防府市付近における尾根ぞいの凅れ谷発達について

抄録

I, We have often observed that a number of gullies tend to develop along some ridges of granitic hills in the vicinity of Hofu City, Yamaguchi prefecture, western part of Japan. The writer studied on the distribution of those gullies in specially gully-developed hill. As a result, it is recognized that most gullies lie on the north-western side of the hill where a settlement is located (Fig. 1).
2. The characters of those gullies are as follows a the width varies from 1 to more than 10 meters, the depth from 0.5 to some 7 meters, the length 20 meters and over and the cross section shows V-shaped.
3. To make clear the cause of these gullying on this area, the writer tried some observations and measurements; the measurement of slope-gradient, effective depth of soil, water content, soil texture, Atterberg's limits, etc. From the results of these observations and measurements, however, he could not obtain any distinctive difference in each character between the vegetated slope and the adjacent bare land in the same ridges (Table 1).
4. He gave an attention to a large variance, however, on the porosity of the upper layer of soil at those respective stations of observation in any ridge, and he made attempts of measurements of infiltra tion-capacity on three adjacent stations, i. e., above-mentioned two sites and on a narrow path, in two ridges. The way of mesurement was as follows: using a cylinder (emptied can) with about 10 cm in diameter, the writer inserted it carefully into soil surface as deep as 10cm and over under natural condition, poured difinite quantity of pure water into the cylinder, and timed how soon the water from the soil surface within the instrument disappeared. He repeated its procedure until a constant infiltration rate was obtained.
5. The results are shown as Fig. 2 and 3; the former shows the results on the two sites, vegetation slope and bare land; the latter on the adjacent three sites, that is, vegetation slope, bare land and path. In any case constant infiltration rates in the last duration of measurement are the highest on the vegetation areas, less higher on the bare land and the lowermost on the path of this hill. That is, on the path and bare land, surface run-off should be much greater than on the vegetation slope. It is considered, therefore, that a great deal of surface run-off on the path and bare land and rain-beat actions on those areas brought about a serious accelerated erosion (gullying action) over there.
6. Fig. 3 obviously shows that the values of the measurements on Hino area (H) are higher than on Akasaki area (A). This character is assumed to be a reflection of difference in land control manner; the Hino area with denser vegetions and smoother slopes has been a private land since the ancient time, whereas the Akasaki area with poor vegetations and rugged topography has been a common land for a long time.
7. As for the cause of gullying along the ridges, he thinks it because of most paths running on the ridges in Akasaki area. It must be easier for one to walk on the ridges sloped by 10_??_15 degrees than on the slope bottoms or in valleys both of which are covered with thick bushes and loose materials in this weathered granitic hill; accordingly, most people would have mainly walked on the ridges.
8. It seems that the gullying started more than 200 years ago.