Large Riprap Design and Construction—Big Bar Landslide

The Big Bar landslide occurred on a remote section of the Fraser River, 64 km north of Lillooet, British Columbia, and created a barrier to the vital seasonal northward Fraser River salmon migration. Intensive efforts were made by Canada Department of Fish and Oceans in 2019 as an emergency response to stabilize the slide source and to remove large boulders and rockfall debris from the river. Early in 2021, Kiewit design teams had nearly finalized plans for construction of a roughened embankment for volitional passage and a concrete fish ladder. However, ongoing rockfall hazards prevented any work commencing on the fish ladder structure. The roughened slope design was largely completed as this embankment was planned to support the concrete fish ladder. In lieu of the fish ladder, the slope of the roughened embankment was extended. Construction of the roughened embankment consisted of large riprap with a characteristic size of 1.5 m with rock material produced from on-site sources. Modeling of the hydraulic performance was accomplished with depth-averaged two-dimensional analysis (USBR SRH-2D). Spatial distributions of shear stress and velocity were computed over the embankment for determination of riprap stability as well as provide an environment for volitional fish passage. A thorough inspection and testing protocol (ITP) for rock material and placement were followed. ITP implementation included use of image segmentation gradation analysis and other testing methods. Heroic construction efforts through the winter of 2020–2021 in the tough environment of the Fraser River completed the roughened embankment in time for the spring freshet. Flow in the Fraser River in June of 2021 peaked at approximately 6,100 m³/s with the roughened embankment remaining completely stable.