The Effect of Sediment Reduction into Wetland on Kushiro Wetland Restoration Project

The Kushiro wetland is the largest wetland in Japan. However, the wetland area has been decreasing over the last 50 years, and thus, project for the wetland restoration commenced in 2003. Decrease of the wetland area 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, as a result of deforestation, farmland development and river channel straightening. It had led the wetland to the desiccation and eutrophication. Therefore, reduction of sediment intrusion was set as the objective of this project. One of the most essential efforts of sediment reduction is the Sediment Retention Basin (SRB) in Kuchoro River, a tributary of Kushiro River. It is intended to reduce sediment in the marginal area of the wetland and prevent to discharge into the center of the wetland. SRB is a part of marsh beside the river which is surrounded with low wall made of wooden boards named artificial kermi. Inundated flood water is impounded in the surrounding low wall and sediments dissolved in flood water are deposited in it. Monitoring was conducted during flooding season after setting 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. We evaluated the performance of sediment reduction in SRB with numerical modelling. The model is developed with 2D shallow water and suspended load equations. The effectiveness of SRB in sediment reduction was evaluated using this model in comparison between pre- and post-setting kermi. The total amount of sediment passing the point of the SRB was estimated to decrease by 30 - 40% based on the numerical model results. The result shows excellent performance of SRB.