Reducing the uncertainty assessment factors in discharge measurement by using the velocity-area method based on ISO standards

The velocity-area method is commonly used to measure discharge in small rivers, typically using wading techniques with acoustic or propeller current meters. However, the accuracy of this method is influenced by several factors, including the number of verticals, the number of measurement points per vertical, measurement duration, and depth. ISO 748 (ISO, 2021) and ISO 1088 (ISO, 2007) offer guidelines for applying the velocity-area method and estimating discharge uncertainty based on these factors. While widely used, the uncertainty values provided in these standards were derived from field measurements conducted in the 1960s using propeller current meters in uncontrolled natural river conditions. These values have since been applied globally without further validation. This study aims to re-evaluate the errors associated with key measurement factors outlined in ISO 748 and 1088 through a controlled, real-scale experiment. The experiment was conducted at the KICT River Experiment Center in a stable, gentle-slope channel (A2) where flow conditions could be precisely managed. Discharge measurements were taken using micro-acoustic Doppler velocimeters (ADV) and total stations to assess the reliability of existing ISO-based uncertainty estimations under controlled conditions.