Which type of dam is constructed as a concrete or masonry wall across a valley?
A) Arch Dam
B) Gravity Dam
C) Embankment Dam
D) Buttress Dam
The correct answer and explanation is :
The correct answer is B) Gravity Dam.
A gravity dam is a substantial structure, typically made from concrete or masonry, that spans across a valley or river. Its primary design principle relies on its own weight to resist the horizontal pressure exerted by the water it impounds. This means that the stability of the dam is achieved through the force of gravity acting on its massive structure, preventing the water from pushing it downstream.
Design and Structure:
The cross-sectional shape of a gravity dam is usually triangular, with the widest part at the base tapering to a narrower crest at the top. This design ensures that the weight of the dam is sufficient to counteract the water pressure at all depths. The thickness of the dam decreases from the base to the crest because water pressure is greatest at the bottom due to the depth of the water.
Materials Used:
Gravity dams are predominantly constructed using concrete or masonry. Concrete is favored for its strength and durability, allowing the dam to withstand immense pressures over extended periods. Masonry, involving stone or brick, is also utilized, especially in regions where these materials are readily available. The choice between concrete and masonry often depends on factors such as local material availability, economic considerations, and specific site conditions.
Functionality and Applications:
The primary function of a gravity dam is to hold back water, creating a reservoir for purposes such as water supply, irrigation, flood control, and hydroelectric power generation. Due to their massive structure and reliance on weight for stability, gravity dams are particularly suitable for locations with wide valleys and solid foundations capable of supporting the immense load. They are also preferred in situations where the geological conditions do not favor other types of dams, such as arch dams, which require strong abutments.
Examples of Gravity Dams:
- Grand Coulee Dam (USA): One of the largest concrete structures in the world, the Grand Coulee Dam is a solid gravity dam on the Columbia River in Washington State. It serves multiple purposes, including hydroelectric power production and irrigation.
- Aswan Low Dam (Egypt): Constructed across the Nile River, the Aswan Low Dam is a gravity masonry buttress dam that was initially completed in 1902 and later heightened to increase its capacity. It was built to provide storage of annual floodwater and augment dry season flows to support greater irrigation development and population growth in the lower Nile.
Advantages of Gravity Dams:
- Stability: Their massive weight provides inherent stability against the forces exerted by the water.
- Durability: Constructed from robust materials like concrete and masonry, gravity dams have long lifespans and require minimal maintenance.
- Versatility: They can be built in a variety of locations, including wide valleys, provided the foundation is strong enough to support the structure.
Considerations and Challenges:
While gravity dams offer numerous benefits, they also come with challenges. The sheer volume of materials required makes them expensive and time-consuming to construct. Additionally, they necessitate a strong and stable foundation to bear the immense weight, limiting their suitability to certain geological conditions. Environmental and social impacts, such as displacement of communities and changes to local ecosystems, must also be carefully considered during the planning and construction phases.
In summary, gravity dams are monumental structures that utilize their own weight to hold back water, making them integral components in water resource management, energy production, and flood control.