# Development of a Practical Evaluation Method for Tsunami Debris and Its Accumulation
## 1 Introduction  
The 2011 Tohoku-oki earthquake tsunami caused not only structural and inundation damage but also fire damage [1]. Tsunami-related fires were also observed in Kamaishi City during the 1933 Showa Sanriku earthquake tsunami [2] and on Okushiri Island during the 1993 Hokkaido Nanesi-Oki earthquake tsunami [3]. Because saltwater conducts electricity better than freshwater, electrical wires short-circuit when they come into contact with seawater, and the resulting sparks can ignite flammable materials, alternatively, a fire may be started when vehicle and boat fuel tanks become fractured during collisions. Subsequently, combustion is sustained and spread by flammable gases, debris, and fuel oil. Although the fire damage caused by the 2011 Tohoku-oki earthquake tsunami has been obscured by the devastation caused by the massive force of the tsunami waves, it was larger than that which occurred during the 1995 Great Hanshin-Awayi Earthquake [4], when 330 fires occurred in 14 prefectures in Japan. Tsunami-related fires differ from fires triggered by strong ground motions. Fires triggered by strong ground motions generally start in individual buildings and their spread depends on weather conditions and land use. In contrast, tsunami-related fires, caused,for example, by drifting objects, such as cars and boats in inundated areas, may spread widely by igniting flammable oil spilled by the tsunami as well as accumulated combustible woody debris derived from destroyed buildings. [PERSON] et al. [5] defined a tsunami-related fire as \"a fire that breaks out or is spread in a tsunami inundation area, including the sea area\". Although the processes by which a tsunami fire is ignited and spread are extremely complicated and have been difficult to quantify for the hazard assessment, available records of the huge number of tsunami-related fires from the 2011 Tohoku-oki earthquake can shed light on some of these processes. As one example of a fire's outbreak and spread, we present the case of the area around Kadonowaki Elementary School in Ishinomaki City. Almost all of the wooden houses in the residential area between the elementary school and the coast were destroyed by the tsunami and reduced to flammable woody debris. This woody debris was trapped against the hill behind the elementary school and accumulated there. Vehicles swept away during the tsunami inundation also accumulated around the elementary school building. Repeated impacts as they accumulated caused their fuel tanks to fracture, and the salt in the seawater caused short circuits in electrical systems, which in turn ignited the spilled gasoline. The burning gasoline is thought to have ignited the woody debris that had accumulated around the school building, thus causing the fire to spread (Figure 1). Once started, fires may also have been spread by liquid propane gas leaked from household cylinders and oil leaked from storage tanks. Thus, the presence of woody debris derived from destroyed wooden buildings and transported and accumulated by the tsunami, which allows tsunami fires to spread, is one indicator that can be used for tsunami fire risk assessment. There have been many studies on the dynamics of tsunami debris and drifted objects since the 1980s [7,8]. In addition, there are some studies on the observation [9] and numerical analysis [10] of the behavior of tsunami debris in the sea after the 2011, off the coast of Tohoku earthquake; however, there are not so many studies on the prediction of the amount of tsunami debris launched on land. [PERSON] and [PERSON] [11] and [PERSON] et al. [12] have proposed models for predicting the generation and accumulation of tsunami debris. In these models, tsunami-related building destruction and the associated generation of debris are analyzed in relation to tsunami fluid force data obtained by numerical analysis.