Abstract
Landslide tsunamis and impulse waves are hazardous events with severe socioeconomic impacts. A long standing problem with simulations of these events is the generation stage, where landslides and water interact. Depth-averaged models like the Saint-Venant or Boussinesq Equations lose their validity for such applications. Therefore, we have to rely on a full treatment of the hydrodynamics, for instance by applying the Navier-Stokes Equations and Computational Fluid Dynamics (CFD). However, applications of fully three-dimensional methods to landslide tsunamis are sparse, and have often been outperformed by depth averaged models when compared to experimental data. In this work, we evaluate the multiphase Navier-Stokes Equations as implemented in OpenFOAM® in terms of impulse wave generation. We focus on a simplified two-dimensional setup where the landslide consists of water, in order to circumvent additional complexities due to treatment of landslide rheologies. We conduct a thorough grid refinement study and compare results to experiments to investigate model convergence, stability, and accuracy. The simulations display good agreement with the experimental data if the Courant-Friedrichs-Lewy (CFL) condition is modified to account for the specific properties of the multiphase system. Further, we use the validated model for sensitivity studies and to review various scaling relations for landslide generated tsunamis. The application of numerical models allows us to perform broad parametric tests and dissect the underlying physics of these predictive equations systematically. We found that the first wave crest may be well estimated by solely the landslide mass in our setting. Including additional properties related to landslide momentum can improve the predictive skill, while other parameters lead to no substantial improvement.
Highlights
Landslides are the second most frequent tsunami source (Harbitz et al, 2014)
A wide range of alternative and mixed methods was investigated: Savage-Hutter model coupled with Navier-Stokes Equations (e.g. Ma et al, 2015), discrete element method coupled with Navier-Stokes Equations (e.g. Shan and Zhao, 2014), smoothed particle hydrodynamics (e.g. Pastor et al, 2008; Heller et al, 2016) or particle finite element method (e.g. Mulligan et al, 2020) are a few of the promising approaches
The landslide was repre sented in an idealized manner by water, allowing us to ignore granular rheology and porosity of the landslide
Summary
Landslides are the second most frequent tsunami source (Harbitz et al, 2014). Recently, the 2018 Anak Krakatoa landslide induced tsunami caused several hundred fatalities (e.g. Grilli et al, 2019). The generation can be influenced by a highly rotational and depth-varying velocity field, a complex water surface (e.g. breaking waves) and other processes that stand in strong contrast to the assumptions of depth-integrated and potential flow models. This is especially the case for tsunamis generated by subaerial landslides that impact the water reservoir, such as the Vajont landslide Depth averaged models have shown to face difficulties related to strong non-linearities (e.g. Løvholt et al, 2013) and steep topographies (Løvholt and Pedersen, 2009) For such cases, the most appropriate approach requires a minimum degree of simplification, which implies solving full three-dimensional continuum mechanical models, i.e. the Navier-Stokes Equations. A wide range of alternative and mixed methods was investigated: Savage-Hutter model coupled with Navier-Stokes Equations (e.g. Ma et al, 2015), discrete element method coupled with Navier-Stokes Equations (e.g. Shan and Zhao, 2014), smoothed particle hydrodynamics (e.g. Pastor et al, 2008; Heller et al, 2016) or particle finite element method (e.g. Mulligan et al, 2020) are a few of the promising approaches
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