Transportation-mechanics of Sakurajima [Japan] -volcanic-ash drifted over the fixed bed open channel.
1992
Cho K. | Kawaharada R. | Tanaka H.
This experimental investigation have been carried out to clarify the transportation mechanics of Sakurajima volcanic ash drifted over the fixed bed open channels, with taking physical properties of ash particles into account. The main findings of the present study can be summarized as follows: 1. The ash particles are very angular and jagged, the particle sizes range approximately 0.1 mm to 1.0 mm in diameter in Kagoshima city. The specific gravity of ash range 2.6 to 2.71, these values are a little large than ordinary sand. The settling velocity of ash particles in water is slightly slower than sand in spite of larger specific gravity than sand. 2. The distributions of the velocity over ashed bed channels show the log-law except the near water surface regions. Ordinary ashed bed channels are classified as a smooth channel surface. 3. The critical tractive force of ash over the fixed bed is little larger than that of sand, because of the difference of particle feature. Smaller ash particles usually set each other and increase flocculation during dry-process, however the difference between ash and sand tractive force can not be seen in this experiment. We consider that ash particle become weak flocculation in being flooded with water. 4. A free surface flow over the ashed generates sediment transportation and ripple form. Ash particles roll and run over the ripple crest and fall at the leeward because of separation, and continue scour and deposition toward the downstream. Ripple length and height do not depend on flow depth, but particle diameter. However ripple form over the fixed bed is distinguish from it over movable bed. 5. Movement speed of ripple changes according to particle diameter. The smaller diameter particle makes slower the speed, and same diameter particle makes faster movement accoding to shear-velocity increase.
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