Debris Flow Monitoring using Load Cells in Sakurajima Island
Takeshi OSAKA1, Rei UTSUNOMIYA2, Satoshi TAGATA3, Takahiro ITOH4* and Takahisa MIZUYAMA5
1 Ohsumi Office of River and Highway, Kyushu Regional Bureau, Ministry of Land, Infrastructure, Transport and Tourism (MLIT) (Now in Reconstruction Agencym, Japan)
2 JFE Advantech Co., Ltd., Japan
3 Nippon Koei Co., Ltd., Japan
4 Research and Development Center, Nippon Koei Co., Ltd., Japan
5 Graduate School of Agriculture, Kyoto University, Japan
*Corresponding author. E-mail: a6556@n-koei.co.jp
INTRODUCTION
Many debris flows have recently taken place in Sakurajima Island due to falling volcanic ash by volcanic activities since 2010, and the number of debris flow occurrences has gradually increased: e.g., 43 times in 2010, 31 times in 2011 and 55 times in 2012. For countermeasure against debris flow hazards, various kinds of measurements have been carried out to understand the flow characteristics of debris flows. In those monitoring, temporal changes of flow depth and discharge were collected using ultrasonic sensors and video camera, and profiles of bed elevations were also collected near river mouth in the sea. Sediment in debris flow bodies tried to be measure using a sampler box. However, it was difficult to obtain data for temporal changes of sediment discharge in floods. Present study proposes new monitoring system for evaluating for data of temporal changes of debris flows using loadcell system.
DEBRIS FLOW MONITORING IN ARIMURA RIVER
Arimura River is in southern-east area in Sakurajima, and is with a watershed area 1.35 km2, bed slope 19% and flow width 20.5 m at the Arimura 3rd sabo dam (Fig. 1). The number of debris flow occurrences in Arimura river has gradually increased: e.g., 6 times in 2010, 6 times in 2011 and 9 times in 2012.
A monitoring system using load cells and a steel plate (McArdell et al., 2007) was installed to obtain temporal changes of flow characteristics of debris flows at the Arimura River 3rd sabo dam. The system mainly consists of (1) four pin-type load cells, (2) a (2 m long and 4 m wide) steel plate, (3) two pressure sensors on the steel plate to measure interstitial water pressure in the bed, (4) an ultrasonic sensor to monitor flow depth and a radio wave velocity meter for velocity, and (5) CCTV
cameras to monitor flow conditions.
Since June, 2012, a monitoring system with pin-type load cells, a steel plate and pressure sensors was installed, and the pin-type load cell independently measures vertical stress and horizontal shear stress. In Arimura River, temporary change of mass density, sediment concentration of coarse and fine component by data of weight, bed pore pressure and flow depth. Excavated sediment volume after debris flow events can be evaluated if the temporal changes of
Fig. 1 Longitudinal bed profiles of Arimura River
0 100 200 300 400 500 600 700 800 900 1,000
0 1,000 2,000 3,000 4,000 5,000
Arimura 1st Sa bo da m
1 13.9
1 14.0
1 5.3
1 8.6
1 1.9
Dista nce from river mouth (m)
Bed elevations (m)
Arimura 3rd Sa bo da m (with loa d cells for debris flow measurements)
sediment discharge and the volume to flow to the sea can be also estimated.
In addition, in order to measure occurrences and run-off of debris flow, many sensors have been installed (Fig. 2), and those are as follows: Rain gauge, X- band MP Rader, ultrasonic water level meter, Radio wave velocity meter, wire sensor, falling ash gauge, acceleration vibrograph and CCTV camera.
After installing the monitoring systems, debris flow took place nine times in the Arimura River in 2012, and temporal changes of debris flows such as flow depth, flow discharge, sediment concentration of coarse and fine sediment particles were obtained on June 15th and 21st, 2012. In two debris flow events, especially there are two kinds of debris flows observed on June 15th and 21st, 2012.
CONCLUSIONS
On June 21st, debris flow had one surge with peak sediment concentration of coarse sediment at peak discharge rate (117m3/s). On the other hand, on June 15th, debris flow had several surges during flood (Fig. 3). Temporal changes of coarse sediment concentration have at least three peaks in a flood, though fine sediment concentration had one peak. It seems that this flood show interesting sediment transportation during flood, supposing that small sized landslide dams formed and broke at confluence section and so on.
The results indicate the necessity of evaluation of bed variations in upstream reach before/after debris flow event. The debris flow measurement by present method is useful for evaluating sediment runoff and it could be management for excavating after every debris flow events.
Keywords: Debris flow, Monitoring, Loadcell, Sediment concentration
Fig. 2 Parts of Debris Monitoring equipments in Arimura River basin
Fig. 3 Temporal changes of discharge, sediment concentration of fine and coarse components obtained by loadcell system (On June 15th, 2012)
Origin is 18:14:00 in 15th June , 2012 Time (sec.)
Sediment discharge rate (Total, Coarse and fine components ) (m3/s) Debris flow discharge rate (m3/s)
Debris flow discharge
Debris flow discharge Total sediment discharge rate of debris flow
Sediment discharge rate of coarse components
Sediment discharge rate of fine components
Fine component Coarse component
Total components (Coarse and fine)
X-Band MP Ra der (Upstrea m a rea of Arimura river)
Falling ash ga uge (Arimura No.1)
Rain ga uge
Accelera tion vibrogra ph
Fa lling a sh ga uge (Arimura No.2)
River mouth
Off-limits line for eruptions Debris flow mea surement
using loa d cells
