3. Approach Flow

Poor flow conditions in the area just upstream from the measuring device can cause large discharge indication errors. In general, the approaching flow should be tranquil. Tranquil flow is defined as fully developed flow in long, straight channels with mild slopes, free of close curves, projections, and waves. Venturi meters require 10 diameters of straight pipe approach. By analogy, open channel flow would require 40 hydraulic radii of straight, unobstructed approach.

A good example of practical approach criteria taken from Bos (1989) follows:

Deviation from a normal transverse or vertical flow distribution, or the presence of water­surface boils, eddies, or local fast currents, is reason to suspect the accuracy of the measuring device. Errors of 20 percent are common, and errors as large as 50 percent or more may occur if the approach flow conditions are very poor. Sand or gravel bars, weed growths, or slumped riprap obstructions along the banks or in the flow area can cause nonsymmetrical approach flow. Inadequate distance downstream from a drop, check, or slide gate will concentrate flow locally and cause error. A bend or angle in the channel just upstream from the measuring device or a rapid expansion in the flow section can cause secondary flow or large eddies, which tend to concentrate the flow in part of a cross section.

Figure 5-1 shows an example of a poor flow distribution in the approach to a weir. The high­velocity, turbulent stream is approaching the weir at a considerable angle. The high­velocity approach flow and the waves on the surface hinder head measurement. With this poor approach flow, the weir will not produce the same head-discharge relationship as its standard equation and calibration table.

Figure 5-1 -- Example of poor approach flow conditions upstream from weir.

Standard weir proportions for rectangular, Cipoletti, and 90-degree V­notch weirs are shown on figure 5-2. The approach velocity toward weirs should be less than 0.5 foot per second (ft/s). This velocity value is equivalent to a head error of 0.005 ft. Velocity of approach can be estimated by dividing the maximum discharge by the area at a point 4 to 6 measuring heads upstream from the blade. Excess approach velocity is commonly caused by violating the criteria specified in chapter 7.

Figure 5-2 -- Weir proportions.