The existing methods for calculating critical depths of open channels were summarized and their characters have been clarified. In order to identify simple, universal, accurate explicit equations for calculating the critical depth in open channels with typical crosssections, two dimensionless variables related to channel crosssection geometry and discharge were defined. Then, those explicit equations available for five sorts of open channels with typical crosssections in literature were expressed by the dimensionless variables, thus the most appropriate ones could be selected by comparing their complexity, accuracy and application range. Also, by applying the best approximations algorithm, a new explicit equation expressed as a series of piecewise power functions was developed for the openchannel with standard citygate crosssection. For the new equations, the results of error analyses indicate that all the maximum relative errors in critical depth are less than 1% in the channel normal application range. The results of the study may provide a reference for the design and hydraulic calculations of typical open channels applied in drainage and irrigation.
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