|Budget Amount *help
¥7,200,000 (Direct Cost : ¥7,200,000)
Fiscal Year 1999 : ¥1,100,000 (Direct Cost : ¥1,100,000)
Fiscal Year 1998 : ¥6,100,000 (Direct Cost : ¥6,100,000)
Effect of mechanized thinning operation carried out by highly efficient forestry machine on forest stand and forest floor was studied. The thinning operation was based on dense forest road (128m/ha). Turbidity of run-off water from the concerned forest is taken as an index to study the effect.
First of all, turbidity of water at two upper and lower observation points in a stream which drain water from logged forest, indicated equivalent high value in a moment during rainfall. But average turbidity during the same rainfall period decreased by about 1/2 of the upper observation point in a short distance (about 300m) in down stream.
Water drained from logged forest showed higher turbidity value compared with the water from unlogged forest, when there was no rainfall. But, with the beginning of thinning operation in unlogged forest, difference of turbidity was not observed. In some cases, it was just reverse. Based on the precise investigation of drainage net work in logged and unlogged fore
st it can be inferred that the cause of increase in turbidity is repeated occurrence of soil inflow originated from the forest roads. Inflow soil deposits in down slope, which has low infiltration capacity. Depending on the extension of such soil deposit along the slope suspended sediment containing surface run-off water increases in the down stream. Appropriate arrangement and continuing maintenance of drainage facilities after the completion of operation for a long time is necessary to attain the enhancement of soil and water conservation function of the forest, which is one of the main purposes of thinning operation.
Particle size analysis of scattered soil, which was excavated during construction of forest road, was carried out. Scattered soil samples were collected from various sites where forest roads were constructed from 1 to 30 years ago. The scattered soil just after construction is composed of various sizes of soil particles. But with the lapse of years fine soil particles continue to flow away with run-off water during rainfall and only more coarse soil particles remain in the same slope. It is inferred that 4 to 10 years after roads construction the slope seems stable by thick vegetation cover, but fine soil particles are continuing to flow down with water for more than 30 years, specially fine soil particles less than 2.0 mm in size. In such cases if there is a stream used as water resource in the down stream of scattered soil, appropriate counter measures should be implemented to prevent the flowing down of fine soil. Less