Composite modelling of the effect of the water body geometry on landslide-tsunamis

Subaerial landslide-tsunamis (impulse waves) are generated by mass movements such as landslides, rock falls or glacier calving interacting with a water body. Preliminary landslide-tsunami hazard assessment is commonly based on empirical equations derived from wave channel (2D) or wave basin (3D) experiments. It is crucial to select the most appropriate set of empirical equations for a particular case as the difference in the far-field wave height between 2D and 3D may exceed an order of magnitude. The present study systematically investigates the effect of the water body geometry on the wave characteristics. Physical model tests were conducted in 2D and repeated in 3D, involving two water depths, three rigid slides and different subaerial slide release positions. The waves were found to decay in 2D considerably slower with distance x ‒0.30 than in 3D with radial distance r ‒1.0. The 3D wave heights in the slide impact zone can be identical large as in 2D for a large slide Froude number F, relative slide thickness S and relative mass M. However, for small F, S and M, the 3D waves are considerably smaller, both in the near- and far-field. Empirical equations are presented to transform wave parameters from 2D to 3D. One 2D-3D test pair, involving a solitary-like wave, is investigated in detail regarding the slide kinematics, water surface elevations and slide-water interaction power. This power is derived from pressure measurements on the slide front and the slide kinematics. The identical test pair is then used to calibrate the Smoothed Particle Hydrodynamics SPH code DualSPHysics and to numerically investigate the wave features in five intermediate geometries between 2D and 3D. For a “channel” geometry with diverging side wall angle of 7.5°, the wave amplitudes along the slide axis were found to lie approximately halfway between the values observed in 2D and 3D. At 45°, the values are practically identical to those in 3D. These findings support preliminary landslide-tsunami hazard assessment

Scale effects in shallow-water vortices

A widely applied model strategy in experimental fluid dynamics is to conduct laboratory experiments at reduced scale in the Reynolds number R invariant regime to ensure that the turbulent behaviour in the field situation is correctly modelled. This study investigates R invariance and quantifies R scale effects in dissipative-type shallow-water vortices where R invariance can naturally not be maintained. A laboratory scale series of monopole shallow-water vortices was conducted in a circular domain with rotating bottomless cylinders. Froude scale ratios were applied to carefully scale all experimental parameters between three scales, apart from the kinematic viscosity. Surface particle image velocimetry was conducted to record the vortex decay. The radial-averaged azimuthal velocity over radial distance and the ensemble-averaged mean azimuthal velocity, Reynolds number and dimensionless vorticity decays are presented. A similar pattern in the initial turbulent regime is observed in all three scales for the vorticity whilst the decays deviate in the transition and laminar regime. Such results help to quantify scale effects and to improve the modelling of shallow-water vortices in Froude models. The results reveal several interesting research questions which will be addressed in the near future

Composite modelling of the effect of the water body geometry on landslide-tsunamis

Subaerial landslide-tsunamis (impulse waves) are generated by mass movements such as landslides, rock falls or glacier calving interacting with a water body. Preliminary landslide-tsunami hazard assessment is commonly based on empirical equations derived from wave channel (2D) or wave basin (3D) experiments. It is crucial to select the most appropriate set of empirical equations for a particular case as the difference in the far-field wave height between 2D and 3D may exceed an order of magnitude. The present study systematically investigates the effect of the water body geometry on the wave characteristics. Physical model tests were conducted in 2D and repeated in 3D, involving two water depths, three rigid slides and different subaerial slide release positions. The waves were found to decay in 2D considerably slower with distance x ‒0.30 than in 3D with radial distance r ‒1.0. The 3D wave heights in the slide impact zone can be identical large as in 2D for a large slide Froude number F, relative slide thickness S and relative mass M. However, for small F, S and M, the 3D waves are considerably smaller, both in the near- and far-field. Empirical equations are presented to transform wave parameters from 2D to 3D. One 2D-3D test pair, involving a solitary-like wave, is investigated in detail regarding the slide kinematics, water surface elevations and slide-water interaction power. This power is derived from pressure measurements on the slide front and the slide kinematics. The identical test pair is then used to calibrate the Smoothed Particle Hydrodynamics SPH code DualSPHysics and to numerically investigate the wave features in five intermediate geometries between 2D and 3D. For a “channel” geometry with diverging side wall angle of 7.5°, the wave amplitudes along the slide axis were found to lie approximately halfway between the values observed in 2D and 3D. At 45°, the values are practically identical to those in 3D. These findings support preliminary landslide-tsunami hazard assessment

Impulse wave run-over: experimental benchmark study for numerical modelling

This research intends to provide a detailed data basis for numerical modelling of impulse waves. Three tests are described involving a rectangular wave channel, in which a trapezoidal ‘breakwater' was inserted to study wave run-over. In addition, a reference test is also described, in which the breakwater was removed. Two-dimensional impulse waves were generated by means of subaerial granular slides accelerated by a pneumatic landslide generator into the water body. Wave propagation and run-over over the artificial breakwater are documented by a set of high-quality photographs. Water surface profiles were recorded using capacitance wave gages upstream and downstream of the breakwater, and velocity vector fields were determined for the run-over zone by means of Particle Image Velocimetry. The measurements are compared with predictive formulae for wave features and wave non-linearity. The present data set involves both simple channel topography and wave features to allow for numerical simulations under basic laboratory condition

Numerical modelling of landslide-tsunami propagation in a wide range of idealised water body geometries

© 2019 Elsevier B.V. Large landslide-tsunamis are caused by mass movements such as landslides or rock falls impacting into a water body. Research of these phenomena is essentially based on the two idealised water body geometries (i) wave flume (2D, laterally confined wave propagation) and (ii) wave basin (3D, unconfined wave propagation). The wave height in 2D and 3D differs by over one order of magnitude in the far field. Further, the wave characteristics in intermediate geometries are currently not well understood. This article focuses on numerical landslide-tsunami propagation in the far field to quantify the effect of the water body geometry. The hydrodynamic numerical model SWASH, based on the non-hydrostatic non-linear shallow water equations, was used to simulate approximate linear, Stokes, cnoidal and solitary waves in 6 different idealised water body geometries. This includes 2D, 3D as well as intermediate geometries consisting of “channels” with diverging side walls. The wavefront length was found to be an excellent parameter to correlate the wave decay along the slide axis in all these geometries in agreement with Green's law and with diffraction theory in 3D. Semi-theoretical equations to predict the wave magnitude of the idealised waves at any desired point of the water bodies are also presented. Further, simulations of experimental landslide-tsunami time series were performed in 2D to quantify the effect of frequency dispersion. This process may be negligible for solitary- and cnoidal-like waves for initial landslide-tsunami hazard assessment but becomes more important for Stokes-like waves in deeper water. The findings herein significantly improve the reliability of initial landslide-tsunami hazard assessment in water body geometries between 2D and 3D, as demonstrated with the 2014 landslide-tsunami event in Lake Askja

Grain Reynolds number scale effects in dry granular slides

©2020. The Authors. Scale effects are differences in physical behavior that manifest between a large event and a geometrically scaled laboratory model and may cause misleading predictions. This study focuses on scale effects in granular slides, important in the environment and to industry. A versatile 6 m long laboratory setup has been built following Froude similarity to investigate dry granular slides at scales varied by a factor of 4, with grain Reynolds numbers Rein the range of 102 to 103. To provide further comparison, discrete element method simulations have also been conducted. Significant scale effects were identified; the nondimensional surface velocity increased by up to 35%, while the deposit runout distance increased by up to 26% from the smallest to the largest model. These scale effects are strongly correlated with Re, suggesting that interactions between grains and air are primarily responsible for the observed scale effects. This is supported by the discrete element method data, which did not show these scale effects in the absence of air. Furthermore, the particle drag force accounted for a significant part of the observed scale effects. Cauchy number scale effects caused by unscaled particle stiffness resulting in varying dust formation with scale are found to be of secondary importance. Comparisons of the laboratory data to that of other studies and of natural events show that data normalization with Re is an effective method of quantitatively comparing laboratory results to natural events. This upscaling technique can improve hazard assessment in nature and is potentially useful for modeling industrial flows

Tsunami Science and Engineering II

Earthquake-tsunamis, including the 2004 Indian Ocean Tsunami and the 2011 T?hoku Tsunami in Japan, serve as tragic reminders that such waves pose a major natural hazard. Landslide-tsunamis, including the 1958 Lituya Bay case, may exceed 150 m in height, and similar waves generated in lakes and reservoirs may overtop dams and cause significant devastation. This book includes nine peer-review articles from some of the leading experts in the field of tsunami research. The collection represents a wide range of topics covering (i) wave generation, (ii) wave propagation, and (iii) their effects. Within (i), a tsunami source combining an underwater fault rupture and a landslide are addressed in the laboratory. Within (ii), frequency dispersion with the nonlinear shallow-water equations is considered and a detailed account of the 1755 Lisbon earthquake, tsunami, and fire in downtown Lisbon is presented. Two articles involve all three phases (i) to (iii), including runup and dam over-topping. Within (iii), a new semi-empirical equation for runup is introduced and the interaction of tsunamis with bridges and pipelines is investigated in large laboratory experiments. This state-of-the-art collection of articles is expected to improve modelling and mitigate the destructive effects of tsunamis and inspire many future research activities in this challenging and exciting research field

Scale effects in subaerial landslide generated impulse waves

Hydraulic scale modelling involves scale effects. The limiting criteria for scale models of subaerial landslide generated impulse waves including solid, air, and water are discussed both based on a literature review and based on detailed two-dimensional experimentation. Seven scale series based on the Froude similitude were conducted involving the intermediate-water wave spectrum. Scale effects were primarily attributed to the impact crater formation, the air entrainment and detrainment, and the turbulent boundary layer as a function of surface tension and fluid viscosity. These effects reduce the relative wave amplitude and the wave attenuation as compared with reference experiments. Wave amplitude attenuation was found to be more than 70 times larger than predicted with the standard wave theory. Limitations for plane impulse wave generation on the basis of the present research are given by which scale effects can be avoide

Composite modelling of subaerial landslide-tsunamis in different water body geometries and novel insight into slide and wave kinematics

This article addresses subaerial landslide-tsunamis with a composite (experimental-numerical) modelling approach. A shortcoming of generic empirical equations used for hazard assessment is that they are commonly based on the two idealised water body geometries of a wave channel (2D) or a wave basin (3D). A recent systematic comparison of 2D and 3D physical block model tests revealed wave amplitude differences of up to a factor of 17. The present article investigates two of these recently presented 2D-3D test pairs in detail, involving a solitary-like wave (scenario 1) and Stokes-like waves (scenario 2). Results discussed include slide and water particle kinematics and novel pressure measurements on the slide front. Instantaneous slide-water interaction power graphs are derived and potential and kinetic wave energies are analysed. Solitary wave theory is found most appropriate to describe the wave kinematics associated with scenario 1, whereas Stokes theory accurately describes the tsunami in scenario 2. The data of both scenarios are further used to calibrate the smoothed particle hydrodynamics (SPH) code DualSPHysics v3.1, which includes a discrete element method (DEM)-based model to simulate the slide-ramp interaction. Five intermediate geometries, lying between the ideal 2D and 3D cases, are then investigated purely numerically. For a “channel” geometry with a diverging side wall angle of 7.5°, the wave amplitudes along the slide axes were found to lie approximately halfway between the values observed in 2D and 3D. At 45°, the amplitudes are practically identical to those in 3D. The study finally discusses the implications of the findings for engineering applications and illustrates the potential and current limitations of DualSPHysics for landslide-tsunami hazard assessment

Large-scale investigation into iceberg-tsunamis generated by various iceberg calving mechanisms

© 2020 Elsevier B.V. Mass balance analysis of ice sheets is a key component to understand the effects of global warming with iceberg calving as a significant contributor. Calving recently generated tsunamis of up to 50 m in amplitude endangering human beings and coastal infrastructure. Such iceberg-tsunamis (IBTs) have been investigated based on 66 unique large-scale experiments conducted in a 50 m × 50 m large basin at constant water depth h. The experiments involved five iceberg calving mechanisms: A: capsizing, B: gravity-dominated fall, C: buoyancy-dominated fall, D: gravity-dominated overturning and E: buoyancy-dominated overturning. The kinematics of the up to 187 kg heavy plastic blocks mimicking icebergs was measured with a motion sensor and the wave profiles were recorded with wave probes at up to 35 locations. The IBTs from the gravity-dominated mechanisms (B and D) are roughly an order of magnitude larger than from mechanisms A, C and E. Empirical equations for preliminary hazard assessment and mitigation for the maximum wave height, amplitude and period for both the near- and far-field are derived for the five calving mechanisms individually and combined. The relative released energy, Froude number and relative iceberg width are the most influential dimensionless parameters in these equations. A maximum wave height decay trend close to (r/h)−1.0 is observed, with r as the radial distance, in agreement with the theoretical wave decay from a point source. The empirical equations are applied to a past event resulting in a good agreement and the upscaled wave periods to typical Greenlandic conditions overlap with the lower spectrum of landslide-tsunamis. However, empirical equations for landslide-tsunamis were found to be of limited use to predict IBTs in the far-field supporting the need of the newly introduced empirical equations for IBT hazard assessment and mitigation