The change of the relationship between rivers and lakes affected by the Three Gorges Reservoir has been a major focus in recent years. The backflows from the Yangtze River to the Poyang Lake is the main feature of the relationship, and its variation characteristics and occurrence conditions are still controversial. The hydrological data since 2003 of the Jiujiang, Balijiang and Datong hydrological station located in the Yangtze River and the Duchang, Kangshan and Hukou hydrological station located in the Poyang Lake were used. The relationship between the Yangtze River and the Poyang Lake after the operation of the Three Gorges Reservoir was analyzed from the aspects of water level, flow rate, inter-annual variation and annual distribution of runoff. The internal mechanism of the backflows from the Yangtze River to the Poyang Lake based on the basic law of energy balance was discussed and the theoretical formulas were deduced. The results showed that: 1) Poyang Lake had the characteristics of both river channel and lake. The characteristics transforms from river channel to Lake when the water level of Duchang station was above 15 m. The lake characteristics was presented when the water level of Kangshan station was above 17 m;2) The backflows from the Yangtze River to the Poyang Lake occued mainly in the flood season from July to September. In the total days of backflows, 41.6%,24.8% and 28.8% belonged to July, August and September respectively. The intensity of backflows was weakened to a certain extent by the flood control operation after the 175 m deep experimental impoundment of the Three Gorges Reservoir.3) The conditions of water level of Yangtze River were higher than that of Poyang Lake, large floods in the Yangtze River, and total inflow of the Poyang Lake receding or remaining stable were necessary for the formation of backflows. Furthermore, the magnitude and duration of the backflows mainly depended on the Yangtze River flood peak. The proposed methods and conclusions can provide a reference for further understanding the relationship between rivers and lakes and their interaction with each other,and provide scientific evidence for the comprehensive management of the Poyang Lake.

In order to slove the problem that the representative particle size method is difficult to accurately quantify the roughness of the gravel bed surfaces (GBS), the statistical theory was introduced to study the roughness properties of natural GBS. A series of loose-stacked GBS were prepared by using particles with different size and composition. Based on the analysis of the laser scanning data of the above GBS and the existing test results, the relationship between the statistical parameters of bed roughness and particle size was discussed, and the elevation variation characteristics of gravel bed profiles (GBP) and GBS was analyzed. The results showed that the frequency distribution of bed elevation had a negative skewness, the curve shape was steeper than the normal distribution, and the kurtosis Ku is greater than 3 which belongs to high narrow peak. The standard deviation σz increased with the increase of the median particle size d50, and the skewness Sk decreased with the increase of d50. In the case of the same particle size d50, the standard deviation σz and skewness Sk of unworked GBS were less than that of water-worked GBS, but there was no significant difference in kurtosis Ku. The one-dimensional structure function of GBP satisfied the variogram spherical model, whose parameters included the range, nugget and abutment value. The range showed a trend of first decrease and then increased with the increase of d50 and σz, and nugget and abutment value increase with the increase of d50 and σz. The trend couied be fitted with second order polynomial curves. The distribution pattern of two-dimensional structure function was closely related to sampling scale hx and hy. When hx, hy and d50 were equivalent, the distribution of the two-dimensional structure function was close to the circular shape and the rough GBS was isotropic, with the distribution law of the structural function being consistent with that of the water-worked GBS. With the increase of hx and hy, the complexity of the distribution of the two-dimensional structure function was enhanced obviously. The structure function values of different quadrants were different, and there was no longer the distribution law of the water-worked GBS and the rough GBS was anisotropic.