A combined sewer network serves the dual purpose in the same network to collect and transport both the daily ow of wastewater and the wet weather runoff from an urban catchment. Corresponding high ow rates must therefore be managed during rain events. When the hydraulic capacity of an interceptor or a trunk sewer is exceeded during a runoff event, overow of excess water from the network into adjacent receiving water must take place upstream of this point. Combined sewer networks are therefore equipped with structures where overow can take place. Such overows, combined sewer overows (CSOs), mean that a mixture of untreated wastewater, runoff water from the catchment, and materials eroded from deposits in the network itself is discharged. The consequence of CSOs is a potential pollution and deterioration of downstream receiving water systems.