The traditional methods for computing critical water depth of open channel with trapezoid section (h k) are trial and error method, graphic method, approximate solution method and iteration method, whose shortcomings are rough precision, large computation amount and complex resolving problem.

This paper discusses the calculation error of traditional dividing section method in complex section channel by theoretical analysis,and produces a new dividing section method which the velocity of dividing section is equal.

Flow pattern of compound section river bend;