Class Notes
1. Introduction to Diversion Headworks
Definition:
Diversion headworks are hydraulic structures constructed across a river to divert water from the river into canals or other conveyance systems for purposes such as irrigation, drinking water supply, or power generation.
Purpose of Diversion Headworks:
- Water Diversion: Diverting a portion of the river flow into a canal or other conveyance system.
- Flow Regulation: Controlling the flow rate to ensure a consistent and controlled water supply.
- Sediment Control: Minimizing the entry of sediments into the diversion system to protect canals and irrigation systems.
- Flood Control: Managing flood flows and protecting downstream areas by regulating the release of excess water.
- Intake Protection: Providing stable conditions at the intake points to ensure efficient water extraction.
Types of Diversion Headworks:
- Weirs
- Barrages
2. Weirs
Definition:
A weir is a barrier built across a river or stream to raise the water level on the upstream side. It allows water to flow over its crest in a controlled manner.
Types of Weirs:
Based on Crest Shape:
- Sharp-Crested Weir: A weir with a thin, sharp edge that allows water to flow smoothly over the crest. Used for measuring discharge in channels.
- Broad-Crested Weir: A weir with a wide crest that causes a more gradual overflow. Commonly used in rivers and canals for water level regulation.
Based on Alignment:
- Straight Weir: A weir with a straight alignment perpendicular to the flow.
- Curved Weir: A weir with a curved alignment, often used to align with riverbanks or other natural features.
Based on Purpose:
- Overflow Weir: Designed to allow excess water to spill over during floods, thus preventing upstream flooding.
- Non-Overflow Weir: Designed to divert water without allowing overflow, used mainly for controlled irrigation.
Components of a Weir:
- Crest: The top edge over which water flows. The height of the crest determines the water level upstream.
- Abutments: The supports at either end of the weir that connect it to the riverbanks.
- Apron: A protective layer on the downstream side of the weir to prevent erosion due to falling water.
- Body: The main structure that holds back the water.
Design Considerations for Weirs:
- Height and Length of Crest: Determines the volume of water that can be diverted and the upstream water level.
- Structural Stability: Ensures that the weir can withstand the forces exerted by the water flow, including hydrostatic and hydrodynamic forces.
- Erosion Protection: The apron and downstream protection must be designed to prevent erosion caused by the falling water and high flow velocities.
- Material Selection: Weirs are typically constructed from materials such as concrete, masonry, or rock, chosen based on durability, cost, and availability.
Advantages of Weirs:
- Simple and cost-effective to construct.
- Provide a reliable method of diverting water.
- Effective for flow measurement and regulation.
Disadvantages of Weirs:
- Limited flexibility in flow control.
- Susceptible to sediment accumulation, which can reduce efficiency.
- Can cause flooding upstream if not designed to handle high flows.
Example:
Dufferin Weir, India
- Located on the Mutha River, Pune, India.
- Built in the 19th century to divert water for irrigation and municipal supply.
- Features a masonry structure with a series of sluice gates for controlled water release.
3. Barrages
Definition:
A barrage is a type of diversion headwork that consists of a series of gates or sluices across a river, allowing for controlled regulation of the river flow and water level. Unlike weirs, barrages are adjustable, making them more flexible in managing water levels.
Types of Barrages:
- Tidal Barrages: Used to control the flow of water from the sea or estuaries. They can generate power using the tidal movement.
- River Barrages: Built across rivers to control the flow and divert water into canals for irrigation, power generation, and water supply.
Components of a Barrage:
- Gates/Sluices: Movable barriers that can be raised or lowered to regulate water flow. They control the water level and discharge downstream.
- Piers: Vertical supports that hold the gates and form the main structural component of the barrage.
- Head Regulator: A structure that controls the water entering the canals. It includes gates and shutters to regulate the flow into the canal system.
- Spillway: A section of the barrage that allows excess water to pass safely over or through the structure to prevent flooding.
- Navigational Lock: A chamber with gates that allows boats and ships to pass from one level of the river to another.
Design Considerations for Barrages:
- Flow Regulation: The number and size of gates must be designed to handle varying flow rates, including low flows for irrigation and high flows during floods.
- Foundation Stability: A strong foundation is essential to support the barrage and prevent seepage and erosion. Piles or sheet piles are often used to reinforce the foundation.
- Sediment Management: Sediment traps and sluice gates help manage sediment buildup, ensuring efficient operation and reducing maintenance costs.
- Structural Integrity: The barrage must withstand the forces of flowing water, debris impact, and potential seismic activity.
Advantages of Barrages:
- Provides precise control over water levels and flow rates.
- Facilitates navigation by maintaining a stable water level.
- Helps in flood management and water conservation.
Disadvantages of Barrages:
- Higher construction and maintenance costs compared to weirs.
- Complex operation requiring skilled personnel.
- Potential for sedimentation and debris accumulation, which can affect performance.
Example:
Farakka Barrage, India
- Located on the Ganges River, West Bengal, India.
- Built to divert water into the Hooghly River for navigation and to prevent siltation.
- Consists of a 2,240-meter-long barrage with 109 gates, allowing precise control of water flow.
4. Comparison of Weirs and Barrages
| Feature | Weir | Barrage |
|---|---|---|
| Structure | Fixed crest, overflow type | Adjustable gates or sluices |
| Flexibility | Limited flexibility in flow control | High flexibility, adjustable flow |
| Cost | Lower construction cost | Higher construction cost |
| Operation | Simple, less maintenance | Complex, requires skilled operation |
| Application | Small to medium-scale diversions | Large-scale diversions, navigation |
| Flood Control | Less effective | More effective |
| Sediment Control | Limited | Better sediment management |
5. Case Studies
A. Dufferin Weir, India
Location: Mutha River, Pune, India
Purpose:
- Divert water for irrigation and municipal use.
- Maintain water level in the river for drinking water supply.
Features:
- Masonry structure with a series of sluice gates.
- Provides controlled release of water to downstream areas.
Performance:
Dufferin Weir has been operational since the 19th century, demonstrating the longevity and reliability of well-designed weir structures for small to medium-scale water diversion projects.
B. Farakka Barrage, India
Location: Ganges River, West Bengal, India
Purpose:
- Divert water into the Hooghly River to maintain navigability.
- Prevent siltation in the Kolkata Port.
- Irrigation and water supply.
Features:
- 2,240 meters long with 109 gates.
- Provides precise control of water flow for multiple purposes.
- Includes a navigational lock for ship passage.
Performance:
Since its completion in 1975, Farakka Barrage has played a crucial role in managing the water resources of the Ganges River, supporting irrigation, navigation, and flood control. It highlights the importance of adaptable structures in large river systems.
6. Conclusion
- Weirs and Barrages are essential components of water management infrastructure, each serving specific needs and purposes.
- Weirs are simple and cost-effective solutions for water diversion and level control, best suited for smaller rivers and streams.
- Barrages offer greater flexibility and control, making them ideal for large rivers and complex water management scenarios.
- The choice between a weir and a barrage depends on factors such as the river's characteristics, required water management objectives, and available resources.
Understanding the design, function, and application of these structures is critical for effective water management and ensuring sustainable development in water-scarce regions.