对棱柱体明渠梯形断面下的恒定非均匀流沿程水面线进行深入研究，基于恒定渐变流基本微分方程，通过对该微分方程无量纲化，采用数值分析理论，得到由简单双曲函数组成的新等效恒定渐变流基本微分方程；通过变量分离，直接积分得到流程S与始、末段水深h_1, h₂的解析函数；通过该解析函数可直接计算出沿程水面线。该方法比《溢洪道设计规范》中推荐的分段求和试算法更简单、便捷，特别对水深较敏感段，规范推荐试算法误差较大，并且逐段试算推求水深将导致末端断面水深误差逐步累积，导致误差增大，精度下降。最后通过工程实例计算比较得出：新解析法计算成果与规范推荐法（程序）计算成果基本一致，对于渠长较长时新解析法成果甚至优于规范推荐的分段试算法，完全满足工程实践要求。

A new method to calculate the water surface curve along constant nonuniform flow in trapezoidal cross section of prismoidal open channel is presented. Through numerical analysis and the dimensionless basic differential equation of constant and gradually varied flow, a new equivalent basic differential equation which comprises the simple hyperbolic function is obtained. By variable separation and direct integral, the analytic functions between the flow path S and water depth h₁ and h₂ respectively in the beginning and final part are obtained, through which the water surface profile is computed. This method is simpler and more convenient than the trial-and-error procedure by partition and summation recommended in Spillway Design Standard. Especially for sections more sensitive to water depth, the error of the trial-and-error procedure is bigger owing to the accumulation of errors section by section. Computation example shows that results of this new method is generally consistent with that of the procedure in the Standard, and even more superior for long channels. This method meets the requirements of the project practices.