An environmental flow method applied in small and medium-sized mountainous rivers

In small and medium-sized mountainous rivers, there are usually hydropower stations in upper reaches as well as widened and heightened river sections in downstream reaches that are close to settlements. The environmental flow (EF) ensures river connectivity and the survival of aquatic organisms. The Tennant and wetted perimeter methods were used to calculate the minimum EF, and the R2CROSS criteria were used to evaluate the rationality of hydraulic parameters. The result shows that downstream areas with large cross-sections may suffer from shallow water depths, insufficient wetted perimeters, and poor overall connectivity of the water bodies, even under the standard EF discharges. A novel method was proposed to ensure EF and sustain suitable hydraulic conditions. The minimum EF calculated by the Tennant method is adopted as the design flow, and a small trapezoidal trough channel is excavated on the wide riverbed of an artificial river section. The width and depth of the small channel are calculated with Manning's equation. As a study case, this method was applied in the Fenglingang River in Zhejiang Province of China. A trapezoidal groove with a depth of 0.74 m and a bottom width of 0.52 m was excavated in the center of Fenglingang River to sustain EF and maintain river connectivity. This small channel not only prevents the river from cutoff, but also enables the water depth and wetted perimeter to meet the demand of aquatic organisms.