## 1 Introduction  
Debris flow hazards caused by typhoons and heavy rainfall have significantly increased all over the world. Debris flows with entrained debris, gravel, and boulders mixed with water are extremely fast, have a high impact force, and bring disastrous consequences to urban areas and infrastructure. For this, the installation of a single rigid barrier is a commonly used method for intercepting debris flows [1]. However, such an approach allows debris flows to increase in speed and volume, entraining basal sediments before hitting the barrier [2]. Consequently, multiple structural barriers installed in the flow path of debris flows are necessary, as shown in Figure 1, which demonstrates the cylindrical baffles installed at Umyeon Mountain in Korea and the rectangular baffles installed behind a rigid barrier at Landau Island in Hong Kong, China.  
In mountainous regions with steep slopes, large boulders can be entrained by debris flows. However, because of their rigidity, the rigid structures are prone to damage by large boulders. To reduce potential damage from boulders, installing baffle arrays behind a rigid barrier is also an effective countermeasure because, in the course of debris flow, the baffles can decelerate frontal velocity and filter out boulders [3; 4; 5]. However, a design guideline for the specifications and arrangement of baffles has not yet been suggested. For the engineering design of such structures, a comprehensive understanding of impact force and run-up characteristics is necessary [6]; however, engineering designs have yet to reach complete agreement on impact behavior and impact force estimation. Meanwhile, there have been many experimental studies that have investigated the effect of debris flow on a rigid barrier. [PERSON] et al. (2015a), [PERSON] et al. (2017), [PERSON] and [PERSON] (2020), and [PERSON] et al. (2018) [7; 8; 9; 10] studied the flow-structure interaction, such as the run-up, overflow, and impact force characteristics. However, the effect of the installed baffles behind a rigid barrier has not been extensively studied. Baffles are mainly deployed in multiple groups, and the interaction between a debris flow and baffles with a complex flow mechanism is significantly affected by the height, spacing, and number of rows of baffles. Therefore, to control the mobility of debris flows containing boulders, an appropriate arrangement of baffle arrays behind a rigid barrier is needed.  
In this study, to investigate the effect of cylindrical baffle arrays behind a rigid barrier on debris flow behavior and dynamic impact force, a series of small-scale flume tests was performed according to the varying the baffle height and number of rows. High-speed cameras used to capture the flow interaction with the baffle arrays and the rigid barrier were installed at the top and side of the flume. Load cells employed to record the dynamic impact force of a debris flow against the terminal barrier after the flow has passed through the baffle arrays were also installed behind the rigid barrier. After the test, we analyzed the run-up and impact force characteristics due to the baffle arrays.  
Figure 1: Debris flow baffles [11]: (**a**) cylindrical baffles (Umyeon Mountain in Seoul, South Korea); (**b**) rectangular baffle installed behind a rigid barrier (Lantau Island in Hong Kong, China).