THE VARIATION OF CRITICAL FLOOD INFLOW IN DONGTING LAKE AND ITS INFLUENCE ON FLOOD CONTROL

In recent years, the phenomena of small flood discharge, high water appear frequently in the middle reach of the Yangtze River. The variation of flood regulation capacity of the Dongting Lake is widely concerned. The present study uses the recorded flood information to investigate the characteristics of the change of the flood regulation capacity of the Dongting Lake. By analyzing conflux and flood regulation characteristics of the Dongting Lake in different times over the year, it is found that the flood regulation capacity in high water level period is most important to flood control. The critical flood inflow is defined , which is the minimum of flood inflows whose corresponding flood regulation capacities are all above zero. The flood regulation processes in middle and high water level period may then be separated generally. In the present paper, the relationship between flood regulation and its corresponding flood inflows in different years is analyzed and the one-day information is shown in Fig. 4. The investigation shows that the water inflow at which the Dongting Lake begins to play its flood regulation function has in average decreased by 4 300 m3/s from the 1950's to 1980's. In a year, one-day flood regulation capacity increases with flood inflow and the increasing rate of the flood regulation capacity is more rapidly than that of the flood inflow. With time lapsing, the increasing rate of one-day flood regulation capacity increases rapidly with the flood inflow of the Dongting Lake years by years. Compare the increment of the flood regulation capacity between 1980's and 1950's,if the flood inflow increases 1 × 108 m3 then 1980's will be more than 1950's by 0. 67× 108m3. This is equavalent to 1. 6 cm increase of the water level elevation. The factors responsible for the variation of the flood regulation capacity is also discussed. The serious sediment deposition in the down main reach and the flood area decreasing of the Dongting Lake are the two main reasons for flood level elevation in the area of the Dongting Lake. The variation of the water area of the Dongting Lake is given in Table 2. The ratio of the water level elevation between 1950's and 1980's due to the deposition in down stream main reach and the decrement of the flood area is 4 to 1. Although the flood regulation capacity is increased, it is the result of the continuous sediment deposition in down main reach, and it is at the cost of elevating flood level, worsening the flood control situation in the area of Jingjiang River and Dongting Lake. The most likely efficient ways to handle these problems are to manage and reduce the deposition in down reach and enlarge flood area of the Dongting Lake.