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Watermain Flushing

What is watermain flushing?

If you notice a City crew operating a fire hydrant, flushing water down the street, they are actually cleaning the watermains in the area. Regular flushing is an important component of a comprehensive water management program. The City of Lethbridge is responsible for maintaining and operating 570 km of watermains and more pipe is added each year. Flushing the mains using water under pressure cleans out these pipes, along with the rest of the system. Flushing is generally done between April and October.


What is Unidirectional Flushing (UDF)?

UDF is the preferred method of cleaning watermains within the City of Lethbridge. In essence, the water system is operated in a way that enables the water to be flushed from the system in one unified direction. This preventative maintenance program is the most effective in scouring the water lines and minimizing the amount of water needed.

How does it work?

Traditional flushing involves opening a hydrant and allowing the water to exit the system for an extended period of time. This draws water from all directions within your distribution system and can be quite time consuming and wasteful.
UDF isolates a section of watermain so that water can only be drawn from one direction. By doing this, you actually increase the velocity of the flushing. This in turn will do a better job of removing any natural build-up in the distribution system. The flushing sequences are designed in such a way that service interruptions are minimal and you don’t flush “dirty,” water into “clean,” water.

Why is it needed?

  •  Scouring of the inside of the pipe
  •  Removal of sediments, built up silt and biofilms
  •  Maintain chlorine residuals
  •  Clear the system of any discoloured water
Watermains are designed to handle fire flow, which may be several times larger than typical residential or commercial water flow. Since the velocity of flow (or rate that water flows through pipes) is normally fairly low, material may accumulate on the bottom of the pipes. The problem may be more significant where there are dead-end pipes or areas of low water use. Over time, these deposits can be a source of colour, odour and taste problems in the water if the deposits are stirred up by periodic increases in the flow. Flushing the pipes at high velocities will remove most of the settled material and discoloured or stale water.
The UDF method does a better job of cleaning the water main which reduces the frequency which mains need to be cleaned. It also provides an opportunity to ensure that all valves and hydrants are operational for when they needed in case of an emergency.

What about my water bill?

This will have no effect on your water bill. You water is metered at your house or business, this program focuses purely on water mains in the streets and boulevards, before your water meter. Water usage associated with UDF is measured and recorded to ensure that flushing routes are calibrated in a way that uses as little water as possible.


Environmental concerns

Flushing of the water distribution system is a long standing maintenance program used across the globe. The UDF method actually uses less water because it decreases the need to flush as often. When flushing, the water doesn’t have to be run for as long to clean the pipe and routes are designed to minimize water usage.
Flushing horns are equipped with de-chlorinating ‘pucks’ so that all flushing water entering the storm system and the environment is free of chlorine. Material removed from the pipes is primarily trapped in flushing bags and disposed of at the Waste & Recycling Center.

How will this affect me?

During flushing, customers may notice a drop in water pressure. This drop is only temporary; once flushing is completed your water pressure will return to normal.
In the event that your water appears to be discoloured or has an odd smell, please follow the following procedure:
  • Go to your bath or sink  and turn on the Cold water tap and let the water run until clear (5-10 minutes)
  •  This should clear out your homes water line and correct the problem
  •  If the problem persists, call 311.


Signage will be posted in the area when flushing crews are present. Due to the nature of the work, schedules will only be known approximately one week in advance. Please visit our News Centre for the latest notices.



Q: Why is the flushing water sometimes discoloured?
A: Potable water from the Water Treatment Plant is clear and virtually free of sediment. However, these miniscule amounts sediment can accumulate over time. Coupled with this, pipes made of metal such as cast iron can deteriorate and leave particles in the pipelines. UDF creates a velocity high enough to remove all of this material from the water mains.
Q: Is discoloured water harmful to me?
A: If your tap water is discoloured or cloudy, it is recommended that you flush out your cold water line as indicated above. The water itself is safe to drink; nothing in it will harm you.

For more information

Online: Submit a service request
Phone: 311 or 403-320-3111 (if outside of Lethbridge)
Address:  City Hall, 910 4 Avenue South, Lethbridge, Alberta, T1J 0P6