Ecological Flow Research on Small and Medium-sized Rivers in Mountainous Areas with Staggered Weirs

Small and micro stepped weirs are distributed in the small and medium-sized rivers of the southern mountainous areas in China to meet the water extraction and use needs on both banks, and they affect the spatiotemporal water demand processes of the river ecosystems. Conducting research on the ecological flow process of small and medium-sized rivers in mountainous areas with multiple weirs is of great significance for the protection of such ecosystems. Taking the typical small and medium-sized river in mountainous areas, namely Fujian Jinxi River, as an example, this study used the improved wetted perimeter method to investigate the characteristics of ecological flow in small and medium-sized rivers with multiple weirs in mountainous areas, identified the main control factors, and analyzed their impact. The results indicate that: ① The main control factors affecting the calculation of ecological flow in small and medium-sized rivers with multiple weirs in mountainous areas using the improved wetted perimeter method are flow characteristic values, cross-sectional shape, and the width-to-depth ratio of the cross-section. ② The multi-year average flow is recommended for the characteristic value. ③ The cross-sectional shape is suggested to be parabolic and trapezoidal with a width-to-depth ratio less than 1 000. ④ The width-to-depth ratio of the river channel cross-section is negatively correlated with the ecological flow of the river, and there is a threshold to distinguish whether the river cross-section can meet the minimum requirements of ecological flow, which is negatively correlated with the multi-year average flow. ⑤ The decrease in ecological water demand of rivers is caused by factors such as changes in river water volume and riverbed evolution due to weir construction, which lead to increased river evaporation and calculation result deviations.