The velocity–area method for the determination of discharge in open channels consists of measurements of stream velocity, depth of flow and distance across the channel between observation verticals. The velocity is measured at one or more points in each vertical by current meter and an average velocity determined in each vertical. The discharge is derived from the sum of the product of mean velocity, depth and width between verticals. The discharge so obtained is normally used to establish a relation between water level (stage) and streamflow. Once established, this stage-discharge relation is used to derive discharge values from records of stage at the gauging station.
Not all current-meter measurements, however, are made to establish a stage-discharge relation, and individual determinations or’ spot measurements’ are very often required for management functions. Such measurements may not require the measurement of stage, but otherwise the method of measurement is the same. At...