I’m not familiar at all with the Calgary water supply system but I do have some experience in this general area. There are many reasons why the city can’t just easily re-organize the system a) there is sediment in the pipes that will be stimulated with a change in flow direction or velocity that would lead to water quality issues b) there are pressure management zones which might not be easily reconfigured and overloading other pipelines could lead to a slew of further problems c) de-pressuring pipelines can lead to backlow of contaminants into the pipe and generate further water quality issues.
The city will have to repair the pipe sections, swab the pipe to get rid of any debris, then charge it up to full pressure and do a pressure test, and then chlorinate the pipe and test water samples to make sure it is safe to consume. It all depends on length and pipe diameter but I would guess that filling the main would take at least a full day probably longer, then you have time for swabbing - let’s say another 2 days, then the pressure test which is usually over 48 hours, and then the chlorination which is another couple of days for chlorine contact time. So that whole process could take well over a week in itself never mind the actual time to excavate and carry out the localized repairs.
So that will be one of the reasons that they want to do all repairs now as going back in later would mean having to repeat all of that, never mind the risk that these poor condition areas might fail in the meantime.
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