Author(s): Keisuke Yoshida; Shiro Maeno; Kaho Yamaguchi; Tomohiro Iwaki; Syunsuke Fujita; Yasutaka Hirai
Linked Author(s): Keisuke Yoshida
Keywords: Sand removal; Backfilling; Forestation; Elevation change
Abstract: In recent years, forestation has progressed on bars in the middle reaches of the Asahi River flowing through Okayama city. Forestation reduces the flood flow capacity and increases the water level during flooding. As countermeasure works to mitigate forestation, tree trimming and removal of deposited sand were conducted at the bar of the Gion area in 2005. Although the restored gravel bar was maintained for several years, reforestation was confirmed there in 2015. This background suggests the necessity of evaluating the long-term effects of construction by considering the secular change of elevation change and vegetation dynamics to maintain gravel bars. This study examines means of providing more effective control against forestation, which were assessed as a combination of two countermeasures: sand removal at the bar and sand backfilling in a low-water channel. The annual scale flood flow and the large scale flood were simulated over five years using two models. The effectiveness and persistence of the works were evaluated by comparison of results. Calculation results showed that flood waters flow easily in places where sand of the bar was removed. Results show a different trend of sediment dynamics by combinations of countermeasure works. Long-term effects of control forestation were evaluated using these results.