Abstract:

Leakage water from the bottom layer of a reservoir, which is low in dissolved oxygen (DO) and water temperature (WT), would cause ecological problems if flowing into the downstream channel without any recovery methods. The recovery of DO and WT in leakage water from a reservoir along the stream remains poorly understood, although studies of stratification of WT and DO in such a reservoir have been more numerous in recent years. In this study, field investigation and field experiment were carried out on the 2.25kilometerlong test section of Wenxia River, downstream of the Wenxia Reservoir. DO and WT values were measured along the test reach as well as their diurnal variation. Effects of steppool on channel reaeration abilities and influence of sandbar on the transverse distribution of DO downstream were preliminarily analyzed based on the data collected in the field experiment. Besides, macro invertebrates were sampled to evaluate aquatic ecology. Results show that flow velocity almost has no effect on the diurnal variation of DO and WT for a single spot in the test reach. Both WT and DO increased along the downstream reach of the reservoir and almost got completely recovered at the end of the test reach.WT experienced an 18.7 °C rise and was close to the air temperature at the end of the test reach; DO reached saturated state from 16.1% in saturation through a 1.7kilometerlong reach. Channel width is well correlated with the recovery of both DO and WT, however, WT is more susceptible to channel width than DO.WT experienced a growth spurt but DO did not at where the channel is broadened to a large extent. Reaeration rate had a limited variation along the test reach except at the experiment site, uncorrelated with channel width. For a steppool, the reaeration ability was weaker in the morning than in the afternoon under large discharge, which was opposite to the situations under small discharge or in plane bed. The phenomenon could be explained by different submergence of steppool under different discharges. Submergence affected turbulence intensity and thus had impact on air entrainment. DO is sectional in the downstream area of a sandbar, high at the tail but low in the two branches flowing by. Besides, the difference of DO concentration between the branches and bar tail grew with larger flow velocity difference, indicating a remarkable influence of flow velocity on DO distribution for this river pattern. Biodiversity increased with the reaeration process and a species succession, from species with high tolerance against low DO to species without the capacity, occured with higher DO concentration, implying a controlling effect of DO on the downstream aquatic ecology of the reservoir