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Vegetation transition and coarse sediment movement after gravel bar restoration with two meandering lanes in a steep river

Ikeda, Hirokazu, Iimura, Kosuke, Komura, Satoshi, Kawashima, Chie, Sato, Wataru
Journal of hydro-environment research 2020 v.30 pp. 25-34
Anaphalis margaritacea, Aster kantoensis, Eragrostis curvula, dams (hydrology), gravel, indigenous species, introduced plants, laboratory experimentation, plant communities, rivers, sand, unmanned aerial vehicles, vegetation
Recently, in the mid-upper reach of Japanese rivers, due to artificial impacts such as dam construction, original gravel bar environments and endemic plants have been disappearing, meanwhile alien plant communities have been expanding. As a countermeasure against these issues, gravel bar restoration project started in Kinugawa River. In the present study, vegetation transition was investigated from 2012 to 2014 on a large-gravel bar with two lane meandering in Kinugawa River, where the restoration had just been implemented. It is shown that U-shaped vegetation zone, open to the upstream side, was formed along the main channel and the second channel. This U-shaped zone was mainly composed of Poaceae. The endemic species for gravel bar, such as Aster kantoensis Kitam and Anaphalis margaritacea subsp. yedoensis, and the alien species, such as Eragrostis curvula, inhabited according to the sediment size deposited in/around the U-shaped zone. In 2015, we had a large flood. This gravel bar was greatly deformed and almost all vegetation was flushed away. A field investigation with UAV in 2018 revealed that the U-shaped zone reappeared and the endemic plant communities were widely enlarged in the similar range of relative height through these seven years. Based on the field investigation, laboratory experiments were performed which examined the effect of the U-shaped zone on flow field and coarse sediment movement. It is suggested that fine sand for Eragrostis curvula can be deposited on the internal or downstream area of the U-shaped zone, and that gravel for the endemic species can appear in the outer edge of the U-shaped zone.