Seite - 481 - in Book of Full Papers - Symposium Hydro Engineering

flood plains. The river restoration methods of which the discharge is controlled by a dam upstream include flexible dam operation for flood control including artificial flooding (Kondolf and Wilcock, 1996), a dam removal that has lost its functions and improvement of sediment transport through sediment supply to the dam downstream (Gaeuman, 2014). River restoration improves the river morphology, or reproduces flood plains to connect the river with the protected lower channel habitats or to increase the flood frequency of the flood plains, increasing the diversity of the habitats (Gaeuman, 2014). For a river where the sediment is trapped by a dam, sediment supply to the river upstream is the most common restoration project (Bunte, 2004). Reduction of the sediment inflow in the river by a dam construction upstream may change the geomorphic features of the river or disturb the ecosystem habitats in the dam downstream, affecting the aquatic ecosystem. Decrease of the sediment supply to a dam downstream lowers the bed elevation, changes the sediment size in the bed, separates gravel from fine sediment to form an armored layer and spreads the layer to the downstream. Sediment can be supplied to a river through various methods, and the specific implementation method is determined by following the major objectives of the sediment supply. Bunte (2004) proposed the sediment supply method of placing pebble piles in a river during a dry season before a flood event, in which the sediment supplied to a river may be immediately swept by a flood to the river downstream. Recently, various methods have been employed in South Korea to improve the river environment in dams downstream. In particular, methods of sediment supply have been prepared to improve the river environment by supplying sediment to dams downstream. In this study, a numerical simulation was conducted to investigate the efficiency of the plans as a preliminary study. The effect of sediment supply was evaluated in the Naeseongcheon Stream under discharge conditions that were dependent on the dam operation. 2. NUMERICAL MODEL This study used a physical-based morphodynamic model, Nays2DH, developed by Shimizu(Iwasaki et al., 2015). This model solves the two-dimensional depth-averaged flow equations and calculates sediment transport and sediment sorting, and geomorphic changes considering bank erosion and vegetation. This model has been applied in various rivers. A numerical model is applied to simulate flow field. The governing equations, i.e., the continuity and momentum equations, for water flow are transformed from the Cartesian coordinate system into boundary fitted coordinate system. Continuity equation: 0                               J hu ηJ hu ξJ h t ηξ (1) Momentum equations in the  and  directions: 481