It is a technique of diverting water to the canal, when a weir or barrage is constructed across a perennial river to raise the water level.
Diversion Head Works serves several objectives as follows:
- To raise the water level at the head of the canal.
- To form a storage by constructing dykes on both the banks of the river so that water is available throughout the year.
- To control the entry of silt into the canal and to control the deposition of silt at the head of the canal.
- To control the fluctuation of water level in the river during different seasons.
Components of Diversion Head Works :
1.Weir or Barrage
The Weir is an impervious barrier constructed across the river to raise water level on the upstream side, such that the water level rises upto the required height and the surplus water is allowed to flow over the weir. Weir is generally constructed across the inundation river.
The Barrage is an upgradation of Weir in which adjustable gates are installed over the weir to maintain the water surface at different levels at different times. The water level is adjusted by operating adjustable gates. These gates are operated by cables.
It is a long wall constructed at right angles to the weir or barrage. It may be constructed with stone masonry or cement concrete.
The functions of Divide Wall are as follows :
- It controls eddy current or cross current in front of the canal head.
- It provides a straight approach in front of the canal head.
- It resists the overturning effect on the weir or barrage caused by the pressure of the impounding water.
3.Scouring Sluices or Under Sluices
Scouring sluices are the openings provided at the base of the weir or barrage to allow the flow of slit deposited. These openings are provided with adjustable gates. When the gates are closed the silt gets deposited and after appreciable collection of silt, the gates are opened and the deposited silt flows towards the downstream through scouring sluices.
Due to construction of weir or barrage, the movement of fish gets obstructed and becomes detrimental. To avoid this, fish ladder are provided just by the side of divide wall for free movement of fishes. In fish ladder, the baffle walls are constructed in a zig-zag manner such that the velocity of flow within the ladder does not exceed 3m/s.
5.Canal Head Regulator
It is constructed at the head of the canal to regulate the flow of water. It consists of number of piers which divide the total width of canal into a number of spans known as bays. The piers consist of number of tiers which are adjustable gates, operated from top by suitable mechanical device.
It is found that, when the divide wall is constructed, the still water pocket is formed in front of canal head, where it is found that the lower layer of water contains heavy silt and the upper layer contains very fine silt. Since, the very fine silt is very fertile, it may enter the canal whereas the heavy silt causes sedimentation in pocket. To eliminate the suspended heavy silt, the silt excluder is provided. It consists of a series of tunnels starting from the side of the head regulator up to the divide wall.
7.Dyke or Marginal Embankment
It is an earth embankment which is constructed parallel to river bank on one or both the banks according to the condition.
a) Top width = 3 to 4 m
b) Side Slope = 1.5 : 1 (River side)
2 : 1 (Country side)
The height of embankment depends on high flood level. To resist the erosion effect, wooden piles are driven along the river banks throughout the length of dyke.
- Protects valuable agricultural land
- Protects the town and villages from heavy flood
- Retains the flood water or storage water within a specified section.
It is a structure constructed across both sides of barrage to prevent it from erosion. It is an earthen embankment with curved heads on both the ends.
a) Upstream curved head extends upto 1.5 L
b) Downstream curved head extends upto 0.25 L
c) Crest width varies from 3 to 4 m
d) Free Board – 1.5 m
e) Side slope – 1.5 : 1 (River side)
2 : 1 (Country side)
The river side slope is protected by boulder pitching and the country side by turfing.
- Protects barrage from the effect of scouring and erosion
- Provide straight approach towards the barrage
- Controls the tendency of changing the course of river
- Controls velocity of flow near the structure