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Causes of basement flooding

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Learn how the wastewater and storm systems work, the main sources and causes of basement flooding, and what we’re doing to help reduce the risk in your community.

How the wastewater and storm systems work

In Halton Region, municipal stormwater and wastewater travel through two separate underground systems and they’re not connected. That’s important for a few big reasons:

Health and safety

Keeps untreated sewage from entering public spaces or bodies of water and reduces the spread of waterborne diseases.

Environmental protection

Helps protect ecosystems and wildlife ensuring sewage doesn’t get into rivers, lakes or streams.

Flood prevention

Prevents overflows when stormwater systems are overwhelmed during heavy rain, which can flood streets and homes.

Components

There are three key parts to how water moves in and around your home:

Halton Region is responsible for the wastewater system which includes wastewater mains, maintenance holes and the public side of the wastewater lateral (main to property line). We also manage wastewater treatment plants and pumping stations.

Halton is also responsible for Regional Road storm sewers and catch basins.

Local Municipalities are responsible for stormwater systems and drainage, which includes creeks, storm ponds, culverts, roads, ditches and swales.

Local Municipalities are also responsible for local Municipal local road storm sewers and catch basins.

Homeowners (private side) are responsible for plumbing, foundations drains (e.g., weeping tiles), downspouts, sump pumps and sewer laterals on private property. These connect to either the storm or sanitary system, depending on your set up.

Homeowners are also responsible for any subsurface waterproofing systems, proper lot drainage and maintaining ditches, backyard swales and driveway culverts located on private property.

Note: Local Conservation Authorities are responsible for managing natural watercourses like rivers, creeks, and floodplains. They also monitor flood levels in our region. More information and the current river & stream flood status (external link).

Diagram of house that shows homeowner responsibility, property line, storm lateral, wasterwater lateral, storm sewer main and wastewater sewer main

A wastewater/sewage pipe carries wastewater away from your home before it is treated and returned to the environment. This includes wastewater sources from your toilet, sinks, showers and indoor plumbing fixtures.

A storm/rainwater pipe carries rainwater and melted snow runoff from streets, parking lots, roofs, and other surfaces directly back to the environment – like a river or the lake.

Key causes of basement flooding

Flooding most commonly occurs during extreme rainfall events, when a large amount of water falls in a short time. When stormwater can't drain fast enough, it builds up and can cause different types of flooding, sometimes all at once. This raises the risk of basement flooding and property damage.

Common types of flooding

Surface (overland) flooding happens when rainwater has nowhere to go. It can’t soak into the ground or drain away quickly enough causing water to collect on streets, yards, and around homes and buildings.

Riverine flooding happens when rivers, creeks or streams rise beyond their banks and water spreads into surrounding low-lying areas, streets and properties. This overflow adds to rainwater already on the surface from overwhelmed storm sewers or saturated grounds, intensifying surface flooding.

Storm sewer flooding can happen when the municipal storm water system is overwhelmed and cannot carry away runoff quickly enough, leading to water backing up onto roads, properties or into catch basins. Homes with external drains connected to their plumbing or direct storm sewer connections can also experience backups in their basements.

Wastewater system (sanitary sewer) backup flooding can happen when the wastewater system becomes overloaded, often due to stormwater getting into it, causing sewage to backup into homes through floor drains and plumbing fixtures.

Wastewater flooding and impact of Inflow and Infiltration (I&I)

During extreme rain events, the primary cause of flooding is usually too much stormwater or groundwater entering the wastewater system. This is called Inflow and Infiltration (or I/I).

Inflow

Stormwater (rain and snowmelt) that gets into the sanitary system through direct private storm drain connections (e.g., downspouts and weeping tiles) and vent holes in maintenance hole covers.

Infiltration

Groundwater that leaks into pipes through cracks, joint failures and holes in aging sewer pipes, maintenance holes and sewer laterals (public and private).

The majority of the inflow and infiltration comes from private side/homeowner sources, including:

Improper plumbing connections - foundation drains (weeping tiles), some reverse driveway drains, outdoor stairwell or patio drains, and downspouts (roof leaders) connected improperly or illegally to the wastewater system.

Homes flooded with stormwater - when stormwater floods basements, it has a direct path to the wastewater system through floor drains and other plumbing connections, increasing the downstream community’s risk of basement flooding from wastewater system surcharging with watered down sewage.

Exterior cleanouts with broken or missing caps – surface water can enter into the cleanouts outside the home or near property lines and enter the wastewater system.

Leaky private wastewater laterals – when groundwater enters the private side of wastewater drainage pipes through cracks, holes and joints.

Other sources of stormwater inflow and infiltration to the wastewater system include:

  • Submerged maintenance holes – surface water can enter into maintenance hole lids rims and vent holes if they are submerged by storm water. Groundwater can also enter into the maintenance hole structure through cracks, holes and joints.
  • Leaky wastewater mains – when groundwater enters the wastewater main through poor lateral connections, cracks, holes and joints.
  • Leaky public wastewater laterals – when groundwater enters public side of wastewater drainage pipes through cracks, holes and joints.

Reducing flood risk & how Halton Region is taking action

Halton Region has invested millions of dollars in optimizing the wastewater sewer system throughout the community. Part of our core program includes investments that inspect, monitor and analyze the system to identify system improvements. These improvements help ensure that the Region’s wastewater sewer system is more resilient to more frequent and severe localized wet weather and prevent sewer back ups during dry weather. Learn more about ongoing work to improve Halton’s wastewater collection system to reduce the risk of future basement flooding throughout the community.

Halton Region has partnered with residents that have experienced basement flooding due to lateral blockages to repair or line their laterals.

Halton Region also offers subsidies to help with the cost of home improvements that can reduce flooding risk. Learn more about our Basement Flooding Prevention Subsidy program.

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