Real-time pier scour monitoring and observations at three scour-critical sites in Idaho, water years 2020–22

To observe real-time pier scour at three scour-critical sites in Idaho, the U.S. Geological Survey, in cooperation with Idaho Transportation Department, installed and operated fixed real-time (15-minute interval) bed elevation scour sonar sensors at three bridge locations associated with U.S. Geological Survey streamflow gaging stations for water years 2020 through 2022. Daily mean and peak streamflow conditions during the 3-year study were at or below average except for the peak flow in 2022. Each of the three sites included in the study had a coarse bed with an armored channel. Observed pier scour at each of the three sites was less than 20 percent than the stated minimum depth to the pier pile tip. The below average daily mean and peak streamflow during the study period may have resulted in below average scour.

Observed pier scour data during spring runoff (water years 2020–22) were compared to both Coarse Bed and Hydraulic Engineering Circular 18 (HEC-18) general pier scour design equation estimates to better understand how the observed pier scour data compared to design pier scour equation estimates during the same observational periods. For the 3-year study period, the Coarse Bed design equation generally overpredicted scour by about 2.5 times less than the HEC-18 general pier scour equation. The risk associated with each design equation was summarized using a reliability index to describe how each prediction might be expected to reliably overestimate scour depth. Overall, the Coarse Bed design scour equation provided more reasonable scour depth estimates than the HEC-18 general pier scour equation but with more risk to underestimating scour depth. Because these data are limited (3 sites, 3 years, and during average streamflow conditions), further research is needed to compare observed scour data to estimates predicted by the Coarse Bed design equation and other design equations.

This study demonstrated that real-time pier scour monitoring is a useful method and countermeasure at critical bridge sites. A recently developed rapid deployment real-time pier scour monitoring method may be a useful method to consider for future studies. Real-time monitoring at scour critical sites may be a useful tool to confirm previous scour evaluation estimates where site inspection scour observations conflict with the scour evaluation estimates. Considering alternative scour monitoring and evaluation methods, including the rapid estimation method, and updating pier scour calculations using the most recent coarse-bed pier scour equation may offer a more cost-effective solution to identifying and updating scour critical coding for bridges in Idaho. For scour critical bridge sites, the real-time pier scour monitoring methods used for this study provided an effective real-time local pier scour monitoring countermeasure.