A monitoring and alert methodology ascertains conditions that may result in a combined sewer overflow (CSO) event. The method employs a semantic model of a combined sewer system that includes specification information describing the components of the combined sewer system. Sensors distributed throughout the components of the combined sewer system sense operating parameter information for those components. A monitoring and alerting (MA) information handling system (IHS) receives operating parameter information from the sensors. The MA IHS analyzes the operating parameter information received from the sensors together with the specification information in the semantic model of the combined sewer system to determine if a combined sewer overflow (CSO) event is possible. The MA IHS generates an alert to provide notification of a possible CSO event if the MA IHS determines that a CSO event is possible. In this manner, an operator may take early corrective action before a CSO event occurs.