Lecture Notes: Types of Impounding Structures
1. Introduction to Impounding Structures
Definition: Impounding structures are barriers designed to obstruct, control, or regulate the flow of water in a river, stream, or other water bodies. These structures create a reservoir, or a body of stored water, which can be used for various purposes such as irrigation, water supply, hydroelectric power generation, flood control, and recreation.
Key Functions:
- Water Storage: To store water for domestic, industrial, and agricultural use.
- Flood Control: To regulate the flow of water and prevent flooding downstream.
- Hydroelectric Power Generation: To produce electricity by using the stored water's potential energy.
- Recreation and Wildlife Habitat: To provide areas for recreational activities and habitats for aquatic life.
2. Types of Impounding Structures
Impounding structures can be classified based on their construction material, purpose, and structural design. Here are the main types:
A. Dams
Dams are the most common type of impounding structure, built across rivers to store water. They are classified based on their construction materials and structure.
Gravity Dams
- Definition: Massive concrete or masonry dams that rely on their weight to resist the force of the water. They are typically built on solid rock foundations.
- Example: Hoover Dam, USA
- Features:
- High resistance to water pressure due to their weight.
- Typically have a triangular cross-section with the wide base facing downstream.
- Uses: Hydroelectric power generation, water supply, flood control.
Arch Dams
- Definition: Curved dams that transfer the water pressure to the abutments (the valley sides). They are usually made of concrete and suitable for narrow, steep-walled valleys.
- Example: Hoover Dam, USA
- Features:
- Curved shape that efficiently transfers water pressure.
- Requires strong rock abutments.
- Uses: Water supply, flood control, hydroelectric power.
Buttress Dams
- Definition: Dams that are supported by a series of buttresses or supports on the downstream side. They use less material than gravity dams.
- Example: Itaipu Dam, Brazil/Paraguay
- Features:
- Vertical or sloping upstream face.
- Buttresses provide support, reducing the need for concrete.
- Uses: Hydroelectric power, water storage, irrigation.
Embankment Dams
- Definition: Large dams made of earth or rock-fill materials. They are the most common type due to their adaptability to different site conditions.
- Example: Aswan High Dam, Egypt
- Features:
- Constructed from natural materials like soil, clay, and rock.
- Sloping sides with a waterproof core or membrane.
- Uses: Irrigation, water supply, flood control.
Rockfill Dams
- Definition: Similar to embankment dams but primarily use rock material for construction. They often have a watertight membrane or core.
- Example: Oroville Dam, USA
- Features:
- Stable structure due to the heavy weight of the rocks.
- Permeable outer layers with an impermeable core.
- Uses: Water storage, flood control, hydroelectric power.
B. Barrages
- Definition: Low-lying structures built across rivers to control water levels and regulate flow. Unlike dams, barrages do not store large amounts of water; they divert water for irrigation or maintain navigable water levels.
- Example: Farakka Barrage, India
- Features:
- Series of gates that can be opened or closed to control water flow.
- Built on wide, flat rivers where a significant rise in water level is not needed.
- Uses: Irrigation, navigation, flood control.
C. Weirs
- Definition: Simple, low-head structures built across rivers or streams to raise the upstream water level. They can be fixed or adjustable.
- Example: Thames Weir, England
- Features:
- Typically consist of a solid obstruction with an overflow section.
- May have movable gates or boards to control flow.
- Uses: Water level control, flow measurement, diversion of water for irrigation.
D. Check Dams
- Definition: Small dams constructed across channels to reduce water flow velocity, promote sediment deposition, and control erosion.
- Example: Check dams in the Aravalli Hills, India
- Features:
- Constructed from stone, concrete, or other materials.
- Temporary structures often used in streams or gullies.
- Uses: Erosion control, groundwater recharge, flood control.
E. Reservoirs
- Definition: Artificial lakes created by impounding structures like dams. They store water for various purposes.
- Example: Lake Mead, USA (created by Hoover Dam)
- Features:
- Vary in size from small ponds to large lakes.
- Can be multipurpose (e.g., drinking water, irrigation, power generation).
- Uses: Water supply, hydroelectric power, recreation, flood control.
3. Selection of Impounding Structures
Factors Influencing Selection:
- Topography: Determines the type of structure that can be built (e.g., arch dams in narrow valleys, embankment dams in wide, shallow valleys).
- Geology: Foundation conditions influence the choice of structure. Rock foundations are suitable for gravity and arch dams.
- Hydrology: The amount and variability of water flow affect the size and type of structure.
- Purpose: The intended use (e.g., water supply, hydroelectric power, flood control) dictates the design.
- Environmental Impact: Consideration of impacts on local ecosystems, fish migration, and water quality.
- Cost: Budget constraints influence the type of materials and construction methods.
4. Real-Life Examples of Impounding Structures
1. Hoover Dam (USA)
- Type: Concrete Arch-Gravity Dam
- Purpose: Hydroelectric power, water supply, flood control.
- Features: Provides water and power to Nevada, Arizona, and California; creates Lake Mead.
2. Three Gorges Dam (China)
- Type: Concrete Gravity Dam
- Purpose: Hydroelectric power, flood control, navigation.
- Features: Largest power station by installed capacity; provides flood storage.
3. Aswan High Dam (Egypt)
- Type: Embankment Dam
- Purpose: Irrigation, hydroelectric power, flood control.
- Features: Controls flooding of the Nile River; provides water for agriculture.
4. Farakka Barrage (India)
- Type: Barrage
- Purpose: Divert water to the Hooghly River for navigation, control flow into Bangladesh.
- Features: Vital for maintaining the navigability of the Kolkata Port.
5. Thames Weir (England)
- Type: Weir
- Purpose: Water level control, navigation.
- Features: Regulates flow and water levels in the River Thames.
6. Check Dams in the Aravalli Hills (India)
- Type: Check Dam
- Purpose: Erosion control, groundwater recharge.
- Features: Small dams to slow water flow, reduce soil erosion, and recharge aquifers.
5. Conclusion
Impounding structures are essential for managing water resources, providing benefits such as water supply, flood control, power generation, and environmental conservation. The choice of structure depends on various factors, including topography, geology, hydrology, and intended use. By understanding the different types of impounding structures and their applications, engineers can design effective and sustainable water management solutions.
6. References
Books:
- "Dams and Development: A New Framework for Decision-Making" by the World Commission on Dams
- "Water Resources Engineering" by Larry W. Mays
Websites:
- International Commission on Large Dams (ICOLD): www.icold-cigb.org
- U.S. Bureau of Reclamation: www.usbr.gov
Journal Articles:
- "Sustainable Water Management: A Review of Dams and Reservoirs" - Journal of Hydrology
- "Impacts of Dams on River Ecosystems" - Environmental Management.