Abstract: A water backflow preventing system is provided for a given area having a sewer line for receiving rain and other water where the sewer line is incapable of carrying, without backup, a given fow rate of water. The sewer line is provided at a given point therealong with at least one overflow opening into which opening water which cannot be handled by the sewer line will rise and overflow. Water storage tank means are buried in the ground below the sewer line and into which the overflow water from said opening will drain and be temporarily stored. The storage tank means have a volume to store many thousands of gallons of water which cannnot otherwise flow in the sewer line. A pump is placed in the bottom of the storage tank means. Various float switches are placed in different parts of the system for controlling the pump.