Abstract
Sediment transported by episodic floods forms flood-plain deposits and remains for much longer periods of time such as decades or hundreds of years. The amount of sediment transported by a big flood may be fairly large in comparison with the volume transported by the ordinary stream flow. The objectives of this study were to clarify the characteristics existing in the temporal and spatial distribution of flood-plain deposits. Generally, the flood-plain deposits are stepped, and even-aged forests are established on these steps, which indicate discrete movement in magnitude and frequency. The sediment volume deposited on the flood-plain is determined by the river width which is limited by the proximity of adjacent hillsides. Therefore, large storages are predominantly observed in wide sections of a riverbed, and old sediment is mainly located at these places. Although the magnitude of sediment movements which occur historically are different, deposits are gradually decayed by successive floods. Generally, the total length of flood-plain deposits decreases with advancing in age, whereas the average height increases with age; both relationships are expressed approximately by power functions. The average width of deposits does not change greatly with age. The residence time and erosion-rate of sediment were evaluated from the age-distribution of the total length, average width, and average depth of deposits. The validity of this analysis was confirmed by estimations from aerial photographs.
