Bottom friction dissipation is a key factor for wave attenuation in nearshore environments presenting complex geomorphological structures, such as rocky or coral shores. The present paper reports on a series of laboratory experiments performed in a wave flume with controlled wave conditions and seabed structures. Using the frequency-integrated short-wave analysis and classical models for bottom friction and breaking dissipation, the wave friction factor and the hydraulic roughness parameter were estimated from the experimental data. The former varies from 0.17 to 98 while the latter varies from 0 to 0.291 m. The observations reveal the combined influence of several topographical metrics, including the standard deviation, the skewness, the directionality and the effective slope of the seabed elevation. A metric-based multi-varied model for the hydraulic roughness parameter is proposed and confronted with other field data recovered on coral and rocky shores.

Landslide-generated tsunamis are a relevant hazard. Their low frequency/high consequences character and the complex phenomena related to their generation, propagation and interaction with the shore make the proper modelling of these phenomena a crucial activity to mitigate the related risk. In this article, a new numerical method for modelling tsunamis generated by granular landslides in OpenFOAM® is presented. The approach consists in modelling the granular material by using a Coulomb viscoplastic rheology (non-Newtonian rheology) implemented in the standard solver multiPhaseInterFoam. The proposed approach is simple as it only depends on few physics-based parameters, thus implying less uncertainties than dense fluid models and more flexibility and computational efficiency than Euler-Euler approaches. This numerical framework is applied to reproduce three literature benchmark landslide-tsunami cases: two-dimensional (2D) submerged as well as 2D and three-dimensional (3D) subaerial. Comparing numerical and experimental results, a good agreement is found for granular material behavior, while an overall very good (excellent in some cases) agreement is found as far as fluid behavior and waves characteristics are concerned, testifying that the momentum transfer between granular and fluid phases is well reproduced by this simple rheological model. Qualitative descriptions of the numerical results, in terms of landslide behavior, wave generation characteristics, and velocity field during the generation/propagation process are provided. Moreover, quantitative comparisons between experimental and numerical results by comparing landslide evolution, free surface elevation time series, and runup time series are presented and discussed in the article.

On January 15, 2022, the eruption of the Hunga Tonga-Hunga Ha'apai volcano created a tsunami that reached a height of 20.5 m in the near field. The tsunami impacted the entire Pacific Ocean basin and was the largest tsunami to impact California since the 2011 Tōhoku tsunami. Videos recorded on surfcams during the event were downloaded from three sites along the California coast (Santa Cruz, Pismo Beach, and Santa Barbara Harbor). Airborne Lidar data, shadow projections, and vessel traffic information was used to rectify imagery instead of traditional field methods. Water level was extracted at 2 Hz based on the change in brightness relative to a fixed background object and filtered to remove swell and tidal influences. Tsunami inundation was captured at all three sites, with image extracted maximum tsunami heights measuring 1.42 m at Santa Cruz, 1.07 m at Pismo Beach, and 0.49 m at Santa Barbara Harbor. Comparisons to nearby in-situ water level measurements were made to calculate the accuracy, with a RMSE of 0.41 m, 0.52 m, and 0.17 m at each site, respectively. Analysis of surfcam extracted water levels measurements show promising results, and in combination with providing holistic monitoring of the scene, suggests an alternative platform for future tsunami events.