A Note on the Discharge over Full-width Rectangular Sharp-crested Weirs and Weirs of Finite Crest Length

The head-discharge relationship for sharp-crested weirs is developed based on the energy consideration upstream and at the crest of the weir. The discharge for weirs of finite crest length is estimated by correlating the critical depth and total energy at the upstream of the weir. In both cases, discharge is linked with the total head. Therefore, prediction of discharge for both sharp-crested weirs and weirs of finite crest length requires an iterative solution method. The present technical note formulates the relationship between the discharge coefficients based on water and total heads to estimate the error associated with implementation of head-discharge equation based on the head. The proposed prediction curves are used to convert the iterative solution method based on the total head to a direct solution strategy based on the water depth at the upstream of sharp-crested weirs and weirs of finite crest length with either a sharped-edge or rounded entrance. Based on the similarity of velocity profiles in gate flow, it is concluded that a distance as short as 2.4 times of the water head is suitable enough to measure the upstream water depth. In sharp-crested weirs, the effect of velocity head is negligible for the approach velocity ratio smaller than 0.1. Different correction curves were developed for weirs of finite crest length based on the ratio of water head to the crest length of weirs.