Water Main Loop Madness 1

[Pinwards]

What are the requirements to loop potable water mains in your jurisdictions? Are short (~150 ft) cul-de-sacs allowed to have "dead-end" mains, with a hydrant or other flushing means on the end? We have a local jurisdiction that is now taking their requirement to "loop water mains wherever feasible" to the Nth degree and requiring a full loop around even the smallest commercial buildings, and requiring easements across residential lots to loop it back to another main on even the shortest of subdivision cul-de-sacs. It really seems like a lot of added expense, and added water main for the City to maintain, for very little (if any) improvement in the circulation of those mains. In fact, if you have a single demand, and you are now providing two equal routes for the water to flow from, you have now halved the flow rate in each main from what it would have been if there was just one serving the demand. There are thousands of cul-de-sacs across the US that have similar dead-end mains, and I don't see mass news headlines about everyone that lives on a culdesac getting ill from water main bacteria, so it would seem they're trying to solve a problem that doesn't exist. Yes, dead-end mains can cause stagnant water that can harbor bacteria, but this doesn't seem like the best way of eliminating that possibility. What are your thoughts, and what is commonly done in your area?

Paul

 

[cvg]

600 feet dead ends are usually allowed. the fire hydrants are for fires, they could be used for flushing, but not stinking likely unless somebody complains...

 

[77JQX]

I avoid dead ends if I can. But sometimes, there's no alternative. Our regulators regularly approve dead end mains in 600 to 800 foot cul-de-sacs, but they don't like to. We have been forced to install parallel mains, just as you describe. Luckily for us that regulator has since retired. It seems the regulator's focus is on system reliability, and having two independent supply paths to each service. However, when your most common service interruption is due to dig-ins, having two pipes in the same trench doesn't help, and could result in longer time to repair. Our regulators don't seem to consider the effect on water quality of increasing the pipe volume.

For subdivisions, we try to avoid getting into the dead end pipe problem by working with developers in the early design phase of their projects, where we will look at proposed lot layout etc, and spell out the looping we would require in advance so they can take it into account with their lot, street, and grading plans. This results in a lot fewer crazy layouts. I think the fire department's egress requirements also help us out a lot in having multiple rights-of-way to provide looping in.

For individual buildings or in-fill development, we'd try to avoid having any additional pipe added to the public system, unless the additional pipe in some way improved the system (looped a dead end, improved fire flow, or provided for future expansion, etc.). We push private services over public mains. If the single building needs a fire loop, we'll let them put in two DCDA assemblies, but the loop etc. would be owned and maintained by the property owners. The fewer fixed assets we own, the lower our long term costs will be.

When I design a dead end pipe I like to see the summertime demand on the dead end pipe be able to turn over the volume in a day or two to maintain disinfectant residual. I'm not so worried about turnover time in the winter - our residual is better in the colder water, even with longer retention time.