# Tsunami penetration in tidal rivers, with observations of the Chile 2015 tsunami in rivers in Japan

## TL;DR

This study analyzes how tsunamis propagate in tidal rivers, showing that wave-locked slopes and tidal interactions significantly influence tsunami dissipation and upstream reach, based on observations from the 2015 Chilean tsunami in Japan.

## Contribution

It introduces a new methodology for data processing and provides analytical insights into tsunami dissipation mechanisms related to wave-locked slopes and bottom drag effects.

## Key findings

- Tsunami dissipation depends on wave-locked slope and tidal conditions.
- Small tsunamis can travel upstream with minimal loss under certain slopes.
- Tidal and river currents enhance damping at higher wave-locked slopes.

## Abstract

An extensive data set of water level measurements of the September 2015 Chilean tsunami in rivers in Japan and a new methodology for data processing are used to verify that tsunami dissipation in a river at each instant and locality depends on the tidally-modified wave-locked slope of the river surface. As deduced from the observations, a relatively small tsunami or ocean noise traveling at mild wave-locked slopes can propagate virtually without losses to the upstream locations where observed tidal ranges are a fraction of that downstream; though at the higher slopes, tidal and riverine currents combined efficiently damp the shorter waves. The observed correlations between the tsunami admittance upriver and the traveled wave-locked slopes are explained analytically under the fully-nonlinear shallow-water approximation. It is found that the wave-locked slope in a purely incident wave relates to the bottom drag in the same manner as a steady surface slope does for a stationary flow. For a small-amplitude tsunami in the study rivers, the wave-locked slope in a co-propagating tidal wave defines the background current and thereby friction experienced by the tsunami.