Author(s): Norio Tanaka, Kosuke Matsuo
Linked Author(s): Norio Tanaka
Keywords: Two-layered model, sediment budget, improvement of stream ecology, reduction in friction velocity
Abstract: Sediment supply downstream of a dam by dredging sediment in a reservoir and placing the sediment in a downstream flood basin of the river is recently tested for solving a riverbed environmental problem caused by armoring. The objective of this study is to clarify the dynamics of artificially supplied sand quantitatively in armored bed by proposing a new model for calculating a reduced bed shear stress in the armored, boulder or stone (BS) layer. Quasi-three dimensional (Q3D) model has been developed considering the force balance, i. e. , sheltering effect by boulders and reduction of bed shear stress. The flow is divided into two layers vertically and the velocity in the armored layer is solved by continuity equation and momentum equations in between the two layers. Both the two dimensional (2D) depth averaged flow model and Q3D model were applied downstream of the Futase dam, Japan. For verifying the effectiveness of the Q3D model, middle class flood event in September 2009 was selected because the gravels or stones with different color were set in the three locations before the flood event. The threshold values of gravel/stone movement and the percentage of movement in different gravel/stone were investigated. Simulated results indicated that friction velocity derived in 2D model exceeds the threshold value of movement of the stones in most of the river grids. However, Q3D model showed similar or smaller values of bed friction in most of the river. After the verification, the difference of 2D model and the Q3D model were compared in the flood events in October 2006. The Q3D model can express the sedimentation at the inner side of meandering part of the river well. On the contrary, 2D model cannot express the bed aggradation that actually occurred
Year: 2017