Combined sewer overflows

Because of this widespread flooding, even building CSO control infrastructure to the level of a 2050-25 Year Storm may provide limited public health and water quality benefits. Conceptual design of the facilities needed to control overflows for the 2050-25 Year Storm indicates that large storage tanks and tunnels will be needed. The amount of available land/space for such facilities will be a significant constraint, even including any potential use of Department of Conservation and Recreation (DCR) open space.

However, the Cambridge, Somerville, and MWRA project team have evaluated alternatives that can achieve the 2050 25-Year Storm level of CSO control and detailed information will be available in the upcoming Draft CSO Control Plan.

Updating Stormwater Management Regulations for Private Development

The City's Wastewater and Stormwater Drainage System Ordinance (Chapter 13.16 of the Cambridge Municipal Code), enacted in 1998, was significantly revised in 2008 to establish regulatory authority to require stormwater management standards for private development. This Ordinance is implemented through the Land Disturbance Regulations as well as the Wastewater and Drainage System Use Regulations, which specify criteria for projects that require Stormwater Control Permits.

Since adopting these regulations, Cambridge has consistently applied more stringent project criteria than any overlapping federal or state requirements. Cambridge has also chosen to apply its stormwater management criteria to development projects citywide, in both separated and combined sewer areas.

Projects that meet one or more of the following criteria require a Stormwater Control Permit from DPW: Disturbance of one or more acres of land, Construction exceeding 50,000 square feet of Gross Floor Area, Project parcel(s) size equal to or greater than one acre, Inclusion of outdoor parking for 10 or more cars, Requirement of a Special Permit from the Planning Board, or Determination by the City Engineer that the project may result in an adverse impact on the municipal Sewer, Combined Sewer, Stormwater Drainage Systems or Water Resources.

DPW has issued approximately 200 Stormwater Control Permits since the Land

Disturbance Regulations were established.

Cambridge’s Wastewater and Stormwater Management Guidance Document (2008)

establishes the City’s technical standards for stormwater control. The current standard for the management of stormwater volume requires a permit holder to demonstrate that their property will be able to store the difference between the 2-year storm (3.6 inches) pre- construction, and the 25-year storm (8.2 inches) post construction, using the City’s projected rainfall data for the 2070 storm.

A storage requirement of this magnitude (equivalent to approximately 4.6 inches of rainfall on-site) and the use of projected storm events incorporating climate change are not mandated by any federal or state regulations. Cambridge adopted this rigorous standard to directly support citywide stormwater management improvements.

Due to the density of redevelopment in Cambridge, on-site storage of the 2070 100-year storm (11.7 inches) is not feasible due to the space required to store this volume. Despite this, by enforcing the 2-year to 25-year 2070 storage requirement across a wide range of projects, Cambridge has achieved a substantial level of on-site stormwater management and will continue to require developers to incorporate significant stormwater storage into their projects.

Cambridge also extends stormwater management to even smaller projects requiring a Building Permit, but not a Stormwater Control Permit, including any project with: A building addition greater than 150 square feet, or Any proposed increase in impervious area.

For these projects, DPW requires stormwater controls to the maximum extent practicable, including evaluation of storage for the 2070 2-year to 25-year storm difference. DPW reviews an average of 320 building permit applications annually. Of those, an average of 60 reviews each year include a review of civil site plans and stormwater management. The goal of these reviews is to identify opportunities for smaller projects to make improvements that contribute to the goals of the City’s stormwater management program.

DPW continually evaluates existing regulations and project thresholds to align with changes in development and City zoning. In the summer of 2025, DPW initiated a detailed review of project criteria for Stormwater Control Permits, evaluating recent changes to zoning and recent development projects (parcel size, project criteria triggering a Stormwater Control Permit, etc.). As a result of this review, DPW plans to revise its criteria for projects that require a Stormwater Control Permit, in order to maintain the effectiveness and impact of this regulatory program.

In parallel, DPW is revising the Wastewater and Stormwater Management Guidance

Document to clarify requirements for smaller projects. Proposed updates include requiring all projects undergoing DPW site plan review for a Building Permit to store the difference between the 2-year (pre-construction) and 25-year (post-construction) 2070 storm events to the maximum extent practicable, with a minimum on-site storage requirement this is still being evaluated.

Cambridge’s stormwater regulations already exceed all federal and state requirements and set a high standard for stormwater storage in an urban setting. The proposed revisions to project criteria will allow the City to incorporate additional private development projects into the City's program and support stormwater improvements citywide.

Implementation of Green Stormwater Infrastructure

As a standard practice, DPW evaluates roadway and capital improvement projects for opportunities to incorporate green stormwater infrastructure where feasible. The City’s Green Streets Guidance document outlines the City’s approach to integrating green infrastructure and stormwater management into the design of public rights-of-way while balancing other public infrastructure needs. The City integrates green infrastructure features into roadway, sidewalk, and open space improvements whenever practicable.

Examples of green stormwater infrastructure implemented across Cambridge include the Alewife stormwater wetland which provides stormwater quantity and quality control implemented during previous CSO control work in West Cambridge. The City has also constructed porous bike paths, porous pedestrian walkways, bio-basins, rain gardens, subsurface infiltration systems, tree filters, infiltrating catch basins, and drywells.

Subsurface infiltration system at Longfellow Park.

Bio-basin on Lakeview Ave.

Porous pavement bike path along Galileo Galilei Way.