## 5 Summaries and Conclusions  
In this study, the laboratory SSCB model tests were performed to efficiently evaluate the water-shielding performance of the CB system. Unlike in previous studies, our SSCB system was characterized by setting the drainage condition in the flow direction of the inclined sand layer as the lateral no-flow condition. Through the laboratory SSCB model tests, we tested three different rainfall intensities (i.e., 20, 50, 100 mm/h). To evaluate the effectiveness of the SSCB model test, the diversion lengths were estimated by the empirical equation for the lateral flow condition based on the physical and water retention characteristics of sand and gravel; then, these diversion lengths were compared to the results of the SSCB model test. The conclusions derived from the results of this study are summarized below. * The inside of the SSCB model proposed in this study was 45.5 cm long, 47.0 cm high, and 15.0 cm wide. Notably, the amount of drainage water under the lowest tested rainfall intensity (I = 20 mm/h) reached a steady state within 6 h. We can hence infer that the production work and the testing results of the SSCB model were better when the testing time was relatively short. Moreover, since this model was smaller than the large-scale models described in previous works, it was expected that its production work and testing would have a lower cost. * The following rainfall intensities were achieved during the SSCB model tests: 20 mm/h (Case 1), 50 mm/h (Case 2), and 100 mm/h (Case 3). The time required by the sand layer to reach the saturation state in each of the above cases was 180 min, 85 min, and 45 min, respectively; moreover, breakthroughs occurred at 210 min, 85 min, and 45 min, respectively. Thus, the rainfall intensity was very closely related both to the time required by the sand layer to reach the saturation state and to the time of breakthrough occurrence in the CB system. * The diversion lengths in the SSCB model test of this study were measured based on two criteria (i.e., LUD1: at the interface between the sand and the gravel layers, and LUD2: the occurrence point of the finger flow at the height of the soil moisture sensors (No. 3 and No. 4) installed in the gravel layer), because water infiltration occurred irregularly from the interface between the sand and the gravel layers. The LUD1 values obtained for Case 1, Case 2, and Case 3 were 13.5 cm, 11.7 cm, and 0 cm, respectively; meanwhile, the LUD2 values were 24.8 cm, 11.2 cm, and 0 cm, respectively. These results indicate that LUD1 and LUD2 were 13.3% and 54.8% lower, respectively, in Case 2 (I = 50 mm/h) than those in Case 1 (I = 20 mm/h). Finally, in Case 3, under the extreme rainfall intensity of 100 mm/h, our CB system collapsed. * The diversion lengths (LUD) under the lateral flow condition were estimated by the empirical equation of [PERSON] et al. [28] based on the hydraulic conductivity functions and soil-water characteristic curves of the sand and gravel. In Case 1, Case 2, and Case 3, they were 145.5 cm, 56.8 cm, and 27.7 cm, respectively. However, considering that the lengths of \(\Delta\)L\({}_{1}\) and \(\Delta\)L\({}_{2}\) would be reduced by the occurrence of breakthroughs in the SSCB model test, the effective diversion lengths (i.e., LED1 and LED2) for the real CB system were derived. The LED1 and LED2 values in Case 1, Case 2, and Case 3 were 114.2 and 125.5 cm, 23.7 and 23.1 cm, and \(-\)17.1 and \(-\)17.1 cm, respectively. Here, negative LED1 and LED2 values (i.e., \(-\)17.1 cm and \(-\)17.1 cm in Case 3) indicated that there is no water-shielding performance of the CB system under extreme rainfall (i.e., I = 100 mm/h). * The diversion lengths in the CB system were compared: LD (estimated by the empirical equation) and LED1 and LED2 (estimated based on the SSCB model test).