土地改変に伴う加速的侵食 : 侵食の速さと沖縄島北部の事例

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This paper is a preliminary note for the world-wide comparative studies of man-induced erosion from the viewpoint of environmental geomorphology. The “geologic norm” or the background level of the rates of natural erosion and the order of the magnitude of accelerated erosion induced by mans impact on physical environment are summarized for the whole world and Japan based on previously published data. As a result, it is tentatively concluded that the rates of natural erosion which are used as the background level may be in the order of 100-500 m3/km2-yr for mountainous regions except for high relief mountains and 10-50m3/km3·yr for hilly lands and uplands. Based on the data obtained from the measurement at the experimental plots, the rates of erosion on the farmlands of Japan are estimated as 50-500 m3/km2·yr for ordinary crops, 500-1500m3/km2·yr for orchards without mulching and 1500-2000m3/km2·yr for bare lands. These values are generally smaller compared those reported for the croplands in the United States in 1930 s when a large-scale uncontrolled monoculture such as cotton and maize predominated. It is worthy to note that in Japan well-managed and terraced puddy fields have greatly contributed to the prevention of accelerated soil loss from cultivated hillslopes since ancient times. Since 1960 s soil erosion and accompanying sedimentation due to large-scale urbanization and highway construction have caused serious problems in many countries, particularly in the United States and Japan. The rate of sediment yield from the small catchments affected by the construction works increases from 5 to 500 times and more that associated with the same terrain under cover of forest and rural land use, and often reaches on the order of 10000 m3/km2·yr (WOLMAN, 1975). Based on the data on the land use change from agriculture or forestry to urban and forestry to agriculture, extraction of mineral resources including gravels and sands and coal mining, it is estimated that the total amount of direct denudation caused by man may be about 1.8 billion m3/yr for whole Japan for the year of 1970-1972. This is equivalent to the annual removal of earth materials 4800 m3/km2·yr (4.8 mm/yr) and is seven times higher than the mean rate of erosion in mountainous regions of Japan (680m3/km2·yr). An example of man-induced erosion under humid subtropical environment (mean annual temperature 22°C, mean annual rainfall 2400-2600 mm) is reported for the northern part of Okinawa Island, Ryukyu Islands. Since 1960 s highly accelerated erosion of reddish soils derived from deep weathering of phylites and slates and Plio-Pleistocene sand and gravel deposits has occurred due to a large-scale pineapple cultivation in the hilly lands and uplands. In addition, sediment yield has greatly increased by the construction of housing lots, roads, etc. since 1970s. Observations on the bulldozer-shaped slopes on which B-horizon or reddish weathered zone expose, show that both bare land under construction and farmland cultivated with first-year pineapples in rows downslope produce a great amount of sediment, estimating 10000-50000m3/km2·yr and more. In the case of reclaimed land built on the former small valleys, filled-in soils are rapidly eroded away by large gullies which follow former stream lines and the amount of eroded materials even by a single gully often exceeds 10000m3/yr. Red-colored suspended sediment derived from such source areas has been transported to the sea within a short time, resulting in silting problems along the coral reef coast.
社団法人東京地学協会の論文

土地改変に伴う加速的侵食 : 侵食の速さと沖縄島北部の事例

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