Reducing stormwater flooding in Philadelphia

Much of Philadelphia’s sewer and stormwater infrastructure is combined, making some neighbourhoods subject to flooding during intense rain events. Reducing flood risk in these neighbourhoods mitigates physical damage to property, minimises financial impacts and protects human health.

The neighbourhoods of the Northern Liberties and South Kensington are close to Philadelphia’s historic centre and beside the Delaware River. There has been a history of both surface flooding and sewer backups when the infrastructure carrying both stormwater and sewage is overwhelmed.

It is one of the largest capital improvement programmes for storm flood relief that PWD has undertaken in recent years.

Gary Snyder

Senior vice president, Mott MacDonald

To tackle this, Philadelphia Water Department (PWD) is undertaking a six-phase programme, the more than US$93.5M Cohocksink storm flood relief project, which will double the capacity of the neighbourhoods’ combined sewer system. “It is one of the largest capital improvement programmes for storm flood relief that PWD has undertaken in recent years,” says Mott MacDonald senior vice president Gary Snyder, who has been involved in the project since 2008.

Named after the Cohocksink Creek that once flowed through the Northern Liberties and Kensington, and is now buried, the project comprises six phases, five of which are now complete. The work involves installing new box sewers and chambers to replace or expand on elements of the existing system. With much of the phasing chosen to mesh with ongoing private development and transport infrastructure work near the city’s waterfront, the project also required complete or partial street closures, as contractors dug trenches for the new concrete sewers.

Weaving new sewer capacity through the existing utilities and historic remnants of a 341-year-old city requires liaison with multiple asset owners and other stakeholders. And the picture is constantly changing: the Northern Liberties and South Kensington are metamorphosing into a thriving mixed-use residential and entertainment district, with new buildings constantly rising.

A whole-city approach

With Philadelphia’s sewers among the first to be built in the US, many carry both sewage and stormwater. During drier periods, both rainwater and sewage flow to a treatment plant for removal of pollutants before being discharged into the Delaware River. But when rainfall is heavy the combined system – which serves approximately 60% of the city – becomes overwhelmed, and excess flows may be directed to waterways via designated and permitted combined sewer outfalls.

As well as affecting fish and aquatic plant life, this makes waterways unsafe for swimming or water sports immediately after a discharge. While not ideal, these designed overflow points are necessary at times of heavy rain to prevent the vast majority of excess flows from flooding neighbourhoods.

Despite these designated overflows, the existing infrastructure contains old inefficiencies and bottlenecks that can prevent the excess flows from reaching the waterways, causing streets and basements in certain neighbourhoods to flood. PWD has identified these areas as requiring special attention, initiating flood relief projects to rebuild the area’s infrastructure so that it has the capacity to prevent backups.

To tackle the issue on a broader level, PWD has developed a strategy, approved by the city in 2011, to slow down rainwater before it reaches the sewers.

Green infrastructure

Green City, Clean Waters is a 25-year plan to introduce a host of green infrastructure elements across Philadelphia, creating parks, planted areas, porous paving and tree pits to soak up water and stop it whooshing straight into the combined sewer system.

Creating green spaces is very important, not only to slow down stormwater flows but also because they are important for the community.

Gary Snyder

Senior vice president, Mott MacDonald

Some new green infrastructure is being created by private developers, who are required and incentivised to do so, while some is being created by the city authorities. For instance, the Cohocksink storm flood relief project includes small elements of green infrastructure.

“Although it is a very small part of this project, creating green spaces is very important, not only to slow down stormwater flows but also because they are important for the community,” says Gary.

The Green City, Clean Water strategy will also see extensive tree planting in the streets and the rejuvenation of green spaces such as riverbanks for public amenity.

Flood relief over six phases

Mott MacDonald has been the designer for phases two, four, five and six, including geotechnical design to determine where piles would be required to support the box sewers. We supported programme planning efforts by the PWD to refine key hydraulic design criteria.

The Cohocksink flood relief project consists of six phases

5m wide original main sewer

525ha stormwater drainage area

35cm anticipated sea level rise

A rising challenge

In the 19th century, the main Cohocksink Creek was encased in an arched brick sewer, 5m (16ft) wide. Hidden below the busy streets of the Northern Liberties and Kensington, it takes stormwater from a drainage area of 525ha (1300 acres) as well as an increasing wastewater load.

Homes and businesses in the Northern Liberties and South Kensington feed into the Cohocksink sewer. As the area was developed, this brought an increase in hard surfaces – roofs, roads, paving – which cause heavy rainfall to rush down drains rather than partly soaking into the earth.

PWD is working to mitigate this by creating green infrastructure such as rain gardens, tree pits and planters, under its Green City, Clean Waters initiative (see ‘Green infrastructure’). The department offers programmes that provide incentives for stormwater management.

Climate change is another factor. The Delaware River at Philadelphia is tidal, so rising sea levels are affecting outfalls along the river. According to the National Oceanic & Atmospheric Administration – the US meteorological office – sea levels on the East Coast are set to rise by up to 35cm (14in) over the next 30 years.

Keeping it all flowing

With 1.6km (1 mile) of box sewer structures and several chambers to shoehorn into the changing underground landscape of the Northern Liberties and South Kensington, the design had to be pragmatic.

Modelling by PWD had determined the system would be fully gravity driven where extra flow capacities were necessary. Designs for the upgrades had to translate these flows into box sewers and identify where best to accommodate new chambers – while minimising disruption to traffic and neighbourhoods and still allowing the existing sewer system to flow.

1 mile of box sewers to construct

Some sections of sewer required an additional conduit to be built alongside the old one. For others, a completely new sewer had to be constructed. With the ground varying from soft, reclaimed land near the river to hard rock further upstream, phases one through four needed piles. These were mostly timber.

All the sewers were installed using open-trench construction. Structures for phases two and four were cast in situ, with the use of precast elements for the phase five box sewer. Where speed of construction was paramount to limit disruption, a travelling formwork was deployed.

Complex design and construction

Some sizeable structures are involved. For instance, a 18m x 61m (60ft x 200ft) chamber under Canal Street includes a dry weather outlet (DWO) and dam to collect and send flow to the wastewater treatment plant. The chamber incorporates three tide gates to prevent water from the Delaware River from entering the DWO.

“That was really complex in terms of both design and construction,” recalls Gary. “The design also had to consider the spaghetti of existing utilities.”

Some utilities could be worked around, but others had to be moved to make way for the excavations in which the box sewers and chambers were built. At Canal Street, the team deployed historic photos (see panel ‘Deep into history’) that showed how the canal had been altered and eventually filled in over time. “Historical photos were an important reference in lieu of detailed drawings,” says Gary.

Construction winds its way through the Northern Liberties and South Kensington streets

Construction winds its way through the Northern Liberties and South Kensington streets

Construction winds its way through the Northern Liberties and South Kensington streets

Often the excavation revealed a different layout of underground utilities than expected. Our engineers would then visit the site to make design modifications – either in the field or back in the office, depending on the complexity.

Even as the project developed, new changes required further accommodation. When design began for phase five, the route passed by four empty lots. By the time work started on site, buildings were under construction on all four. “The contractors had to share space and share a construction entrance, and then it got even tighter,” says Mott MacDonald engineer Melissa Adamson. One of the new buildings abutting the sewer construction required underpinning, with our team reviewing the contractor’s designs.

Deep into history

The many creeks flowing through the Northern Liberties and Kensington area made it home in the 17th century to early industries such as grain processing, corn processing and textile manufacture. That industrial trend continued, with many of Philadelphia’s 19th century factories located in the area.

“In the subsurface you can find just about anything – and we probably did,” says Gary. “There are natural materials, all sorts of fill, coal ash from days past, old 1600 and 1700 era structures. The interesting part was that between 152m and 309m [500ft and 1000ft] from the waterfront is made land, so we were looking into history as we assessed the ground.”

In trying to work out what lay beneath the city’s surface, our designers were aided by an amazing archive of photos collected and digitised by local historical consultant Adam Levine.

Archive photo of the Cohocksink sewer (Credit: Philadelphia Water Department)

Approaching the finish line

With the sixth and final phase yet to be tendered, there have been more changes to incorporate into the design. When the city’s streets department noted that full-width reinstatement works would follow the installation of the box sewer, it saw an opportunity to advance Philadelphia’s bike lane masterplan works.

“The streets department approached the PWD and asked if it would be interested in adding a bike lane to the job,” says Melissa. “We were at 70% design, so we paused to assess the design criteria.”

The bike lane sits alongside the pedestrian sidewalk, so the curb was moved between 3m and 4.3m (10ft and 14ft) into the road in places. Adding a bike lane meant losing some parking bays, so the changes also had to be discussed with the local community.

Although there will be fewer changes to anticipate as the project’s final phase comes into view, there are still hurdles to clear – not least the disruption from excavating a narrow residential street to install the new box sewers.

Once all the work is complete, residents of the Northern Liberties and South Kensington will benefit from the reduction in flood risk. As well as preventing the damaging effects of flooding after heavy rainfall, the increased capacity and greater system controls will reduce sewage outflow into the Delaware River, improving the environment for both aquatic life and humans.