关键词: 临界水深, 直接计算法, 梯形明渠, 单位水面宽度, 相对形状系数

Abstract: By introducing nondimensional parameterunit surface width, an iterative formula on critical water depth of open trapezoidal channels is proposed according to mathematical transformation. Combined with right initial value，direct calculation formula of critical water depth of trapezoidal section is obtained. The greatest relative error is less than 0.082% within all the sections of trapezoidal channel. Examples of error analysis and calculation show that its direct formula format is shortcut, effective and correct.

Key words: critical water depth, direct calculation method, open trapezoidal channel, unit surface width, relative form coefficient