Study Of Environmental Flow Process In Braided Channel

Environmental flow is the basis for protecting aquatic ecology, maintaining river health and developing of water resources. Braided channel is one common type of natural channels, but we are not familiar with the ecological flow demand in this type of rivers, and the related environmental flow methods have not been well developed. In this paper, the hydraulic characteristics and habitat environment of braided channelwere analyzed. Several common calculation methods for environmental flow were extended to braided channels, and were used to determine the environmental flow process for the middle reach of Kashi River in the Ili River Basin in Xinjiang. Wetted perimeter method is an important hydraulic method. Compared with single cross section, we found the average velocity was a relatively better hydraulic evaluation indicator for the braided channel section. Taking the feasible average velocity and the minimum instream flow as objectives for ecology protection and water resources development, respectively, the environmental flow for braided channel sections were estimated using the ideal point method for multi-objective planning. The results agreed well with the results of wetted perimeter method for single cross sections. When the environmental flow in studying river reaches 15.4m~3/s, or about 13% of the average annual flow, it could maintain 82% of average velocity in braided channels sections and 73% of average wetted perimeter in single cross sections.Physical Habitat Simulation System (PHABSIM) is suitable for the assessment of flow requirement for particular species and special life stage with clear ecological significance, which is especially applicable for seasonal rivers. Based on habitat simulation of braided channels, we found that the weighted available habitat area (WUA) of some species preferring to shallow and supercritical flow increases almost monotonously with the flow, or obtained its peak value at a large flow. Accordingly, this paper proposed that we should focus on the cumulative effects of WUA in a longer period. Using WUA expectation of a particular life stage as the reference standard, we could select the appropriate environmental flows by setting conservation objectives, such as 60% of the reference value for WUA.In addition, two hydrological methods, the Tennant method and the Simplified 7Q10 method, were also used to the study area and the results of the instream flow were 11.8~47.4m~3/s and 23.5m~3/s, respectively. Results obtained from the above mentioned methods were compared and analyzed, and the environment flow process for the middle reach of Kashi River was determined, which is 82.9~125.8m~3/s for period from April to August and 23.5m~3/s for the rest months. Annual runoff corresponding to the recommended environmental process is 1.97 billion m~3, accounting for 53.6% of the average annual runoff. The results can be further used in the regulation of water resources and the protection of aquatic ecology.