JNTU B.Tech IV IRRIGATION AND HYDRAULIC STRUCTURES Previous Question Paper (February/March - 2022 - R18)

Reservoirs are artificial storage basins created by constructing a dam across a river to store water for various beneficial uses. They are classified based on their purpose, storage capacity, and location.

Classification of Reservoirs:

1. Storage Reservoirs (Conservation Reservoirs): These are designed to store water during periods of excess flow (monsoon) for use during periods of low flow. They serve purposes like irrigation, water supply, and power generation.
2. Flood Control Reservoirs: Specifically designed to store floodwater temporarily to prevent downstream damage. They may have uncontrolled outlets or gated spillways to regulate discharge.
3. Multipurpose Reservoirs: These are large-scale projects designed to serve more than one primary function simultaneously, such as irrigation, flood control, hydropower generation, navigation, and water supply.
4. Distribution Reservoirs: Small storage units located near the point of consumption to ensure a constant supply of water during peak demand hours in municipal water systems.
5. Retarding Reservoirs: These have no gates at the spillway and are designed to automatically store peak flood flows, releasing them slowly to reduce the flood peak downstream.

Multipurpose Reservoirs:

A multipurpose reservoir is a comprehensive water resource development project engineered to satisfy multiple objectives through a single dam structure. By integrating various needs, these reservoirs optimize the utilization of water resources and improve the economic viability of the project.

Key Aspects of Multipurpose Reservoirs:

• Integrated Planning: The storage capacity is divided into specific zones: Dead Storage (below the lowest outlet), Active Storage (for primary uses like irrigation), and Flood Control Storage (reserved for flood moderation).
• Conflict Resolution: Since different uses often have competing demands (e.g., flood control requires empty space, while irrigation requires full storage), reservoir operation schedules are developed to balance these requirements throughout the year.
• Economic Efficiency: The high capital cost of dam construction is justified by the combined benefits of power revenue, agricultural production, and flood mitigation, making them essential for regional development.

The selection of a reservoir site is a critical engineering decision that balances economic viability, safety, and operational efficiency. The primary factors influencing this selection include:

1. Geological Suitability: The foundation and the reservoir basin must be geologically stable. The site should be free from major faults, shear zones, or soluble rock formations like limestone which could lead to excessive seepage or reservoir leakage.
2. Topography: An ideal site features a narrow valley at the dam axis to minimize construction costs, coupled with a wide, deep basin upstream to provide large storage capacity with a minimum surface area to reduce evaporation losses.
3. Hydrology: The catchment area must provide a reliable and adequate yield of water. Engineers analyze long-term rainfall and runoff data to ensure the reservoir can be filled consistently while managing peak flood flows through spillway design.
4. Water Tightness: The reservoir basin must be impermeable to prevent the loss of stored water through percolation. This involves assessing the porosity and permeability of the soil and rock strata within the basin boundaries.
5. Sedimentation: The catchment area should ideally have low erosion rates to minimize the inflow of silt. High sediment loads significantly reduce the effective life and storage capacity of the reservoir over time.
6. Cost and Accessibility: The site should be accessible for the transport of construction materials, machinery, and labor. Proximity to the area of water use (e.g., irrigation command area or urban center) reduces conveyance costs.
7. Environmental and Social Impact: The selection must consider the extent of submergence of valuable land, forests, and settlements. Minimizing the displacement of local populations and protecting biodiversity are essential regulatory and ethical requirements.
8. Spillway Requirements: The site must offer suitable natural conditions for the construction of a safe and economical spillway to pass excess floodwater without endangering the dam structure.