Diversion headworks is a structure constructed across a river for the purpose of raising the water level in the river so that it can be diverted into the off taking canals. It is also known as canal headworks and performs the following functions.
- It raises the water level on upstream side.
- Other uses of this is to regulates the supply of water into canals.
- It controls the entry of silt into canals.
- It provides some pondage creating small pond.
- It helps in controlling the vagaries of river.
Watch the video for the detail of a Diversion Head works
Component parts of Weir/Barrage
A diversion headwork ( or weir) usually consists of the following components.
- Weir ( or barrage) proper
- Under sluice
- Divide wall
- Fish ladder
- Control head regulator
- Silt excluder, silt ejector
- River training works: guide banks, marginal bunds
- A typical layout of weir or barrage is shown in the figure
Weir or barrage
A weir is a raised concrete ( or masonary) crest wall constructed across the river width. It may be provided with a small shutter on its top. Most of the raising water (ponding) is done by solid wall and very little by shutters.
Types of weir depending upon floor design criteria
- Gravity weir
Uplift pressure is resisted entirely by weight of wall.
- Non Gravity weir
Uplift pressure is resisted by bending action of reinforced concrete floor.
Types of weir depending upon materials used
- Masonry weir with vertical drop
This type of weir consists of a masonary wall with either both u/s and d/s faces vertical or both faces inclined or only u/s face inclined. It has an impervious horizontal floor or apron. Cutoff walls are provided at both u/s and d/s ends of the floor as shown in figure.
- Rock fill weir with sloping apron
This type of weir consists of a masonry wall and dry pack boulder laid in the form of slope [i.e. 1:20 at d/s and 1:4 at u/s] with weir wall at middle and few intervening core walls. The d/s slope is generally made very flat. It requires very large quantity of stone and would be economical only where stone is available in abundance in close proximity.
- Concrete weir with d/s glacis
In this type of weir, sheet piles of sufficient depths are provided at u/s and d/s ends of the floor as shown in figure. Intermediate piles are also provided in some cases. Its design is based on modern concept of sub-soil flow, i.e. Khosla’s theory. This type of weir is constructed on pervious foundation.
If ponding of water is achieved by shutters or gates then it is called barrage. It has low crest wall with high gates.
The weir proper I constructed in the middle portion of diversion head works. At the ends under sluices are provided adjacent to the anal head regulators. A comparatively less turbulent pocket of water is created near the canal head regulator by constructing under sluice portion of the weir.The undersluices are the openings provided in the weir wall with their crest at low level. These openings are controlled by gates.
Functions of under sluices
- To accertain well maintained river channel near canal head regulator.
- To scour away silt deposited in front of heat regulator;
- To pass a portion of flood (10 to 20%) of design flood during rainy season.
- Help in impounding fair amount of flood to secure full storage.
- They are used for quick lowering the u/s high flood evel.
The divide wall is masonary or a concrete wall constructed at right angle to the axis of the weir and sepatares the weir proper from the under sluices. It extends from beyond the end of the head regulator on u/s side to loose protection of the under sluice on d/s side.
Functions of divide wall:-
- To separate the under sluice portion from weir proper portion.
- Increase the effectiveness of the under sluices portion.
- To prevent cross current and flow parallel to the weir.
- Divide wall incidentally acts as one of the side walls of the fish ladder.
- To isolate pocket u/s of head regulator to facilitate scouring operation.
Large rivers are generally inhibited by several types of fish, many of which are migratory such fish has found to be moving from u/s hill to d/s in the beginning of winter season in search of warmer water and return to their spawning ground u/s, slightly before monsoon in May and June. If no arrangement is made in weir or a dam to enable their migration their life goes in danger. So, for easy moment of the fish from u/s to d/s and again from d/s to u/s fish ladder is constructed. Typical plan of fish ladder is shown in the figure.
Canal head regulator
A canal head regulator is provided at the head of each main canal off taking from diversion headwork.
It should be so aligned that its axis makes an angle of 90o to 120o with the axis of weir as shown in the figure.
Functions of canal head regulator
- It regulates the supply of water into canal.
- It controls entry of silt into canal.
- It prevents the river flood from entering the canal.
It can be used to stop the canal supply when the silt in the river water exceeds a certain limit.
Silt excluder is a structure constructed in the bed of river, u/s of head regulator to attack the river bed water, and divert the same into the d/s of the river. Its main function is to prevent te entry of silt into the canal. A typical silt excluider is shown in the figure.
They are also called silt extractors and are provided to extract silt from canal water after the silted water has traveled a certain distance in the off taking canal. These works are therefore constructed on the bed of the canal and a little distance d/s from the head regulator. They consist of curved tunnels located across the canal which starts along the axis of the canal and turn towards the bank into the escape channel. The silted water is discharged into the d/s side of the river from the weir.