The flow needed to reach a target scour velocity in a water main — or the velocity a given flow achieves — plus pipe volume, flush duration, and total water used. Runs entirely offline; inputs persist in this browser via localStorage.
Use limits: Velocity is what scours a main, not flow — and it takes real velocity: roughly 2.5 ft/s to move loose sediment and air, and ~5 ft/s for the unidirectional scour that lifts tuberculation and biofilm. A single 2½-inch hydrant outlet passes only so much (often ~1,000–1,200 gpm at usable residual pressure), so a large main may not reach scour velocity from one hydrant — that's why unidirectional flushing isolates valves to force the flow down one pipe.
Results
Flow
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Velocity
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Pipe volume
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Flush duration
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Total water used
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How the Numbers Are Derived
The formulas
Velocity and flow in a full pipe: V (ft/s) = 0.4085 · Q (gpm) / D (in)², and inverted, Q = 2.448 · V · D². Pipe volume is 0.0408 · D² gallons per foot of length (e.g. 2.6 gal/ft for 8-inch, 5.9 gal/ft for 12-inch). The flush duration to pass N pipe volumes at flow Q is t = N · (volume) / Q, and the total water used is N · volume.
Field notes
It's velocity that scours, and velocity scales with flow over diameter squared — doubling the pipe size quarters the velocity at the same flow. Big mains are hard to scour; that's the whole reason for unidirectional flushing.
New-main disinfection (AWWA C651) calls for flushing at a velocity that removes air and debris — 2.5 ft/s is the usual floor — both before chlorination and to remove the heavily chlorinated water afterward. Pair this with the chlorine dosing calculator.
Heavily chlorinated flush water must be dechlorinated before it reaches a storm drain or stream — the total-water figure is what you'll be neutralizing and disposing of.
Open and close valves and hydrants slowly. Slamming a hydrant on a distribution system spikes velocity, dislodges too much at once, and turns into discolored-water complaints across the neighborhood.
Scour velocity from one hydrant is limited by available flow and residual pressure — verify with a hydrant flow test if you're near the edge, or isolate valves to concentrate the flow.
References
AWWA C651 — Disinfecting Water Mains (flushing velocity and procedure).