A remote-controlled boat equipped with a sonar unit can maneuver around bridge piers and abutments during a high-water event for safer, more efficient inspections. The Michigan Department of Transportation has begun using the boats for bridge inspections across the state.

LANSING – It’s been raining. Hard. The water level in local rivers and streams is rising and there are concerns about the impact on highway bridges. Inspectors need real-time data on what’s happening beneath the turbulent surge but getting into the water for a visual inspection is risky.

This kind of scenario is one bridge inspectors for the Michigan Department of Transportation (MDOT) face regularly. The challenges and risks have the department evaluating small remote-controlled boats for safer and more efficient bridge inspections.

“Any time we have a major storm event, inspectors are required to go out and monitor these bridges to make sure they’re safe and that nothing catastrophic is going to happen to them,’’ said Chad Skrocki, assistant bridge engineer for MDOT’s Bay Region.

Scour, the erosion or degradation of the streambed around a bridge’s substructure, is a leading cause of bridge failure. Scour happens when fast-moving water carries off sediment around the substructure of a bridge, typically exposing the footings or piles. When the streambed is washed away, bridge support can be compromised, creating a hazard. MDOT has identified more than 400 bridges on the state highway system and around 1,200 on local roads that are ‘’scour critical,’’ and in need of close monitoring during high-water events.

MDOT routinely inspects bridges for scour around abutments and piers. Typically, this involves inspectors launching a boat and probing the channel bottom with metal rods, weighted tape measures or sonar devices. During high-flow events, this can be a dangerous task.

“In the past, scour inspections were difficult, especially during periods where water was surging,’’ Skrocki said, “We wanted to come up with a method that was safe and easy for inspectors to use.’’

Since it lurks underwater, scour can be difficult to detect. Ideally, inspectors look for signs of scour during high-water flow events as it’s happening. However, turbulence and fast-moving debris in the water could endanger workers using traditional tools like piloted boats and under-bridge inspection equipment. Also, these methods can’t be used in locations with limited vertical clearance, such as beneath a bridge when water levels are high, and they take time to deploy.

MDOT began a research project in 2016 to look for alternatives that met the agency’s needs. One promising alternative emerged: an unmanned surface vessel (USV), a remote-piloted drone boat, equipped with sonar and a camera to allow inspectors to take measurements and capture images from the shore. MDOT deployed USV units in the field to measure bridge scour and inspect bridges and culverts where access is limited. Results show it could be safer, less labor-intensive and, in some cases, less costly than traditional techniques.

After reviewing available devices, interviewing vendors, and conducting site visits, investigators recommended a USV called Sonar EMILY (EMergency Integrated LanYard). This powerful, durable, maneuverable water-jet-powered buoy is about 4 feet long, 2 feet wide and a foot tall, and is equipped with a sonar unit that can measure water depths and produce side-scan and down-scan images of bridge substructures and streambeds. The system also has a topside camera to view the underside of bridges.

“The environment this vessel is going to operate in is fairly extreme,’’ said Brian Schroeder of Ayres Associates, the principal researcher, “It needs to go fast, gather the data we want and be reasonably durable. EMILY checked all the boxes.’’

EMILY, an outgrowth of a partnership between the U.S. Navy’s Small Business Technology Transfer program and a private company, was originally developed for marine mammal research. However, it can be configured as a lifesaving device, for water sampling, underwater mapping, and many other potential uses. The manufacturer, Hydronalix, has outfitted the platform with sensors to conduct a variety of missions. The versatile platform has communicated with swimmers during triathlons and mapped a safe channel into Marsh Harbor in the Bahamas after Hurricane Dorian’s devastation.  

This system, along with post-processing software called SAR Hawk, was found to be the most effective and cost-efficient setup to meet MDOT’s requirements. The cost of the USV, laptop control unit and running gear was estimated at $50,000.

“Using the USV is much safer and less labor-intensive than traditional inspection methods for detecting scour,’’ said Skrocki, MDOT’s project manager for the research. ‘’It provides a great deal of information, in real time, to the inspector about what is occurring to the channel bottom below the water surface around the bridge substructures.’’

The research showed that a USV equipped with a sonar unit can accurately transmit real-time water depth readings and underwater images during high-flow events to bridge inspectors who are safely on shore. The equipment provides a consistent and accurate stream of data to inspectors. In addition, the project found that the USV can be used for other types of inspections, including difficult-to-access locations such as bridges and culverts with limited clearance between waterline and structure.

The rugged Kevlar-hulled EMILY unit can be transported in a pickup and easily deployed by a two-person team. It does have its limitations, though. The vehicle, transmitter, cameras, and sensors are all battery powered, which limits operating time. The wireless signal for control and data collection is generally line-of-sight. Eddies, currents and debris in the water can make for difficult driving conditions. Piloting a remote craft is difficult in general; an understanding of basic boat operations might help pilots, Skrocki noted.

MDOT is strategically placing four USVs equipped with sonar units throughout the state to inspect bridges for scour during high-flow events. In addition, the agency plans to use the technology for other applications as appropriate: getting images of bridge substructure below water, viewing the underside of bridges, and documenting performance characteristics. Schroeder said he has heard Washington’s DOT is interested in using similar technology but he isn’t aware of any other state transportation departments that have deployed them to date.

To support the use of USVs, MDOT trained bridge inspectors in April. The equipment is relatively straightforward. Personnel without prior knowledge of this specific equipment learned to operate the vessel and understand the data. Project investigators also wrote a user manual to help with future training.

Schroeder predicts unmanned vehicles will gain more widespread use for inspections as transportation departments learn more about them and work out the deployment.

“The preliminary inspections can become far more efficient and then we can use manned inspections to verify the things we’ve found with the USVs,’’ Schroeder said.