Study on debris flow monitoring in Mt. Fuji

. The Osawa River, which has its headwaters at the top of Mt. Fuji, has one of the largest failure in Japan, the Osawa Failure, in its uppermost part of the catchment. The Osawa Failure produces more than 100,000m 3 of sediment every year by erosion and collapse, which causes debris flows and slash avalanches. Since its opening in 1970, the Fuji Sabo Office has been observing the sediment dynamics of the Osawa River through field surveys, observation equipment and aerial photogrammetry, aerial laser survey, and Japanese pipe hydrophone. In 2021, the largest debris flows since the start of observations in the Osawa River occurred twice in March and August. In the March 21 debris flow, 416,000m 3 of sediment flowed down from the Osawa Failure, and 482,000m 3 was deposited in the alluvial fan. In the August 18 debris flow, 421,000m 3 of sediment was deposited in the alluvial fan. Our observations from CCTV cameras, rain gauges, and other equipment installed in the Osawa River indicate that the rainfall characteristics during the two debris flows were different, and that the conditions for debris flows differed depending on the season.


Introduction
The Osawa Failure (Fig 1) is one of the largest collapse sites in Japan; its dimension is about 2,000m in length, its maximum width is 500m, and its maximum depth is 150m. It is located on the upper western slope of Mt. Fuji, the highest mountain in Japan (3,776m). At the Osawa Failure, an average of over 100,000m 3 collapses occur annually, and the sediment deposited on the streambed causes debris flows and slush avalanches. Since its opening in 1970, the Fuji Sabo Office has been conducting aerial photogrammetry and field surveys to monitor the sediment dynamics at the Osawa River. In recent years, with the development of aerial surveying technology, monitoring has been carried out using various observation methods such as airborne LiDAR. In 2021, large-scale debris flows occurred at Osawa River on March 21 and August 18. The two debris flows occurred in different seasons and had different rainfall characteristics observed at the time of occurrence. We analysed the characteristics of the debris flow using data observed by CCTV cameras, rain gauges, Japanese pipe hydrophone, load sensors, and other equipment installed in the Osawa River.

Analysis of airborne LiDAR data
The Fuji Sabo Office has been continuously conducting airborne LiDAR measurements since 2007 to generate DEMs with 1m grid. Annual sediment budgets have been calculated using the differences of DEMs of two consecutive periods. In 2021, five airborne LiDAR measurements were taken between April and October to determine the amount of sediment discharged.

Installation of observation equipment
In the Osawa River, observation equipment is installed at the Ochudo observation site (elevation 2,300 m), which is closest to the Osawa Failure, Otaki observation site (elevation 1,480 m), Iwadoi observation site (elevation 900 m), and Osawagawa-bashi observation site (elevation 500 m). For example, at the Iwadoi observation site (Fig. 2), CCTV cameras, ultrasonic water level gauge, velocity gauge, and load sensor are installed to observe debris flows.

Debris flow on March 21, 2021
In the debris flow that occurred on March 21, analysis of airborne LiDAR measurement data revealed that 416,000m 3 of sediment flowed form Osawa Failure, the largest single debris flow since observations began in 1970. It was also revealed that approximately 482,000m 3 of sediment flowed from canyon part into alluvial fan. The maximum hourly rainfall of 21mm (3/21 11:00-12:00) and cumulative rainfall of 97mm (3/21 03:00-13:00) were observed at the Ochudo observation site. To determine the status of debris flow at the Iwadoi observation site, we divided the video taken by CCTV camera into 10-minute segments and read the water level and velocity for one minute of an average-scale run-off scene in each image frame by frame (1 frame = 0.033 seconds). Based on the readings of water level and flow velocity, the peak flow rate of the debris flow was approximately 290.4m 3 /s (water level: 1.9m, flow velocity: 16.9m/s), and the total flow rate during the analysis period (270 minutes) was approximately 880,000m 3 . The load sensor measures about 4.0 tons before the debris flow peak (Fig.3, Fig.4).

Debris flow on August 18, 2021
In the debris flow that occurred on August 18, analysis of airborne LiDAR measurement data revealed that 241,000m 3 of sediment flowed form Osawa Failure. It was also revealed that approximately 421,000m 3 of sediment flowed from canyon part into alluvial fan. The maximum hourly rainfall of 79mm (8/18 06:00-07:00) and cumulative rainfall of 1,089mm (8/12 15:00-8/18 10:00) were observed at the Ochudo observation site. We calculated the peak flow rate of the debris flow based on the water level and velocity readings from the CCTV camera images at the Iwadoi observation, it was approximately 255.2m 3 /s (water level: 2.9m, flow velocity: 9.2m/s), and the total flow rate during the analysis period (270 minutes) was approximately 587,000m 3 . The load sensor measures about 1.5 tons after the debris flow peak (Fig.5, Fig.6).

Rainfall at the time of debris flow
Debris flow observations that have continued since 1970 have indicated that the amount of rainfall at which debris flows occur varies with the season in Osawa River. Therefore, we analysed the relationship between rainfall data (Ochudo observation site) and the time when debris flows occurred. To analyse rainfall data during debris flow events, we divided the year into summer season (Jun-Oct) and the rest of the year (Nov-May). The relationship between the timing of debris flows and maximum hourly rainfall (Fig. 7) indicates that debris flows occur at a maximum hourly rainfall of 50 mm/hr or higher during the summer season. On the other hand, during the non-summer months, it occurs at 25mm/hr or less, and especially during the severe winter season from February to April, several occurrences at 20mm/hr or less have been observed. The relationship between the timing of debris flows and cumulative rainfall (Fig. 8) indicates that debris flows occur at a cumulative rainfall of about 300mm or more from June to October, and especially in September and October, it may not occur even when rainfall exceeds 600mm. On the other hand, it has been confirmed that debris flows occur from February to April and November and December when rainfall is less than 200mm. Hourly rainfall and cumulative rainfall during summer season debris flows are higher than during non-summer debris flows, suggesting that the conditions under which debris flows occur vary with the season.  As shown in Figure 9, it is inferred that the possibility of debris flows occurring increases in summer when the maximum hourly rainfall exceeds 50mm/hr and cumulative rainfall 300mm. On the other hand, the results indicate that debris flows may occur in nonsummer season even when the maximum hourly rainfall is 25mm/hr or less and cumulative rainfall is 230mm or less. Fig. 9. Relationship between maximum hourly rainfall and cumulative rainfall during mudslide occurrence.

Conclusions
We conducted detailed and quantitative analysis of the two large-scale debris flows that occurred in 2021 by analysing airborne LiDAR data and observing with CCTV cameras and other equipment. The largest erosion since 1972 occurred at uppermost part of the Osawa River (Fig. 10), which is presumed to have destabilized the riverbed and surrounding slopes, and there is a possibility that active sediment discharge will occur again. The results also indicated that there were seasonal differences in the relationship between the occurrence of debris flow and rainfall. This is presumably due to the existence of a frozen layer in the riverbed at high  elevations, where debris flows occur, and the depth of the frozen layer changes with the season. We will continue to accumulate data from existing equipment and study observation methods for Osawa Failure, where there are many unknowns regarding the actual conditions of sediment discharge, in order to analyse the actual conditions of sediment discharge in the Osawa River.