Lituya Bay Case Rockslide Impact and Wave Run-up

On July 8, 1958, an 8.3 magnitude earthquake along the Fairweather fault triggered a major subaerial rockslide into Gilbert Inlet at the head of Lituya Bay on the South coast of Alaska. The rockslide impacted the water at high speed creating a giant nonlinear wave and the highest wave run-up in recorded history. The soliton like wave ran up to an altitude of 524 m causing forest destruction and erosion down to bedrock on a spur ridge in direct prolongation of the slide axis. Total area between trimline of forest destruction on shores of Lituya Bay and high-tide shoreline covered about 10 km*. A cross-section of Gilbert Inlet was rebuilt at 1:675 scale in a two-dimensional physical laboratory model at VAW. The subaerial rockslide impact into Gilbert Inlet, wave generation, propagation and run-up on headland slope were considered in a geometrically undistorted Froude similarity model. A novel pneumatic landslide generator was used to generate a high-speed granular slide with controlled impact characteristics. State-of-the-art laser measurement techniques such as particle image velocimetry (PIV) and laser distance sensors (LDS) were applied to the decisive initial phase with rockslide impact and wave generation. PIV-measurements of wave run-up on headland slope were conducted to complement wave and run-up gage records. PIV provided instantaneous velocity vector fields in a large area of interest and gave insight into kinematics of wave generation and run-up. The whole process of a high speed granular slide impact may be subdivided into two main stages: a) Rockslide impact and penetration with flow separation, cavity formation and wave generation, and b) air cavity collapse with rockslide run-out and debris detrainment causing massive phase mixing. Impact stages overlap and their transition from wave generation to propagation and run-up is fluent. Formation of a large air cavity similar to an asteroid impact in the back of the rockslide is highlighted. The laboratory experiments confirm that the 1958 trimline of forest destruction on Lituya Bay shores was carved by a giant rockslide generated impulse wave. The measured wave run-up perfectly matches the trimline of forest destruction on the spur ridge at Gilbert Inlet. Back-calculation of wave height from observed trimline of forest destruction using Hall and Watts (1953) run-up formula equals measured wave height in Gilbert Inlet. PIV-measurements of wave run-up indicate that enough water ran up the headland slope to cause the flooding observed in Lituya Bay as estimated by Mader (1999) with numerical simulations of the whole Lituya Bay. ’ Research Engineer, VAW, ETH-Zentrum, CH-8092, Switzerland 2 Professor, VAW, ETH-Zentrum, CH-8092, Switzerland ’ Professor, Director of VAW, ETH-Zentrum, CH-8092, Switzerland Science of Tsunami Hazards, Vol 19, NO. 1 (2801) page 3