Looking for Trouble Beneath the Water Surface

SAFL tests scour monitoring system to protect Minnesota bridges

The recent flooding on the Minnesota and Mississippi Rivers due to rain events have yielded some interesting data for researchers conducting automated scour monitoring on two bridges in Minnesota. Scour—the erosion of sediments around bridge piers and abutments—accounts for the majority of bridge failures in the United States.

The primary purpose of the three-year implementation study is to develop systems which can provide automated scour warnings to district-level Minnesota Department of Transportation (MnDOT) bridge crews. Typically, designated river stages (water surface elevations) or discharges associated with extreme floods trigger bridge crews to manually measure scour with sonar or other devices at the bridge site. This is to ensure that excessive erosion, which could create a risk of failure, is not occurring at the bridge. Manual monitoring can be difficult, inaccurate and dangerous with the high currents associated with extreme flood events. This project builds upon the results of a previous study conducted by St. Anthony Falls Laboratory (SAFL) titled, “Bridge Scour Monitoring Technologies: Development of Evaluation and Selection Protocols for Application on River Bridges in Minnesota,” and funded by MnDOT through the Center for Transportation Studies (CTS) at the University of Minnesota.

Sponsored by MnDOT through CTS, and in collaboration with ETI Instrument Systems , SAFL engineers, Matt Lueker, Jeff Marr and Chris Ellis, designed and installed fixed scour monitoring systems on two bridges in Minnesota rivers in October 2011. The first site is located on the Minnesota River in Mankato, MN and includes a sonar sensor on one pier, float-out devices on a second pier and a radar river stage sensor. The second site is located on the Mississippi River in Winona, MN and includes sonar sensors on two piers, a pier tilt meter and a submerged pressure sensor to measure the water surface elevation or stage of the river. Both scour sensor systems are solar powered and data is automatically logged via cellular modem to St. Anthony Falls Laboratory’s field monitoring database.

With the new systems in place, MnDOT employees can be alerted to dangerous scour levels and log onto a web tool to view the historical and real-time scour at the bridge site. Additional automated alerts are used to ensure continuous operation of the system. A second purpose of the deployment is to determine if installation of fixed scour monitoring systems is feasible. The two installed systems act as case studies for any future deployments. Importantly, the data logged and captured by the monitoring system can be used to validate scour calculations at field scale.

Early study results
The first six months of data will be used to characterize the scour behavior of the two bridges and set the appropriate alert levels for MnDOT personnel. The alerts need to balance significant event warnings while not giving false alarms. The dry 2011 fall and winter did not provide significant spring runoff flows on either the Minnesota or Mississippi River, but the rainy 2012 spring and associated river discharges have caused the monitoring systems to measure scour events. Figure 1 shows a screenshot of the web tool used to show historical scour conditions on the Winona Bridge. The March runoff event resulted in a three-foot, long-term scour event (dropping from 625 to 622 feet) and the June rain event yielded a four-foot scour event (dropping from 622 to 618 feet) that refilled with active bedload (sand and gravel constantly being moved along the river bottom) in the days immediately following the event. Because the scour hole was relatively quickly refilled, the June scour event would have gone unnoticed without this scour monitoring equipment. Additional data collection will determine if the bed level of the river rises to its previous winter levels over the course of the summer.

Challenges to deploying fixed scour monitoring equipment include river debris and ice which can destroy the systems. SAFL engineers focus primarily on efforts to mitigate these risks as much as possible. All conduit connecting sensors to dataloggers follows the profile of the pier as closely as possible to avoid snagging debris. Furthermore, sensors are located at elevations typically above or below the ice layer. For the Mankato Bridge, this involved mounting the sonar sensor above typical winter water surface elevations. The Minnesota River water surface elevation at this location rises dramatically with increase in discharge.  This stage increase submerges the sonar sensor. On the Winona Bridge, a downstream dam controls water surface elevations causing little variation, so the sonar and stage sensor was mounted five feet below the lowest possible water levels. This keeps the senor out of the ice layer and debris.

Next steps
Following development and implementation of an alert system, SAFL researchers will provide training and support to the MnDOT bridge staff that monitor scour on these bridges. The system will provide them with automated warnings and remove the need to monitor scour on these bridges manually. In addition, Lueker, Marr and Ellis will continue to monitor the deployed fixed scour monitoring system including any design issues, technology failures, impacts of debris or ice on the system, required maintenance and any additional instances which might impact the monitored results or potential future deployments. Outcomes from the study will determine the effectiveness and feasibility of this system for installation on additional bridges throughout Minnesota, ensuring the safety of bridges and, most importantly, motorists, during significant flow events in rivers.