A City with a Plan is a City with a Vision. Developing the City-Wide Sewer Separation Master Plan in Chelsea, MA

The City of Chelsea, MA is strategically located as a gateway between downtown Boston and North Shore communities and is home to a vibrant, diverse population and industry that contribute significantly to the overall wealth of the region. Chelsea is served by the Massachusetts Water Resources Authority (MWRA) for water supply and wastewater collection. Many MWRA interceptors and a major regional headworks and pumping station are located in Chelsea. Like many other coastal communities in New England, Chelsea is serviced by a mix of combined and separated sewer systems that were built more than a century ago and are at the end of their service life. This outdated infrastructure hinders new development and growth, especially in the context of increased precipitation and coastal-induced inundation risk due to climate change. As a response to this concern, Chelsea has embarked on a city-wide master plan for sewer separation and infrastructure upgrades to systematically work towards attainment of the following three main goals: 1. Allow for future growth by progressively upgrading drainage infrastructure able to accommodate future population and increased stresses from climate change; 2. Eliminate CSOs by separating existing combined sewer areas and; 3. Reduce inflows to sanitary pipes from public and private properties that currently decrease available system capacity and increase operating costs to the City. In order to create this ambitious program, a significant modeling effort was carried out following a multi-step process. The first step consisted of building and calibrating a city-wide drainage model to help understand points with weak level of service and estimate peak flow and volumes reaching the MWRA interceptors and combined sewer overflows during dry and wet weather events. The City-wide model was built in Infoworks ICM using pipe network data from multiple sources such as record drawings, GIS, field inspections, and recently collected ground-level LiDAR information captured city-wide. The second step of the process consisted of developing conceptual level pipe layouts with the goal to achieve sewer separation city-wide and improve level of service. The conceptual level plans were developed using a 'best case scenario' approach (i.e. assuming an ideal drainage network can be built without necessarily accounting for other factors such as utility conflicts or permitting issues for new outfalls). The reasoning behind this approach was the desire to delineate a network able to attain optimum hydraulic performance based on natural topography and street layout and then work backwards, if needed, when conflicts identified during later phases of the process require this layout to be modified. The conceptual storm and sanitary systems were then incorporated into an Infoworks ICM model, which was used to fine tune the conceptual network (e.g. pipe slopes, pipe sizes necessary to provide the desired level of service or flow velocities for a given event). The third step was a planning level engineering effort where a street-by-street evaluation at each sewershed was performed. The feasibility and actions needed to go from the current condition to the proposed condition in the conceptual model were evaluated and alternative actions proposed when identified conflicts could not be resolved. The fourth and final step consisted of packaging and phasing the execution of the sewer separation and inflow reduction projects based on feasibility, City's priorities and constraints, as well as interim system conditions. This product is the City's new Master Plan for Sewer Separation and Drainage Infrastructure Upgrades, which will provide a systematic approach to drainage infrastructure improvements over the next few decades.