Uncertainty in open-channel discharges measured with the velocity-area method

A large part of the discharge measurements conducted in open-channels are performed using the velocity-area method, which consists of sampling flow velocity and depth throughout the cross-section for discrete integration of discharge. To address the limitations of the method proposed by the ISO 748 standard, a generalized approach is introduced for computing the uncertainty associated with velocity-area discharge measurements. Direct computation methods are suggested for estimating the uncertainty components related to the vertical integration of velocity and to the transversal integration of velocity and depth. Discharge extrapolations to the edges and in the top/bottom layers are explicitly taken into account, as well as the distribution of the verticals throughout the cross-section. The new uncertainty analysis method was applied to varied stream discharge measurements, and the results are discussed and compared with the results obtained following the ISO 748 standard method. Similar results were obtained for standard measurements conducted in natural sections, while more variable and more relevant uncertainty levels were computed for less standard cases, especially in man-made canals with regular shapes and low width-to-depth ratios. The new method appears to be more versatile than the ISO 748 method, while as simple and robust. It can be easily implemented in usual discharge computation software. Some perspectives are drawn for improving the assessment of instrumental and environmental error sources, including errors due to variable discharge. Further validation tests still need to be conducted, but the method already provides interesting results, especially in terms of the contribution of the different error sources. It constitutes a useful practical tool to conduct sensitivity analysis and to plan and improve the measurement strategy. (C) 2012 Elsevier Ltd. All rights reserved.