Journal of the Japan Society of Erosion Control Engineering,
Vol.57,No.6,2005
Characteristics of the bedload and suspended sediment in a small torrent
- Evidence from intense field sampling for understanding sediment transport mechanism -
Fumitoshi IMAIZUMI, Toru YAMAMOTO, Satoshi TSUCHIYA, Okihiro OHSAKA
Abstract
Rapid changes in bedload and suspended transport occur in mountainous torrents because of drastic changes of flow conditions with respect to precipitations. Although understandings of bedload and suspended sediment transport in steep mountainous torrents are important for estimating rate and extent of sediment transport in channel networks, relatively few field data have been collected compared to downstream counterparts. Therefore, the main objective of this study is to demonstrate bedload and suspended sediment transport based on intensive field samplings. Then, we discussed the mechanism of bedload and suspended sediment transport in a mountainous torrent. During the selected five storm events from 2000 to 2001, we collected bedload (by a mesh basket) and suspended sediment (grab sampling) with 5 ] 15 minutes interval in the Yanagisawa River, Southern Shizuoka city. Precipitation and stream discharge were also monitored during the study period. All size classes of bedload sediment tend to show counterclockwise hystersis loop, whereas clockwise trend was predominant for suspended sediment. The shapes of suspended sediment loops differed to ones of bedload even in the same event. In counterclockwise hysteresis event of bedload, maximum rate of coarser sediment appeared sooner after the peak discharge and maximum rate of the finer sediment appeared relatively later to the peak discharge. The changes in the bedload rate of each grain size sediment suggested that the characteristics of bedload transport would be influenced by changes in grain size distribution of bed surface material. In contrast, clockwise hystersis loop of suspended sediment suggested that characteristics of suspended sediment would be affected by sediment supply conditions around the channel. Our findings will help to comprehend sediment dynamics in mountainous torrents as well as in channel networks for developing sediment control plan.
Key words:bedload, suspended sediment, mountainous torrent, Yanagisawa River, field sampling, hysteresis curve
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