When you are working with a 1800 gpm demand and multiple interconnections, you really can't separate the development site from the rest of the system and expect to get accurate results. The interactions are just too complex to get by with some of these shortcuts you can use in other situations. I suggest you get together with the City water distribution engineers and discuss how to model this. Ideally you would get a copy of the City model (at least this pressure zone) and build your model on top of it. Or you can get their distribution maps and build very simple skeletal model of the system. You need to model back to some real boundary condition. I'm trying to think how you might get by with some shortcut. One, somewhat far fetched, test might be to run (assuming you have 6 connection points) 6 simultaneous flow tests. You couldn't run all the hydrants wide open (unless you have a very strong system) without lowering the pressure too much. So, if your demand is 1800 gpm, you could run 300 gpm at each flowed hydrant. This isn't as good as running a model of the full pressure zone but at least it gives you an idea of what a 1800 gpm demand will do to the City system. Good luck.
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