Effect of crest height on the stage-discharge formula for triangular broad-crested devices

Abstract

Flow discharge can be measured by weirs through the upstream flow depth and a stage-discharge formula. Triangular broad-crested weirs represent valid measurement devices as they are characterized by accurate measures, can convey large discharges avoiding a significant gain of the upstream water depth, and are not characterized by downstream submergence up to critical water depth. However, the flow behavior and device accuracy can be influenced by the presence of the crest height p. In this study, a theoretical stage-discharge equation, recently proposed for triangular broad-crested weirs that do not have crest height (p = 0), is initially verified by literature data obtained for the same device characterized by p > 0. This analysis demonstrated that the stage-discharge equation for p = 0 tends to underestimate the discharge measured for p/B > 0 with errors greater than the suggested accuracy limit of ±5 %. Then, a specific analysis was conducted for determine the influence of p on the scale coefficient a of the stage-discharge equation which is characterized by errors in discharge estimate lower than ±3 % for 81.8 % of cases. Finally, to determine the effect of p on the discharge estimate accuracy, the stage-discharge equation was modified introducing the dimensionless group (1-p/B) and was calibrated by the available measurements (p/B = 0, 0.28, and 0.35). The results show that including this ratio in the equation reduces the errors in the estimate of discharge, which are always less than ±3 %.