Abstract

In the case of huge tsunamis, such as the 2004 Great Indian Ocean Tsunami and 2011 Great East Japan Tsunami, the damage caused by ground scour is serious. Therefore, it is important to improve prediction models for the topographical change of huge tsunamis. For general models that predict topographical change, the flow velocity distribution of a flood region is calculated by a numerical model based on a nonlinear long wave theory, and the distribution of bed-load rates is calculated using this velocity distribution and an equation for evaluating bed-load rates. This bed-load rate equation usually has a coefficient that can be decided using verification simulations. For the purpose, Ribberink’s formula has high reproducibility within an oscillating flow and was chosen by the authors. Ribberink’s formula needs a bed-load transport coefficient that requires sufficient verification simulations, as it consumes plenty of time and money to decide its value. Therefore, the authors generated diagrams that can define the suitable bed-load coefficient simply using the data acquired from hydraulic experiments on a movable bed. Subsequently, for the verification purpose of the model, the authors performed reproduced simulations of topography changes caused by the 2011 Great East Japan Tsunami at some coasts in Northern Japan using suitable coefficients acquired from the generated diagrams. The results of the simulations were in an acceptable range. The authors presented the preliminary generated diagrams of the same methodology but with insubstantial experimental data at the time at the International Society of Offshore and Polar Engineers (ISOPE), (2018 and 2019). However, in this paper, an adequate amount of data was added to the developed diagrams based on many hydraulic experiments to further raise their reliability and their application extent. Furthermore, by reproducing the tsunami simulation on the Sendai Natori coast of Japan, the authors determined that the impact of total bed-load transport was much bigger than that of suspension loads. Besides, the simulation outputs revealed that the mitigation effect of the cemented sand and gravel (CSG) banks and artificial refuge hills reduced tsunami damage on Japan’s Hamamatsu coast. Since a lot of buildings and structures in the inundation area can be destroyed by tsunamis, building destruction design was presented in this paper through an economy and simplified state. Using the proposed tsunami simulation model, we acquired the inundation depth at any specific time and location within the inundated area. Because the inundation breadth due to a huge tsunami can extend kilometers toward the inland area, the evaluation of building destruction is an important measure to consider. Therefore, the authors in this paper presented useful threshold diagrams to evaluate building destruction with an easy and cost-efficient state. The threshold diagrams of “width of a pillar” for buildings or “width of concrete block walls” not breaking to each inundation height were developed using the data of damages due to the 2011 Great East Japan Tsunami.

Highlights

  • Research for predicting coastal scour and erosion by waves or flow has been performed all over the world

  • By reproducing the tsunami simulation on Japan’s Sendai Natori coast, the suspension load showed a smaller effect on topography change as the total bed-load

  • Since the local government constructed coastal banks of 13 m in height made of cemented sand and gravel (CSG) and some evacuation soil mounds, we examined the effect that CSG banks and artificial refuge hills had on reducing tsunami damage

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Summary

Introduction

Research for predicting coastal scour and erosion by waves or flow has been performed all over the world. Takahashi et al (1993a, 1993b) [1,2] proposed a formula to estimate bed-load transport to predict scour due to tsunamis. They assumed that the bed-load transport was proportional to power n of the Shields number. Ca, Yamamoto, and Charusrojthanadech (2010) [9] presented a two-dimensional numerical simulation model that calculated inundation velocity, inundation depth, and topographical change on an inundation area. This equation requires a bed-load transport coefficient that is usually decided after using verification simulations. The evaluation method using threshold diagrams was found to be easy, economical, and useful in countries like Japan

Numerical Model for Fluid Motion
Numerical Model for Topographical Change
Rational Evaluation Method of Bed-Load Transport Rate
Hydraulic Model Experiment Method
Threshold Width of Columns in Reinforced Concrete Buildings
Design Destruction
Findings
Conclusions
Full Text
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