6. Velocity Head in Approach

As flow approaches a weir, the water surface becomes lower due to acceleration of the flow by the force of gravity (figure 5-6).

Figure 5-6 -- Weir approach flow.

The water surface is considerably lower at the weir blade than it is at 5 ft upstream. The elevation difference between the two circled points on the surface of the approach flow is called the velocity head and represents the potential required to produce the increase in velocity between the points.

A drop in water surface of 0.1 ft is common just upstream from a weir and (from the equation above) represents an increase in velocity of 0.8 ft/s. If the head on the weir is measured too close to the weir, the head measurement can be up to 0.1 ft too small. For a weir 6 ft long, with a head of 0.45 ft, a discharge of 7 ft3/s is indicated. If you measured the head too close to the weir, such that the head was reduced by 0.1 ft, a discharge of 5 ft3/s would be indicated. This difference amounts to an error of about 35 percent based on the reported discharge; and, more importantly, the water provider would be giving away 2 ft3/s.

Standard weir tables are based on the measured head of the weir (velocity head is negligible) and do not compensate for excessive velocity head. Any increase in velocity above standard conditions, therefore, will result in measuring less than the true head on the weir. Therefore, more water will be delivered than is measured. Causes of excessive velocity head include inadequate pool depth upstream from the weir, deposits in the upstream pool (figure 5-7), and poor lateral velocity distribution upstream from the weir.

Figure 5-7 -- Sediment deposits in weir pool.

Figure 5-7 shows sediment deposits, which have reduced the depth of the weir significantly and increased the velocity of the approach to well above the desirable level. Other problems exist as well; the head gage should not be located this close to the weir blade, the weeds should be removed, and the "edge" of the weir should be sharp. Discharges over this weir will be larger than indicated in "standard" tables.