General Requirement for the Setting and Operation of Weirs
Properly constructed and installs weirs proved most accurate devices for measuring flow. However improper setting and operation may result in large errors inn discharge measurements with weirs.
To ensure reliable results in measurement, he following precautions necessary in the use of weirs
1. The weirs should be set at lower end of a long pool sufficiently wide and deep to five smooth flow having velocity less than 15cm/sec.
2. Baffles may be put in weir pond to reduce velocity.
3. The weir wall must be vertical [not leaning to upstream or down stream].
4. The center line of the weir should be parallel to the direction of flow.
5. The crest of weir should be level so that water passing over it will be of same
Depth at all point along the crest.
6. Notch should be of regular shape and its edge must be rigid and straight.
7. The weir crest should be above the bottom of the approach channel.
8. The crest of weir is placed high enough so that water will fall freely below weir.
9. The depth of water flow over the rectangular weir should not less than about 5 cm and not more than about 2/3 crest width.
10. The scale or gauge used for measuring the head should be located at a distance of about four times the approximate head. Zero of scale should be exactly a same level s crest level of all weirs.
Limitation in the use of weirs:
1. Weirs are not always suitable for measuring flow.
2. They are not accretes unless proper conditions are maintained
3. They require a considerable loss of head which is mostly not available in channels on flat grads.
4. Weirs are not suitable for water carrying silt
5. Weirs are not easily combined whit turnout structures.