Environmental Impacts of High Water Turbidity of the Niulan River to Dianchi Lake Water Diversion Project

The water quality of Dianchi Lake has deteriorated in the last 50 years due to excessive human activities. The Niulan River to Dianchi Lake Water Diversion Project (NRDLWDP) was implemented in 2013 to improve the water environment by enhancing the water exchange and transport in the lake. Since the start of its operation, the water quality of the lake has been improved except in flood seasons, when the turbidity of the water pumped from the water source of the project, the Deze Reservoir, exceeded the regulatory standards. The characteristics of flow and sediment transport near the intake and its upper reaches along the Niulan River and its tributary were first analyzed based on field observations in the Deze Reservoir. The effectiveness of the check dams and multi-water-level intake on reducing the sediment concentration of the intake was evaluated by using a three-dimensional model SCHISM (semi-implicit cross-scale hydroscience integrated system model). The results revealed that the original design of placing the intake near the riverbed in order to be able to take in water at low water levels overlooked the possibility that the very fine incoming sediments and the frequently occurring density currents tended to occur near the bottom where the intake is located. The construction of check dams can encourage the deposition processes upstream and thus decrease the sediment concentration or water turbidity at the intake. However, the reduced turbidity is still too high to meet the standard of 17.1 NTU. Therefore, a multi-water-level intake should be adopted to solve the sediment problems of the NRDLWDP because it can be fully adapted to the stratified characteristics of the sediment concentration at the intake of the Deze Reservoir. With the multi-water-level intake, the turbidity of the pumped water can be substantially reduced when compared with the current approach of a single-water-level intake and the construction of check dams.