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Flood Prevention


Lethbridge receives approximately 275 millimetres of rain each year, most of which falls from May to October. Our spring and summer storms tend to drop large amounts of rain over short periods of time. During major rainstorms the sh​eer volume of water can back up drainage systems and flood basements.

Municipal governments prevent flooding by maintaining the sewage systems. If you experience drainage problems, begin by ensuring that the source of flooding is not on your property. Take steps to flood proof your home. First, learn how and why flooding occurs.


Basic Information

What is a Sanitary Sewer?

A sanitary sewer is a pipe, located in the street that is designed to transport wastewater from your home. This consists of water from sanitary fixtures and floor drains inside your house, as well as groundwater from weeping tiles around the foundation of your home.

What is a Storm Sewer?

A storm sewer is a pipe, located in the street that is designed to carry storm related water runoff. Storm sewers are normally much larger than sanitary sewers because they are designed to carry much larger amounts of flow.

What Causes Sewer Backup?

Extra storm-related water (from sources other than wastewater and groundwater) should flow into the storm sewer or soak slowly into the ground without entering the sanitary sewer.

Sewer backup insurance is available on most homeowner policies for a small premium. Check with your insurance agent or broker.

If excess storm water does enter the sanitary sewer system, it causes a supercharged sewer flow. An eight-inch (20 cm) sanitary sewer can handle wastewater from up to 500 homes; however, it takes only a few unexpected water sources to overload this kind of system.

How can a Supercharged Sanitary Sewer Cause Basement Flooding?

A supercharged sewer flows at a greater than normal level. Basement flooding can occur if
the home has sanitary fixtures or floor drains below the supercharged level.

Downspouts and Roof Drainage?
Most homes are equipped with downspouts which discharge the water collected by eaves troughs directly into the ground. Excess water runs into the front street where it enters the storm sewer. It is very important that this water does not enter the sanitary sewer. 

If your downspout drains too close to the side of your house, this water can drain into the sanitary sewer through the weeping tile adjacent to your house foundation. Damage or sanitary sewer surcharging can occur if rainwater drains too close to your house on ground that may not be tightly compacted. The excavation for your basement may have been dug a few feet wider on all sides to allow working room during construction of the basement walls. When this extra space was backfilled, the soil may not have been tamped down as tightly as the original soil, making it more likely to settle and trap surface water.

Surface water soaking down to your foundation can create problems: it can damage your foundation; seep through cracks in your basement wall, causing dampness; or overload the sanitary sewer by draining through weeping tiles, causing a sewer backup.


The City of Lethbridge wants to be proactive in communicating with residents about how to protect their homes from flooding. We want to share with you the common reasons for basement flooding and the many things you can do to prevent basement flooding.


Common reasons ​for bas​ement flooding

  • Overloading of the wastewater and stormwater collection systems. For more detailed description click here.
  • Poor lot grading may contribute to basement flooding.
  • Plugged or damaged eavestroughs and downspouts
  • Downspouts not being extended
  • Water from window wells
  • Cracks in basement floor and foundation wall
  • Leaking weeping tile
  • Sump pump and backwater valve not working
  • Gutters and stormdrains plugged with debris.

 Sump Pump Design - Criteria for design


What can you d​o to help prevent​ basement flooding? 

  • If you are constructing a new home and plan to build on a slab, or install a washroom in the basement, consider taking the necessary precautions to prevent sewage and water from backing up into your house through the sanitary drains.
  •  You may also consider taking these precautions in an existing home, but these changes vary in complexity and cost.
  • You may be able to do some “flood proofing​” tasks yourself, while other changes need a qualified contractor or tradesman.
  • One aspect of flood protection involves the installation of backflow valves on toilets, floor drains, washing machine drains, rain downspouts, sump pumps and any sink drains in the basement. Main sewer lines and septic connections should also be considered. These are designed to prevent sewage and water from backing up through these waste lines. Some backflow valves operate automatically while others may have to be closed by hand.
  • In most cases the backflow valves may need to be installed by a qualified plumber or contractor according to existing building codes. Be sure you understand the type of backflow valves installed, including their operation and maintenance.
  • Your municipal office can advise on flood proofing in your geographic location.

Sewer Backup Protection Can Help

  • Talk to a qualified plumber before you install any sewer backup protection devices. Plumbing fixtures such as toilets, sinks, showers, floor drains and washing machines that are set below ground level require special protection from sewer backup. Read about the following devices and decide if they can help you.
    • Screw Cap. If you’re having trouble with your floor drains, a screw cap installed upstream of the trap seal of the floor drain may be all you require.
    • A Canadian Standards Approval (CSA) Inline Sewer Backwater Valve should prevent sewer backup if you maintain it properly and have it checked regularly. Your plumber can provide you details on maintenance.
    • A Gate Valve. Where a backwater valve isn’t sufficient protection, you may need a gate valve and a backwater valve for basement plumbing fixtures that are subject to backflow.  Sewer Backup Insurance. This insurance is available on most policies for a small premium. Check with your insurance agent or broker.

Take special note of the preparations you must take to protect your home and major electrical and gas-fired appliances.

  • Seal hazardous materials such as weed killers and insecticides in plastic garbage possessions from the basement to an upper floor. bags, and move them into safe storage or dispose of them safely

Things to check Inside your Home include:

  • ​Sump pumps
  • Backwater valves
  • Drainage






Things to check Outside your Home include:

  • ​Lot Grading
  • Eaves troughs and Downspouts
  • Window Wells





What do I do if my ​​basement is flooded?

Basement flooding-Next steps 

Proceed with caution if you have experienced flooding in your home

 If you smell gas, leave the house immediately. Contact the local utility for assistance. Do not touch any electrical fixtures, light switches or telephones in your home.

Do not go into a flooded basement until the fire department, utility company or licensed electrician has checked that it is safe to do so.

Electrical Appliances

  • At the first warning turn off electrical power and leave it off. If the basement is already wet, be cautious. If you are confident that you can do it safely, stand on a dry wooden chair or box and use a dry board or stick to turn off the main switch.
  • Water usually ruins electrical motors. Move all portable electric heaters, power tools and electrical appliances to a safe place.
  • If this isn’t possible, and if you are qualified to do so, consider removing the motors, controls and switches from furnaces, refrigerators and washers.
  • If you can, consider moving all thermally insulated appliances such as freezers and refrigerators to upper floors of the house. This precaution should save you the expense of replacing insulation if it gets wet.

Gas-fired Appliances

  • At the first warning of flooding, turn off all gas-fired appliances and put out the pilot light.  Then turn off the gas inlet valve to your home, which is located at the gas meter. This will prevent gas from escaping when the inlet valve is re-opened, in the event that any burner controls had been accidentally been left open.
  • Do not remove any gas-fired appliances without ensuring that the main gas inlet valve has been closed.
    There should be no smoking or open flames of any kind in the area. To avoid causing sparks, use a flashlight taped in the “on” position.
  • For a forced air furnace, move the fan motor and fan to a dry storage place if possible.
  • Do not drain the hot water tank — it will serve as ballast to secure the tank in place. If gas appliances are removed, the gas outlets should be made tight by means of a plugged valve, a cap, or a plug on the piping system.


  • Remove any basement toilet bowls. Plug these outlets and all other basement outlets, such as floor drains, sinks, laundry drains, etc., to prevent floodwater from entering them.
Source: Emergency Management Alberta

Who do I call?

  • Call your insurance company as soon as possible. Your insurance company will advise you on clean-up and processing a claim.
  • Once it is safe to do so, take photos of damaged areas and items before you repair or discard items.
  • Keep receipts for all emergency repairs, cleanups and damaged items.
Emergency: 911
ATCO Gas Emergency: In an Emergency 1-800-511-3447
Phone: 311 or 403-320-3111 (if outside of Lethbridge)

What can I do? 

  • When it is safe to do so, salvage your belongings as soon as possible to prevent rotting, mildew and warping. Thoroughly clean and disinfect all rooms and contents reached by flood waters.
  • Ensure area is dried out and has good ventilation as soon as possible  to avoid mold development. Open doors and windows and use air blowers to help this process. This could take several days.
  •  Contact your insurance company for more information. The City of Lethbridge does not clean up flooded basements.

Other Information and Resources

Protect Your Home From Flooding-A guide for Lethbridge residents

Handbook for reducing basement flooding - Designed for Safer LivingR is a program endorsed by Canada's insurers to promote disaster-resilient homes. Find out more about how you can reduce your chances and your neighbours' chances of having basement flooding.

For more information

Phone: 311 or 403-320-3111 (if outside of Lethbridge)
Online: Submit a service request