- 著者
-
林 千夏
池田 宏
HAYASHI Chinatsu
IKEDA Hiroshi
- 出版者
- 筑波大学陸域環境研究センター
- 雑誌
- 筑波大学陸域環境研究センター報告 = Bulletin of the Terrestrial Environment Research Center,the University of Tsukuba (ISSN:13463381)
- 巻号頁・発行日
- no.1, pp.43-50, 2001-02
The Shimanto River, in the southwest of Shikoku Island, forms incised meanders. It has been suggested that gorges are formed as meanders incise bedrock during tectonic uplift, or a lowering of the baselevel of erosion. However, it is known that the formation of incised meanders is dependent on many other factors.The Shimanto River is composed of a main stream and two major tributaries. Incised meanders occur especially in the middle section of the main stream. It is clear that scarcely any lateral erosion occurs here, as the meanders have very small terraces.The study site is located in the Shimanto Belt, which is composed of interbedded sandstone and mudstone. The sandstone is resistant to erosion, but the mudstone is easily eroded and transported by the river in suspension. There is little sediment load in the river because the area has not experienced recent uplift. Also, since the sediment load is mainly mud, there is very little bedload in the river, which results in a rough, bedrock channel floor. It is proposed that the lack of bedload supplied to the channel and the rough channel floor influence the lateral erosion of the channel and lead to the formation of incised meanders.To test this hypothesis, a flume experiment was conducted using a channel with fixed meanders and varying degrees of roughness of the channel bed. It was found that the greater the roughness of the channel bed, the weaker the concentration of streampower at meanders. It is therefore concluded that little lateral erosion is produced at channel bends over a rough bed. It seems likely that incised meanders are formed in the middle section of the Shimanto River because there is little bedload in the channel, resulting in a very rough, bedrock channel floor and a reduction in the efficient streampower for cutting banks erosion at the bends. This in turn reduces lateral erosion.