Evaluation of Froude Methods in Determination of Critical Depth in Compound Cross Sections Using Specific Energy

Document Type : Technical Note



Different approaches have been proposed for estimation of the number and the correct amount of critical depths in compound cross sections and these ideas resulted in the definition of the Froude Number. These models include unit cross section, modified unit cross section, energy method based on energy correction coefficient; momentum method based on characteristics lines and weighted Froude Number method. The researchers have tried to present a method based on the Froude number that is able to determine the points of changes in flow regimes of specific energy curve correctly. It has been stated in the previous researches that among existing methods, only momentum and energy methods have high capability in estimation of critical depths in compound cross sections, and therefore are reliable. However, an investigation of correctness of estimation of the number and the amounts of critical depths in these cross sections using all Froude Number methods in comparison to specific energy methods shows that the proposed relationships do not have the capability yet. This study shows that in non-homogenous compound cross sections with high relative roughness and also in compound cross sections with wide flood plain, the number of estimated critical depths with these two methods is more than the number of points of changes in flow regime of specific energy curve. Therefore, use of these two methods is not recommended under all conditions.