Union Square Stormwater Mitigation Program & Popular Street Pump Station

As the City of Somerville navigates aging infrastructure, regulatory pressures, climate change and redevelopment opportunities, innovative and cost-effective approaches for urban flood relief are being implemented under its Union Square Stormwater Mitigation Program (Program). The City is in its 10th year of program implementation. This project demonstrates how stormwater authorities and consultants can adapt their integrated planning over the course of a project life-cycle. It also features how municipal urban flooding can be creatively solved by engaging in inter-agency cooperation during major public transit projects. Flood mapping and construction photographs of the various projects construction will be featured. BACKGROUND The Union Square area is a commercial district with dense adjacent residential development that lies at the bottom of a large, combined sewer catchment. Presently, stormwater is conveyed by municipal sewers to the Massachusetts Water Resource Authority (MWRA) regional combined sewer system. The MWRA system is capacity-limited downstream of this connection. Flooding is experienced during wet weather equivalent to an existing 2-year / 24-hour design storm or greater. The flooding originates from a historical filling of the Millers River in 1879 and impacts many minority-owned small businesses, the City's Public Safety Building and multiple neighborhoods designated as Environmental Justice Neighborhoods for Minority and Income. INTEGRATED PLANNING Planning originated with an Evaluation of Flood Reduction Alternatives in Union Square (2013). This hydraulic modeling and project prioritization study evaluated the different surface runoff management and storage options throughout the watershed to mitigate or eliminate flooding in the Union Square area. To avoid maintenance-intensive combined sewage facilities, the preferred scheme was to separate sewers and overflow excess stormwater to detention facilities of various sizes. The first project planned was the Nunziato Field Stormwater Storage Tank ($13.6 Million, 2017) being a 1.6 MG storage facility located below a park with rain garden. The project was designed with sewer separation of adjacent streets on a hillside. It functions to capture peak flows and is pumped out post-storm event. The park's design also includes a interactive rain garden for stormwater quality and public education. During design of the Nunziato Tank project, an opportunity arose during expansion of the MBTA public transit system. Upon seeing that the project included stormwater conveyance for 50-year design storm events, City planners quickly recognized that the infrastructure would be underutilized when Union Square flooded. By making a connection to it, the City could regain a stormwater outfall in the Millers River watershed. To validate this plan, the City integrated MBTA's hydraulic models into its own citywide hydraulic model (InfoWorks ICM). The raised railway bed meant that stormwater would be pumped to it, which risked exceeding the available capacity of the MBTA system. Different combinations of pump capacity and storage volumes were evaluated with different degrees of sewer separation tributary to Union Square. The modeling performed convinced all parties that a maximum pumping capacity of 50 MGD could be allowed. An agreement was then negotiated for the City to discharge into the MBTA drainage system. Upon conceiving this stormwater pump station, the City began to modify its Program: -Climate change design storms for the Year 2030 time horizon were established, enabling resiliency planning for intense precipitation. A future 5-year design storm was established comparable to an existing 10-year event. -Design and construction of the Spring Hill Sewer Separation ($23.4 Million, 2021), a 71-acre project involving complete streets and green infrastructure improvements, became the new priority to unlock more stormwater to be conveyed by the future pump station. -Construction of the Nunziato Tank project was re-sequenced to occur after the pump station to allow further optimization and phasing. POPLAR STREET PUMP STATION Now, four years after developing the concept, the Poplar Street Pump Station ($80 Million, 2023) is beginning construction on a 2-acre brownfield site. The project includes a 6,100 SF building that centralizes stormwater operations across the City, a 4 MG underground storage tank, and high-visibility green infrastructure consisting of rain gardens, infiltrating bio-basins and three blocks of infiltrating tree ways to emphasize stormwater management and public education of the facility. The site is also being co-developed as a public park themed around art and urban agriculture, with adjacent complete street improvements. The pump station site lies in the center of the Brickbottom district, where its own microcosm of integrated planning required a non-traditional approach to wet weather engineering. Over the course of three years, an inter-discipline team of planners and engineers successfully resolved the facility's design with a host of complex interfaces: 1.Historic Arts Community — local citizenry passionate about maximizing the park site 2.Brickbottom Vision Plan — prioritizing alternate modes of transit next to the site (pedestrians, biking, buses and rail) 3.GLX Community Path Connection — incorporating ADA pedestrian ramps, bike lanes and a public plaza above the force main discharge 4.McGrath Boulevard — an ambitious DOT plan to eliminate an elevated highway next to the site 5.MBTA Bus Network Redesign — incorporating a prominent bus stop into the site 6.Utility Renewal — upgrade of aged water and gas mains along the force main alignment Wet Weather Management & Control During pump station start-up in late 2025, its operation will be unmanned and rely on telemetry shared between City and MBTA facilities. Level sensors monitoring storm drains along the railway will signal when the MBTA drainage system has reduced capacity, generally coinciding with the existing 5-year design storm event. Variable-speed drives are controlled to slow pumping and match the reduced capacity. For larger storms approaching an existing 10-year design event, no capacity may be available during the peak of the storm. It is these situations when the 4 MG underground storage tank activates as a peak shaving facility and is later pumped out post-event. This combination of pumping and storage eliminates stormwater flooding along the Somerville Avenue corridor for the existing 10-year design storm, providing a stimulus for large scale redevelopment. Recently enacted City stormwater regulations further require large developers to reduce their stormwater runoff into the public right of way (piped and/or overland) such that the 10-yr proposed peak flow is less than the existing 2-yr peak flow. Privately operated storm water storage tanks are being constructed in this area, enhancing the flood resiliency of Union Square. Another aspect of the pump station is how it fits into the National Pollutant Discharge Elimination System (NPDES) permitting framework. The City is studying whether the pump station discharge should be listed as a co-permittee under the MBTA's NPDES General (agency) permit or for it to be part of the City's NPDES MS4 (municipal) permit. Stormwater quality will factor into this decision.