Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Impacts of Discharge Timing of Tsunami Debris on its Distributions and Hazardous Area
in Osaka Bay (Japan): A Case Study by Considering Inundation
Masayasu Irie, Taishi Yoshino

Department of Civil Engineering, Osaka University, Osaka, Japanand other debris and fires may affect the facilities soon after a tsunami.
Natech will damage industries and endanger the continuity of businesses.

ABSTRACTAs the behavior of tsunami debris in an enclosed sea area is dominated bytime-dependent external forces such as tide, wind, and freshwater inflow,
the timing of discharge of tsunami debris is important for the analysis ofdrift destination. Discharge and drifting processes were modeledconsidering inundation to understand variations in the drifting behavior
and map hazardous areas where secondary disasters induced by tsunami
debris under different meteorological conditions in Osaka Bay can occur.
Results show that the hazardous area prone to fire on the sea surface
substantially depended on wind direction and freshwater flow rate.

Many studies on tsunami debris have been conducted based on pasttsunamis. Prasetya et al. (2012) focused on the inundation caused by theSumatra tsunami, analyzed debris behavior using a numerical model, andcompared it with damages in the field. Many studies in recent years havefocused on tsunami debris that resulted from the Great East Japan
Earthquake and analyzed their accumulation in the North Pacific Oceanand on the coast (e.g., Lebreton and Borrero, 2013; Kawamura et al.,2014; Maximenco et al., 2018). Secondary damage caused by tsunamis
on industrial facilities has also received attention. Cruz et al. (2011)
investigated the potential for oil spills from inundated oil refineries basedon tsunami scenarios. Nishino and Imazu (2018) modeled tsunami fire in
the Kesennuma Bay and showed the movement of debris with flames andthe spread of fire. The risk of tsunami debris in Osaka Bay has also beenassessed. Kyaw et al. (2017) constructed an oil-spill scenario andanalyzed three-dimensional oil distribution in the Senboku area in OsakaBay. Nishino and Takagi (2020) have examined the risk of tsunami firecaused by the damage to the oil complex in the port of Osaka.

KEY WORDS: Tsunami debris; secondary damage; fire on sea; threedimensional model; inundation model; Osaka Bay; SCHISM.
INTRODUCTION
The Great East Japan Earthquake that occurred in 2011 caused enormousdamage. The massive tsunami generated by the earthquake inundated anarea of about 560 km2 (Geospatial Information Authority of Japan, 2011)
and washed many ships, containers, vehicles, and other debris ashore.
Hiroi et al. (2012) reported that tsunami debris and oil spilled fromdamaged oil complexes caused fires on the sea surface, and the fire
expanded to the affected area in Kesennuma Bay, Miyagi Prefecture. In
most other ports, tsunami debris caused secondary disasters such as
blockage of shipping routes, leading to shortages of goods.

Most studies on tsunami debris estimate their movement for the first few
hours after the arrival of tsunami; therefore, a two-dimensional model isoften used to analyze because it is adequate to consider only the flowcaused by the tsunami. However, to evaluate the risk of tsunami debris
over several days, a three-dimensional analysis is necessary because thedrifting behavior of debris in an enclosed sea area is strongly affected byexternal forces such as tides, winds, and freshwater inflow. Since theflow field in an enclosed sea area is unsteady, the area of secondary
disaster risk due to tsunami debris and drifting behavior of debris dependon the occurrence time of the tsunami. Modeling the tsunami run-up(inundation) and the discharge of debris according to inundation depth
or flow velocity can enable us to consider the timing of the start of debris
drift. Therefore, it is possible to discuss the time dependency of apotential secondary disaster risk by analyzing the behavior of tsunami
debris considering various meteorological and oceanographic conditions
and the inundation process. In this paper, we focus on the differences in

Natural disaster sometimes causes damages to industries, known as
natural-hazard-triggered technological (Natech) accidents. Osaka, one of
the largest cities in Japan, is expected to be attacked by a tsunami
generated by the probable Nankai Trough earthquake with a 70 to 80%
chance within the next 30 years. A five-meter tsunami is predicted in thecoastal area of Osaka Bay, inundating an area of about 50 km2. Large
ports with many wharves and various industries that have oil tanks andflammable-product storages operate in the coastal area. Secondary
disasters such as the blockage of shipping routes caused by sunk vehicles

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