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Around 1 meter per second. See, for instance, this discussion thread on the design of water supply systems: (we're talking about pipes of six to eighteen inch diameter here)
As a rough rule of thumb, velocities above 0.75m/s are often specified to prevent sedimentation in dirty water lines. The exact figure would be dependent on the size and density distribution of entrained solids. Pumped line velocities up to 3m/s, and gravity lines around 1m/s are usually reckoned to be suitable from the POV of headloss (subject to confirmation by hydraulic calcs). Thrust blocks etc are then usually designed around the resulting forces.
5 feet per second -- about 1.5 meters per second -- is fast enough to dislodge built-up sediment from pipes, so many water utilities periodically flush water through pipes at about this speed to remove the sediment. Here is some information about mains system flushing in Tucson, Arizona.
This means that if your water is served from a reservoir that is, say, thirty kilometers away, the water will be resident in the pipes for a few hours (30,000 seconds is a bit under ten hours).
(Thanks to Rob Partington for correcting a, frankly, embarrassing arithmetical error in the above.)
Copyright (c) 2003 Chris Lightfoot. All rights reserved.