防風垣前後の風に関する研究 (8)

林帯の幅及び遮蔽度と防風機能

抄録

It is desirable that the width of wood belts is narrow in a small farm land. But it is not clear what is the relation between the width of the wood belt and its reductive effect of wind velocity, and when we expect to prevent wind erosion in the inland, what is the relation between the number of belts in the shelter-hedges and the reductive effect of wind erosion. In order to find out these, the following researches have been made.
In the wind tunnel experiment we used hedges of 1 to 8 belts made of Japanese cypress twigs 7cm. high and 25 to 30 covering degree and investigated U′/U×100 the wind velocity ratio between in front of and behind the hedges. (U′: for the wind velocity when there are hedges. U: for that when there is none.)
In the experiment in a cultivated field, we used hedges with 1 to 6 belts made of pine twigs, a meter high and 65 to 70 covering degree and investigated V′/V×100 the wind velocity ratio. (V′: for the wind velocity at each station. V: for the standard wind velocity at a meter above the ground.)
The results of these experiments are as follows.
1. In both experiments, the wind velocity ratio just behind the hedges decreases with increasing of the number of belts, but at the distance of 15 times the hight of the hedge (X/H) the recovery of wind velocity ratio begins to grow quicker.
2. In both experiments, the more the belts of hedges, the higher gets the point of the maximum wind velocity, and the bigger the wind velocity ratio.
In proportion as the number of belts increases, the point of the maximum wind velocity gets father to the leeward of hedges in the wind tunnel experiment, but in the outdoor experiment, it was almost fixed (constant).
3. As to the relation between the number of belts and the covering degree, when a three-belted hedge with 25 to 30 covering degree and a single one with 80 to 90 covering degree are comparedd with each other, the maximum wind velocity ratio is bigger, but the recovery of wind velocity ratio on the lee is quicker in the latter.
4. As to the relation between the width (the number of belts) of the wood belts and the reductive effect of wind velociy, the wind velocity ratio remarkably decreases with increasing of the number of belts, but even if the number increases further, it does not decrease so much. And so, it seems that no bigger reductive effect of wind velocity can be expected.
5. The results of the wind tunnel experiment and that in the cultivated field are fairly in accord with each other.
6. As to the relation between the width of the wood belts and the covering degree, a single hedge with big covering degree rather than with small covering degree and with belts increased in number, gets quicker recovery of wind velocity ratio at the distance of 10 times the hight of the hedge and farther on the lee, though the decrease of wind velocity ratio is big about the hedge on the lee. But in order to prevent wind erosion, a single hedge with big covering will satisfactorily serve the purpose, the preventive function of the hedge being around to 12 times the height of the hedge on the lee.