Subject: Irrigation Engineering

Module: IIIDIVERSION HEADWORKS

Prepared by: Khushbu B. Bhojak

Diversion HeadworksDefinition: A diversion head-works is a structure

constructed across a river for the purpose of raising water level in the river so that it can be diverted into the off taking canals. It is also known as canal head works.

Purposes of diversion headworks: It raises the water level on its upstream side so that

commanded area can be increased. It regulates the supply of water into canals. It controls the entry of silt into the canals. It provides some storage of water for a short period. It reduces fluctuations in the level of supply in the river. There are two types of diversion headworks; Temporary: They are temporary in nature and required to be

constructed after flood. Permanent. It consists of permanent concrete (masonry)

structure. E.g Weir and Barrage

Diversion Headworks

Diversion Headworks

Location of Diversion Headworks

The location of the diversion headworks depends on the stages of flow in the river. Most of the large rivers in our country have the following four stages.

1. Rocky stage or Hilly stage (Mountainous stage)2. Boulder stage3. Trough or alluvial stage4. Delta stage

Rocky stage or Hilly stage (Mountainous stage)In this stage river is in the hills. The bed slope and velocities are high. It is generally not suitable for the location of a diversion headworks. Advantages: A sound rocky foundation is usually available at the site. Thus cost of construction of weir is less. High heads are available for hydroelectric work. Due to high velocities of flow, there are no chances of the supply channel getting silted. Disadvantages: The discharge in the river is low because of small catchment. The land in the hilly area is not suitable for agriculture The ground has a steep slope and therefore a number of canal falls are required. The weir constructed in the mountainous stage would be located away from the commanded area of the canal and therefore the idle length of the canal would be large. the water in this region does not contain silt and thus devoid of fertilizing materials. The river is usually very flashy and there is sudden rise and fall of the water level. The number of cross drainage works on the canals is usually very large.

Boulder StageFrom the rocky stage the river passes on to the boulder stage. It is also called sub-mountainous stage. In this stage the bed and banks of the river are composed of boulders and gravels. The width of river is small and the river has well-defined boundaries. The bed slope and velocity are less than those in the rocky stage. There is large subsoil flow in this region. Advantages: The length of weir is generally shorter in boulder stage. Because the banks are high, the cost of river training works is low. Construction materials such as stone, aggregates, sand, gravel are locally available. Since the ground slope is quite steep, the falls on the canals can be utilized for hydropower generation. The silt charge is less and the associated problems of excessive silt are small. Disadvantages: There is a large loss of stored water due to sub soil flow at the site of diversion headworks. Idle length of canal is more. More cross drainage works are required. In the head reaches of canal, seepage losses are high. The demand of irrigation water is low because the land is not fertile.

Trough or Alluvial stageFrom the boulder stage the river passes on to the alluvial plain created by itself. In this stage the cross section of river is made up of alluvial sand and silt. The bed slope and velocity are small. Advantages: Sub soil flow is comparatively less.The land is fertile demand of irrigation water is high. The site is near the commanded area and hence the length of the canal is small. The seepage losses are less in the canal.Less number of cross drainage works required. The water contains silt which has manurial value. Disadvantages:The river section is quite wide and hence the length of the weir structure is large. Construction material is usually not available locally. The cost of headworks is usually more due to poor foundation Because of meandering tendency more river training works are required. There is problem of silt in the canal.

Delta stageFrom the trough stage the river passes on to the delta stage as it approaches the ocean.

The delta stage is not suitable for location of a diversion headworks because the river section is excessively wide, and the river has a shifting tendency.There is no suitable compact commanded area.Since the water table is also high, there is not much need of irrigation.

Therefore the choice is usually between the locations of diversion headworks in the boulder stage and in the trough stage.

Components of Diversion headworks

1.Weir or barrage

2. Scouring sluices or Under sluices

3. Divide wall

4. fish ladder

5. Canal head regulator

6. Silt excluder

7. Marginal bunds and Guide banks

WeirThe weir is a solid construction placed across the river to raise the water level in the river and divert the water into the canal.

Based on materials of construction, design features and types of soil foundation the weir may be of different types. Such as:

1.Vertical drop weir

2. Rock fill weirs

3. Concrete weir with sloping glacis

Vertical drop weirsA vertical drop weir consists of a masonry wall with a vertical (or nearly vertical) downstream face and a horizontal concrete floor. The shutters are provided at the crest, which are dropped during flood so as to reduce afflux. The water is pounded up to the shutters during the rest of the period.

The weir floor is designed as gravity section. Immediately at the upstream end of the floor a block protection and at the downstream end a graded inverted filter are provided. Launching aprons are provided at the u/s and d/s ends of floor to safeguard against scouring action. This type of weir is suitable for hard clay foundation as well as consolidated gravel foundation, and where the drop is small.. Such weirs are obsolete now a days.

Vertical drop weirs

Rock fill weirsIt consists of a main masonry weir wall and a number of core walls. The space between the core walls is filled with the fragments of rock called rock fill. The rock fill is usually given a slope of 1 in 4 on u/s and a slope of 1 in 20 on d/s.

A rock fill weir requires a lot of rock fragments and is economical only when a huge quantity of rock fill is easily available near the weir site. It is suitable for fine sand foundation. Such weirs are more or less obsolete now a days.

Concrete weir with sloping glacisConcrete weir with sloping glacis are of relatively recent origin. The crest has glacis (sloping floors) in u/s as well as d/s. There are sheet piles (or cut off walls) driven up to the maximum scour depth at the u/s and d/s end of the concrete floor. Some times an intermediate pile is also driven at the beginning of the u/s glacis or at the end of the d/s glacis.

There is a concrete block protection in the continuation of the concrete floor both at the u/s and d/s side. On the u/s of the u/s concrete block protection as well as on the d/s of the d/s block protection, there is talus. This type of weir may be constructed in pervious foundations and are commonly adopted these days.

BarrageThe function of barrage is similar to that of weir, but the heading up of water is effected by the gates alone.

During the floods, the gates are raised to clear off the high flood level. When the flood recedes, the gates are lowered and the flow is obstructed, thus raising the water level at the u/s of the barrage.

Barrage

Under sluices The under sluices are the openings provided in the weir wall with

their crest at lower level.. These openings are provided with adjustable gates. The weir proper is constructed in the middle portion of the diversion headworks an at the ends under sluices are provided adjacent to the canal head regulators. If canal takes off only from one side, the under sluice section is provided near that end only.

There is a divide wall between the weir proper and the under sluice section to separate the two portions and to avoid cross flows.

The under sluice section is similar to the sloping weir section, but its

crest is at a lower level. Most of the dry weather flow passes through the under sluices because of the lower bed level.

Since relatively clear water is supplied to the canal, some silt gets deposited in the pocket just u/s of the head regulator near the under sluice. The deposited silt is periodically washed through the under sluices.

Under Sluice

Under sluicesFunctions of under sluices: They maintain a clear and well defined river channel in front of the head regulator. They are used to scour away the silt deposited in front of the head regulator. They pass low floods without dropping the weir crest shutters. They control the silt entry into the canal They provide greater waterway for floods, thus lowering the flood levels.

Capacity of under sluices: The under sluices should be able to pass dry weather flow and low floods for which the crest shutters on the weir proper need not e dropped. The width of pocket in front of the under sluices should be quite large to induce the settlement of the suspended silt if a silt excluder is provided in the under sluices portion. The capacity of the under sluices is usually kept at least twice the maximum discharge in the off taking canal to ensure good scouring capacity for removing the deposited silt. The crest level of the under sluices is generally kept at the average bed level of the river. The crest level of proper weir (or barrage) section is usually kept 1.0 to 1.50 m higher than the crest level of under sluices.

Components of diversion head works

Divide wallA Divide wall is a long masonry or concrete wall protected on all sides by stone or concrete blocks, constructed at right angles to the axis of the weir to separate the weir proper section and the under sluices section. If there are under sluices at both the sides, there are two divide walls.

The divide wall extends on the u/s side up to a distance little beyond the beginning of the canal head regulator and on the down stream side up to the end of the launching apron of under sluices.

Functions of divide wall:It separates the floor of the scouring sluices from that of the weir proper which is at a higher level. It provides a silt pocket in front of the canal head regulator so that silt gets deposited in it and relatively clear water enters the canal. It isolates the pocket upstream of the head regulator to facilitate scouring operation. It prevents formation of cross currents and the flow parallel to the weir axis. It provides a straight approach through the pocket and thus helps to concentrate scouring action of the under sluices for washing out the silt deposited in the pocket. It serves as one side of the fish ladder.

Fish Ladder A fish ladder is a passage provided adjacent to the divide wall, on the weir side, for the fish to travel from the upstream to the down stream of the weir and also in the reverse direction.

In north Indian rivers, the fish generally travel from the cold water in hills to warmer water in plains during winter. In the months of May and June, they move in the reverse direction in search of clear water.

It has been established that most types of fish can travel upstream against a flow velocity of about 3 to 3.5 m/s. If no fish ladder is provided at the weir site and only a gap is left, even the strongest fish will not be able to travel upstream.

In a fish ladder, the head is gradually dissipated so as to provide smooth flow at sufficient low velocity. Suitable baffles ( or Staggering devices) are provided in the fish passage to reduce the flow velocity.

Fish LadderThe plan and section of fish ladder is shown in figure: It consists of an inclined chute, (or channel) with a slope not exceeding 1 in 10. The difference of water levels on the upstream and downstream of the weir, which is usually up to 5 or 6 m, is split up in steps by means of baffle walls constructed across the chute. The compartment of bays of fish ladder should be sufficiently large so that the fish do not collide with the sides of the bay when ascending. Grooved gates are provided at the entrance and at the exit of the fish ladder for closing it. The water supply to the fish ladder should be adequate for the fish to travel at all times.

(b) Section A-A

Silt Excluder Silt excluder is a device by which

silt is excluded from water entering the canal. It is constructed in the bed in front of the head regulator at right angles to the axis of canal (or parallel to the axis of head regulator)

A silt excluder is consists of a number of rectangular tunnels resting on the floor of the under sluice pocket. The bottom of the tunnels is formed by the floor of the under sluice pocket. The top level of the roof of the tunnels is kept same as the level of the crest (or sill) of the canal head regulator.

The tunnels are of different lengths. The tunnel nearest to the crest of the canal head regulator is of the same length as the width of the canal head regulator. The length of the tunnels gradually decreases as the distance of the head regulator increases.

Silt Excluder This arrangement separates the water into two clear layers.

The top layer (above the roof of the under tunnels) enters the head regulator while the bottom layer, containing relatively heavier silt charge goes to the under sluices and discharges to the d/s of the river.

The capacity of these tunnels is kept about 20% of the canal discharge, and the tunnels are so designed that a minimum velocity of 2 to 3 m/s is maintained.

Thus the required cross sectional area can be obtained by dividing the discharge by the velocity.

Guide banks and Marginal Bunds

Guide banks: The guide banks are provided on both sides of the diversion head works in alluvial soils to guide the flow so as to confine it in a reasonable width of the river. It force the river into a restricted channel, and thus, ensuring a smooth and an almost axial flow.It controls the velocity of flow near the structure near the site and thus protects the barrage from the effect of scouring and erosion.It provides a straight approach towards the barrage.It controls the tendency of changing the course (path) of the river.

Guide banks and Marginal BundsMarginal Bunds: The marginal bunds

are earthen embankments which are constructed parallel to the river bank on one or both the banks according to the condition. The top width is generally 3 m to 4 m. The side slope on the river side is generally 1.5: 1 and that on the country side is 2:1.

It prevents the flood water or storage water from entering the surrounding area which may be submerged or may be water logged.

It retains the flood water or storage water within a specified section.

It protects the towns and villages from destruction during the heavy flood.

It protects valuable agricultural lands.

Construction of these bunds would be justified only, when the value of land saved is more than the cost of the marginal bunds.

Canal Head RegulatorA structure which is constructed at the head of the canal to regulate flow of water in the off taking canal is known as canal head regulator. It consists of a number of piers which divide the total width of the canal into a number of spans which are known as bays. They are operated by gates provided for regulation of flow into the canal. The crest level is usually kept about 1.2 to 1.5 m higher than that of the under sluices. However, if a silt excluder is provided, the crest level is further raised by at least 0.75 m. The span between the piers is usually kept between 8 to 12 m for large canal.

Canal Head Regulator

The axis of the head regulator usually makes an angle of 900 to 1200 with the axis of the weir

Canal Head Regulator The gates are operated form the top by suitable mechanical

device. A platform is provided on the top of the piers for the facility of operating the gates. Again some piers are constructed on the down stream side of the canal head to support the roadway. The maximum height of these gated openings will be equal to the difference of Pond Level and Crest Level of the regulator. But during high floods, the water level in the river would be much higher than pond level, and the flood water may spill over the gates. To prevent such spilling of flood water into the canal, an RCC breast wall between the pond and HFL, and spanning between adjacent piers is always provided. 

Gate

Canal Head Regulator

Functions of Canal Head Regulator:

It regulates the supply of water entering the canal It prevents the river-floods from entering the canal The entry of silt into the canal is controlled by keeping the crest

of the head regulator higher than the crest of the under-sluices. Silt gets deposited in the pocket, and only the clear water enters

the regulator bays. The deposited silt can be easily scoured out periodically, and removed through the under-sluice openings.