<p>This study aims to test the capacity of Flow-3D regarding the simulation of a rockslide impacting a water body by evaluating the influence of the extent of the computational domain, the grid resolution, the corresponding computation times and the accuracy of modelling results. A detailed analysis of the Lituya Bay event (1958, Alaska, maximum recorded run-up of 524 m a.s.l.) is presented. A focus on the tsunami formation and run-up in the impact area is accomplished with the numerical model. Several simulations with a simplified bay geometry are accomplished in order to test the concept of a <q>denser fluid</q> for the impacting rockslide material compared to the water in the bay. Further, a real topography and bathymetry of the impact area are set up. The observed maximum run-up in the impact area can be reproduced using a uniform grid resolution of 5 m, where the wave overtops the hill crest on the opposite side of the impact area, then flowing diagonally along the slopes. The model is enlarged along the entire bay to simulate the propagation of the wave. The tsunami trimline is best reproduced when using a mesh size of 15 x 15 x 10 m. The trimline mainly results from the primary wave, but in some locations also from reflected waves. The <q>dense fluid</q> is a suitable, simple concept to recreate a sliding mass impacting a water body, in this case with impact velocities of 94 ms<sup>−1</sup>. The tsunami event and the related trimline are well reproduced using the 3D-modelling approach with the density evaluation model available in Flow-3D.</p>