Research On Poyang Lake's Pondage Action Under Extreme Flood Encountering

Poyang Lake is the largest freshwater lake in China. Located in south-central China, the lake is a tributary of the Yangtze River and directly exchanges and interacts with it. Study on the lake regulation law is crucial to recognize interactions of the Yangtze River flow and hydrologic processes of the Poyang Lake and the relationship of Yangtze River and Poyang Lake. In recent years extreme drought and extreme floods occurred frequently, which restrict the development of the regional society, ecology and environment. This paper used the theoretical foundation:Flood Frequency Analysis, Runoff Generation and Routing Theory, Water Balance Principle and Flood Routing Model. This paper calculated the frequency of flood season precipitation in Poyang Lake Bsain and frequency of maximum 30-day discharges in Hankou station. On the analysis of the Poyang Lake area, this paper built a semi-distributed hydrological model to simulate the runoff. After analysis on the characteristics of the pondage actions of the Poyang Lake to the flood of Poyang Lake Basin and Yangtze River, the pondages of Poyang Lake are calculated. At last this paper simulated the Poyang Lake's pondage action under extreme flood encountering. The main achievements are shown as follows:(1) The formation and water level of Poyang Lake basin's flood are controlled by both Poyang Lake and Yangtze River. The flood from Poyang Lake makes the water level rise, and the flood peak level is determined by the flood from Yangtze River. The velocity of Poyang Lake's flood subsidence is controlled by Yangtze River's flood.(2) On the basis of the characteristics of the pondage actions of the Poyang Lake to the flood of Poyang Lake Basin, the flood storage capacity and rate of storage capacity are calculated. The semi-distributed hydrological model calculated the runoff of the Poyang Lake area. The modelwas calibrated against daily observed runoff at Zifang station for the period 1982, and the data in 1985 and 1986 were used for model validation. The goodness of fit using the Nash-Sutcliffe(1970) efficiency factor(R2) was from 0.58 to 0.94, and the water balance coefficient(RE) was from 0.61 to 0.79, which illustrates the ablityof the model to simulate the hydrological behavior of the Poyang Lake area. Chosen five typical years by empirical frequency calculation, the pondage of Poyang Lake to the flood of Poyang Lake Basin is calculated:The pondage of Poyang Lake is large as the Yangtze River's flood is small, but it is limited as the Yangtze River reached peak level. The rule of Poyang Lake's pondage to Poyang Lake basin is closely related to the distribution of precipitation in the year. The strong pondage effect to Poyang Lake basin acts as Poyang Lake basin's flood season.(3) Hydrological flood rooting model for Yangtze River (Hankou station to Hukou station) was used to calculate the pondage actions of the Poyang Lake to the flood of Yangtze River. The daily water levels of Hukou station were simulated by flood rooting model. The model was calibrated against daily observed water level at Hukou station from May to September, which showed the overall simulation was attached to observed water levels, but the peak levels were less than observed peak levels. When analyzing the pondage action to the flood of Yangtze River, the lake basin of Poyang Lake was made a differential treatment. The results showed that Poyang Lake's pandage action subducted peak levels from 0.37m to 0.39m (0.37m for 1998), which relieved flood pressure for the middle and lower reach. The characteristics of pondage action are that pondage is smaller as the Yangtze River's flood is larger. The peak levels followed the order of 1998>1995>1996, and the Poyang Lake's pondage action followed the order of 1998<1995<1996.(4) Based on the hypothesis that the key factor effecting Poyang Lake's flood was the climate and human activity's effect was ignored, four extreme flood scenes were assumed. The pondage actions to Yangtze River were simulated in four scenes. The initial water level was 12.64m(1st, May,1998). Simulation time was from 1st May to 30th Sep.. The results showed that simulated peak levels in four scenes were higher than peak level in 1998. The pondage actions were less than that in 1998, which were from 0.36m to 0.37m. The Yangtze River's flood had a larger impact on the peak level in Hukou station. When large floods have occurred in Poyang Lake Basin, close attention should be payed to changes in the Yangtze River flood and flood control work should be ready in time.