一、问题的提出永定河上游水土流失严重。在山西省界以上流域面积为17744平方公里的桑干河上,悬移质天然年输沙量达3686万吨,平均年侵蚀模数2077吨/平方公里。在强烈侵蚀区的源子河、恢河两支流上,年侵蚀模数可达5000～6000吨/平方公里以上。可以说,仅次于黄河中游。这样在水利工程的规划设计中,要重视泥沙对水库寿命的威胁。绝大多数的水库不得 First, the problem raised The upper reaches of Yongding River soil erosion is serious. On the Sanggan River with a catchment area of ​​17744 square kilometers above the boundary of Shanxi Province, the suspended sediment transport capacity is 36.86 million tons of natural annual sediment, with an average annual erosion modulus of 2077 tons / km 2. In the strongly eroded area of ​​the source of the river, the two streams on the Hui River, the annual erosion modulus of up to 5000 ~ 6000 tons / square kilometers or more. It can be said that after the middle reaches of the Yellow River. This planning and design of water conservancy projects, we should pay attention to the threat of sediment on the reservoir life. The vast majority of reservoirs may not