River landscape with sandbar is composed of several elements such as vegetation, bare area, side-pool and so on. In recent years, it has been reported that sandbar (vegetation in particular) has water purification function, but the quantification of purification is still unknown. The purpose of this study is to examine how the nitrogen is trapped and retained by vegetation of sandbar through numerical calculation of subsurface flow based on field observation. The numerical simulation was performed under the discharge and the topographical conditions of the Yahagi river. The main results of this study are that the vegetation area of the sandbar has higher potential of denitrification than bare area. Also, it was clearly showed that the distribution of vegetation affected nitrogen dynamics. Thus, it has become possible to propose the effective distribution of vegetation for the maximization of the water purification function by using the numerical simulation.
