# Development of a Practical Evaluation Method for Tsunami Debris and Its Accumulation
## 4 Application to Predictive Analytics  
In this section, we describe the application of the developed evaluation method for tsunami debris accumulation to Owase City, Mie Prefecture, in central Japan, which has repeatedly suffered damage from huge tsunamis caused by past Nankai Trough earthquakes. The way in which the tsunami source scenario is determined affects tsunami damage predictions. Since the 2011 Tohoku tsunami, tsunami damage prediction has been based on either the disaster prevention level for a 100-year tsunami in the target area of the prediction, or the disaster mitigation level for the maximum possible tsunami according to the Japan Cabinet Office [21]. Here, we used 11 earthquake source fault models (Cases 1 to 11) for Nankai Trough megathrust earthquakes published by the Cabinet Office. Vertical displacement associated with one of these models (Case 7) is shown in Figure 15.  
To calculate tsunami propagation, we used the JAGURS tsunami analysis code [22], based on the nonlinear long wave equation, as the governing equation. The spatial grid spacing including the tsunami source was changed stepwise from 810 m to 270, 90, 30, and 10 m for a high-resolution bathymmetric model. The computation time step was set to 0.1 s to satisfy Courant-Friedrichs-Levy conditions in each region. In the part of the damage prediction area, we used a grid spacing of 10 m.  
To predict debris accumulation in Owase City, the amount of debris generated must first be estimated. For this estimation, we calculated the maximum inundation depth using the conventional tsunami inundation computation method, and we used Equations (4) and (6) to calculate the damage rate of wooden buildings, because we assumed that only buildings that were completely damaged or washed away would be reduced to debris. Although earthquake strong motions also produce debris from totally collapsed buildings, that effect was not included in the calculation. The amount of combustible debris in each wooden building was calculated by a method proposed by MLIT [23], which separates combustible from noncombustible components of wooden buildings and from other components, such as fixtures and furniture. The state of woody combustible materials turned into debris by a tsunami was calculated by the method proposed in [24]. In estimating the total debris volume by these methods, we assumed that 50% would remain on the land, as estimated for the 2011 Tohoku-oki earthquake tsunami (Table 1). Figure 14: Fragility functions for building loss in Sendai City. The solid curve was obtained by fitting Equation (4) to observed building damage data. The dash–single dot curve represents the function when \(\mu\) = 3.57 and \(\sigma\) is represented by Equation (6), for washed away buildings, and the dash–double dot curve represents the function when \(\mu\) = 3.02 and \(\sigma\) is represented by Equation (6) plus 0.2 (the safe-side design fragility function for washed away buildings). For the prediction of tsunami debris, we calculated \(L_{\text{R}}\) (Figure 16) and \(SI\) (Figure 5) in the tsunami inundation area for each tsunami scenario. The range of parameters used for debris prediction in these 11 cases, according to the extent of the inundation area, as \(0.5<AS<0.8\), \(0<i_{\text{M}}<20\), and \(0<D_{\text{*}}<1.0\), satisfied the applicable range of Equations (1)-(3). The probability of debris accumulation, \(P_{\text{db}}\), and its thickness, \(DT\) were evaluated by using Equations (1)-(3). Debris accumulation was defined by \(P_{\text{db}}>\chi\) (\(\chi\) of a uniform random number between 0 and 1). We conducted 100 debris evaluation trials and used the average as the representative prediction value for debris accumulation. Figure 16: (**a**) Ten-meter-wide belt transects from the inundation boundary to the shoreline in Owase City. (**b**) A portion of the inundated area showing belt transects in relation to the tsunami inundation boundary (\(L_{\text{R}}=0\) m).