Study of quality maintenance of fish habitats in small- and medium-sized mountain rivers with low flow rate

Abstract

Many large-scale southwestern mountain rivers have intensive hydropower development in China, and it is difficult for the mainstream habitats to meet the needs of fish; thus, the replacement of tributary habitats has become an effective measure for river ecological protection. Appropriate ecological flow is an important tool used to repair fish habitats and plays an important role in the ecological protection of tributaries, but this practice has caused conflicts with the economic interests of hydropower owners, which has made the implementation of restoration work more difficult; therefore, it is imperative to ease the contradiction between the two. Based on field investigation and monitoring, this paper designs remedial measures for the horizontal and vertical sections of the river section, namely, dredging the river horizontally and constructing a water-retaining weir and underwater dam vertically. A two-dimensional hydrodynamic model was used to numerically simulate the hydrodynamic conditions before and after different ecological restoration measures, and the corresponding weighted usable area (WUA) was calculated to evaluate the different restoration effects on habitats. The results show that the suitable ecological flow rate of the study section is 16.80 m3/s (35% average annual flow), and the WUA increased by 317.37% compared with the initial state of 2.39 m3/s (5% average annual flow); when discharge flow is 4.80 m3/s (10% average annual flow), the improvement measures of habitat quality include the combination of river dredging and the construction of water-retaining weir, construction of water-retaining weir, construction of underwater dam, and river dredging. The initial state increased by 129.96%, 177.19%, 226.51% and 329.21%, respectively. The partial excavation and dredging have a complementary effect with the construction of the water trap, and the increase in the WUA caused by the combination of the two exceeds the increase caused by the appropriate ecological flow. In summary, targeted restoration measures can achieve the goal of maintaining a high level of habitat quality at lower flow levels.