2.9.3 Measuring Water Discharge
Generally the stream cross-section is divided into 20 sections (verticals). At each vertical the
depth and velocity are measured. The velocity is measured using a vertical-axis rotor with cups
(Price is typical), horizontal-axis rotor with vanes (Ott is typical) or a magnetic (Brien-Macnertry is
typical) velocity meter. If the depth is less than 0.8 m (2.5 ft) the velocity is measured at 0.6 the
depth below the water surface. If over 0.8 m (2.5 ft) the velocity is the average of the velocity
measured at 0.2 and 0.8 the depth below the water surface. In complex velocity distributions,
additional point velocities are taken, plotted, and the average velocity determined. The average
velocity in the vertical (one point, two points, and etc measurement) is multiplied by the sub-area
to obtain a unit discharge. The sub-area is determined by multiplying the measured depth by 0.5
times the width to the adjacent sections (Figure 2.35). Total Discharge is the sum of all the unit
discharges.
Figure 2.35. Definition sketch of computing area and discharge at a gaging station (from
Buchanan and Somers 1968b).
2.60
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