## 3 Results
### Impact Force  
The impulse load induced by boulders is an important parameter for designing debris flow countermeasures [4; 26]. Figure 7 shows a comparison of impact force on the terminal rigid barrier versus time during the experiment, and the impact force was measured under various baffle configurations (H0, H60-R1, R2, and H120-R2, R2). The initial response of the rigid barrier against debris flow impact was set to t = 0.25 s, and the total testing time was 3.0 s. The peak values of the impact force are also marked in each figure. We further investigated the importance of the dynamic and static components of the total impact load recorded at the load cells versus the time. The peak impact force of the debris flow on the terminal rigid barrier showed a distinct change with changes in the baffle configuration. The lower plates of the rigid barrier showed a greater impact force than the upper plates because the debris flow was mainly concentrated at the bottom of the rigid barrier. The case with no baffles (H0) exhibited the largest peak impact force because of the absence of the interference of baffles. Meanwhile, from the case with one and two rows of baffles, a rapid reduction in impact force was observed at the rigid barrier. Figure 8 shows a comparison of the dynamic peak impact force change for all the tests. One row of baffles (H60-R1) exhibited a 85% impact force reduction compared with the case with no baffles. Two rows of baffles (H60-R2) created an additional 47% impact force reduction compared with one row of baffles (H60-R1). This was because the additional staggered row of baffles produced a greater reduction in the gradual mobility of boulders and in the velocity of debris flow through the repeated suppression of the flow. The taller baffles in one row (H120-R1) exhibited an 82% impact force reduction compared with the case of no baffles; however, the impact force showed a negligible difference compared with the case with the shorter baffles (H60-R1). This was because the installed baffles temporarily suppressed the mobility of the boulders, whereas the flow freely passed between baffles without any interference after the baffles. However, increasing the baffle height from 60 mm (H60-R2) to 120 mm (H120-R2) with two rows of baffles decreased the impact force by up to 43%. As observed in previous studies on the debris flow dynamics with baffles by numerical analysis [27; 28], the velocity and the impact force of flow significantly decreased with the increase in height and row number of baffles due to the increased flow impedance of baffles. Thus, the energy dissipation of the flow becomes even more obvious for higher heights and more rows of baffles. Figure 9 shows a comparison between the residual static impact force at various locations of the rigid barrier. Because of the formation of dead zones behind the rigid plate, the static force of the lower plates (LL4, LL5, and LL6) was much higher than that of the upper plates (UL1, UL2, and UL3). However, the static force in each test showed relatively few differences compared to the impact force. Comparing the results of the static force and the dynamic force, the baffle arrays had a major effect on the reduction in the dynamic force, while the baffles did not have a significant effect on the dead zone formation and the resultant lower reduction in the static force. As observed in the series of tests, the formation of dead zones for all the tests were similar regardless of the presence of baffle arrays.  
Figure 7: _Cont._  
Figure 8: Comparison of the dynamic peak impact force.  
Figure 7: Impact force for different baffle configurations with time: (**a**) without baffles (H0); (**b**) H60-R1; (**c**) H60-R2; (**d**) H120-R1; (**e**) H120-R2.