[HGM03-P06] Characteristics of roundness of cobble–pebble deposited on refreshed gravel bar affected to typhoon 1721 in Tama River, central Japan
Keywords:gravel, roundness, dune, Tama River, Typhoon 1721
We examined the relationship between the riverine environment and the roundness of gravels deposited on refreshed gravel bar in Tama River affected to Typhoon 1721 occurred on October 2017.
Cobble (64–128 mm in long axis) and pebble (16–32 mm in long axis) of sandstone, one of dominant lithology, were obtained from the two sites (bars) in the middle reaches of Tama River. Distance between the upper and the lower bars is ca. 600 m. At each bar, lines were set perpendicular to usual river flow direction from near the river. Approximately 50–55 gravels of each size fraction were randomly extracted from 1 m × 1 m square along the lines and the roundness based on the Krumbein roundness chart (Krumbein 1941) and the three axes of gravels were measured.
The upper gravel bar was divided into the high-water level riverbed and the usual riverbed. Near the river, i.e., on usual riverbed, cobbles (average roundness: 0.58) were rounded than pebbles (average roundness: 0.52–0.53) at both upper and lower gravel bars. While, at the upper bar, the roundness of both grain size fraction were almost similar values on the high-water level riverbed. This tendency might be that breaking and abrasion mechanisms acting on gravels are different at between those high-water level riverbed and usual riverbed. Therefore, the representing roundness of gravels measured at usual riverbed can be obtained in a gravel bar. This conclusion is reasonable to Shirai et al. (2018) examined at the middle reaches of Sagami River.
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