Contents preface (VII) introduction 1—37

13
CANAL HEADWORKS
13.1. GENERAL
An irrigation channel takes its supplies from its source which can be either a river (in case of main canal) or a channel (in case of branch canals and distributaries). The structures constructed across a river source at the head of an offtaking main canal are termed “canal headworks” or “headworks”. The headworks can be either diversion headworks or storage headworks.
Diversion headworks divert the required supply from the source channel to the offtaking channel. The water level in the source channel is raised to the required level so as to divert the required supplies into the offtaking channel. The diversion headworks should be capable of regulating the supplies into the offtaking channel. If required, it should be possible to divert all the supplies (at times of keen demand and low supplies) into the offtaking channel. The headworks must have an arrangement for controlling the sediment entry into the channel offtaking from a river. By raising the water level, the need of excavation in the head reaches of the offtaking channel is reduced and the command area can be served easily by flow irrigation.
Storage headworks, besides fulfilling all the requirements of diversion headworks, store excess water when available and release it during periods when demand exceeds supplies.
Most headworks in India are diversion headworks which can be either temporary or permanent. For temporary diversion headworks, bunds are constructed every year across the source river after floods. Sometimes, temporary diversion headworks are constructed in the beginning and when the demand for irrigation has developed sufficiently, they are replaced by permanent headworks. The Upper Ganga canal was run with temporary headworks for about sixty years before the permanent headworks were constructed. In order to further increase the irrigation capacity of the Upper Ganga canal system, these permanent headworks have now been replaced by new structures. For all important headworks, only permanent headworks should be constructed.