地形分類を基礎とした日本と韓国の河成平野の比較研究

抄録

We have conducted a comparative research on the fluvial plain between Japan and South Koreabased on geomorphological land classification. There are distinct regional differences in thegeomorphology of Japan and South Korea even though they are adjacent to each other. The fluvialplains in Japan are depositional plains, but in South Korea they are basically erosional plains, eventhough they are thinly covered with sand and gravels.
Based on the combination of geomophological elements, the plains of Japan and South Korea areclassified as follows:
<Japan> (“On” group =basic form) Fan+Natural Levee (Back Marsh)+Delta.
(“Od” group) small fan+small natural levee+Delta no fan+no natural levee+Delta.
<Korea> (basic from) Pediment+Pen-pediment+Natural Levee (Back Marsh)+Tide land.
(R. Naktong) Pediment+Peri-pediment+Natural Levee (Back Marsh)+Delta+Tide land.
The above outlined differnces were formed by the following factors:
1) In Japan, erosion in the mountainious region is pronounced due to the steep topography andbecause of the torrential rainfall.
Sand and gravels were supplied to the valley bottom by land collapses and transported to the plainby rivers during the flood season. They were deposited at the boundary between the mountainousregion and plains, and formed fans.
In South Korea, a great deal of fine-grained material is produced because of the wide distributionof gneisses and granitic rocks which are easily weathered and by mechanical weathering in themountainous region due to the low temperatures during the winters. The debris which was formed onthe surface of the mountanious regions was washed away by torrential rainfall during the summerseason. The gravels were deposited on the gentle piedmont slopes, especially the lower parts ofpediments which were thinly covered with sand and gravels were formed as peri-pediments.
2) The coefficients of river regimes are large in the rivers of both countries. The ratio in Korea islarger than that of Japan. The longitudinal profile of rivers in Japan is steeper than that of Korea.
Due to the above mentioned features of the rivers, a great deal of sand and gravel was transportedin Japan and sand were transported in South Korea, so that natural levees are developed better in South Korea than in Japan.
3) In Japan, the deposition of sand and gravels a t the lowest reaches is remarkable. Because manyrivers pour into inland bays, many big deltas are formed in the lowest reaches.
In South Korea, the deposition of sand and gravels in the lowest reaches is difficult partly becausethe rivers pour into the open sea directly, and partly because of the very wide tidal range, for example 8.1m at the lowest reach of the Han River.
4) The differences between the two groups based on the combination of geomorphologicalelements in Japan are related to whether a river has intermontane depressions and gorges in the upperreaches or not: the “On” group has no intermontane depressions or gorges in the upper reaches, whilethe “Od” group has.
The differences in the combination of geomorphological elements in Korea are based on naturalconditions at the river mouth.