THE EFFECTS ON SUBSURFACE FLOW TO SURFACE FLOW PROFILE AND HABITATES IN FLUVIAL FAN

Abstract

In Japan, at the upstream end of a fluvial fan, the water is led from the river to irrigation and the instream flow discharge is remarkably decreased. Therefore, we have to face the problem that how much instream water is necessary at environmental point of view. In a fluvial fan river, when we focus on reach scale process, some surface flow penetrates into the sand bar, which leads to water surface drop at riffle area. As for the process in segment scale, the river water leaks to the fluvial fan in upstream area and returns to the river in downstream area. In this paper, the exchange process of surface flow and sub-surface flow is investigated, (i) between thalweg and sand bar in reach scale (ii) between instream and the fluvial fan in segment scale. Both laboratory experiments and numerical simulations are conducted. On one hand, in the experiment, the piezometric heads are measured in both the bar areas and fluvial fan area. On the other hand, 2D horizontal numerical calculation is conducted. Finally, significance to estimate the critical flow discharge that keeps instream flow continuous is shown.