Integrating flood storage and green infrastructure into a net zero school

Tobin School – Stormwater Storage Tank

We were retained by the City of Cambridge to lead the design of a neighborhood flood relief project to be located at the site of the present Tobin Montessori and Vassal Lane Upper Schools Complex. This project continues a series of recent upgrades to the city’s sewer and drain systems, part of the Alewife Brook sewer separation program, also engineered by Stantec.

This project was built with an emphasis on resilience. We designed 1.25 million gallons (4.7 million litres) of underground stormwater storage and added 100,000 gallons (378,541 litres) of green stormwater infrastructure (GSI) surface storage capacity, all while coordinating with reconstruction of the school complex as a net zero emissions building.

The tank is gravity fed from existing connection points from the stormwater mains located at Vassal Lane and Concord Ave. The construction manager at risk (CMAR) approach required integrating the tank design into three school construction packages, with shared electrical systems, including an

emergency generator. The tank was engineered to mitigate flooding associated with projected climate change impacts during the 10-year, 24-hour storm event anticipated in 2070. Our Hydraulic Modeling team conducted an updated hydraulic analysis to support the design.

The facility is located on a brownfield site which presented challenging soil conditions. It required a deep foundation system extending approximately 90 feet (27 metres) below grade to bedrock. The cast-in-place concrete tank was sited beneath the school’s bus loop so that it would not interfere with the critical path of the project and to also increase constructability and ease of operation and maintenance.

Preparations for this facility began in the 2010s during the final stages of implementation for a regional combined sewer overflow long term control plan and highlights how dense urban communities can co-mingle civic and utility infrastructure for multi-use benefit with proper planning. Resilient flood protection of the new school, surrounding residences, commercial business, and the busy Alewife Brook Parkway was achieved without new land acquisition or added disruption to abutters.