## 5 Discussion  
Debris flows are affected by the characteristics of the sediment-water mixture (magnitude, \(C_{V}\), and grain-size distribution) and channel shape (\(\alpha\), width, and curvature) [3; 6; 9]. In this study, various \(\alpha\) and \(C_{V}\) values were therefore evaluated in flume tests along with changes in channel shape due to the installation of a berm in the middle of the channel. The experimental results showed that the development of a flow was influenced to occur at \(\alpha\) of 15\({}^{\circ}\) (Table 6), which has been mentioned as the slope that causes debris flow in previous studies [3; 9]. [PERSON] [9] mentioned that the debris flow pattern suddenly changes when its \(C_{V}\) is less than 0.55 because of active particle separation, and that a debris flow cannot reach the outlet of its channel when the \(C_{V}\) exceeds 0.58. In this study, the development of the debris flow was also observed to differ around a \(C_{V}\) of 0.55; at a \(C_{V}\) of 0.60 in the single-berm channel test, the debris flow stopped in the channel before reaching the outlet (Table 6). In the straight channel test, however, it appears that \(\alpha\) had a larger influence on the flow. Thus, debris flow occurred when \(\alpha\) was 15\({}^{\circ}\) or higher, even when \(C_{V}\) exceeded 0.58. As \(\alpha\) increased, the debris flow velocity and mobility ratio were both observed to increase (Figures 6a and 10a). The changes in the flow velocity and mobility ratio differed around an \(\alpha\) of 15\({}^{\circ}\). The flow depth consistently decreased as \(\alpha\) increased when no berm was installed, but it suddenly increased at an \(\alpha\) of 25\({}^{\circ}\) when the berm was installed (Figure 7a). This appears to be because the debris flow moved along the steeply sloped channel with considerable momentum, and then the flow suddenly changed under the influence of the channel cross section geometry where the berm was installed. The installation of the berm caused the Froude number to increase when \(\alpha\) was less than 25\({}^{\circ}\), but it decreased when it was 25\({}^{\circ}\) (Figure 8c). In other words, it was confirmed that the 15\({}^{\circ}\) slope known to cause debris flow indeed affects the debris flow velocity and mobility ratio, and that an \(\alpha\) of 25\({}^{\circ}\) affects the flow depth and Froude number in single-berm channel. Figure 10: (a) Mobility ratio according to \(\alpha\) (at \(C_{V}\) = 0.40–0.55); (b) mobility ratio according to \(C_{V}\) (at \(\alpha\) = 10\({}^{\circ}\)–25\({}^{\circ}\)); (c) percent decrease in mobility ratio due to berm according to \(\alpha\) (at \(C_{V}\) = 0.40–0.55); (d) percent decrease in mobility ratio due to berm according to \(C_{V}\) (at \(\alpha\) = 10\({}^{\circ}\)–25\({}^{\circ}\)). As \(C_{V}\) increased, the debris flow velocity and flow depth decreased (Figures 6b and 7b), but the mobility ratio increased (Figure 10b). A \(C_{V}\) of 0.50 marked important the changes in the development of debris flow depth and Froude number, and a \(C_{V}\) of 0.55 marked important changes in the development of the debris flow velocity and mobility ratio. At a \(C_{V}\) of 0.60, the debris flow did not reach the outlet at any \(\alpha\) when the berm was installed, but it reached the outlet if \(\alpha\) was 15\({}^{\circ}\) or higher when no berm was installed. Furthermore, it was confirmed that the \(C_{V}\) dominated the flow resistance coefficients \(\mu\), \(\xi\), \(n\), and \(C_{1}\), while \(C_{2}\) was not affected by the \(C_{V}\), likely because it was defined using a numerical analysis, unlike the other flow resistance coefficients, which were defined using the experimental results. Similarly, [PERSON] [16] mentioned that \(C_{2}\) is appropriate for the numerical analysis of unsteady debris flows. The Froude numbers obtained in this study ranged from 4.14 to 10.79. This range is similar to those reported in studies conducted using an experimental setup with a channel length of 2 m or less [2; 35].