On sediment extent and runup of tsunami waves

Abstract

Tsunami sediments play an important role in tsunami hazard assessments. Recent tsunami events and their associated tsunami deposits have produced a wealth of sediment data. Surveys of recent tsunami events observed that maximum inland tsunami inundation exceeds the maximum inland extent of sand deposition. A driving question of tsunami sedimentology is whether the difference between maximum tsunami inundation and inland extent of sand can be employed as a measure of inundation for past events for which only the extent of the sand is known. Attempts to infer the causative flow characteristics from these sediments often suffer from uncertainties in the data and from restrictive assumptions made in inversion models. The latter demonstrates that the interaction between the tsunami flow and the movable bed is not fully understood. In our contribution, we employ numerical simulations of tsunami propagation and inundation with MOST and a simple model for deposition. In our simulations, we vary the onshore slope, the initial amplitude, which can be interpreted as offshore wave amplitude, and the grain size. While the initial amplitude and onshore slope influence the difference between maximum inundation and sand extent, the grain size has little effect. The link between the onshore slope, initial amplitude and the difference between the maximum tsunami inundation and inland sediment extent can be employed as a first-order quantitative tool to estimate tsunami flow characteristics and maximum inundation from tsunami deposits. We anticipate these results will improve tsunami hazard assessments and narrow the gap between field observations and numerical simulations.