Irrigation Engineering

Canals draw supplies from rivers. Rivers do not have constant flow; the maximum being during monsoon months and minimum in winter months but the canals require continuous and dependable supplies for irrigating the command areas. For this purpose, perennial canals are essentially provided with permanent head known as headworks. Headworks comprise a weir or a barrage on the river and head regulators on the offtaking canals, the former for local pondage and raisitig of water level for diversion of supplies into the offtaking canals as well as passing regulated supplies in the river downstream while the latter is meant to draw the water supply into the canals. A weir is defined as a barrier constructed across a river to raise the water level in it course, with or without low shutters.
The functions of a barrage or a weir are (i) Divert water from river into canal, (ii) Effect pondage and raise water level in the river in the order to feed the offtaking channel during low supplies, (iii) Effect control over fluctuations in water level of the river, i.e., impart permanency to the arrangement unaffected, within working limits, by vagaries of the river, (iv) Command the irrigable area by gravity flow, (v) Regulate the supplies into the offtaking canal and the river downstream, (vi) Pass extra supply into the canal when required and prevent flooding the canal during high floods in the river, and (vii) Control silt entry into the offtaking canal.