Combined Efforts for Combined Sewers: Benefits and Challenges of Combining the Collection System Models of the Massachusetts Water Resources Authority and the Cities of Cambridge and Somerville, MA to Support Development of Coordinated Updated CSO Control Plans

The Massachusetts Water Resources Authority (MWRA) and two of their member communities, City of Cambridge, MA and City of Somerville, MA, are required to submit Updated CSO Control plans for their permitted CSO outfalls that discharge to the Charles River, Upper Mystic River, and Alewife Brook. Each entity will need to submit its own individual plan, but the plans need to be coordinated as the Cambridge, Somerville, and MWRA systems are hydraulically interconnected. MWRA had originally developed a system-wide collection system model in support of its 1997 CSO Control Plan, which covered its four member communities with CSOs (Cambridge, Somerville, Chelsea and Boston). Since then, MWRA has maintained and updated its model over the years, while Cambridge and Somerville have also developed and maintained their own collection system models in parallel. The Cambridge and Somerville models were developed to a higher level of detail in their respective systems compared to the MWRA version. Not surprisingly, the three models produce somewhat different results at certain CSO locations. In order to provide coordinated Updated CSO Control Plans with consistent modeled performance, MWRA, Cambridge and Somerville decided as a group that the best path forward was to combine the individual models into one Unified Model. With this approach, one model would be used to develop baseline conditions and evaluate alternatives for all three Updated CSO Control Plans, providing consistency in the assessment of performance and upstream or downstream hydraulic impacts. The purpose of this presentation is to walk through the steps taken to combine these three collection system models and document lessons learned throughout the process. The approach and lessons learned would benefit other agencies/communities where hydraulic connectivity across municipal boundaries can complicate the hydraulic evaluation of alternatives to improve system performance. This effort required very close coordination between MWRA, Cambridge, and Somerville as well as each of their consultants that maintain their models. Bi-weekly meetings, sometimes supplemented with additional meetings, were held throughout this two-year process to discuss each step that would need to be taken to combine the models and to define the run parameters so that each entity could run the model and get consistent results. As each entity and its consultants had their own preferences for running their models, compromises were required to establish consistent modeling parameters such as model time steps, the number of run threads during a simulation, rainfall smoothing, and ground water. Similarly, agreement was also needed for post processing procedures including interevent times, volume thresholds, and statistical templates. The meetings also served as a time to identify the roles and responsibilities and to discuss methods for troubleshooting model run issues such as run time, model instabilities, reconciling differences in model results or file size. Following the combining of the model networks and agreement on the parameters described above, it became apparent that a model naming convention would be necessary to organize the different versions of the model. To address this need a model naming convention was developed and a model log was posted to a shared drive where those working on the model could make updates as needed. The basic steps for combining the model networks included the following: 1.Identify the locations where the models would be connected. 2.Prepare the models by checking for duplicate names for model nodes, conduits, and subcatchments, as well as unique names for land use, runoff surfaces, and wastewater profiles. 3.Combine the model networks. 4.Develop Design Conditions (future rainfall, sea level, and evaporation) 5.Have each team run the model independently to check that all three entities are producing similar results and check that the results are consistent with previous results for each entity's CSOs. 6.Each team added planned projects to create a version of the model representing Future Baseline Conditions (FBC) With the efforts of the entire team the MWRA, Cambridge and Somerville models were successfully integrated into one Unified Model. Each team was able to run the Unified Model independently and produce similar results. The Unified model was compared with previous model results and the differences were investigated and explained. Each team then added projects that were planned to be constructed to create an FBC version of the model. The FBC version of the model is now ready to be used for the next steps of the project. It is anticipated that the Unified Model will continue to be updated as work on this project progresses.