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Researchers test low-cost thermal method to monitor bridge scour

Bridge scour is the enemy. Scour is the process by which water erodes channel-bed material due to its flow being disturbed either by an obstruction or constriction. The same process can deposit channel-bed material depending on the flow and channel-specific geometry/hydrology. The fluctuation of streambed material can affect the stability of bridge piers, levee foundations, or structure footings.

The ITD Scour Committee typically assesses the scour risk of a bridge based on measurements made during annual or bi-annual inspections. To address the risk of bridge failure due to scour, the committee monitors real-time flows under scour-critical bridges using U.S. Geological Survey, Next-Generation Radar, and Snow Telemetry information through the BridgeWatch™ software.

"Current methods for scour detection, which may include ring rods, acoustic or sonar technology, are too complicated, too costly or just impractical. Measurements of streambed elevation are difficult to obtain using the current methods until after high flows have subsided, and when flows subside silt may be deposited potentially masking a scour problem," Lotwick Reese explained. Reese, state hydraulic engineer, is the ITD project manager for the research.

The Scour Committee requested and the Research Program has funded a project with the University of Idaho to field test a new inexpensive method to accurately measure streambed elevations accessible in real-time.

Along with Reese, the Technical Advisory Committee includes Scour Committee members Tri Buu, Dan Gorley, Jake Legler, Bryan Martin, and Luis Zarate; Research Program Manager Ned Parrish; and FHWA Bridge Engineer Ed Miltner.

Researchers Daniele Tonina and Frank Gariglio at the University of Idaho, and Charlie Luce, of the U.S. Forest Service, developed this low-cost thermal scour-deposition sensor device, in collaboration with graduate student Tim DeWeese and undergraduate student Aston Carpenter.

"The thermal-sensor device measures scour and deposition in the streambed based on the differences in temperature between surface and streambed waters," said Tonini. The method uses temperature sensors in a tube buried vertically in the streambed. Analysis of amplitude and time lag between the temperature signals measured in the surface water and in the streambed provides information on the change in streambed elevation. Equations to analyze the data are simple and have been published in peer-review literature, he said.

"This research will test the accuracy of this method in a field-case study," Legler explained. This will be accomplished by testing the new method at four scour-critical bridges across the state, and at one stable bridge (to provide a control "subject"). Researchers are currently installing the devices at the selected bridges while water levels are low, he said.

Pictured above: Thermal scour chain installation along the North Fork Payette River. To the left is graduate student Tim DeWeese. To the right is undergrad Aston Carpenter.

Scour measurements from the new method will be paired with a scour chain to compare maximum scour with manual survey measurements. Temperature data will be collected with data loggers and transmitted with a cellular connection to an internet server where it can be accessed and analyzed. The project also aims to develop a warning procedure in case critical scour is reached at a pier.

This research will improve the efficiency of detecting bridge scour, thereby reducing field work and providing more timely data. There is also the potential to detect critical conditions remotely, which would provide a tool for avoiding potential catastrophic situations. Avoiding catastrophic bridge failure not only saves lives, but can also significantly mitigate costs associated with repairing or replacing damaged structures.

"This research directly addresses these vital problems and has the potential to reduce efforts in monitoring scour and improve safety for the traveling public," explained Parrish.

The bare cost of the materials for the sensor is about $300. The cost for installation depends on the configuration of the bridge, depth of the water, and the depth of the sensor device beneath the streambed.