扩散波方法被理论和实际洪水演算证明是一种既具有足够精度又相对简单的方法 ,并且广泛地应用于河道洪水演算之中。长期以来针对河道的上下边界条件均为流量或水位过程线的研究进行得较为深入 ,而对边界条件为水位流量关系的研究较为少见。基于河道下边界的水位流量关系和圣维南方程组中动力方程的联立求解 ,利用小扰动分析方法 ,导出了第三类下边界条件。利用 L aplace变换法以及数值求解 L aplace逆变换的 Crum p方法 ,得到了扩散波方程在该边界条件下的解析解。研究示例表明 ,可利用该法进行洪水演算 The diffusive wave method is proved by theory and actual flood calculation to be a method which is both accurate and relatively simple, and is widely used in flood calculation of the river course. For a long time, the study on the upper and lower boundary conditions of the waterway is the process of flow or water level is more in-depth, but the research on the relationship between the boundary conditions and water level flow is relatively rare. Based on the relationship between the water level and discharge at the lower boundary of the river and the simultaneous solution of the equations of momentum in the Saint-Venant equations, the third type of lower boundary conditions are derived using the method of small perturbation analysis. The analytical solution of the diffusion wave equation under the boundary conditions is obtained by using the L aplace transformation method and the Crum p method for solving the inverse Laplace transform. The research shows that this method can be used to calculate floods