Study On The Flow Properties And The Water Environment Characteristics In Poyang Lake

Poyang Lake located at the middle and lower region of Yangtze River, is the largest dynamic shallow lake in China. The Poyang Lake had a substantially varying water level, intricacy lakebed topography, and flourished wetland vegetation. Well know the laws of water flow movement, water environment evolution, hydrological relationship between the Poyang Lake and Yangtze River was the important precondition and foundation for the wetland ecological system protection and scientific utilization of resources. In this thesis, the flow properties and the water environment characteristics were studied, main contents are summarized as follows:(1) To study flow properties (when water level was high, it looked like a lake, while when water level was low, it looked like a river), the flow numerical simulation model was established on the basis of the peculiarity of Poyang Lake. In view of the complexity of space scale of computational domain for the Poyang Lake, an Adaptive Structured Grids-based Model (ASGM) was used in order to characterize different types of topography such as deep-water and shoaly land in the lake and improve computational efficiency and accuracy. Based on the hydrological water ecological monitoring system, five flow numerical simulation programmes under different conditions including water level, lacustrine facies, fluvial facies, rising flow and falling flow were drawn up. Each programme simulation lasted two days and the corresponding results mainly contained the water level, water depth, velocity, and flow at each numerical grid. The rules and effects of the water level, water depth, velocity, flow, and localized region flow field (including dished lake, Beng Lake, Songmen Mountain, northeast bays, delta front of Gan River in the south, middle and north of the estuary) were analyzed and summarized.(2) To investigate water environment characteristics, the exergy theory was applied to assess the water environment of lake ecological system. The exergy assessment model for chemical matter in water areas of large-scale dynamic changing shallow lake was established. Based on the Szargut model, the pure water was taken as the reference environment (RE), and the target of water quality protection of Poyang Lake (GB3838-2002, III level of lake water) was treated as the assessment criterion. Then the water environment characteristics of Poyang Lake were evaluated and predicted by adopting the single factor and integrated factor quality and quantity exergy, respectively. (3) Several conclusions which had a practical significance were drawn from the studies of flow properties and the water environment characteristics:1) East waterway channel may be obstructed under low water level conditions.2) The flow fields in the situation of flow rising in lacustrine facies were more complicated than those in the situations of flow falling in lacustrine facies, and both falling and rising flow in fluvial facies.3) The differences of velocity of bilateral flows around Songmen Moutain were significant in the situations of flow rising in lacustrine facies, and clockwise circulation appeared around Songmen Moutain (while counterclockwise circulation appeared and in the situation of flow falling both in lacustrine facies and fluvial faciess).4) In the situation of flow rising in lacustrine facies, the flow fields from main channel flow backward northeast bays, the velocity was small, which was adverse to diffusion and reduction of pollutants.5) In the situation of flow rising in lacustrine facies, the flow at delta front of Gan River in the south, middle and north of the estuary was jacked by the flow of Fu River and Xin River. The velocity was small, and flow direction was pointed to the estuary of Gan River, which were adverse for diffusion and reduction of pollutants.6) The quality and quantity of water environment of permanganate and ammonia nitrogen in lacustrine facies were better than those in fluvial facies, which in flow falling were worse than those in flow rising for lacustrine facies, while which in flow falling were better than those in flow rising for fluvial facies. In contrast, the quality and quantity of water environment of total phosphorus in lacustrine facies were worse than those in fluvial facies, which in flow falling were worse than those in flow rising for both lacustrine facies and fluvial facies. The quality and quantity of water environment of total nitrogen in lacustrine facies were worse than those in fluvial facies which in falling flow were better than those in rising flow for both lacustrine facies and fluvial facies. The total exergy of the lake increased as the water level increased. The water level of12m could been taken as a turning point. The total exergy increased slowly when water level did not reach12m, while it increased quickly when water level was higher than12m.7) The total phosphorus and ammonia nitrogen had always been known as the main pollutants in Poyang Lake. The research results indicated that the main pollutants in Poyang Lake were total phosphorus and total nitrogen, followed by ammonia nitrogen.8) Besides the sanitary sewage, industrial wastewater, and other regular discharge pollutants come from the Gan River, the Fu River, the Xin River, the Rao River, and Xiu River, the main reason of the water quality of Poyang Lake exceeding the III level of lake water was the pollution of surface source from the runoff after the precipitation.9) The diffusion and the distribution of pollutants were directly affected by the flow direction and flow velocity. The quality and quantity of water environment was not necessarily better when the water level and the flow became higher. The results showed that only the water area under the standard in lacustrine facies was larger than in fluvial facies when the water level and the flow raised. The total nitrogen and total phosphorus exceeded the standard at the estuary of south branch of Gan River, Lean River, and west branch of Xin River in lacustrine facies. In contrast, total nitrogen and total phosphorus exceeded the standard at the main channel in fluvial facies, which could be diffused to the water area around Songmen Mountain. Total nitrogen and total phosphorus exceeded the standard almost for all the year at Dustpan Lake because a large number of feeds and fertilizers were thrown into the Lake for the aquaculture.