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EXPERIMENTAL APPROACH ON MEASUREMENT OF IMPULSIVE FLUID FORCE USING DEBRIS FLOW MODEL
Nobutaka Ishikawa㧝, Ryuta Inoue㧞, Kenjiro Hayashi㧟, Yuji Hasegawa㧠 and Takahisa Mizuyama㧡
INTRODUCTION
Recently many sediment disasters have occurred at the mountainous area in Japan by local downpour based on the global warming. These disasters may be caused by the impulsive loading of debris flow. To this end, the hydrodynamic test was first performed by using the water in stead of debris flow in order to appear the impulsive fluid force and to measure it accurately as a preliminary test. Second, three materials of debris flow model, i.e. sediment with water, gravel with sediment water and beads with water were used as the quasi-debris flow by using channel test with a steep slope. Third, the pumice produced at Sakurajima volcanic mountain was used as the quasi-debris flow by flowing it naturally. The final test resulted in showing the impulsive load-time relation by representing the surge in front wave.
PRELIMINARY TEST
The hydrodynamic test for only water was performed in order to examine the measurement accuracy of impulsive fluid force by using both the force component meter (700Hz) and the pressure sensor (2500Hz). Figure 1 shows the fluid force-time in case of slope 1/50 and fluid velocity 2.6m/sec. It is confirmed that the fluid force measured by the force component meter is almost good agreement with the one by the sum of pressure sensors (PA,PB,PC from the bottom). Therefore, the fluid force can be measured by the force component meter. It is also found that the rise time to the maximum load is slow (about 0.15sec).
CHANNEL TEST WITH STEEP SLOPE
Figure 2 shows the channel test with steep slope (17$) in which the mixture of water, gravel ---
1Professor Emeritus of National Defense Academy, Research Adviser, Society for the Study of Steel Sabo Structures,6-20-68,Kugo-cho,Yokosuka,238-0022,Japan(e-mail;cgishikawa@m4.dion.ne.jp)
2Civil Engineer, Kyosei-Kiko, 1-23-1 Shinjiku, Shinjiku-ku,160-0022 Japan (e-mail;inoue@kyosei-kk.co.jp)
3Associate Professor, Department of Civil and Environmental Engineering, National Defense Academy, 1-10-20 Yokosuka, 239-8686,Japan (e-mail ;hayashik@nda.ac.jp)
4Rearcher, Civil Engineering Research Laboratory, 904-1, Tohigashi, Tukuba-shi, Ibaraki, 300-2633, Japan (e-mail; hasegawa@crl.or.jp)
5Professor, Department of Forestry, Graduate School of Agriculture, Kyoto University, Kitashirakawa, Oiwake-cho, Sakyo-Ku, Kyoto,606-8502, Japan(e-mail;mizuyama@kais.kyoto-u.ac.jp)
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It is found that the rise time is very slow (about 0.20 sec) and the peak load is about 50N. The reason why the rise time became slow may be caused that the consistency is not reached to the equilibrium and the head of flow shape becomes to the wedge type. It is found that it is difficult to get the surge shape in front wave by using the mixture of gravel, sediment and water.
DEBRIS FLOW MODEL TEST USING PUMICE STONE
(1)Figure 3 shows the fluid force- time by using the pumice stone in which the peak load is about 83N and rise time is 0.09 sec which becomes quite faster than other materials. This may be caused by forming the surge shape due to the effect of coming up to the surface.
(2) The ratio of peak load (83N) and stabilized load (47N) after the peak load is quite large (1.7-1.8) and as such, this phenomenon is called as the impulsive load which acts on the structure larger than the static design load (44N) of thick line in Fig.3.
CONCLUSIONS
(1) It is confirmed that the fluid force measured by the force component meter is almost good agreement with the sum of pressure sensors.
(2) It is found that it is difficult to get the impulsive loading in cases of sediment +water, gravel + sediment +water, even if the channel slope becomes steep, because the rise time is slow.
(3) It is interested to note that the front wave of debris flow model using pumice stone shows the surge shape and as such, the ratio of the peak load and the stabilized load after the peak load became quite large (1.7-1.8).
(4)The rise time in fluid force-time relation using pumice stone became faster than other debris flow model materials. This may be due to the effect of forming the surge shape.
(5)These phenomena will be simulated by using the particle method which may be used for the Sabo dam design in the near future.
Key words: debris flow model, impulsive fluid force, pumice stone, hydrodynamic test
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Fig.1 Fluid force-time relation
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(a) Fluid force -time relation(b)Pressure-time relation Time(sec)
Fluid force (N)
Time(sec)
Pressure Load cell
Pressure (Pa)
Component meter
Sum of pressure sensors
Fig.4 Fluid force-time relation ( Water, slope of 1/50, flow velocity of 30
25 20 15 10
Fluid force (N)
Time (sec)
(a) Fluid force-time relation
(b) Pressure-time relation Fig.㧝 Fluid force-time relation (water, slope 1/50, flow velocity 2.6m/sec)
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Fig.3 Fluid force-time relation (pumice) [thick line: design load]
Fig.2 Fluid force-time relation at steep channel (gravel with sediment including water)
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Keywords:
