Ultrasonic Devices Protect Bats from Bridge Repairs


Researchers have teamed up with the Minnesota Department of Transportation to deploy ultrasonic deterrence devices at bridge construction sites in the hopes of reducing bat activity during repairs. The project comes as concerns grow about the U.S. bat population from White-Nose Syndrome (WNS) and increased disturbance of habitats.

MnDOT reports that regulatory requirements that protect bats, including the threatened northern long-eared bat familiar in Minnesota, can limit how crews can paint, clean or repair structures, as well as disrupt timelines and budgets.

The team examined ultrasonic devices, which are also used in wind turbines, to temporarily deter bats from bridge sites.

“We were very happy to identify potential solutions for MnDOT. This technology temporarily deters bats without causing harm,” said Basak Bektas, Assistant Professor, Minnesota State University, Mankato Department of Mechanical and Civil Engineering.

“This project was innovative. We worked with a technology that wasn’t really on the market yet for real-world applications in anticipation of its availability,” said Christopher Smith, Wildlife Ecologist, MnDOT Office of Environmental Stewardship.

Bats in Minnesota

According to the Minnesota Department of Natural Resources, there are seven species of bats in the state. Lori Naumann, Minnesota DNR Nongame Wildlife Program Information Officer, explained that the bats eat thousands of mosquitoes in one night and are crucial to the state’s ecosystem.

"They are very common in Minnesota," Naumann said in an interview. "They're just great little creatures."

WNS, a disease that causes a white fungus to grow around a bat’s muzzle and its body, has reportedly killed millions of bats across North America since it was discovered in 2006/2007. The infection was first confirmed in Minnesota bats in winter 2016. The state’s DNR reports that as of 2021, the number of bats hibernating at Soudan Underground Mine and Mystery Cave has declined by over 90%.

Bats typically like to roost in the expansion joints in bridges during construction season. MnDOT explained they do not want to permanently keep the bats away because the bat population is already in decline, also due to habitat loss.

"If the work were to start when the bats were present on the bridge and they have all their babies they would often leave those babies behind then those babies would likely not survive," Smith said.

"We care a lot about bats and want to do our part to protect them.”

The Research

MnDOT teamed up with the Technical Advisory Panel, the Minnesota DNR and the U.S. Fish and Wildlife Service for the project in summer 2019.

For the research, the technology was installed at two different sites in Minnesota, an area on U.S. Highway 61 in Red Wing and a single-span bridge on State Highway 43 near Rushford Village.

Four battery-operated echolocation recording devices were installed to monitor bat activity before, during and after the use of deterrence devices. The ultrasonic deterrence devices were then used for a short-term and long-term trial at site one and site two, respectively.

"When we project this sound and create this jamming of their signal, they then look for other places to go," Smith said. 

At site one, deterrence ran for 10 days. Prior to the deterance, 6,008 to 7,227 bat calls were captured, with 22 calls detected during deterrence. At site two, deterrence ran for 21 days, was shut off for a week and then turned back on for 24 hours. Prior to that, 6,308 to 9,734 calls were reported, with eight calls detected during the first period and none in the second period.

Once projects are complete, the deterrence devices can be removed to allow the bats to return safely.

Call numbers ranging from 3,162 to 5,564 were heard at site one after removal, and 10,432 to 14,826 were heard at site two.

"They came back pretty quick, anywhere from the very next day to a few days later," Smith said. 

MnDOT stated that they hope to implement this technology in the future, as well as research the devices in different configurations and environments around the country and conduct cost-benefit analysis.

The full research report can be viewed here.

Bat Conservation Bridges

In 2017, a bridge being built over a newly constructed roadway in County Galway, Ireland was designed specifically to help bats—lesser horseshoe bats, a protected species that inhabits the area—cross safely, according to the Galway Independent.

The landscaping, shape and location of the “green bridge,” in Coole Green, was designed to help maintain an already-established flight path of the horseshoe bats, according to DirectRoute, the public-private partnership that’s building the new Gort-to-Tuam road over which the bat bridge stands.

According to DirectRoute, the bat bridge doubles as an “accommodation overbridge”—a bridge that will route traffic from an existing, smaller road over the new highway. Its design and construction, though, were optimized to help low-flying bats across the high-speed motorway.

It’s not the first bridge designed specifically for wildlife—in fact, it’s not the first bat bridge. In 2013, two bridges built for bats over a road in Germany sparked controversy when some citizens questioned the hundreds of thousands of dollars that funded them.

In The Netherlands in 2015, another bat-friendly bridge was opened, designed both for pedestrian and cycle traffic. The Vlotwatering Bridge, in the town of Monster, has nooks and crannies that encourage bat occupancy, and is made to maintain an ideal climate for the flying mammals.

With its diverse sheltered roosting spots for the mammals, the final design is meant to provide an “ideal habitat” for a variety of different bat species so that a large colony can be supported near the bridge.

While the 25-meter (about 82-foot) concrete arch is said to contribute a “stable and pleasant climate” for bats, all of the bridge areas were built to offer roosting spots to meet different seasonal needs.

The abutment at the north side of the structure provides a source of shelter for the winter months, when the bats will hibernate. Masonry walls within the interior cavity divide the space into smaller spaces for roosting.

Both the deck and brick balustrade include small openings that make for ideal spots for summer perches.

The underside of the bridge features 300-by-20mm (12-by-0.8-in.) slits that create access points for the bats. The slits line up with the vertical slats of the wooden screen wall above.

Along the brick wall, small openings provide access to cool spots for summer roosts, as the brick protects the animals from the heat during the summer months.

Here and in the winter roosts, the architects made sure that the openings were small enough that the bats could enter, but their predators—cats, owls, humans—could not.

This combination of functionality for both humans and the bat population, as well as its aesthetics, earned the Batbridge a spot in the ARC15 Detail Award competition, hosted by Dutch magazine de Architect.


Tagged categories: Bridges; Bridges; Construction; Department of Transportation (DOT); Environmental Controls; Environmental Protection; NA; North America; Program/Project Management; Rehabilitation/Repair; Research; Research and development; Technology

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