$40 million
invested in clean water
88%
reduction in overflows
3.5 million
gallons CSO storage
The Back Cove South Combined Sewer Overflow Storage Facility is a prime example of how expertise, creative problem solving, and public-private collaboration can make an immense impact on the environmental health of a community.
Working closely as the designer/engineer for design-builder Sargent Corporation, Woodard & Curran performed project design for the new 3.5-million-gallon Back Cove South Combined Sewer Overflow (Back Cove South CSO) below-grade storage facility. Our engineering work included site/civil, environmental, structural, hydraulic and conveyance, mechanical, electrical, plumbing and instrumentation, and controls disciplines. In addition to critical stormwater infrastructure, we incorporated resilience measures to enhance and protect valuable public amenities on the project site.
For nearly three decades, the City of Portland has focused on sewer separation projects in pursuit of long-term stormwater quality controls as part of its CSO abatement program. In 2013, the City developed its Tier III Long Term Control Plan (LTCP). In addition to continued separation efforts, the plan identified opportunities for wet weather storage tanks throughout Portland, with a significant focus on Back Cove.
This highly visible, low-lying area west of the city’s peninsula and east of its mainland neighborhoods features commercial developments and a popular park and walking trail. In addition to the issue of combined sewer discharge to the cove, the area is prone to flooding. Winter storms over the last decade have brought heavy winds and wave activity, resulting in erosion along the public waterfront.
In partnership with Sargent Corporation, Woodard & Curran was selected by the City of Portland to deliver the Back Cove South CSO project via design-build. The project team needed to meet stringent requirements throughout the bidding and contracting process to comply with city and federal policies. After navigating the complexities of authorization and design, the project received notice of award in August 2019 and notice to proceed in February 2020; we issued construction documents by November 2020.
invested in clean water
reduction in overflows
gallons CSO storage
Early conceptual designs for the Back Cove South CSO facility centered on installation of a single box conduit in a major commercial area. To avoid the complications posed by siting the project beneath a busy city road dotted with businesses, a new location was selected: beneath a public soccer field. However, this site posed its own challenges. Its proximity to a tidal mudflat and elevated highway, Interstate 295, created significant geotechnical and structural concerns.
To mitigate global stability concerns during excavation, the design team altered the facility layout to include four separate, sequentially filling, 875,000-gallon tanks. Our plan called for two-phase installation to ensure the stability of the site and surrounding features: This approach reduced the risk of rotational slope failure related to the elevated highway and provided the construction crew with more surface space to work without disrupting traffic or the nearby public trails. This alternative also offers substantial cost savings over the City’s original single-tank plan.
The project team also had to address the challenge of capturing inflow from three separate, tidally influenced CSOs. With a diurnal tidal range of approximately 10 feet (or greater during extreme conditions) and the existing pipe elevations four to five feet below mean sea level, the system hydraulics dictated that the bases of the storage tanks would need to be at least 20 feet below mean sea level.
The design team used a dynamic stormwater management model (SWMM) to account for the tidal cycles, interconnecting outfalls, backflow prevention, storage tank connections, and the losses imparted by diversion and access structures. Additionally, the team leveraged 3-D hydraulic modelling to confirm that the hydraulic design of CSO diversion structures meets the City’s CSO control requirements These results were used in conjunction with the system wide SWMM model to assess impacts to upstream collection systems and set design elevations for the storage facility.
A cyclist passes behind Ethan Hipple, Parks, Recreation & Facilities Department Director for the City of Portland, during his speech at the facility’s ribbon cutting ceremony in September 2025.
When redesigning the public amenities situated above the CSO tanks, our plan called for installation at a higher elevation. This improved the long-term resilience of the playing field and abutting paths and obstacle course, all of which are heavily used by the community.
This highly visible project has engaged and impacted the public at a scale rarely achieved by comparable initiatives. Its scale and location have made the Back Cove South CSO storage facility project impossible to miss, presenting a unique opportunity to inform and accommodate community members.
The socio-economic wellbeing of the Greater Portland Region is tied intrinsically to the health of Casco Bay. The city’s classic New England waterfront and proximity to maritime industry and recreation are a major draw for residents, businesses, and visitors. The Back Cove South CSO project significantly reduces combined sewer overflow, leading to cleaner water and a healthier environment that are paramount to a thriving community.
On a smaller scale, when redesigning the playing field situated above the CSO tanks, our plan called for installation at a higher elevation. This not only ensured the project incorporated maintaining this valuable public recreation space but also improved the long-term resilience of the playing field and abutting paths and obstacle course. The non-field surface area adjacent to the shoreline was also revegetated with native plants.
Now complete, the Back Cove South CSO storage facility is actively reducing overflows to Casco Bay. As intended, this integrated stormwater management measure is providing flexibility in flow management for the City’s overall sewer system and decreasing cumulative overflow volumes by up to 88 percent. The project was awarded an Engineering Excellence Award by ACEC Maine in November 2025.
