Geir Kleivstul Pedersen

• Grue, John; Pedersen, Geir Kleivstul & Sætra, Øyvind (2022). Free Wave Effects in Meteotsunamis. Journal of Geophysical Research (JGR): Oceans. ISSN 2169-9275. 127(2). doi: 10.1029/2021JC017669. Fulltekst i vitenarkiv Vis sammendrag

  Free wave generation due to a ship or storm moving past a shallow, great depth change of the water, is measured on the shore (coast), modeled and compared. The free waves are generated at the depth change where the forced wave and velocity field attached to the moving pressure system adjust to the new depth. The wavenumber is a factor 1/12 smaller in the meteotsunami case compared to the ship case. The subcritical depth Froude numbers are similar in the two cases. The meteotsunami that occurred on the Norwegian Coast on 29–30 June 2019 was driven by a supercell thunderstorm moving at speed 110 km/hr. A localized, strong high pressure feature of width of 60 km and crest of 120 km obtained from weather forecast was used as input for a set of simulations of the water-level response including realistic bathymetry. At the transition between the North Sea and the Norwegian Trench, the storm generated a free depression wave. This arrived at the coast 24 min ahead of the depression attached to the storm. The calculation fits to a period of 23 min, of a series several oscillations of height of 0.3–0.4 m, as measured by the water-level gauge. An impulsive start-up generates an additional forerunning elevation wave. Short waves of period of 1/3 of the main ship-driven waves may originate from the steep gradients of the bow and stern. Similarly, short waves of period 6–7.5 min (0.002–0.003 Hz) are observed in the measured water-level on the coast.

• Pedersen, Geir Kleivstul (2021). Asymptotic, Convergent, and Exact Truncating Series Solutions of the Linear Shallow Water Equations for Channels with Power Law Geometry. SIAM Journal on Applied Mathematics. ISSN 0036-1399. 81(2), s. 285–303. doi: 10.1137/19M1305860. Fulltekst i vitenarkiv Vis sammendrag

  The present study was originally motivated by some intriguing exact solutions for waves propagating in nonuniform media. In particular, for special depth profiles reflected waves did not appear and ray theory became exact. Herein, geometrical optics is employed to obtain asymptotic series for waves of general shapes in nonuniform narrow channels, within the framework of linear shallow water theory. While being kept simple, the series incorporate higher order contributions that may describe the evolution of waves with high accuracy. The higher orders also contain a secondary wave system. For selected classes of geometries and wave shapes explicit solutions are calculated and compared to numerical solutions. Apart from the vicinity of shorelines, say, higher order expansions generally may provide very accurate approximations to the full solutions. The asymptotic series are analyzed for different wave shapes and are found to be convergent for cases where the basic wave profiles have compact support (finite length). A number of new, closed form, exact solutions are also found. The asymptotic expansion is put into a context by employing it for the transmission of waves from a uniform channel section into a nonuniform one. Additional results and side topics are presented in a supplement.

• Zengaffinen, Thomas; Løvholt, Finn; Pedersen, Geir Kleivstul & Harbitz, Carl Bonnevie (2020). Effects of rotational submarine slump dynamics on tsunami genesis: new insight from idealized models and the 1929 Grand Banks event. Geological Society Special Publication. ISSN 0305-8719. doi: 10.1144/SP500-2019-201. Fulltekst i vitenarkiv Vis sammendrag

  Sediment slumps are known to have generated important tsunamis such as the 1998 Papua New Guinea (PNG) and the 1929 Grand Banks events. Tsunami modellers commonly use solid blocks with short run-out distances to simulate these slumps. While such methods have the obvious advantage of being simple to use, they offer little or no insight into physical processes that drive the events. The importance of rotational slump motion to tsunamigenic potential is demonstrated in this study by employing a viscoplastic landslide model with Herschel-Bulkley rheology. A large number of simulations for different material properties and landslide configurations are carried out to link the slump's deformation, rheology, its translational and rotational kinematics, to its tsunami-genesis. The yield strength of the slump is shown to be the primary material property that determines the tsunami-genesis. This viscoplastic model is further employed to simulate the 1929 Grand Banks tsunami using updated geological source information. The results of this case study suggest that the viscoplastic model can be used to simulate complex slump induced tsunami. The simulations of the 1929 Grand Banks event also indicate that a pure slump mechanism is more tsunamigenic than a corresponding translational landslide mechanism.

• Zengaffinen, Thomas; Løvholt, Finn; Pedersen, Geir Kleivstul & Muhari, Abdul (2020). Modelling 2018 Anak Krakatoa Flank Collapse and Tsunami: Effect of Landslide Failure Mechanism and Dynamics on Tsunami Generation. Pure and Applied Geophysics (PAGEOPH). ISSN 0033-4553. 177, s. 2493–2516. doi: 10.1007/s00024-020-02489-x. Fulltekst i vitenarkiv Vis sammendrag

  The 2018 Anak Krakatoa volcano flank collapse generated a tsunami that impacted the Sunda Strait coastlines. In the absence of a tsunami early warning system, it caused several hundred fatalities. There are ongoing discussions to understand how the failure mechanism of this event affected landslide dynamics and tsunami generation. In this paper, the sensitivity to different failure scenarios on the tsunami generation is investigated through numerical modelling. To this end, the rate of mass release, the landslide volume, the material yield strength, and orientation of the landslide failure plane are varied to shed light on the failure mechanism, landslide evolution, and tsunami generation. We model the landslide dynamics using the depth-averaged viscoplastic flow model BingClaw, coupled with depth-averaged long wave and shallow water type models to simulated tsunami propagation. We are able to match fairly well the observed tsunami surface elevation amplitudes and inundation heights in selected area with the numerical simulations. However, as observed by other authors, discrepancies in simulated and observed arrival times for some of the offshore gauges are found, which raises questions related to the accuracy of the available bathymetry. For this purpose, further sensitivity studies changing the bathymetric depth near the source area are carried out. With this alteration we are also able to match better the arrival times of the waves.

• Verschaeve, Joris C. G.; Pedersen, Geir Kleivstul & Tropea, Cameron (2018). Non-modal stability analysis of the boundary layer under solitary waves. Journal of Fluid Mechanics. ISSN 0022-1120. 836, s. 740–772. doi: 10.1017/jfm.2017.825. Fulltekst i vitenarkiv
• Smith, Lisa; Jensen, Atle & Pedersen, Geir Kleivstul (2017). Investigation of breaking and non-breaking solitary waves and measurements of swash zone dynamics on a 5° beach. Coastal Engineering. ISSN 0378-3839. 120, s. 38–46. doi: 10.1016/j.coastaleng.2016.11.004. Fulltekst i vitenarkiv Vis sammendrag

  This study presents an experimental investigation of plunging breakers on a sloping beach with an inclination of 5.1°. The incident waves are solitary waves with various amplitudes from non-breaking waves to plunging breakers, and the area investigated is the swash zone. PIV (Particle Image Velocimetry) is performed on images captured at four different field of views (FOV). Shoreline position and maximum runup are measured, and are repeatable in both time and height, although cross-sectional variations of the shoreline shape are observed at maximum runup. For non-breaking waves the runup and fluid flow is computed by a boundary integral technique combined with a boundary layer model. Then, there is excellent agreement between the experimental and the computed velocity profiles at the lower region of the beach, while the boundary integral technique overpredicts the maximum runup height severely. For breaking waves the experiments indicate that the motion becomes more irregular as we move further up the beach. In addition, there are more irregularities present for waves with larger amplitude. Length and velocity of air bubbles entrapped by the plunging breakers are extracted from an image series captured with a large FOV. The images showed that a large air bubble remains intact for a time period during runup for the breaking waves.

• Kim, Jihwan; Pedersen, Geir Kleivstul; Løvholt, Finn & LeVeque, Randall J. (2017). A Boussinesq type extension of the GeoClaw model - a study of wave breaking phenomena applying dispersive long wave models. Coastal Engineering. ISSN 0378-3839. 122, s. 75–86. doi: 10.1016/j.coastaleng.2017.01.005. Fulltekst i vitenarkiv Vis sammendrag

  The nonlinear shallow water model is widely used in the study of tsunami propagation, but an increasing number of studies are dedicated to the dispersion dynamics of tsunamis. If the wave dispersion becomes important, Boussinesq-type models are often used. In this work, a general purpose Boussinesq solver, BoussClaw, is introduced for modeling non-linear dispersive tsunami propagation, taking into account inundation. The BoussClaw model is an extension of the GeoClaw tsunami model. It employs a hybrid of finite volume and finite difference methods to solve Boussinesq equations from the literature, which are based on the depth-averaged velocity and include enhanced dispersion properties. On the other hand, in the selected formulation only some non-linearity is retained in the dispersion term. In order to validate BoussClaw, numerical results are compared to analytic solutions, solutions obtained by pre-existing models, and laboratory experiments. Even though the equations of BoussClaw are not fully nonlinear they perform far better than standard Boussinesq equations with only linear dispersion terms. Furthermore, the wave steepening and breaking are carefully scrutinized, and we demonstrate that the point of wave breaking may be wrongly identified in many of the commonly used Boussinesq models.

• Jeschke, Anja; Pedersen, Geir Kleivstul; Vater, Stefan & Behrens, Jörn (2017). Depth-averaged non-hydrostatic extension for shallow water equations with quadratic vertical pressure profile: equivalence to Boussinesq-type equations. International Journal for Numerical Methods in Fluids. ISSN 0271-2091. 84(10), s. 569–583. doi: 10.1002/fld.4361. Fulltekst i vitenarkiv
• Løvholt, Finn; Pedersen, Geir Kleivstul & Harbitz, Carl Bonnevie (2016). Tsunami-Genesis Due to Retrogressive Landslides on an Inclined Seabed. I Lamarche, Geoffrey; Mountjoy, Joshu; Bull, Suzanne & Hubble, Tom (Red.), Submarine Mass Movements and their Consequences: 7th International Symposium. Springer Publishing Company. ISSN 9783319209791. s. 569–578. doi: 10.1007/978-3-319-20979-1_57. Fulltekst i vitenarkiv Vis sammendrag

  Clay-rich landslides commonly involve retrogressive mass and momentum release mechanisms. Motivated by the retrogressive behaviour of major landslides offshore Norway, previous studies have demonstrated substantial effects of the release rate on the generation of the tsunami. However, the few existing models are limited to overly idealized conditions. In the present study, we explore further the wave generation due to a continuous retrogressive landslide model, quantifying the effects of the wave model, landslide configuration, and the continental slope. In the present examples, we find that the landslides involve large accelerations that may be crucial for tsunami-genesis. Tsunami footprints due to individual short blocks comprising the landslide are smeared out by dispersion. Keeping landslide material properties constant, we investigate mobilised landslide mass and maximum tsunami crest elevations for three different slopes: 1°, 1.5°, and 2° respectively. In the present examples, the smaller volume landslides are stronger tsunami generators than the larger ones because they are situated in shallower water, thereby clearly demonstrating the importance of the water depth on the tsunami generation.

Se alle arbeider i NVA

• Birknes-Berg, Jørn & Pedersen, Geir Kleivstul (2017). The “Chain of Markers” code applied to large scale problems; solitary waves, sloshing and a plunging wave. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  There are many different numerical methods for solving the Navier-Stokes equations for problems with a free surface. Some examples are volume of fluid (VOF), Marker and cell (MAC), level set, smooth particle hydrodynamics (SPH), boundary element methods (BEM) and weighted residual methods. One important element when solving problems with a free surface is how the free surface is treated. One of the most popular techniques is the VOF method. Another approach is to have markers on the free surface and this method is pursued in this study.

Se alle arbeider i NVA