At the stage of depopulating society, the trade-off relationship exists between securing the water power for extinguishing fires and the preservation of residual chlorine density, and also, it may be difficult to correspond with the matters using only conventional methods at the process of reconstruction for pipeline, therefore we need to add and adopt the new ideas. This report guides the concrete and multiple methods to resolve the problems occurring from the plural requirements including trade-off relationship at the stage of depopulating society. As for the pipeline feature which is economic and performance scale merit, it can be attained, and compatible by the implementation of actual study of pipe size ratio and combination, and by the maximum exertion of this feature, it can improve the water transporting capability, as it were, backup capacity increase, secure of residual chlorine, direct water supply area expansion, and secure of extinguishing water. And also, this feature can be achieved by the maintenance or reinforcement of main pipe water transporting capability and moreover, by the size reduction of branch pipeline.This study verified its effect by the theoretical demonstration, and also the simulation using existing actual pipeline data. Besides, as we need the plenty of operating time to get the suitable water hydraulic analysis result from the plural requirement matters, we reaped the harvest from the study and improvement of efficiency for water hydraulic analysis technology.

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