Numerical modelling of gravel transportation by a tsunami with the extended XBeach-G

Estimating coastal erosion by a tsunami is essential for land use planning, assessing hazards for current structures (e.g., coastal nuclear power plants), and for paleotsunami reconstruction. Such estimations are currently available only for sandy beaches, using sand sediment transport models, which are not applicable to gravel beaches, which are the most common beach type in high-latitude settings. This study extended the one-dimensional cross-shore XBeach-G model to account for two-dimensional gravel transport by a tsunami. First, this study confirmed that the extended XBeach-G model can simulate a time series of waveforms of solitary waves during laboratory experiments. The proposed model was then applied to gravel transport by the 2011 Tohoku-oki tsunami at Koyadori, Japan, and found that the simulation results were consistent with observations of gravel deposits in previous studies. It was revealed that infiltration and exfiltration have an impact on morphological change caused by a tsunami on gravel coasts. In the simulation, inundation depth over land by the tsunami increased due to groundwater exfiltration, which increased the onshore deposition volume of gravel tsunami deposits. The groundwater flow calculation has not been incorporated so far for tsunami modelling, but this is important for modelling tsunami inundation at gravel beaches and gravel sediment transport by a tsunami. However, choosing appropriate values for the sediment friction factor and multiplier in the equation for gravel transport is more critical to reproducing the deposition of gravel sediments by a tsunami because these parameters are more sensitive than the parameter of groundwater flow. Although the presented model has been developed for tsunami simulation on any gravel beach, further testing and validation are recommended.