Maximum sand sedimentation distance after backwash current of tsunami — Simple inverse model and laboratory experiments

Abstract

Abstract Estimating the magnitude of a past tsunami is important in tsunami hazard assessment and protection. In particular, sand deposition data provide important information in such processes. The effects of the backwash current of a tsunami on the sand distribution should be considered in estimating the historical tsunami magnitude by field survey data. A simple inverse model was developed to estimate the maximum distance of a sand deposit from a tsunami considering not only sedimentation but also re-erosion by the backwash current. Laboratory experiments were conducted to understand the characteristics of the effects of backwash current and estimate the parameters needed for the model development. The proposed simple model consists of several macroscopic characteristics of both non-tsunami terms, such as land slope and sand characteristics, and comprehensive tsunami properties, such as the energy gradient of the backwash current and tsunami height. It was assumed that the velocity and water depth of the backwash current are linearly approximated by the law of tsunami current conservation on a land slope. To estimate the decrease in sand layer depth due to re-erosion by the backwash current, the tractive force of the current and the drag force of the sand removed were related with one empirical parameter ( e ). Laboratory flume experiments were conducted to calibrate the parameter. Sensitivity analysis confirmed the effect on the changes in the value of e on the sand deposition distance to be smaller than that of other parameters. In spite of many simple assumptions, the estimate agreed well with the sand deposit distance of the 2011 Great East Japan Earthquake tsunami. Finally, the limitations and effectiveness of the model are discussed. This study developed a useful method for measuring backwash in a relatively extensive land slope where the effect of the backwash current is large and regional topography and land use effects are small.