12. Methods of Determining Mean Velocities

The following methods are used to determine mean velocities in a vertical line with a current meter:

The two-point method consists of measuring the velocity at 0.2 and then at 0.8 of the depth from the water surface and using the average of the two measurements. High accuracy is obtainable with this method, and its use is recommended. However, the method should not be used where the depth is less than 2 ft.

The six-tenths-depth method consists of measuring the velocity at 0.6 of the depth from the water surface and is generally used for shallow flows where the two-point method is not applicable. The method gives satisfactory results.

The vertical velocity-curve method consists of measuring the velocities at enough vertical positions so that the velocity profile is defined well enough to calculate a sufficiently accurate mean velocity. The method is very accurate, depending upon the number of data points measured for profile, but is time consuming and costly.

The subsurface method involves measuring the velocity near the water surface and then multiplying it by a coefficient ranging from 0.85 to 0.95, depending on the depth of water, the velocity, and the nature of the stream or canal bed. The difficulty of determining the exact coefficient limits the usefulness and accuracy of this method.

The depth or traveling integration method is performed by observing the velocity along a vertical line by slowly and uniformly lowering and raising the meter throughout the range of water depth two or more times. The method is not accurate and should be used only for comparisons or quick, rough checks.

The two-tenths, three-point, and one-point continuous methods are special procedures based on a relationshipCpreviously established for the sectionCbetween the true discharge and the velocities observed by these methods. These methods are generally reliable for sections which undergo no serious changes because of erosion, sedimentation, or other deformation. They are discussed in detail in USGS (1965) and USGS (1980). Of the methods cited in this section, the two-point method and the six-tenths-depth method are most used in canal work.