Cincinnati's SWMM model: A journey through time

The Metropolitan Sewer District of Greater Cincinnati (MSDGC) provides sewerage collection and treatment services to approximately 230,000 residential and commercial users in Hamilton County, Ohio. It serves an area covering approximately 290 square miles through approximately 3,000 miles of sanitary and combined sewers, seven major wastewater treatment plants, three package treatment plants, and more than 120 pump stations.  The treatment system has a dry weather capacity of 200 MGD.  The largest Mill Creek plant has a wet weather capacity of 440 MGD. 

The MSDGC models have been developed over a 15-year period.  In 2003 MSDGC completed a three-year project to develop a calibrated hydraulic model of the sewer system using EPA SWMM4 modeling software.  The project was completed jointly by MSDGC and a nine-firm consultant team.  Referred to as the System Wide Model (SWM), with 42,206 nodes and 26,629 subcatchments this was one of the largest models of its kind ever constructed at the time.  It provided MSDGC with an extraordinary tool to evaluate existing and projected conditions within the sewer collection system.  SWM included real-time control (RTC) modeling capability.  Modeled sewer network data was derived from District’s Geographic Information System (GIS) called the Cincinnati Area Geographic Information System (CAGIS).  In developing the SWM, MSD advanced the state-of-the-art in collection system modeling in a number of areas including the size and scope of the model, the application of radar rainfall data, the use of water consumption data to estimate dry-weather flows and use of GIS for model development.  Due to sheer size of the model and the computational speed available at the time, long term continuous simulations to produce CSO and SSO statistics were run using the Kinematic Wave solution to overcome long run times. 

Since 2003 both modeling software and computing speed have significantly advanced, and MSDGC has consistently added detail and upgraded the model.  The old SWMM4 model has been converted to new SWMM5 version.  The physical characterization of the collection system has been improved through field investigations of the system and modifications in the way which regulators are represented, amongst other changes. Calibration and validation of the model have been supported through a monitoring program that has gathered a substantial amount of flow data both in the number of locations monitored and duration of monitoring. The level of detail incorporated into the model equals or exceeds that of many models developed for similar applications, i.e., large metropolitan areas under CSO/SSO consent decrees.  With over 51,000 nodes in the current version, MSDGC’s SWM continues to be one of the largest SWMM model.

This paper will elaborate on the above summary.  It will provide the history, features, and applications of MSDGC SWM.  It will describe the massive ongoing model calibration and validation work and how the SWM is being used for the planning and design of Consent Decree projects.  Challenges and lessons learned will be shared.