Abstract
As located in a mountainous region, the Shimanto River has little or no plains alongside both its upper and lower banks. In case of torrential rains caused by typhoons in July-September periods, extremely large quantities of water flow down to the river and the flow of 17, 000 m3/sec., which was the greatest in this country, was recorded by the heavy rain on August 29, 1935. As this water passed through narrow valley plains, the water surface was extremely raised upward. This means that its surface was raised up by 18.5 m above the average surface of 7.5 m. The figure of 18.5m may be regarded as the biggest overflow in the past 50 years or so in this country.
Kawanobori is a tiny agricultural village located 22 km upstream the Shimanto River. This village is on a natural levee formed by the Shimanto River and is seem-ingly free from the danger of overflow. In actuality, however, the natural levee was flooded 4.5 m deep on August 29, 1935, nevertheless it is located 7.5m higher than its backmarsh. From these reasons, this natural levee is apt to be misunderstood as either a terrace or a sand dune, However, we understand that this natural levee is neither a terrace, since the deposit is formed by sand mixed with fresh clay of more than 11 m in thickness, nor a sand dune from the analysis of the shape of grains of the deposit. We also understand that the natural levee with a height of 7.5 m above the backmarsh, the biggest figure to be seen in this country, implies that the level of flood along the Shimanto is exceptionally high and neither mud nor sand is supplied from the mountain sides by landslides. Although there were 45 houses in Kawanobori located on the natural levee, all of them were totally washed away by the flood of 1935.
The extraordinarily high floods of the Simanto River strongly influences the floods by the Nakasuji River located a little further downstream. As the Nakasuji River is exceptionally almost level, large quantities of water flowed into it from the main stream of the Shimanto, not from upper stream, in the case of the flood of 1935. As a result, a depth of 7-10m of water overflowed the low terrain of 2-5m high.
The fact that the level of the natural levee along the Nakasuji River goes down from down to upper stream is explained by the reverse overflow of the main stream into the tributary. The deposit of the natural levee is almost similar to that of the backmarsh and the contents are mainly silt mixed with clay. However, in 1964, the confluence with the main stream was changed about 7km downstream from the former point. In this way it was successful in lowering the water level by about 5m at the time of flood.
