Yasukazu KOSUGE, Masayuki SHOZAWA, Yuji HASEGAWA, Yoshifumi SATOFUKA and Takahisa MIZUYAMA
Abstract
It is believed that when cobbles on the bed of a mountain river are transported
downstream while repeatedly rolling, sliding and bouncing, their diameter decreases,
they become round and polished, and fine sediment is produced. This phenomenon
is important for the integrated management of sediment in a watershed, and a
reevaluation of Sabo facilities is expected to take place because of it. Kosuge
et al. (2010) clarified the crush and abrasion characteristics of mountain cobbles
and gravels ranged at about 200 mm in diameter. Here we limit ourselves to four
types of rocks|characteristic mudstone, chert, granite and andesite. We aim
to clarify1) the effect of particle diameter on the crush and abrasion of cobbles
and gravels, and2) the procedure for transforming the experimental process of
crush and abrasion with rotation to the natural process of that using the straight
experimental channel. Our findings are as follows. 1) Some riverbed cobbles
and gravels rock types have a high rate of weight reduction as the rotations
increase. Among of these the rate of weight reduction of 100|mm and 50|mm cobbles
and gravels becomes lower than that of 200|mm cobbles, and the effect of particle
size on crush and abrasion is recognized. However, types of rock for which the
rate of weight reduction of cobbles and gravels is low are mainly subject to
abrasion, and the effect of particle size is not recognized. 2) The rate of
production of silt and clay sediment measuring 0.1mm and less, which is produced
because of abrasion that accompanies an increase in rotations, shows no effect
of particle size. 3) The effect of particle size on the production rate of sand
particles measuring 0.1|2mm and cobbles measuring 2mm and over resulting from
crushing is recognized. 4) The shape and change tendency of the sediment grain|size
accumulation curve that is produced along with the increase in the number of
rotations is characteristic depending on each type of rock, and is influenced
more by the rate of weight reduction of the cobbles and gravels than by particle
size. 5) A test on the crush and abrasion of 200|mm cobbles using an experimental
channel showed the same change tendency for each type of rock when the Los Angeles
machine was used, and about the same or a lower rate of weight reduction. 6)
A test on the crush and abrasion of cobbles and gravels using an experimental
channel was more similar to the flow of cobbles and gravels in an actual river
than the test on the crush and abrasion of cobbles and gravels using the Los
Angeles machine. 7) We applied Sternberg's law and multiplied the conversion
rate by the crush and abrasion coefficient in the case of using the Los Angeles
machine, and calculated the crush and abrasion coefficient obtained from an
actual river (i.e. when an experimental channel was used) to show that the rate
of weight decrease can be estimated.
Key wordsFcrush and abrasion test, rate of weight reduction, crush and abrasion
coefficient, conversion rate, particle size effect