Urban flooding: the challenges of combined sewage systems in coastal cities facing climate change and sea level rise

Older coastal cities, such as Philadelphia, New York, and Boston, face the risk of being overwhelmed by untreated sewage during floods. Due to the design of their combined sewer systems and rising sea levels, these cities could face a growing public health crisis as climate change also leads to more extreme rainfall, according to researchers at Drexel University studying urban drainage systems. The group recently published research modeling the potential scope of the problem in a part of the coastal city of Camden, New Jersey, and the effectiveness of one proposed solution to protect these communities.

A Complex Problem
Beginning in 1855, many American coastal communities were designed with combined sewer systems. In these systems, stormwater and sewage are collected using the same pipes. Initially, these pipes discharged water into streams and rivers; later, they were directed to wastewater treatment plants. However, the pipes can carry only a certain flow. During rainy weather events, to avoid overwhelming the wastewater treatment plants, some of the flow still spills over into natural water bodies via functions known as combined sewer overflows (CSOs).

While the Pollution Control Act and the Clean Water Act have prompted communities to upgrade infrastructure and take steps to reduce CSOs, climate change brings a whole new dimension to this compliance challenge.

When the water level in the receiving water body is high, CSO flaps that normally prevent river water from flowing back into sewer pipes cannot easily open. Without these valves fully open, combined sewage generated during rainy weather can back up into the system, even spilling out onto streets or into people's basements.

As climate change brings more intense rains and higher river levels, the problem worsens and cannot be mitigated by conventional stormwater management approaches.

Montalto's team is working closely with the Camden County Municipal Utilities Authority (CCMUA) to study potential solutions to this problem.

Seeking a Better Answer
In their research, recently published in the Journal of Water Management Modeling, the group reported on the results of their detailed hydrological and hydraulic models of flooding and combined sewer overflows in a part of Camden called Cramer Hill. This flood-prone area of the city is located very close to the largest CSO point on the eastern side of the Delaware River.

After calibrating their models to historical conditions, they used them to simulate how flooding and CSOs will change in the future as the climate changes. The same models are also used to assess the potential effectiveness of various conceptual solutions.

"CCMUA has been working for years to reduce environmental injustice in Camden," Montalto said. "They have worked on reducing odors from their wastewater treatment plant and reducing the frequency and pollution associated with CSOs. It's exciting to work with them now to develop solutions that can also reduce flooding and make Camden's neighborhoods more resilient to climate change. Our modeling will support CCMUA in developing multifunctional infrastructure strategies."

Drexel's model is unique because it is an "all-pipe" model built by integrating many different geospatial data sets into one computational model. This allows the team to simulate stormwater flows through nearly every surface, stormwater catch basin, and pipe in the area.

To verify the accuracy of the modeling program, researchers compared the predicted annual volume of combined sewer overflow discharges with CCMUA records. Simulated flood patterns were compared with photos of actual floods taken by the research team during storms in the summer of 2021. "It was important to conduct a thorough validation process because we will rely on this model to simulate future climate and infrastructure conditions," Montalto said. "Not every municipality records all the necessary data to create a complete model, so part of this research is showing that the ad-hoc process we developed can reliably validate our model without some of the data that would otherwise be needed."

Projecting Future Challenges
Montalto's team used the validated model to simulate what would happen if rainfall increased by up to 30% and if sea level rose by up to 1.8 meters. They simulated each of these climate changes separately and together.

The model predicted that increased rainfall would result in overflow discharges 21-66% above the current annual discharge volume. And, although each of the sea level rise scenarios resulted in a reduction in the number of overflows and the annual discharge volume, the duration of flooding increased with each compounded factor.

Testing the Theory
One key strategy Camden is considering for water management challenges in Cramer Hill involves diverting stormwater from the northern part of its sewer system. With Drexel's modeling program, the municipality was finally able to test this idea.

Called the "Pennsauken disconnection," the proposal is to divert stormwater generated in Pennsauken, New Jersey, located just northeast of Camden, away from Cramer Hill's combined sewer system via an intermediate pumping station.

The team found that the diversion would help in all future climate scenarios. However, even with the diversion, the effects of climate change and sea level rise still resulted in an increase in the number of flooding events and a significant increase in the duration of flooding under sea level rise conditions.

Setting a New Course
The overall results suggest that increased rainfall events due to climate change will cause more combined sewer overflows. And sea level rise will make it harder for these systems to discharge into nearby water bodies. About 40 million people currently live in areas served by combined sewer systems, so this is an urgent issue that could affect a significant number of people across the country.

Montalto's group plans to continue refining their Cramer Hill model as they gather information about water flow through the sewer network and surface flooding. They will also model other stormwater management infrastructure interventions.

Source: Drexel University