16. Weir Selection

If, after applying the concepts in chapter 4, the selection of a weir is indicated, discharge capacity and range should be considered next. Each weir has characteristics suited for particular operating conditions.

To conserve delivery head and reduce head loss, the rectangular and wide V-notch weirs rated by Kindsvater methods should be selected. These weirs provide the capability to use shorter heights from the approach channel invert to the crest. However, the downstream water surface should be sufficiently below the crest to prevent the nappe from clinging to the blade and to provide proper ventilation. If higher head through pipe outlets must be handled, then the short weir box can be used to reduce concentrated velocities and measure the flow.

For higher accuracy, a rectangular weir or triangular weirs down to a 20-degree V-notch weir should be used. Because the V-notch weir has no horizontal crest length, the head required for a small flow through it is greater than that required with the other weir types. This greater head is an advantage because nappes of smaller discharges will spring free of the crest. Nappe clinging to the crest of any type of weir makes measurement inaccurate. Although sharp-crested Cipoletti and rectangular weirs of 6-in crest length are sometimes used for measuring small flows, they are not as accurate or as sensitive as the V-notch weir for such flows and are not recommended where the V-notch weir can be used.

Cipoletti weirs have modified end contractions and have not been investigated experimentally as thoroughly as the rectangular and V­notch weirs. However, they have been usefully adopted by some water districts.

The range of flows to be measured by a weir usually can be estimated in advance. With this range in mind, other factors should be considered in selecting type and size of weir. If regulation is needed, then movable weir crests, including the overshot weir discussed in section 4, can be used.

The weir should be sized to prevent measuring at heads less than 0.2 ft to prevent surface tension effect on discharge and to keep the nappe from clinging to the crest. Also, at smaller depths, gage readings accurate enough to calculate reliable flow quantities are difficult to obtain.

Designing a weir for head as high as the allowable one-third of the crest length is not necessarily a good practice because the higher head may require a larger stilling basin and riprap protection downstream. A sill at the downstream end of the basin will help prevent bottom shear flow on the channel bed downstream from the basin. Bos (1989) has discussion of designing basins for water measurement structures and designing riprap with proper underbase material.

Chapter 4 has further information on weirs but may indicate selection of a device other than a weir.