Multi-phase flow simulation of impulsive waves generated by a sub-aerial granular landslide on an erodible slope
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Cheng-Hsien Lee I Zhenhua Huang
Multi-phase flow simulation of impulsive waves generated by a sub-aerial granular landslide on an erodible slope
Abstract This study uses a multi-phase flow model to examine the impulsive waves generated by collapse of steep slopes at a water body. Possible effects of slope’s erodibility on the motion of the landslide and the resulting waves are investigated by comparing a sub-aerial landslide on a rigid slope to that on an erodible slope. Key parameters that are relevant to the generation of the impulsive waves are examined, including the time series of the elevation and location of first wave crest, the location and thickness of granular flow front, submerged granular volume, and potential and kinetic energy for water and granular landslide. Before it is free from the influence of the granular front, the generated wave is a forced wave traveling at the speed of the granular flow front. The first wave crest is about to leave the granular flow front when the speed of the granular front is close to the celerity at which a free long wave can propagate in the water above the granular front, and the maximum height of the first wave crest is reached at the time when the first wave has completed its transition from a forced wave to a free wave. It appears that the erodible slope can increase (1) the volume of the granular material plunged into water, (2) the thickness of granular flow front, and (3) the height of the first wave generated by the landslide. However, the total volume of the granular material plunged into water is not directly related to the height of the first wave. Keywords Steep slopes . Slow subaerial landslides . Water waves . Tsunamis . Numerical simulations Introduction Sub-aerial landslides can generate large impulsive waves in lakes (Evans 1989), reservoirs, or bays (Muller 1964), causing adverse impacts on human lives and the economy in affected areas. One of the most famous events is the 1958 Lituya Bay tsunami (Miller 1960; Fritz et al. 2009), which resulted in the destruction of the forest over a total area of 10 km2 (Miller 1960). As far as the generation of impulsive waves is concerned, subaerial landslides can be grouped into two main types: fast subaerial landslides because of the high speed of the landslide at impact and slow sub-aerial landslides due to the collapse of steep granular slopes on a coast or at a lake. Most of the existing studies of impulsive waves generated by sub-aerial landslides focused on fast sub-aerial landslides. To study the impulsive waves generated by fast sub-aerial landslides in laboratories, researchers have used rigid blocks (Noda 1970; Kamphuis and Bowering 1972; Walder et al. 2003; Heller and Spinneken 2013; Huang et al. 2014; Heller and Spinneken 2015) and granular assemblies sliding down rigid slopes to generate impulsive waves. Granular assemblies that have been used in the past experimental studies include natural cobbles with diameter d = 100 mm (Huang et al. 2014), natural pebbles with diameter d = 5–50 mm (Lindstrøm 201
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