Triggers and consequences of landslide-induced impulse waves – 3D dynamic reconstruction of the Taan Fiord 2015 tsunami event

Natural multi-hazards as landslide-induced tsunamis require a multi-disciplinary approach to analyze the cascade effects that pose a significant threat to mountain communities and the surrounding territory. This paper comprises a detailed study of both the landslide evolution and the wave dynamics of the October 2015 Taan Fiord (Alaska) tsunami event, which represents a highly valuable case study for generic methodology development of single code applications using the numerical software Flow3D and testing its applicability for cascading wave hazard evaluation. First, a geomorphological analysis of the unstable slope is performed by elaborating diverse digital elevation models, where a significant vertical displacement of −90 m is observed before the final collapse, and the influence of listric faults within the slide body results in a bulging of the glacier at the toe. Data from time-series analyses suggests that the glacier retreat (and the reduction of local buttresses) critically destabilized the slope leading to the October 2015 catastrophic failure. The reconstructed landslide volume is estimated to be 49.4 Mm3, where 26 Mm3 entered the fiord and triggered the tsunami. Second, wave dynamics are recreated with Flow3D. Both dense fluid and granular media models are used and compared to verify their performance in initiating the impulse wave, where a measured impact speed ranging between 32 and 49 ms−1 triggers a maximum wave amplitude of about 95–99 m. the maximum run-up of 193 m at the Hoof Hill Fan is recreated with both approaches, but general overestimations (about 9–12%) compared to the observations, in the impact area, are computed for the inundation process. A good approximation of the observed run-up along the entire length of the fiord is found for wave propagation models using the granular media approach. Beyond the Taan-Fiord case study and for evaluation of cascading landslide-induced hydraulic hazard in other settings, this work points out (i) the necessity of using a high temporal resolution of digital elevation models to analyze the multi-stage slope failure and to properly estimate the landslide volume, (ii) the applicability of the applied numerical models to reproduce the wave dynamics of a landslide-induced tsunami event on one code only, and (iii) how these models can be adopted to develop hazard maps related to potential wave hazards in natural basins.