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Massive landslides can generate large tsunamis and endanger human lives and properties. While a number of laboratory experiments on either subaerial or submarine landslide‐tsunamis have been conducted, few studies have focused on partially submerged events. The present study aimed to investigate the characteristics of partially submerged landslide‐tsunamis and to develop a predictive equation that can estimate their heights through three‐dimensional laboratory experiments that used 114 different conditions. The experimental results showed that, while waves generated in the landslide sliding direction had around 1.7 times higher amplitude than those propagating laterally in the near‐field region, the wave period, celerity, and wavelength did not alter significantly according to the propagation angles. The effects of wave dispersion were also shown to be significant during the propagation process. Such wave characteristics were consistent with those reported in existing subaerial landslide tsunami experiments conducted by other researchers. Based on the data set obtained, the authors also derived a predictive equation which was confirmed to be able to estimate the results of other experimental studies on partially submerged landslide‐tsunamis with an error of around ±30%, and thus can be helpful for a rapid tsunami hazard assessment. The developed equation indicated that the volume of a landslide could have more than twice the impact on the height of a partially submerged landslide‐tsunami than other parameters. The effects of the landslide size were also shown to be more significant than in the case of a subaerial landslide‐tsunami, though the wave attenuation process was similar in both types. Plain Language Summary: Massive landslides can generate large tsunamis and endanger human lives and properties. While many laboratory experiments on landslide‐generated tsunamis have been conducted, few studies have investigated on partially submerged landslide‐tsunamis. The present study conducted three‐dimensional laboratory experiments on partially submerged landslide‐tsunamis to investigate the characteristics of such waves and to develop a predictive equation of their likely height. The results showed that waves generated in the landslide sliding direction were higher than those generated in the lateral direction, though other wave properties did not change greatly (which is similar to that reported by other researchers for other types of landslide tsunamis). In addition, the results show that partially submerged landslide tsunamis have similar wave propagation properties as those of other types of landslide tsunamis. Based on the experimental results, the authors also derived a predictive equation and confirmed that it can accurately estimate the heights of partially submerged landslide‐tsunamis. The derived equation can be helpful when conducting a rapid tsunami hazard assessment. It was also found that whereas the generation process of a partially submerged landslide‐tsunami is different from a subaerial landslide‐tsunami, the propagation process is similar in both types. Key Points: Three‐dimensional laboratory experiments were conducted on partially submerged landslide‐tsunamisWhile the amplitudes of the generated waves differed according to the propagation angles, other wave properties were not clearly differentThe derived predictive equation is able to estimate the height of the partially submerged landslide‐tsunamis well [ABSTRACT FROM AUTHOR]

In the present study, three different types of landslide-generated tsunamis (subaerial, partially submerged and submarine) were investigated through laboratory experiments that used 261 different experimental conditions. The results show that, for the near-field region, the subaerial landslide generated a higher leading wave, while the submarine landslide generated a higher second wave. However, frequency dispersion considerably influenced their propagation, with the leading wave decaying and the trailing waves being enhanced. The present study also revealed that the leading wave has higher values for wave celerity, period, and length than the second wave, regardless of landslide type. It was also shown that the celerity of the leading wave and second wave can be approximated by the solitary wave theory and third-order Stokes theory, respectively. Using the extensive experimental dataset obtained, predictive equations to estimate the maximum crest amplitudes generated by partially submerged and submarine landslides were also developed. A comparison with the experiments of previous researchers and field survey data from the 2018 Palu Tsunami indicated that these equations can estimate reasonably well the height of the generated tsunami. It was also shown that the relative mass and initial position of the landslide are influential nondimensional parameters to determine the tsunami amplitudes generated. [ABSTRACT FROM AUTHOR]