7.2 Flushing & Unidirectional Flushing

7.2 Flushing & Unidirectional Flushing

Quick Answer: Flushing moves water through mains at higher-than-normal velocity to clear sediment, stale water, and biofilm. Conventional flushing simply opens a hydrant; unidirectional flushing (UDF) closes selected valves to force a single, high-velocity flow path that scours the pipe wall. UDF is planned, sequenced, and documented.

Why Flush?

Water sitting in dead-ends and low-flow mains loses disinfectant residual, accumulates sediment, and grows biofilm on the pipe wall. Customers at the end of those mains see cloudy water, taste and odor complaints, and sometimes coliform hits. Flushing is the simplest, cheapest tool for restoring water quality in those areas. It also lowers disinfectant decay by removing the biofilm that consumes chlorine, and it restores a chlorine residual in the flushed zone.

Conventional Flushing

Conventional flushing (also called dead-end or spot flushing) opens a hydrant at the end of a main or at a low-flow point and lets it run until the water clears. It is fast, needs no valve operation, and is the standard maintenance practice on dead-ends. Its limitation is velocity: at typical hydrant discharge the velocity in a 6- or 8-inch main is often below the threshold needed to scour the pipe wall, so sediment lifts but biofilm stays. Conventional flushing is still valuable — it removes settled sediment and stale, low-residual water — but it does not strip biofilm or tuberculation.

Unidirectional Flushing (UDF)

Unidirectional flushing is a planned program that closes selected distribution valves to force water down a single, predefined path at a higher velocity. With valves closed on the cross-streets, the entire flow from a source main is squeezed through one segment, and velocity rises — the target is ≥3 ft/s — enough to scour biofilm, tuberculation scale, and sediment off the pipe wall. The result is a cleaner main than conventional flushing can achieve.

Why Velocity Matters

The cleaning force at the pipe wall is shear stress, and shear stress rises with the square of velocity. Doubling velocity quadruples the scouring force on the wall. That is why closing valves to concentrate flow into one segment is so much more effective than simply opening a hydrant. At 3 ft/s most loose biofilm and sediment lift off; at higher velocities even older tuberculation can be loosened, though operators stop short of velocities that would damage coatings or cause pressure complaints. Velocity in a segment is estimated from the flow rate and the inside pipe diameter — V = Q / A — and the UDF plan precalculates the hydrant flow needed to hit the target in each pipe size.

Sequencing: Cleanest to Dirtiest

A UDF program is sequenced from the cleanest part of the system toward the dirtiest — usually starting at the treatment plant or a clearwell-fed main and working out toward the periphery. The reason is straightforward: if you scour a dirty section first and then push that dirty water into a section you cleaned next, you recontaminate the clean section. Each segment is flushed until the discharge runs clear and the chlorine residual returns to normal.

Dechlorination of Discharge (AWWA C655)

Flushing discharge is often chlorinated. Chlorinated water reaching a storm drain, creek, or wetland is toxic to aquatic life. AWWA C655 governs dechlorination of water-main discharge. Common field methods include:

• Dechlorination tablets in a diffuser or basin
• Spray or cascade through a sodium thiosulfate or ascorbic acid solution
• Neutralizing mats at the discharge point

Operators measure chlorine residual at the point of discharge to confirm it is at or near zero before the water reaches the receiving water. Flow, chlorine residual, and the dechlorination method are all documented. Some agencies also need a discharge permit or a stormwater pollution prevention plan (SWPPP) before flushing to a surface water.

What to Record

A complete flushing record — UDF or conventional — includes:

• Hydrant ID and location
• Date and crew
• Start and end time
• Flow rate (gpm) and residual pressure
• Initial and final chlorine residual
• Initial and final turbidity
• Dechlorination method and discharge location
• Any issues (closed valves, customer complaints, low pressure)

Scheduling and Customer Notice

Flushing is best scheduled in cooler months when demand is lower and the system can spare the water. Crews should notify customers in advance in the flushed area — flushing can stir up sediment and cause short-term cloudy water, and a notice reduces complaint calls. During the work, the crew watches for residual pressure to stay above 20 psi; if flushing pulls the system pressure too low, the crew throttles back or stops and reschedules.

Water-Quality Benefits in Plain Terms

The biofilm that coats a pipe wall consumes chlorine. By removing it, UDF slows disinfectant decay in the flushed zone, so a free chlorine residual survives further from the booster station. Customers see fewer cloudy-water complaints, taste and odor complaints drop, and total coliform hits fall. For these reasons UDF is also a recognized tool for distribution system water quality optimization, not just for sediment. Many systems that adopt a routine UDF program see disinfectant residual improve across the entire system within a single cycle.