Hydrant Flushing

A hydrant testing and flushing program are very important to the maintenance of a domestic water distribution system. Each year, city and/or private water technicians may systematically open hydrants throughout the City to gauge flows and to allow water to flow at a high velocity through the distribution main lines.

Flushing helps maintain the safety of residents drinking water. Flushing a water system twice a year helps remove sediments, bacteria, parasites, pesticides, and other pathogens, and contaminants.

The two main types of flushing programs are conventional flushing and uni-directional flushing.

Conventional flushing involves flushing one hydrant at a time, often during hydrant inspections and flow testing. 

Unidirectional flushing involves the sequential closing of water valves to maximize water flow velocity to create a pipe scouring effect.

Regular flushing serves the following purposes:

  • Enhances long-term water quality by removing sediments from inside the mainline and flushing them out through the hydrant.

  • Identifies malfunctions of the hydrant and related valve issues.

  • Helps determine weaknesses in the water distribution system.

  • Identifies inadequate water volumes and pressures in the mainlines.

  • Helps determine fire flows at the hydrant.

Unidirectional Flushing (UDF) Program

The purpose of conducting a flushing program is to improve water quality, conserve water when flushing, gather information, and provide knowledge to conduct proper management for years to come.

The key to planning a successful UDF program is to have accurate information on all water assets and functional critical assets. These assets include GIS maps, hydraulic models, pipes, valves, and hydrants. GIS and hydraulic models must be complete, accurate, up-to-date, and integrated for the UDF flushing sequences to be functional. Without fully functioning water system assets and accurate information, field crews conducting the flushing sequences will experience failures that range from inadequate system pressures and low flows to incomplete flushing zones, nonsensical sequences, and areas unable to be flushed adequately.


Information from the GIS and a distribution system model is used to divide the distribution system into sections/loops to conduct daily flushing sequences. These loops should be established to assure the following:

  • Flushing always moves water from clean to “dirty” areas.

  • The size of each loop represents an area that can be completed by a flushing crew in one working day.

  • A minimum flushing velocity (>5 fps) can be maintained in the pipeline to be flushed.

  • Sufficient flow and pressure (typically 20 psi residual) are maintained in surrounding areas to support basic service and fire flows.


The following items are also developed for each sequence: minimum flushing time, total flushing volume and pipeline length, flushing velocity of every pipeline in each sequence, and the minimum number of valves to operate, in addition to available flow at minimum residual pressures. The hydraulic impact of each flushing sequence is also monitored to ensure the desired minimum pressure is maintained throughout the flush zone while maintaining the desired flushing velocity.

Benefits of flushing at higher velocities: scouring of sediment, biofilm, corrosion products, tuberculation, and can improve taste and odor issues. UDF is a more effective way of cleaning than conventional flushing and uses less water than conventional flushing.

Information needed to update and plan a UDF program:

  • Hydrant and valve count for each location

  • All current as-builts if not already in GIS

  • Access to GIS data

  • Past hydrant and valve survey data

  • Repairs or construction which may interfere with a UDF program

  • What is the historical and/or current water quality data?

  • Age of infrastructure

You may encounter the following conditions during hydrant flushing operations:

  • A temporary drop in water pressure to your residence or place of business.

  • Rusty brown, or cloudy water. (These conditions will subside after the flushing is completed.)

Helpful Hints to Remember:

  • Flushing of water system main lines and water lines within your property improves tap water by removing dangerous contaminants.

  • Before flushing operations begin, notify customers in areas where flushing is to occur

  • Do NOT wash white or light-colored laundry during the flushing operations

After the flushing operation is completed:

  • Open the cold water faucets and let the water flow until it is clear. Turn off the faucets.

  • Open the hot water faucets and let the water flow until it is clear. Turn off the faucets.

  • When both hot and cold water are clear, the water is ready for use as normal.

  • Check the faucet screens for trapped particles.

  • Wash a load or two of dark-colored clothes first.


Why is hydrant flushing necessary?

It enhances water quality by removing sediments from inside the mainline and flushing them out through the hydrant; identifies malfunctions of the hydrant and related valves; helps determine weaknesses in the water distribution system; identifies inadequate water volumes and pressures in the mainlines and helps determine locations where flow pressures may require valve inspections.

What should I do when hydrants are being flushed in my area?

  • Mainly, avoid using the dishwasher, washing machine and don’t turn on the faucets (hot or cold). Wait until the operation is completed.

  • If you are driving in the work zone where hydrants are being flushed, please drive carefully.

Why does my water look rusty or cloudy after hydrant flushing?

When a hydrant is opened, the water in the mainline will flow out at a high velocity. This creates a scouring action in the pipe and dislodges fine sediment particles that have accumulated in the pipe. The fine sediment mixes with the water, turning the water a cloudy or rusty brown color. This mixture is discharged out of the hydrant.  Remember, after the hydrant flushing operation, run your hot and cold faucets to let the water clear before you use it.