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As one of flood control measures, installation of a fixed weir is scheduled at the bifurcation point between the Asahi River and its distributary, the Hyakken River. Forestation has recently been of concern in these rivers, because it might cause a change of diverging discharge at the weir. Therefore, it is necessary to clarify the effect of vegetation on distributary for river management. This study describes flow simulations for two floods of different scales in these rivers, and demonstrates the relation between vegetation conditions, longitudinal variation of water levels and diverging discharges. Results showed that vegetation growth contributes little to increase of water levels because these rivers were mostly covered by trees whose heights were taller than water levels during these floods. Additionally, simulated water levels are approximately consistent with observations, except for situations when the Asahi River discharges were recorded at less than 2000 m 3 /s. Such discrepancy is supposedly attributable to low accuracy of the rating curve.

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非会員 岡山大学環境理工学部 環境デザイン工学科（同上）Forestation in rivers generally causes reduction of river flow capacity. Therefore, we have to manage a vegetation condition in the river properly. The object of this study is to forecast the vegetation condition with enough accuracy for effective management tasks. We had conducted a field observations of an annual vegetation growth rate, hydraulic conditions of wash out and lodging of vegetation, and bed deformation at Ohara area in the Asahi River from 2008 to 2013. Then, we developed a vegetation dynamic model based on the field observations. We simulated the vegetation condition at Ohara area in 2013 by using this model, and also examined the model by comparing the simulation results with an aerial photograph taken at Ohara area. The simulation results showed that the model can basically reproduce vegetation condition in Ohara area.