Looking beneath the surface to understand explosive air-water interactions

// Civil & Environmental Engineering

A city’s stormwater infrastructure doesn’t often draw the public eye, but when conditions are right, it can put on a show.

Photo of Dan Wright
Daniel Wright

When a storm hit Belo Horizonte, Brazil, it created just the right mix of trapped air and water within a stretch of the city’s stormwater drains. And that created an explosive display of water erupting along the side of a street. Though not always so spectacular, these events—called adverse multi-flow interactions—happen around the world, from Chicago to Taiwan, the United Kingdom and New Zealand.

Daniel Wright, a hydrologist and assistant professor of Civil and Environmental Engineering at the University of Wisconsin-Madison, says these interactions occur when air gets trapped with water in storm pipes. If there’s nowhere for the air to escape as more water flows into the pipes, an air pocket can compress, storing energy like a spring.

“That air pocket is going to rebound and push the water back where it came from,” Wright says. “If there’s a manhole shaft, it can blow water and sewage into the air. That can be dangerous, especially if it sends a manhole cover flying into the air.”

Now Wright is part of a multi-institutional team, with Jose Vasconcelos at Auburn University and Ben Hodges at the University of Texas at Austin, that’s seeking to better understand what causes these events and how we can improve stormwater systems to minimize the risk. Their project is funded through the National Science Foundation’s Civil Infrastructure Systems Program.

When it rains, city stormwater systems are tasked primarily with moving huge quantities of water safely and efficiently. To that end, Wright says, they generally perform very well, and engineers have rules of thumb for matters such as what sort of pipe works best in a certain part of a system to move water. But adverse multi-flow interactions present a different challenge that most stormwater systems aren’t designed to handle, and Wright says most systems are laid out with the assumption that air within them can generally be ignored.

“Our hypothesis is that these interactions follow the beat of a different drummer, in terms of the timescales at which they develop,” Wright says. “We think it can be much faster than the usual concern for these systems, which is lots of water moving downstream where it might adversely affect someone. If our rules of thumb don’t apply for these events, we need to develop new ones.”

Wright, who is an expert on extreme rainfall events and their impacts on urban areas, says the collaboration brings together different areas of expertise: Vasconcelos has long focused on the physics of air pockets within pipes and Hodges is experienced with city-level storm sewer analysis. Team members will bring all of their combined expertise to bear to better understand the timescales at which adverse multi-flow interactions form and the complicated interactions among stormwater pipe networks that are at play in their creation.

They’ll also likely employ computational tools—because it can be difficult observing these events in the field without a way to consistently predict their occurrence—though Wright notes there are challenges in attempting to model a stormwater system that might be made up of thousands of individual pipes.

“We’ll see if we can identify indicators of when these interactions might pop up based on metrics like the amount of water coming down a pipeline, or the length and diameter of the pipeline,” he says. “We’re looking for some simple combination of metrics that seems to be indicative that an adverse multi-flow interaction is going to occur.”

Author: Alex Holloway