ABSTRACT
The Kushiro wetland is the largest wetland in Japan. The wetland area has been decreasing over the past 50 years, and the vegetation transitioned from reed or sedge meadow to alder forest. One of the assumed major factors of vegetation transition is the increasing sediment intrusion into the wetland. Because of this, the wetland restoration project was commenced in 2003 and some practical efforts have been conducted for reduction of sediment intrusion into the central area of the wetland. One of the most essential efforts of sediment reduction is the old channel restoration in Kayanuma district in Kushiro River. This is the re-meandering work that switched from straightened channel to old abandoned channel to enhance the river environment and to make floodwater inundate easily to reduce the amount of sediment discharge into the central wetland. The switching work was carried out in 2010. After this work was completed, inundation occurred around the channel during annual floods, especially in the huge flood that caused a large amount of sediment accumulation in 2016. The amount of overflowed water and sediment was estimated by field observation. We developed the 2D numerical simulation model of shallow water flow and suspended sediment load transportation and diffusion. The results showed that the sediment inflow to the wetland was reduced by approximately 40% after re-meandering work.
