Why Are There No Short Arch Dams?

Summary

Flaming Gorge Dam, an iconic arch dam, serves as a key example of the unique engineering challenges and solutions involved in dam construction. Unlike more common gravity and embankment dams, arch dams rely on their shape and structural efficiency to hold back water in narrow, steep valleys. The video explains how dams handle the immense forces of water, comparing different types of dams and exploring their failure modes. The focus on the advantages and complexities of arch dams highlights why they’re used for the tallest and most iconic structures, including Hoover Dam, while emphasizing the importance of design in specific conditions.

Takeaways

• 😀 Arch dams are rare, with around 50 in the U.S., making up less than a tenth of a percent of the total 92,000 dams listed in the national inventory.
• 😀 Arch dams are considered iconic due to their massive size and unique engineering, often doubling as tourist attractions.
• 😀 Gravity dams rely on weight and friction to resist the force of water, but they can fail in two main ways: sliding and overturning.
• 😀 Friction is the main stabilizing force in gravity dams, while the destabilizing force comes from hydrostatic pressure exerted by the reservoir water.
• 😀 A shift in weight distribution on gravity dams can significantly affect their stability, and moving the weight upstream can improve stability.
• 😀 Uplift is a significant issue for gravity dams, as water can seep underneath the dam and reduce its stability, counteracting its weight.
• 😀 Gravity dams need to handle pressure that increases as the depth of the water rises, which means taller dams require much more material and higher costs.
• 😀 Engineers often try to reduce the mass of dams by leveraging the resistance from the canyon sides, but this relies on strong, competent rock at the site.
• 😀 Arch dams use the compression-only properties of arches to transfer loads more efficiently, allowing for lighter, cheaper, and longer-lasting structures.
• 😀 The main challenge with arch dams is the need for strong, stable rock at the abutments to handle the horizontal thrusts created by the arch.
• 😀 The Hoover Dam is a prime example of a gravity-arch dam, combining mass resistance with arch action to handle the forces from the water.
• 😀 The tallest dams in the world often incorporate arch designs, which are especially effective in narrow, deep valleys and steep gorges.

Q & A

What makes the Flaming Gorge Dam a quintessential example of a dam?

The Flaming Gorge Dam is considered quintessential due to its iconic image as a large concrete wedge rising from the Green River. It represents the classic image of a dam, with its graceful yet massive structure stretching across a narrow valley.

Why are arch dams considered rare despite their iconic status?

Arch dams are rare because they require specific conditions to be effective, especially in narrow canyons. They make up less than a tenth of a percent of the 92,000 dams in the US, largely due to the need for steep, narrow valleys and strong abutments.

How do gravity dams resist the force of water?

Gravity dams resist the force of water primarily through the weight of the structure, which relies on friction between the particles that make up the dam. This friction helps prevent the dam from sliding or overturning under hydrostatic pressure from the water.

What are the two major failure modes of gravity dams?

Gravity dams can fail by sliding or overturning. Sliding occurs when the frictional resistance is not enough to counter the hydrostatic pressure, while overturning happens when the torque generated by the water pressure exceeds the stabilizing moment created by the dam's weight.

Why does the distribution of weight matter in gravity dams?

The distribution of weight in a gravity dam is crucial because it affects the dam's resistance to overturning. By concentrating weight on the upstream side, the dam can better resist the rotational forces caused by hydrostatic pressure, increasing stability.

What is the role of uplift in gravity dams?

Uplift occurs when water seeps beneath the dam, creating upward pressure on the bottom. This counteracts the dam's weight, potentially destabilizing it. Uplift forces are more significant in taller dams, and engineers must manage them with drainage systems and cutoff walls.

How do arch dams differ from gravity dams in terms of structural efficiency?

Arch dams are more structurally efficient than gravity dams because they use the shape of the arch to transfer the load into compression, requiring less material and weight. This design is ideal for narrow, steep canyons where the abutments can support the thrust generated by the arch.

What are the primary challenges in building arch dams?

The main challenges in building arch dams are ensuring strong, competent rock in the abutments to withstand the horizontal thrusts and dealing with the complexity of three-dimensional structural behavior. They are also less effective in resisting uplift forces, which requires robust foundation drainage systems.

Why don't we build dams out of materials like aluminum flashing, as seen in the demo?

Aluminum flashing is too flexible and would bend under the pressure of the water, unlike concrete or masonry used in real dams. These materials are designed to handle compressive forces effectively, ensuring stability over long spans and under immense pressure.

How does the combination of mass and arch action make certain dams more economical?

By combining the mass of a gravity dam with the arch action, such as in the Hoover Dam, engineers can reduce the size and weight of the structure while still effectively resisting the water pressure. This hybrid design allows for more efficient use of materials, making it economically viable in certain sites.