Author(s): Taro Yamamoto, Norio Ishida, Toru Ozawa

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Keywords: Wetland restoration; Sediment retention; Reed and sedge; Alder forest;

Abstract: The Kushiro wetland is the largest wetland in Japan. The wetland area has been decreasing over the last 50 years and that was considered to be the vegetation transition from reed or sedge meadow to alder forest. The major factors of vegetation transition were assumed that sediment intrusion into the wetland had been increasing. Then project for the wetland restoration commenced in 2003 and some practical efforts have been conducted for reduction of sediment intrusion into center of the wetland. One of the most essential efforts of sediment reduction is the Sediment Retention Basin (SRB) in Kuchoro River, a tributary of Kushiro River. SRB is a part of marsh beside the river which is surrounded with low wall made of wooden boards named Artificial Kermi. In 2016, a huge flood occurred in northern Japan. Flood water spread in the SRB and large amount of sediments were deposited. Sediment accumulation observed in sediment traps set on the ground surface resulted in around 6 cm thickness of fine sand at the maximum point in SRB. The effectiveness of SRB in sediment reduction was evaluated using numerical model in comparison between pre- and post-setting SRB. The total amount of sediment passing the point of the SRB was estimated to decrease by 30 - 40% based on the numerical model results.

DOI: https://doi.org/10.3850/38WC092019-0457

Year: 2019