OK, I may be the only one who doesn’t know, so I’ll ask. What is the city doing when they periodically come around and open up a fire hydrant and let it gush for several hours? I’ve seen this all my life (I noticed one today).
My best WAG is that (even though I can’t see them) there is a work crew in an area that is affected by the water pressure at the hydrant in question and they need to lower the local water pressure to attempt whatever job they need to do. Anyone?
Note the verb flush in your expanded question. If not periodically flushed out, water lines will silt up, reducing pressure and sometimes clogging completely.
In addition, if your city has hard water, (either iron or calcium), the deposits can not only block the pipe, but can freeze the valve so that the hydrant could not be opened in an emergency.
But flushing is what they are doing.
Wow…that was like 12 minutes, and its the same answer I had (which, of course, is the correct one…).
If you look in the local paper, there should be some type of notice saying that flushing is taking place. Flushing a line stirrs up all sorts of nasty stuff in your pipes that stains laundry (rust particles and manganese around here). Water companies (around here, at least) give a notice that flushing is taking place.
All kind of crud builds up in pipes. There it sits happily with water flowing in one direction. It will keep building up (reducing flow capacity of the pipe* and increasing the risk of bacterial outbreaks) until the city opens a hydrant and gets the water to flow in the other direction to flush out a bunch of that crap. You’ll notice that the water coming out of the hydrants is tan/brown.
** crud = mineral/organic build-up plus bacterial plaque plus dirt, etc.