Comment on “Direct solution for discharge in circular free overfall by Z. Ahmad, H. Md. Azamathulla” J. Hydrol., in press. doi: http://dx.doi.org/10.1016/j.jhydrol.2012.04.025

Recently, it is proposed [by Ahmad and Azamathulla (2012)] a closed form equation for discharge in terms of end depth in subcritical flows by simulation of the free overfall in a circular channel with a sharp-crested weir. As shown in this discussion, the theoretical discharge relationship can be accurately integrated with the aid of popular mathematical software and series expansion method is not required. According to this study, the theoretical end-depth ratio (EDR) is almost constant with an average value of 0.766 over the entire practical range. Using the numerical results which are accurate to six significant digits, this discussion shows that the proposed power equation by the authors for theoretical discharge in subcritical flow regime is not very accurate (the errors increase up to 12%), thus it is proposed a new accurate equation (with maximum error less than 0.35%) for prediction of theoretical discharge in circular channel with a single measurement of end flow depth.