Understanding about Dams | Civil Engineering Optional Notes for UPSC PDF Download

What is a Dam? 

Purposes of Dams 

• Supply of Water
• Irrigation
• Flood Control
• Hydroelectric Power Generation
• Enhancing Water Quality
• Managing Sediments
• Storage of Mine Waste
• Facilitating Inland Navigation
• Supporting Fish Farming

Types of Dams 

• Classification Based on Materials of Construction
• Classification Based on The Flow Over its Top
• Classification Based on The Use of The Dam
• Classification Based on the Mode of Resistance Offered by the Dam against External Forces

Classification of Dams Based on Materials of Construction

1. Earth Fill Dams

• Constructed from compacted layers of earth (soil).
• Impervious materials form the core, with more permeable materials upstream and downstream.
• Crushed stone facing prevents erosion, and a concrete spillway guards against washout.
• Typically built in wide valleys with flat abutments and can be constructed on weaker foundations.
• Height depends on foundation strength.

2. Rock Fill Dams

• Made of large boulders and rock fragments.
• An impervious membrane, usually asphaltic or cement concrete, is placed upstream to reduce seepage.
• Can have an impervious earth core to prevent seepage, supported by dry rubble.
• Require stronger foundations than earth fill dams.
• Side slopes typically maintained at 1.4:1 or 1.3:1.

3. Concrete Dams

• Constructed from plain or reinforced concrete cement.
• Types include gravity dams, buttress dams, arch dams, and other forms using concrete.

4. Masonry Dams

• Built from stone and brick, sometimes mortared together.
• Can be gravity or arch-gravity type.
• Example: Nagarjunasagar Dam in India, the largest masonry dam in the world.

5. Steel Dams

• Consist of a steel framework and upstream steel skin plate.
• Types: Cantilever-type and direct strutted steel dams.
• Cantilever truss or inclined struts transmit water pressure to the foundation.

6. Timber Dams

• Main structural components made of wood, primarily coniferous species like pine and fir.
• Used for small heads (2-4 meters, rarely up to 8 meters).
• Sluices, piers, buttresses, and posts divide the sluice into multiple openings, covered by wooden shields.

Classification Based on the Flow Over its Top

1. Overflow Dams

• Water flows over the top once storage capacity is reached.
• Primarily made of concrete or masonry.

2. Non-Overflow Dams

• Top kept above the highest possible flood level; water does not flow over the top.
• Often a combination of overflow and non-overflow dams in river valley projects.

Classification Based on the Use of the Dam

1. Storage Dams

• Built to store water during high flow periods for use during dry seasons.
• Provide water for irrigation, hydroelectric power, and flood control.

2. Diversion Dams

• Divert river water into canals or conduits.
• Typically low height with a small storage reservoir upstream.

3. Detention Dams

• Control flooding by storing flood water and releasing it gradually to protect downstream areas.

Classification Based on the Mode of Resistance Against External Forces

1. Gravity Dams

• Made of stone or concrete, relying on their weight to resist water pressure.
• Suitable for broad valleys or confined gorges; require solid bedrock foundations.

2. Buttress Dams

• Use triangular concrete walls (buttresses) to transfer water pressure to the foundation.
• Spaced evenly across the dam site, supporting a reinforced concrete slab (deck).

3. Arch Dams

• Curved in plan with a convexity facing upstream.
• Utilize arch action to transfer water pressure to abutments.
• Suitable for small canyons with strong flanks.

Classification Based on Rigidity of the Dam

1. Rigid Dams

• Made of materials like concrete, masonry, steel, and wood.
• Do not deflect or deform significantly under pressure.

2. Non-Rigid Dams

• Made of rock and earth fill.
• Experience more settlement and deformation compared to rigid dams.

Site Selection Criteria for Dams

• Adequate Foundation: Ensuring the presence of a suitable foundation is imperative.
• Optimal Dam Length: The dam's length should be minimized for economic viability, while maximizing water retention capability for a given height.
• Proximity to Spillway Location: An appropriate location for the spillway in close vicinity is essential.
• Elevation Discrepancy: The bed level at the dam site should surpass that of the river basin to facilitate drainage and reduce dam height.
• Accessibility of Construction Materials: Easy availability of necessary construction materials in the area is crucial.
• Reservoir Basin Integrity: Ensuring the reservoir basin's reasonable watertightness is vital.
• Accessibility and Connectivity: The dam site should be easily reachable and economically linked to major urban centers.
• Living Conditions: Availability of space for labor colonies and a conducive environment in the nearby vicinity is important.

Advantages of Dams

Exploring the Benefits of Dams

• Dams play a crucial role in generating a steady supply of electricity through hydroelectric power.
• They serve as reservoirs during periods of excess water, aiding in flood control measures.
• Stored water can be released from dams during dry spells to cater to various needs.
• Water held in dams can be channeled for irrigation by diverting it through canals and channels.
• Dams contribute to the regulation of water flow in rivers and streams, safeguarding communities downstream during floods.
• By trapping harmful sediments and reducing flood risks, dams help in environmental protection by preventing the spread of pollutants.
• Dams also offer recreational prospects, attracting visitors to sites like Maithon Dam and Bhakra Nangal Dam for leisure activities.

Disadvantages of Dams

Apart from the advantages, the construction of dams in an area has some drawbacks as well. These are as follows:

• The primary issue associated with constructing dams is the displacement of people living in that area.
• Reservoirs often release a significant amount of greenhouse gases.
• Frequently, local ecosystems are negatively impacted and even destroyed.
• Dam construction can lead to disruptions in the groundwater table.