $2 million investment in source control methods would have a greater impact on flood reduction than a $10 million investment in upgrading protective infrastructure. As a result of this study, the City of London decided to embark on a pilot program of disconnecting inflow sources. They had previously enacted a sump pump program in the 1990’s, and increased the payments again in 2009, but had received very little uptake from citizens. Given this, it was decided that the city would need to undertake substantial efforts in order to gain citizen participation in this program. After all, weeping tiles and other inflow sources are primary located on private property, which severely limits the City’s legal rights to enforce change. Computer modelling selected 65 homes in the target Sherwood Forest neighbourhood and the affected homeowners were notified by mail, phone, and public meetings.

Implementation

In order to enhance citizen cooperation levels, a full-cost recovery was allocated, as well as an additional $1000 payment to help offset future maintenance needs. Working with private-side sewer connections provided several major complications to the project. It required working directly with homeowners as their express permission was required in order to conduct operations on their property; this, in turn, required the city to take additional measures to ensure compliance such as police record checks for all contractors and their staff, additional requirements for liability insurance, and comprehensive insurance coverage for all employees. Obtaining buildings permits for each individual house also required coordinating with the city’s building division. Working inside private homes also required that several individual agreements needed to be signed prior to each retrofit, and that contractors were able to work under a flexible schedule.

Outcomes and Monitoring Progress

Since the completion of the program, there has been a notable decrease in the total volume of water in the city’s sewer during extreme rainfall events and, additionally, none of the participating homes have experience a sewer backup event since conducting the operation. One of the notable lessons learned, as reported by contacted staff, was the necessity of spending time educating the homeowners about the project and ensuring that they fully understood what was required from them, what work was being conducted, and what the anticipated benefits were.

Resources

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Using climate change projections enables better adaptation decisions, as it allows you to better understand how the climate may change. To learn how to choose, access, and understand climate data, visit ClimateData.ca’s Learning Zone.
For more information on variables that may be useful in work related to stormwater and wastewater management, visit ClimateData.ca and click “Explore by Variable”. Here you will find pertinent future climate projections related to:

  1. Historic and climate change scaled IDF data
  2. Total precipitation
  3. Maximum 1-day total precipitation
  4. Wet days (>1mm, >10mm, >20mm)