Why does it happen? The wall of a dam is broad at its base.

The design of a dam with a broad base is primarily intended for stability and safety reasons. Dams are structures built to impound and control the flow of water in rivers and streams, and they serve various purposes, including flood control, water supply, irrigation, and hydroelectric power generation. Here's why the base of a dam is made broad:

Stability: The primary purpose of a dam is to hold back a significant amount of water. The broad base of a dam provides a stable foundation that can support the weight of the water behind it. The force exerted by a large volume of water can be substantial, and a broad base helps distribute this force over a larger area, reducing the risk of structural failure or collapse.

Counteracting Uplift Pressure: Water exerts an upward force known as uplift pressure on the base of a dam. This force can potentially lift the dam off its foundation. By making the base broad, the dam's weight counters this uplift force, ensuring that the dam remains firmly in place.

Preventing Sliding: A broad base increases the friction between the dam and the foundation material (usually rock or soil). This enhanced friction helps prevent the dam from sliding or shifting, even under the pressure of a massive reservoir behind it.

Distributing Loads: Dams often support various loads, including the weight of the water, the weight of the dam structure itself, and other dynamic forces such as wind and seismic activity. The broad base helps distribute these loads evenly, reducing stress on any specific part of the dam.

Stability Against Erosion: Water can erode the soil or rock beneath a dam's foundation. A broader base provides a larger surface area for the dam to interact with the foundation material, making it more resistant to erosion.

Resisting Overturning: A dam must also resist overturning, especially when it is subject to external forces such as floods or seismic events. The broader base increases the leverage the dam has against these forces, making it more stable and less likely to topple over.

In summary, the broad base of a dam is a critical design feature that enhances its stability and safety, ensuring that it can effectively perform its intended functions of water control and management. The specific design considerations and dimensions of a dam's base will depend on factors such as the size of the reservoir, the type of foundation material, the local geological conditions, and the engineering standards and regulations in place.