Geomorphic assessment at three real-time bridge scour monitoring sites in Idaho, 2020-2021

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. 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. As part of the larger study, site-specific geomorphic data and other observations were collected during a single visit to each bridge. Geomorphic data collected during each visit included a Wolman pebble count (Wolman, 1954) to define the median diameter of the streambed material, an estimate of the flow angle of attack (during peak flow conditions), an assessment of pier shape and dimensions, observations of the floodplain, and observations of bridge scour countermeasures. In addition, a GNSS site survey was completed to update the gage datum water surface elevations and determine the elevation of each bridge structure (road deck and low chord elevations). Real-time (15-minute) hydrologic data were available at each USGS streamflow gaging station (both prior to and during this study) and included real-time discharge and water-surface elevation data (U.S. Geological Survey, 2016, 2024a, 2024b, 2024c). The historic USGS discharge measurement data were used to develop peak flow velocity estimates at each site where the velocity is depth averaged over the cross-section. For each peak flow, the velocity was linearly interpolated using observed measurement data collected from each bridge. Depth for each peak flow condition was computed using the difference between the water surface elevation and the computed channel bed elevation at each site. Geomorphic site surveys provided site specific parameters required for the hydraulic assessment.