3. Learning Objectives
1. Types of Diversion Head Works
2. Weirs and Barrages
3. Layout Diversion Head Works
4. Causes of Failures of Weirs and Barrages
on Permeable Foundations
5. Silt Ejectors and Silt Excluders
4. Types of Diversion Headworks
Any hydraulic structure which supplies water to the off-taking canal is called
a Headwork
Types:
1. Storage Headwork A Storage headwork comprises the construction
of a dam on the river. It stores water during the period of excess supplies
and releases it when demand overtakes available supplies.
2. Diversion Headwork to divert required supply to canal from the river
a) Temporary Spurs or bunds which are temporary and constructed
every year after floods
b) Permanent Weirs and Barrages
5. Diversion Headwork
Diversion headworks are generally constructed on the perennial rivers
which have adequate flow throughout the year and, therefore, there is no
necessity of creating a storage reservoir.
Purpose:
1. It raises the water level in the river so that the command area can be
increased
2. It regulates the intake of water into the canal
3. It controls the silt entry into the canal
4. It reduces fluctuations in the level of supply in river
5. It stores water over small periods of short supplies
6. Location of Headworks
1. The torrential, rocky or mountainous stage
Very steep bed slope and high velocity
2. The sub-mountainous or boulder stage
Strong sub-soil flow & more percolation losses
3. Trough stage or alluvial plain
Most suitable – bed slope is small and velocity is gentle
4. Delta stage
7. Selection of Site for Diversion Head Works
At the site, the river should be straight and narrow
The river banks should be well defined.
The valuable land should not be submerged when the weir or barrage is
constructed.
The elevation of the site should be much higher than the area to be
irrigated.
The site should be easily accessible by roads or railways.
The materials of construction should be available in vicinity of the site.
The site should not be far away from the command area of the project,
to avoid transmission loss.
8. Component parts of a Diversion Headwork
1. Weir or Barrage
2. Divide wall or Divide Groyne
3. Fish Ladder
4. Pocket or Approach Channel
5. Scouring Sluices
6. Silt Prevention Devices
7. Canal Head Regulator
8. River Training Works (Marginal Bunds and Guide Banks)
9.
10. Weir
The weir is a hydraulic structure constructed across the river to
raise its water level and divert the water into the canal
If a weir also stores water for tiding over small periods of short
supplies, it is called as storage weir.
The main difference between a storage weir and dam is only in
height and duration for which supply is stored
11. Types of Weir
Classification based on design of floors
1. Gravity Weir uplift pressure due to seepage of water
below the floor is resisted entirely by the weight of floor
a) Vertical Drop Weir
b) Sloping Weir
i. Masonry or Concrete Slope Weir
ii. Dry Stone Slope Weir
c) Parabolic Weir
2. Non-Gravity Weir Thickness of floor is less. Uplift
pressure is largely resisted by RCC floor
12.
13.
14. Barrage
Similar to weir, but the heading up of water is effected by
the gates alone.
No solid structure across river
The crest level is kept at a low level
Flood flow is controlled by gates
Less silting and better control over the water levels.
Costlier than the weirs
18. Divide Wall or Divide Groyne
The divide wall is a masonry or concrete wall constructed at right angle to the
axis of the weir.
The divide wall extends on the upstream side beyond the beginning of the canal
head regulator; and on the downstream side, it extends up to the end of the
loose protection of the under-sluices.
Functions:
It separates the ‘under-sluices’ with lower crest level from the ‘weir proper’ with
higher crest level.
It helps in providing a comparatively less turbulent pocket near the canal head
regulator, resulting in deposition of silt in this pocket and, thus, to help in the
entry of silt-free water into the canal.
19.
20. Fish Ladder
In big rivers, fish will always move up and down respectively in search of clear and
warm water. It is, therefore, essential for the provision of some space in the
construction of weir.
It is usually located between the weir and divide wall.
It is suited near the divide wall as there is always some water in the river section
below the scour sluices
It consists of an inclined trough with baffles with holes provided in them.
The baffles reduce the velocity and provide compartments for the fish to rest.
To have effective control, grooved gates are provided at the extreme upstream
and downstream end walls.
The fish ladder should be designed to have a velocity of 3 m/s or less in the
trough.
21.
22. Scouring Sluices
They maintain a deep channel in front of head sluice and dispose off heavy
silt and a part of flood discharge on the downstream side of the barrage or
weir.
Functions:
To preserve a clear and defined river channel approaching the regulator.
To control the silt entry in to the canal.
To scour the silt deposited in the river bed above the approach channel.
To help in passing low floods without dropping the shutters of main weir.
To provide additional waterway for floods, thus lowering the flood levels.
23. Silt Prevention Devices
The entry of silt into a canal, which takes off from a head works, can be reduced by
constructed certain special works, called silt control works.
Types:
1. Silt Excluders
Silt excluders are constructed on the bed of the river upstream of the head regulator.
The clearer water enters the head regulator and silted water enters the silt excluder.
In this type of works, the silt is removed from the water before in enters the canal
2. Silt Ejectors
Silt ejectors, also called silt extractors, are those devices which extract the silt from
the canal water after the silted water has traveled a certain distance in the off-take
canal. These works are, therefore, constructed on the bed of the canal, and little
distance downstream from the head regulator.
26. Head Sluice or Canal Head Regulator
A head regulator is structures constructed at the head of a canal off take
from a reservoir behind a weir or a dam. It may consist of a number of spans
separated by piers and operated by gates.
Functions:
To make the regulation of supply in the canal easy.
To control the silt entry in to the canal.
To shut out river floods.
To provide full supply required for irrigation at moderate velocities with
sufficient allowance.
27.
28.
29. River Training Works
River training works are required near the weir or barrage in order to ensure
a smooth and an axial flow of water when the length of a weir or barrage is
smaller than the width of a river
Types:
1. Guide banks
2. Marginal bunds
3. Spurs or Groynes
30. Guide Banks
When a barrage is constructed across a river which flows through the
alluvial soil, the guide banks must be constructed on both the approaches to
protect the structure from erosion.
Guide bank serves the following purposes:
1. It protects the barrage from the effect of scouring and erosion.
2. It provides a straight approach towards the barrage.
3. It controls the tendency of changing the course of the river.
4. It controls the velocity of flow near the structure.
31. Marginal 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.
The marginal bunds serve the following purposes:
1. It prevents the flood water or storage water from entering the
surrounding area which may be submerged or may be water logged.
2. It retains the flood water or storage water within a specified section.
3. It protects towns and villages from devastation during the heavy flood.
4. It protects valuable agricultural lands.
32. Spurs or Groynes
These are temporary structures permeable in nature provided on the curve
of a river to protect the river bank from erosion.
These are projected from the river bank towards the bed making angles
60o to 75o with the bank of the river.
The length of the spurs depends on the width of the river and the
sharpness of the curve.
The function of the spurs is to break the velocity of flow and to form a
water pocket on the upstream side where the sediments get deposited.
Thus the reclamation of land on the river bank can be achieved.
33.
34.
35. Causes of Failures of Weirs on Permeable Foundations
Main Causes are:
1. Due to Seepage or Sub-surface Flow
a) Piping or Undermining
b) Rupture of Floor by Uplift Pressure
2. Due to Surface Flow
a) By Suction due to Hydraulic Jump
b) By Scour on the u/s and d/s of the weir
Remedies:
Decrease Hydraulic gradient i.e, increase path of percolation by providing
sufficient length of impervious floor
Providing curtains or piles at both upstream and downstream
Remedies:
Providing impervious floor of sufficient length of appropriate thickness.
Pile at upstream to reduce uplift pressure downstream
Remedies:
Piles at greater depth than scour level
Launching aprons
Remedies:
Additional thickness
Floor thickness in one concrete mass instead of masonry layers
