Part iii-d – Dam Failure



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Arch dams are best suited to narrow canyons where they divert the force of the water behind the dam to the sides of the cavern in order to help support the weight. Therefore, arch dams need not be as thick as gravity dams since the dam itself supports less weight making them less expensive to construct. Arch dams may carry less weight than other dams; however, they are affected by the same kinds of force. These forces must be considered when building one, and include pressure of the water, weight of the water, and weight of the dam.


Pressure of the Water

Pressure applied by the water: For any type of dam, the water applies pressure to the dam that is proportional to the depth of the water. Therefore, the pressure exerted by the water in the reservoir must be considered before building a dam. The force of the water pushing on the dam is proportional to the depth of the water contained behind the dam. So, as the depth increases, the pressure increases. At the bottom of a dam, the pressure is always the highest.


Weight of the Water and Weight of the Dam

The weight of the water and the weight of the dam must be considered to ensure that the foundation can support the dam. The earth supporting the dam must be strong enough to support the weight of the dam. Therefore, the normal force (N) must be sufficient to support the weight (W) of the dam and the water. A well-known example of an arch dam is Hoover Dam, which sits in the Colorado River on the border of Nevada and Arizona. Dams are also used to generate energy by harnessing the potential energy of water contained in the reservoir.


Buttress Dams

Buttress dams can also be called Ambursen dams after the American engineer who used this type of dam in the early 20th century.


Originally, buttress dams were used in areas requiring irrigation, but where the land was not capable of supporting the size and weight of other types of dams. Generally, buttress dams are built in wide valleys. The name "buttress" dam comes from the structure of the dam itself. The dam is supported at intervals by several buttresses, concrete slabs reinforced with steel, which form a watertight seal against the river. There are two main types of buttress dams: flat slab and multiple arch dams. Multiple arch dams are generally more expensive and time consuming to build. This is because the front of the dam consists of several arches that face upstream instead of just a flat front.


Physics Involved in Buttress Dams

As with the arch dam and the gravity dam, the same forces must be considered for a buttress dam. The weight of the dam is also another factor involved. This is the downward force exerted by the concrete. To determine this, the specific weight of the concrete is used. This is the volume of concrete times the specific weight of the concrete. The specific weight of the concrete and the ratio of the density of concrete are compared to the density of water at 4 degrees Celsius. For concrete, the specific gravity is 23.6 N/m3. The weight of the dam times the gravity is the total downward force that is exerted by the dam.


The normal force is the upward force that is exerted by the Earth. Since the dam is not in motion, the normal force provides equilibrium for the weight. Thus, the normal force for a dam is equal but opposite in direction to the weight of the dam times gravity.


Gravity Dams

Gravity dams serve the same purposes as arch and buttress dams; however, they differ in structure and method of retaining water. This type of dam is solid and triangular in shape; therefore, it requires a large amount of concrete or other construction material. The immense weight of the concrete provides stabilization and allows the dam to maintain control of the water. As you can see, gravitational force holds the dam to the ground, hence the name. Some well-known examples of Gravity Dams are Prado and Friant Dam.



Forces Acting on Gravity Dams

The forces acting on gravity dams include the force of the water in the reservoir (acting horizontally and vertically), the weight of the concrete (acting in a downward direction), and the uplift force that results from the water pressure under the dam pushing in an upward direction (a result of the buoyant force of water) if there is not a drainage source. The overall force acts one-third of the way from the bottom of the dam, which is its center of gravity.



Pressure Acting on the Dam

Gravity dams must be able to withstand the pressure of the reservoir water. As a result of the triangular shape of the dam, the pressure of the water is also distributed in the shape of a triangle. The amount of pressure on the dam increases proportionally to the depth of the water; therefore, the pressure is at a maximum at the bottom of the reservoir. A portion of the weight of the dam is cancelled by the water pressure resulting from the uplift force, so if the dam is not properly constructed, it could be uplifted and overturned.



Energy Production

As far as energy production is concerned, gravity dams are also used in the generation of hydroelectric power by the conversion of the potential energy of the water into mechanical energy.


Embankment Dams

Embankment dams in the US prior to 1930 had a poor track record. Of those over 490 feet high, almost 10% failed, usually due to overtopping in a flood. Overtopping is when the water level in the reservoir reaches maximum height and begins to flow over the top of the dam. The South Fork dam in Johnstown, PA was one of the first to use rockfills, or loose rocks, on the downstream face. This dam failed after being overtopped in 1889, killing over 2,000 people.


Embankment dams are massive dams made of earth or rock. Embankment dams usually have some sort of waterproof interior (called the core), which is covered with earth or rock fill. Grass may even be grown on the earth fill. Water will seep in through the earth or rock fill, but should not seep into the core. They rely on the weight to resist the flow of water, just like concrete gravity dams.


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