Integrated Collection Systems Model for Hartford, Connecticut

Individual collection system models traditionally assess conditions in one component among combined sewers, sanitary sewers, storm drains, and open channel drainage. While some models integrate the different sewer types, and some integrate open and closed channel drainage, very few address complete urban collection systems. For The Metropolitan District Commission (MDC) of Hartford, Connecticut, CDM built a model of the full urban collection system that drains to the Hartford Water Pollution Control Facility (WPCF) and to the Connecticut River at the City's eastern edge. The model will help the District in planning studies to implement its Combined Sewer Overflow (CSO) Long Term Control Plan and reduce flooding of sanitary sewers and storm drains. A 4,000-conduit USEPA SWMM model integrates a new model of pipes in Hartford with three existing models of sanitary sewers in outlying communities and a regional drainage model of the 75 mi² (200 km²) Park River system. The models are being used to plan nearly two billion dollars of improvements to the collection system, which serves 250,000 people. The model is calibrated and validated to numerous datasets. In spring 2009, MDC installed temporary flow meters at 27 locations throughout the combined, sanitary, and stormwater systems. These data are supplemented by depths measured continuously at 80 CSO regulators since 2002, long-term continuous flows at the WPCF, 12 permanent flow meters at Hartford's borders installed in 2008, daily flow records on eight streams collected by USGS between 1936 and 1986, detailed flow metering in the outlying communities from 2005 and 2009, and 60 years of monthly observations by USGS at a water table well in South Windsor. While the sewer system operates almost entirely by gravity, the drainage system includes five large pump stations that primarily operate in spring when the Connecticut River rises 20 feet (6 m) or more above its summer stage. The drainage system component includes many real time controls that mimic operating protocols for the gate and dike system that protects the city from flooding. The model rigorously represents groundwater infiltration into the collection system using SWMM's groundwater infiltration module. Each model catchment includes a groundwater compartment with levels calibrated to the South Windsor well. This allows robust simulation of the large variation in seasonal dry weather flow to the WPCF, which can vary from 30 million gallons per day (mgd; 1.1 x 10⁵ m³/d) in summer to 60 mgd (2.2 x 10⁵ m³/d) in spring. This approach has been implemented in few cities. Vanrolleghem et al. (2005) discuss a comparable model for a Belgian town of 20,000 people. However, recent advances in computer technology and scientific datasets make it increasingly feasible. Many cities have well-developed computerized databases of collection system assets. Available drainage system data is typically less detailed, but reasonable US drainage system models can be constructed using hydrography and topography datasets available from USGS. Such models are likely to become increasingly important as planners consider the linkage of sewer and drain infiltration, their interaction with surface and ground water resources, and the impacts of collection systems on receiving waters.