Lecture 15
Diversion Headworks
An irrigation canal takes its supplies from a river or a
stream. In order to divert water from the river into the canal it is necessary
to construct certain works or structures across the river and at the head of
the offtaking canal. These works are known as canal headworks or headworks.
Canal headworks are structures built across a river or stream to divert water into an irrigation canal.
They are classified into two types:
1.
Storage headworks and
2.
Diversion headworks
Storage headworks involve constructing a dam
across the river to create a reservoir, storing excess water during high-flow
periods. This stored water is then supplied to the canal as needed.
Diversion headworks - A diversion headworks serves
to raise the water level in the river and divert the required quantity into the
canal.
The various purposes served by a diversion headworks:
(i)
Raising the river's water level to increase the
irrigated area,
(ii)
Regulating water flow into the canal,
(iii)
Controlling silt entry,
(iv)
Providing short-term water storage, and
(v)
Reducing fluctuations in river water levels.
Types of Diversion Headworks
Diversion headworks are categorized into two main types:
1.
Temporary Diversion Headworks: These
involve temporary structures like spurs or bunds built across a river to
elevate water levels and channel water into a canal. These structures often
need annual reconstruction post-floods due to potential flood damage.
2.
Permanent Diversion Headworks: These
consist of durable structures such as weirs or barrages constructed across a
river to raise water levels and divert water into a canal. In many significant
canal systems, permanent diversion headworks are predominantly used.
Location of Canal Headworks
The location of canal headworks is influenced by the flow
stages of a river:
1.
Rocky (Hilly) Stage: The river flows
through hilly terrain with steep slopes and high velocities. The riverbed
consists of rock or large boulders.
2.
Boulder Stage: The river transitions to a
stage where the bed and banks are composed of boulders and gravel. The
cross-section is well-defined, with non-submersible banks close to the main
flow. Bed slope and velocity decrease, but subsoil flow is significant due to high
permeability of materials.
3.
Trough (Alluvial) Stage: The river enters
an alluvial plain formed by its own deposits. The bed consists of sand and
silt, with low slopes and velocities. During floods, the river spreads widely
as banks are distant from the main flow.
4.
Delta Stage: As the river nears the
ocean, its slope and velocity decrease further, causing sediment deposition and
the formation of a delta through branching channels.
Canal headworks are unsuitable in the rocky and delta
stages. In the rocky stage, constructing canals to transport water is
prohibitively expensive. In the delta stage, the available area is limited, and
irrigation needs are minimal. Therefore, headworks are best located in
the boulder or trough stages. Each stage has its advantages and
disadvantages for headworks construction.
Advantages of Locating Canal Headworks in Boulder Stage:
1.
Shorter weir length due to boulder terrain.
2.
High banks reduce the need for extensive river
training.
3.
Local availability of construction materials
like stone and sand.
4.
Potential to harness hydroelectric power from
canal falls.
Disadvantages of Locating Canal Headworks in Boulder Stage:
1.
High water loss from subsoil flow.
2.
Significant seepage losses in the canal due to
similar terrain.
3.
Increased need for cross-drainage structures.
4.
Limited irrigation demand due to higher rainfall
and smaller cultivable areas.
Advantages of Locating Canal Headworks in Trough Stage:
1.
Reduced subsoil flow compared to boulder
regions.
2.
Efficient canal utilization with no idle
sections.
3.
Larger irrigable area served.
4.
Fewer cross-drainage structures required.
Disadvantages of Locating Canal Headworks in Trough Stage:
1.
Higher construction costs due to poor foundation
conditions.
2.
Limited local availability of construction
materials.
3.
Extensive river training works needed for
stability.
Fig. Typical
layout of a diversion headworks
Components of Diversion Headworks
The various components of a diversion headworks are as
follows:
1.
Weir/Barrage: Controls River flow
and raises water levels.
2.
Divide Wall/Groyne: Separates flow
between the main structure and undersluices.
3.
Fish Ladder: Facilitates fish
migration across the structure.
4.
Approach Channel: Directs water
toward the diversion point.
5.
Undersluices/Scouring Sluices:
Regulate sediment flow and prevent silt buildup.
6.
Silt Excluder: Removes sediment
from diverted water.
7.
Canal Head Regulator: Manages
water entry into the canal system.
8.
River Training Works (e.g.,
Marginal/Guide Bunds): Stabilize riverbanks and guide flow.
1. Weir or Barrage
- Function:
A weir or barrage is constructed across the river to raise the water level
and regulate the flow of water.
- Weir:
A low wall or barrier built across the river to raise the water level,
allowing water to flow over its crest.
- Barrage:
A more advanced structure with gates to control the flow of water
precisely. It can store water during low flow periods and release it as
needed.
- Purpose:
Ensures a consistent supply of water to the canal system.
Fig. Barrage
2. Divide Wall or Divide Groyne
- Function:
A wall or groyne constructed parallel to the weir or barrage to separate
the main river flow from the diverted flow.
- Purpose:
- Prevents
cross-flow between the main river and the canal.
- Stabilizes
the flow pattern and reduces turbulence.
- Protects
the canal head regulator from direct river currents.
3. Fish Ladder
- Function:
A structure designed to allow fish to migrate upstream or downstream
across the weir or barrage.
- Design:
Consists of a series of steps or pools that enable fish to swim past the
barrier.
- Purpose:
Maintains the ecological balance by ensuring the natural movement of
aquatic life.
4. Pocket or Approach Channel
- Function:
A channel constructed upstream of the weir or barrage to guide water
smoothly toward the diversion structure.
- Purpose:
- Ensures
a steady and uniform flow of water toward the canal head regulator.
- Reduces
turbulence and sediment deposition near the diversion point.
5. Undersluices or Scouring Sluices
- Function:
Gates or openings located near the bed of the river, typically at the base
of the weir or barrage.
- Purpose:
- Flushes
out accumulated sediment and debris from the riverbed.
- Prevents
silt from entering the canal system.
- Maintains
the efficiency of the diversion structure by keeping the riverbed clear.
6. Silt Excluder
- Function:
A device installed at the canal head to separate and exclude silt from the
diverted water.
- Design:
Consists of tunnels or channels that allow heavier silt-laden water to
flow back into the river.
- Purpose:
Reduces silt entering the canal, preventing clogging and maintaining
irrigation efficiency.
7. Canal Head Regulator
- Function:
A structure located at the head of the canal to control the flow of water
entering the canal system.
- Components:
Includes gates or shutters to regulate the water flow.
- Purpose:
- Controls
the quantity of water diverted into the canal.
- Prevents
excess water from entering the canal during floods.
- Ensures
a stable and controlled supply of water for irrigation or other uses.
8. River Training Works
- Function:
Structures built to guide and stabilize the river flow, protecting the
diversion headworks and surrounding areas.
- Components:
- Marginal
Bunds: Embankments constructed along the riverbanks to prevent
flooding and protect adjacent land.
- Guide
Bunds: Structures built to direct the river flow toward the diversion
headworks and prevent erosion.
- Purpose:
- Protects
the diversion structure from damage due to river currents.
- Prevents
riverbank erosion and flooding.
- Ensures
the river flows in a controlled manner.