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The following points highlight the eight main components of diversion head works. The components are: 1. Weirs 2. Scouring or Under Sluices 3. Divide Wall 4. Fish Ladder 5. Log Chute 6. Main Canal Head Regulator 7. Regulation of Head Regulator 8. Silt Control at Head Regulator.
Component # 1. Weirs:
Weir is a solid obstruction, put across the river to store water on its U/ S. The stored water is diverted to the off-taking canal. Depending upon the criterion of design, the weirs may be gravity type or non-gravity type.
The gravity weir is the weir in which uplift pressure below the weir due to seepage is fully resisted by the self-weight of the weir. In the case of non-gravity weirs the thickness of the floor is kept relatively small and uplift pressure is largely resisted by the bending action of the reinforced concrete floor.
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Depending upon the design features and available construction materials gravity weirs or simply, weirs can be further subdivided into following three categories:
1. Vertical drop weirs.
2. Rock fill weirs.
3. Concrete weirs with sloping glacis.
1. Vertical Drop Weirs:
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This weir consists of a vertical drop wall or crest wall, rectangular or nearly rectangular in shape. The weir may have gates at the crest. Pucca impervious floor is provided below the crest wall and also for substantial lengths both on U/S and downstream side. Cut off piles are also provided both at ends of U/S and D/S pucca floors.
To guard against the scouring, launching aprons are provided, both at D/S and U/S ends of the pucca floors. A graded, inverted filter is also provided on D/S side, in continuation of D/S launching apron, to relieve the uplift pressure of seeping water. This weir can be used over any type of foundation soil.
2. Rock Fill Weir:
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The body of this weir is made by the combination of masonry walls and stone boulder filling. Main weir wall is constructed in required height. A few more masonry walls, also called core walls, are constructed from masonry at suitable interval of D/S side of the main weir wall. The intervening space between masonry walls is filled by hand packed stone boulders.
Boulder packing and masonry walls are arranged in such a way that sloping glacis both on U/S and D/S are formed. D/S slope is generally made very flat. It requires a very large amount of stone boulders and as such is restricted only for the places where stone is abundantly available. Such a weir exists at Okhla, near Delhi.
3. Concrete Weir with Sloping Glacis:
The design of this weir is of recent origin, and its design is done according to Khosla’s theory. This weir consists of a sloping concrete slab. The parts of the slab slope downwards both on U/S and D/S sides. Deep cut-off piles are driven at the ends of U/S and D/S pucca aprons.
Sometimes, even intermediate cut-off piles are also provided. Hydraulic jump is formed at the D/S sloping glacis to dissipate the energy of gliding water. This weir is found suitable for soft sandy foundations. This weir is mostly used when difference in weir crest and D/S river bed is limited to about 3 m.
4. Parabolic Weirs:
It is similar to the spillway section of a dam. Weir wall is made parabolic and is designed as a low dam. A cistern is developed on the D/ S side to dissipate the energy of flowing water. U/S and D/S protection works are provided in the similar way as explained for vertical drop or sloping glacis weirs. This weir is not much in use.
Component # 2. Scouring or Under Sluices:
These are the openings in the weir. They are formed by depressing the weir. They have gates fitted in them. The scouring sluices are located on the side of the weir on which head regulation of the canal, taking off from here, is located. If two canals are taking off on either side of the river two sets of under sluices should be located one on either side of the weir.
Under Sluices perform the following functions:
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1. A clear well defined approach channel is preserved in the river just U/S of the canal head regulator.
2. Silt entry into the off taking canals is controlled.
3. When sluices remain closed for few days, silt gets accumulated in the get just U/S of the sluices. This can be easily scoured to D/S side by opening the undersluices time to time.
4. Low floods can be passed through them without weir shutters being dropped.
5. During high floods, they provide greater water way, and thus rise in the reservoir level can be restricted.
6. They help store fair amount of receding flood water.
Capacity of under sluices depends upon many factors.
However it should be fixed based upon the following considerations:
1. Twice the rated discharge of the taking off canal.
2.10 to 20% of the high flood discharge.
3. Winter floods should pass through them without the weir crest shutters being dropped.
Design of Under Sluices:
In this case, intensity of discharge per metre length, and crest level of the under sluices are different from the weir proper. Hence it is necessary to design them separately. The design of under sluices is done based on the principles of weir design.
Following considerations, however, should be made in the design:
1. Crest level of under sluices should be about 1 m below the crest level of the head regulator of the canal, if no silt excluding device is provided in the river. If silt excluding device, in form of silt excluder, has been provided in the river, the crest of head regulator should be about 2 m above the crest level of the under-sluices.
2. In the case of under sluices, the discharge is more intensive and hence more protective length on D/S side is required.
Total length of apron and Talus on D/S side (l3) should be as follows:
Where q is the intensity of discharge in cumecs/ metre.
3. Length of impervious apron on U/S side of the sluice gate, should be as follows
4. A bridge should be constructed on pillars of under sluices. Winches or cranes can move on this bridge, to lift or drop the gates.
5. D/S floor should be depressed below the D/S bed level. This depression is dependent upon the likely retrogression of the bed level.
Component # 3. Divide Wall:
It is also called groyne, or groyne wall. It is an embankment constructed in the river, U/S of the weir. Its axis is kept at right angles to the axis of the weir. The embankment is protected from all the sides with the help of stone or concrete blocks. The divide wall separates weir from under sluices. It extends a little U/ S of canal regulator and on D/S end upto loose protection of the under sluices. It may be made of concrete or masonry, with top width of 1.5 m to 3 m.
This wall should be designed for following conditions:
(i) Silt pressure upto full tank level on the face opposite to the face lying towards the head regulator and minimum possible or no water on the face lying towards the head regulator.
(ii) During high floods the water level behind the weir should be assumed about 1 m to 2 m above the level of water behind the under sluices.
(iii) Top width is taken 1.5 m to 2 m, while bottom width is found out on the basis that resultant of forces acting on divide wall life within the middle third of the base.
The main purposes of divide wall are as follows:
(i) To separate the floor of channel formed behind under-sluices from the floor level behind the weir proper. Level of channel is at lower level than that of weir proper.
(ii) The area enclosed between under sluices, divide wall and head regulator, and is known as pocket. The divide wall provides a relatively quite pocket in front of canal head regulator. This phenomenon concentrates more silt in lower layers of water entering the pocket. The lower silt laden layers are directly passed through the under sluices on D/S side and thus water entering the canal is relatively clear, having very little of silt.
(iii) Divide wall serves as one of the side walls of fish ladder.
(iv) Divide wall prevents formation of cross currents and thus avoids their erosive effect. For this purpose sometimes more than one divide walls may have to be provided.
(v) When under sluices are not worked, as in case during low water in winter, silt may get deposited behind the under sluices. This silt is easily washed to D/S side by opening under sluice gates and thus approach channel is kept clear of silt.
Component # 4. Fish Ladder:
Fish ladder is a fish pass provided along the divide wall to enable migrating fishes to move from U/S to D/S and D/S to U/S direction, in different seasons. Fish ladders are provided on all such works which hinder their movements. Fish ladder is always located along the divide wall as some water always remains here.
Fish ladder consists of a rectangular trough having sloping floor joining water levels on D/S and U/S of the weir. The difference in water level on U/S and D/S sides of weir is divided into several water steps with the help of baffle walls, constructed across the inclined floor.
The slope of the fish ladder should not be steeper than 1:10 so that velocity of flow in it does not exceed 2 m/sec. To exercise effective control on the flow through the fish ladder, grooved gates should be provided on U/S and D/S ends. All the baffle walls are provided with adequate sized holes so that water and fishes can smoothly move through them.
Component # 5. Log Chute:
It is an inclined platform joining U/S and D/S waters. It is provided to pass the wooden logs from U/S to D/S of the weir. Log chutes are located on diversion works of those rivers only which are being used to transport the timber from jungles. Cut timber is floated into the river, which carries them along the flow.
Diversion works constructed across the river obstruct the flow of logs and as such to utilize the transportation capacity of the river all the diversion works constructed across them, are provided with log chutes. All the floating logs are assembled at the mouth of log chute.
When sufficient timber logs have accumulated, the gate of the log chute is opened and all the logs are floated to D/S side one by one. In the case of log chute, some minimum depth of water on D /S side is very essential, as otherwise logs may hit the D/S floor and damage it. Log chutes may also have side walls so that logs slipping over the inclined floor may not fall at sides.
Component # 6. Main Canal Head Regulator:
It is a masonry or concrete structure, constructed at the head of the canal taking-off from the river. It is constructed U/S of the under sluices and located in one bank. Its alignment is kept at angle varying from 90° to 120° with the axis of the weir. The head regulator consists of a number of spans separated by piers and each span is fitted with a steel gate which can be moved up or down in the grooves made in the piers, with the help of either manual labour or winches. In old regulators, the spans used to be quite small, but the modern trend is to use larger spans of 8 to 18 m.
Following are the functions of a main canal head regulator:
(i) To open or close the discharge in the canal as and when required.
(ii) To check the silt entrance into the canal.
(iii) To prevent river floods entering the canal.
Design of Head Regulator:
1. The water way of head regulator should be adequate. Discharge passing through a head regulator having broad crest and sloping glacis at the D/S is found by following formula –
Q = 1.7 (L – knH) H3/2
Where Q = Discharge in cumecs
L = Water way length of regulator in metres
H = Head causing flow
n = Number of end contractions.
k = A constant, whose value depends upon the shape of the nose of the pier. Its value varies from 0.01 to 0.03.
2. The angle of alignment of head regulator with the axis of weir may be 90° to 120°. Greater angle is preferred from the point of view of smooth entrance of water.
3. Crest level of head regulator should be about 1 m above the crest level of under sluices. If some silt excluder device is to be located U/S of head regulator in the river, this difference may be increased to about 2 m.
4. Design of sloping glacis and impervious floors, is done in the similar way as for weirs. D/S cut off must be provided to keep exit gradient well within the limits.
5. Piers should extend beyond the sloping glacis so as to provide support to the cistern floor against bending.
6. To prevent high flood water, spilling into the canal, R.C.C. breast wall should be provided on the U/S side of the regulator, from reservoir level to well above the H.F.L. This wall is supported on piers and is designed by considering its own weight and the water pressure from U/S.
7. A high bridge is normally provided over the piers and winches and cranes are operated from the bridge for lifting and dropping the gates.
8. Length of D/S Talus should be about 4 to 5 times the depth of water in the canal and its thickness about 1 m. It is made of concrete or stone blocks.
Component # 7. Regulation of Head Regulator:
In order to control silt entry into the canal, following two methods of regulation of a head regular may be adopted:
1. Open flow regulation.
2. Still pond regulation.
1. Open Flow Regulation :
In this method under sluices are kept open to pass the surplus river water D/S. Bottom layers of silt laden water are lead to under sluices and passed to the D/S. The top layers of water having comparatively less silt are diverted to the canal. In this operation the pocket behind the under sluices remains clear of silt.
2. Stilling Pond Regulation:
In this case a separate pocket is formed in the river and head regulator is fixed in this pocket. Only that much water is admitted in to the pocket as is required by the canal. The excess water remains passing D/S over the weir proper. In this case water, in the pocket is almost still.
This pocket is also known as still pocket or still pond. Since velocity of flow here is very small, the silt gets deposited in it. When silt reaches the level of say about 1 m below the crest of the head regulator, the gates of regulator are dropped, and gates of under sluices opened to scour off the deposited silt D/L.
Component # 8. Silt Control at Head Regulator:
Excess silt entering into the main canal has to be prevented by all the possible means. If excess silt gets entered the main canal, it will cause silting of the whole of canal system and ultimately the capacity of the canal will be reduced.
The silt entry into the main canal from river can be controlled by following measures:
1. Divide wall in the river creates a quite pocket behind the head regulator. The quite pocket renders top layers of water silt free and only top layers of water are admitted into the canal.
2. Provide raised crest for the head regulator. This measure automatically eliminates lower silt laden layers of water.
3. Providing wide head regulator, also causes less entry of silt into the canal.
4. Entry of water into the canal should be smooth. Smooth entry does not cause any disturbance and as such less silt enters the canal.
5. By adopting still pond system of operation of the head regulator.
6. Installing silt excluders in the river, U/S of head regulator.
7. Installing silt ejectors in the head reaches of the main canal to withdraw silt laden water from the canal.
The two measures ‘still excluders’ and ‘still ejectors’ have been discussed here in details:
1. Silt Excluder:
It is a device by which silt laden bottom layers of river water are separated from the top comparatively clear layers of water. This device is located in the river bed just U/S of the head regulator. It consists of a number of rectangular tunnels, running parallel to the axis of head regulator and terminating close to under sluices.
The top level of die roof slab of the excluder tunnels is kept at the same level as the head regulator crest. The silt excluder consists of a number of tunnels each of different length. The tunnel near the head regulator is almost of the same length of the width of the head regulator, but each successive tunnel decreases in length.
The water which enters the tunnels is ultimately discharged to the D/S side by the under sluices which are kept partially open upto the level of the roofs slab on the tunnels. Usually two to three days of the under sluices are covered by the excluder. The capacity of the excluder tunnels should be about 20% of the canal discharge and minimum velocity of flow through them tunnels. 2 to 3 m/sec. Total numbers of tunnels can be worked out, once the discharge and velocity of flow are decided for the silt excluder.
2. Silt Ejector:
It is sometimes also known as silt extractor. It is a device by which the silt laden water which has already entered the main canal somehow, is extracted. This is constructed in the canal at some distance D/S of the head regulator. It consists of curved tunnels, located across the canal. Curved tunnels start along the axis of the canal, and then take turn towards a bank.
The bed of the canal, where tunnels for silt ejector are to be located, is lightly depressed. The tunnels are covered by an R.C.C. roofing slab like silt excluder. The top of the roofing slab is kept slightly above the bed level of the canal. Height of tunnels is kept about the 60 cm for sandy rivers and 1.20 m for boulder stage rivers. Velocity of flow is maintained about 3 m/sec. All the tunnels are provided with gates at the exist end.
Silt ejector is located at point where main canal is crossing some natural drainage. The water from silt ejector is discharged in to the drainage, which leads this water back to the river somewhere D/S. In order to accelerate the velocity of flow in the tunnels, sectional area of the tunnels is reduced by streamlined vanes. The radius of bend of the tunnels varies from 10 to 15 times the tunnel width.
Bottom layers of heavily silt laden water of canal, enter the tunnels and get separated from top layers of water, having comparatively less amount of silt. The silt water entering the tunnels is led out of canal and discharged into the natural drainage by opening the gates. Silt ejectors are usually designed for 20% of the canal discharge.