Precipitation washout is a common and effective measure to restore the storage capacity of reservoir. In order to study erosion effects under the condition of different boundary conditions, control water level and flood duration in later sediment retaining period of Xiaolangdi Reservoir flood duration, four groups of precipitation washout tests were conducted. The paper analyzes reservoir erosion effects of the different group. In this paper, the result show that, with more discharge and sediment, lower water level, longer duration and higher deposition surface, the more scour depth and better effect will be obtained. Based of experiment data, the relationship between output sediment discharge rate and input discharge, sediment concentration and water gradient is obtained and verified by measured data of Sanmenxia Reservoir, and the relationship between cumulative erosion and input water and sediment amount and water level before dam are obtained. INTRODUCTION Research of operational mode of Xiaolangdi Reservoir for flood control and sedimentation reduction during later sediment retaining period refer "multi-year sediment regulation and man-made precipitation washout at right occasion". Man-made precipitation washout at right occasion refers to under the favorable reservoir boundary condition, when suffer a large flood process, decrease reservoir water level, scour reservoir sediment, recover storage capacity, prolong the reservoir sediment storage life. At present, the operation of Xiaolangdi reservoir has entered the late sediment retaining period, recovery capacity is an important content. This paper studies the influence of boundary conditions, water and sand conditions, control water level and other factors on the scouring effect, and puts forward the relationship between sediment transport rate and the influence factors.

According to the unique hydrological characteristics of "a flood in high water level, a line in low water level", Poyang lake is partitioned. Based on the hydrological datas, 2003, 2005 and 2010 are selected as typical years of dry year, average year and wet year respectively. The model of the pollutant degradation ability is selected to calculate the degradation coefficient during hydrological periods in typical years. The results show that the degradation ability of pollutants is greater in high water period than in normal water period. There was no obvious rule in the degradation ability because water level and flow velocity are random during low water period in different hydrological year. The degradation ability of river into the lake area is greater than in the north lake district during high and normal water period. The south lake district is the least. The degradation of pollutants depends on hydrological conditions. Physical degradation is main generally.