## 2 Methods
### Test Conditions and Procedure  
Excessive pore fluid pressure is primarily influenced by grain size distribution and the presence of particles in the fluid phase [21]. In this study, to simulate debris flow behavior in a flume, a two-phase debris flow was reproduced with granular materials and fluid mixtures. The debris mixture was composed of 25% gravel (5-10 mm in diameter), 25% coarse sand (2-5 mm in diameter), and 50% medium to fine sand (0.25-2.0 mm in diameter). The debris mixture had a similar grain size distribution as the debris flow hazard site at Samcheock, South Korea, where a debris flow occurred in 2019. According to the Unified Soil Classification System (USCS), the debris mixture in this study and the natural weathered soil collected from the debris hazard site at Samcheock were classified as SP, respectively. Furthermore, glass beads were used to reproduce large boulders entrained by debris flows in nature. The glass beads mixed with the debris mixture had a higher bulk volume and less compressibility, therefore it could be used to represent natural boulders.  
Figure 2: Small-scale test: (**a**) schematic diagram of flume model; (**b**) rigid barrier combined with load cells; (**c**) physical experimental setup.  
The diameter of the glass beads was 40 mm (equivalent to a prototype boulder of 0.8 m in diameter), based on a boulder diameter ranging from 0.5 to 2.0 m in previous studies [22; 23]; Figure 3 shows the debris materials used in the laboratory test. To reproduce the viscosity of a debris flow caused by fine-grained soil in the flume, glycerin was mixed with the debris mixtures with water. The behavior of a two-phase flow with glycerin is similar to natural debris flows and simulates mobility characterized as a non-Newtonian fluid rheology. The volumetric solid fraction of the tested debris flows was determined to be 50%. The flow mass of the debris mixture was 20 kg; the initial bulk density was about 1925 kg/m\({}^{3}\); and the viscosity was about 0.05 pa s. From this, the approaching velocity, flow depth, and inclination of flume for the predetermined Froude number (Fr = 5.0) were estimated as 2.2 m/s, 0.02 m, and 15\({}^{\circ}\), respectively.  
For the small-scale test, firstly, a debris mixture of 20 kg was prepared. Then, the inclination of the flume was set to 15\({}^{\circ}\), and the cylindrical baffle arrays and the rigid barrier with the load cells were installed at the middle of the flume, as shown in Figure 2a. To prevent the rotation and bending of the cylindrical baffles during the test, the cylindrical baffles were fixed by dual-threaded screws at the base of the acrylic flume. To measure the dynamic impact load from debris flows, a data logger connected to load cells was set with a sampling rate of 20 kHz. After the rigid barrier was installed, as shown in Figure 2a, high-speed cameras were also installed at the top and side of the flume. Subsequently, the debris mixture of 20 kg was moved to the storage container, and the debris was continuously stirred by a hand-held electric mixer. Once the debris preparation from the storage container was completed, the debris was released toward the barrier arrays by opening the spring-loaded door by deactivating the magnetic lock. During the test, the flow-structure interaction and impact force were measured by high-speed cameras and load cells, respectively. Particle image velocimetry (PIV) analysis [24] was utilized to investigate the effect of velocity attenuation and the deposition process behind the barrier. Five tests in total were conducted according to various baffle configurations. Each test was repeated two times to eliminate errors and to obtain precise results from the experiments. The test conditions are summarized in Table 1.  
Figure 3: Debris materials.