Continuous Riverbed Monitoring Around Raked Bridge Piles In The Fraser River

NHC sought to better understand scour around raked bridge piles and how bi-directional tidal flow impacts scour at piers. Two Kongsberg dual axis sonars were installed on a floating debris barrier in the middle arm of the Fraser River. A horizontal acoustic doppler current profiler and conductivity and temperature sensor were also installed to continuously record river conditions and enable collection of sound data in real time. Initially, an upward-looking laser was used to record the elevation of the floating debris barrier, but noise issues resulted in the addition of a radar sensor to the installation. Roll and pitch sensors were also used to record the position of the sonars and automatically correct the data movement in the floating barrier. For more than 15 months, each sonar successfully collected bathymetry data every 12 hours over a 30 m by 30 m area. The data collection period has included two snowmelt spring freshets and an exceptional fall flood event. Key observations from the installation include the following: During the winter when incoming tides are the greatest, sand dunes move upriver, and the scour is concentrated on the upstream piles as sediment infilling occurs around the downstream piles. This infilling occurs despite the outgoing tides having faster velocities than the incoming tides. During the summer freshet period, the scour is concentrated on the downstream piles as the dunes infill the piles situated most upstream. NHC observed that the mobility of the dunes and associated sediment deposition may have a greater influence on scour than flow velocity alone. The position of logs at the base of the piles strongly influences the scour that occurs. When logs are in the water column, they increase the scour, in much the same way as adding additional piles to a pile group. Conversely, wood at the base of the piles can reduce scour. Mounting the sonars on floating debris barriers complicated the data collection system much more than if fixed mounts had been used. Logs that accumulated under the debris barrier threatened to damage both the debris barrier and the monitoring equipment and caused the debris barrier to tilt more than the roll sensors were designed to accommodate. Tracking the movement of individual dunes is possible using the dual axis sonars and shows that only a handful of dunes moved past the monitoring station during any one spring snowmelt freshet. During the shoulder seasons when river flows were moderate, dunes did not migrate.