# Development of a Practical Evaluation Method for Tsunami Debris and Its Accumulation
## 2 Practical Method for Evaluating Tsunami Debris Accumulation and Thickness  
In this section, we describe the practical tsunami evaluation method developed by [PERSON] et al. [6]. In this method, four factors that determine the probability and thickness of tsunami debris accumulation are statistically extracted. To identify these factors, areas in Iwate Prefecture, severely affected by the 2011 Tohoku-oki earthquake tsunami, were analyzed: three areas in Yamada town, two areas in Otsuchi town, and all of Kamaishi city (Figure 2). In these areas, high-quality, high-resolution laser imaging detection and ranging (LiDAR) were conducted by the Geospatial Information Authority of Japan immediately after the 2011 Tohoku-oki earthquake tsunami. The resulting digital surface model (DSM) of the inundation area includes the height of the debris generated by the tsunami. The distribution thickness and volume of the debris was calculated from the difference between this DSM and the digital elevation model (DEM) for the corresponding area, based on measurements obtained before the earthquake, following Koshimura and Fukuoka [16]. Figure 2: (a) Over view of analysis area in Japan, (b) Analysis areas for evaluation of tsunami debris. To assess building damage in the tsunami inundation area, information from the Reconstruction Assistance Survey Archive by the Japanese Ministry of Land, Infrastructure, Transport, and Tourism (MLIT) was used. Tsunami inundation depth data were obtained from the Reconstruction Assistance Survey Archive and the 2011 Tohoku Earthquake Tsunami Joint Survey Group [17]. The DSM included the heights of vehicles, remaining houses, and trees at the inundation boundary, which were removed so that debris thickness could be accurately estimated. In addition, since the earthquake caused large crustal deformation in the east-southeast direction along the Iwate Prefecture coast, building locations were corrected by using pre-earthquake positional information. The corrected debris distribution maps showed that tsunami debris tended to accumulate near the inundation boundary and near where buildings had been washed away (Figure 3). Debris accumulated around building foundations and between remaining buildings. The mechanism of debris accumulation was similar to that which causes drifting debris to be trapped by trees [18]. To clarify tsunami debris accumulation trends and amounts, belt transects were defined parallel to the tsunami inundation boundary in each analysis area (Figure 4). Each belt transect was 10 m wide, and the distance from the inundation boundary to the seaward edge of the transect was defined as \(L_{\text{R}}\). Each belt transect was also divided into grids at 10 m intervals. In each grid along each belt transect, the maximum inundation depth \(D\) (m) was used as the index of tsunami magnitude. The average debris thickness, obtained from the difference between the DSM and DEM, was defined as \(DT\) (m), and the maximum topographic slope was defined as \(i_{\text{M}}\) (%). An index for the existence of buildings (including washed away and totally destroyed buildings) weighted by the distance between 10 and 60 m from an arbitrary position was defined as \(SI\) (\(=k_{10}+k_{20}/2+k_{30}/3+k_{40}/4+k_{60}/6\), \(k_{\text{i}}\) is a parameter for the existence of a building, independent of whether it has been washed away or not, with 1 and 0 corresponding to existence or non-existence; the subscript i indicates the distance range in meters), the definition sketch of \(SI\) evaluation is shown in Figure 5. To describe the characteristics of each analysis area, the ratio of the maximum width, \(W\), of the inundation area to the shortest distance from the shoreline to the inundation boundary, \(L_{\text{R0}}\), in each analysis area was defined as \(AS\). Debris volume in each grid was  
Figure 3: The distribution of tsunami debris in Otsuchi area A, estimated by using a DSM and a DEM with 1 m resolution.