In this article we will discuss about the functions and location for canal headwork.
Functions of Canal Headwork:
A canal headwork is an assemblage of various hydraulic structures. They are constructed at the head of a canal at the point of take-off.
The object of a canal headwork is to divert clear water in the canal for irrigation purpose as per requirements. Hence it is called diversion headwork also. The constituent hydraulic structures are designed and constructed in such a way that the required amount of irrigation water is always assured.
To achieve this condition following steps are taken:
(i) A Barrier is Constructed Across the River or a Parent Channel:
It obstructs the flow and diverts it into the canal. It raises the water level at the obstruction. Thus gravity flow can be maintained. It reduces uncertainty of supply by storing the water upto a fixed level all the time. The velocity of stored water is so low that only clean water is diverted in the canal.
(ii) A Regulatory Structure is Constructed at the Entrance of the Canal:
It allows only required quantity of irrigation water to enter into the canal. It prevents silt entry into the canal.
Location or Selection of Site for Canal Headwork:
When a canal is to take-off from a river it is essential to see which reach of the river is suitable for the purpose.
The hilly reach is not suitable for locating the headwork. If located the project becomes costly because in hilly regions cultivated land is not much. Secondly many falls may come across the canal. Thirdly many drainages have to be crossed.
Headwork may be located in sub-mountainous stage.
Points which favour construction of headwork in this stage are the following:
1. The material required for construction is locally available.
2. As the section of a river is narrow magnitude of headwork is small. Extensive river training is not required.
3. In initial stages temporary bund may be constructed for use with locally available material.
The following points are in opposition:
1. In this stage the soil is pervious. It increases losses.
2. Number of cross drainage works to be constructed is more.
3. Demand for irrigation may not be justifiable.
The river plain may also be considered for construction of the headwork. The points which favour the location are the following:
1. Percolation losses in this region are not excessive.
2. Demand for water is sufficiently large.
3. Number of cross drainage works to be constructed is less.
Following Points are in Opposition:
1. Construction material may not be locally available.
2. As the river is broad extensive river training is required.
3. First cost and the cost of maintenance are high.
Delta or tidal reach is also not suitable for locating headwork. The obvious reason is, the surrounding area is not large enough to warrant introduction of costly irrigation project.
Thus the choice of a reach is limited to sub-mountainous and trough (river plain) stages. It is essential to study the merits and demerits of each stage carefully before locating headwork.
Once the reach is selected for locating the headwork next step is to select actual site in that reach.
The site for headwork should be selected after giving proper consideration to the following points:
(a) Construction material should be available in large quantities in close proximity of the site.
(b) The river section should be narrow, straight with high non-submersible banks. It reduces the cost of training works.
(c) Site should be easily accessible.
(d) To reduce the cost of construction of a canal system the site should be close of the culturable area.
(e) Number of cross drainage works required should be minimum.
(f) With moderate height of the obstruction good command should be obtainable.
(g) The canal take-off should be at right angles to the river.
(h) The river reach should have central flow without tendency of scouring or silting.
(i) The canal should give maximum command.
(j) Good foundation should be available at moderate depth.