1989 Fiscal Year Final Research Report Summary
Field and flume studies of heterogeneous sediment transport on an upper-flow regime bed
| Project/Area Number |
62460235
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| Research Category |
Grant-in-Aid for General Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Research Field |
自然地理学
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| Research Institution | The University of Tsukuba |
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Principal Investigator |
TAKAYAMA Shigemi Univ. Tsukuba, Inst. Geoscience, Prof., 地球科学系, 教授 (20062797)
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| Co-Investigator(Kenkyū-buntansha) |
ISEYA Fujiko Univ. Tsukuba, Inst. Geoscience, Research Associate, 地球科学系, 助手 (00193384)
MAITA Hideji Univ. Tsukuba, Inst. Agricultural and Forest Engineering, Assistant Prof., 農林工学系, 講師 (50015864)
IKEDA Hiroshi Univ. Tsukuba, Inst. Geoscience, Associate Prof., 地球科学系, 助教授 (20015986)
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| Project Period (FY) |
1987 – 1989
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| Keywords | Heterogeneous Sediment / Gravel-bed River / Sediment Transport / Torrential Stream / Flume Experiment / Sedimentary Structure / Sediment Sorting / Upper-flow-regime |
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| Research Abstract |
The Higashi-Gochi is a small torrential grave-bed tributary. The catchment got an intense rainstorm in 1982. A 900-mm- rainfall released abundant sediment. In torrential streams of headwaters regions, deposition or erosion depends mostly on the sediment supply, not on the magnitude of flow discharge. This project describes the 1982 Higashi-Gochi deposits and uses three different flumes to discuss the depositional mechanism of heterogeneous sediment at high aggrading rates and at an extremely high boundary shear stress.
Transverse and longitudinal trenchings of one of 1982 terraces schematized the sedimentary structure into two basic units. The lower half was massive, in which boulders, sands, and mud accumulated with few internal structures. In the upper half, a matrix-supported gravel layer and an open-work gravel layer repeated ultimately. Both the thickness and the grain size of each layer became smaller toward the top. Some lenses of cobble gravels, a few grains thickness, also char
… More
acterized the tipper half.
Precise mapping of micro stratigraphy on the terrace surface and many pictures at the study reach that were taken only one day after the rainstorm suggested that the deposits were caused by fluvial processes, never by debris flows as usually expected in those torrents with steep slopes and large sediment supply. A flume experiment which modeled the flow of the Higashi-Gochi successfully created the deposits similar to those of the 1982 deposits. Next three would be critical mechanisms. 1) Where and when does the bed aggrade? : Deposition occurred only when the bed was covered with excess sediment supply or the thalweg that had extremely high concentration of bedload. The flow never covered the entire floor bottom at once, and the thalweg braided and wandered. As the local bed aggraded, the thalweg moved laterally to lower bed surface and thus raised there then moved continually. 2) Grain sorting between coarse and small: Large grains never stop within an intense bedload layer. As the bed aggrades localy, smaller grains thus selectively deposit. Imagine a margin where the thlaweg began to occupy a local bed, large grains that traveled quickly from the upstream bedload layer deposited first, and then covered with smaller grains. 3) Longitudinal time lag for deposition: The lower half sequence of the deposits would be produced at the initiation of sediment supply from the upstream, when bedload tongues gradually filled the stream bed. Less
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