Surging in Urban Storm Drainage Systems

The existence of surge and related problems, namely, sharp‐peaked overflows, manhole, or drop‐shaft cover blow‐off; waterjets into the atmosphere (geysers); and structural damages in urban storm drainage systems, are described conceptually and illustrated with both a mathematical model and field records. A storm sewer tunnel in the system may be pressurized if its conveyance capacity is exceeded or if it is also used for storage. The pressurization process is associated with the movement of a surge that separates the free‐surface flow regime from the pressurized flow regime. Analysis of the surge movement characteristics revealed that the surge strength at the end of the pressurization process determines the extent of the surge problems. Operational and structural methods to reduce the surge strength are proposed and evaluated for the Mainstream System and the O'Hare System of the Tunnel and Reservoir Plan in Chicago, Illinois. They include inflow regulation, initial storage regulation, use of a downstream reservoir, and use of an upstream surge tank.