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= Gravity Dams =

Purpose
Gravity dams are useful for a number of reasons. During the winter and spring, gravity dams are used to help in controlling the flow of melting snow in a river. During the summer, they are used as water storage to help provide water year long to the neighboring land. The reason we would use a gravity dam is because, without them, the snow and rain from the cold months, would build up and eventually melt. This would create a devastating quake of destruction as it flows downhill either to another lake or the ocean. Once all the snow has melted and the water has reached the ocean, the towns and cities between the top of the mountain and a couple miles away from the ocean are left with flooded and destroyed properties. Once the summer comes around, these cities are running short on water. A gravity dam helps in capturing the melted snow and rain in a large area containing the water for use through the entire year and as it overflows, it slowly drains over the dam allowing controlled soft flow of the water preventing any destruction down stream.

Design
Gravity dams are built by first cutting away a large part of the land in one section of a river allowing water to fill the space and be stored. Once the land has been cut away, the soil has to be tested to make sure it can support the weight of the dam and the water. It is important to make sure the soil will not erode over time which will allow the water to cut a way around or under the dam. Sometimes the soil is sufficient to achieve these goals; however, other times certain conditioning need to be done by adding support rocks which will bolster the weight of the dam and water. There are three different test that can be done to determine the foundations support strength:  Westergaard approach, Eulerian approach, and Lagrangian approach. Once the foundation is appropriate to build on, construction of the dam can begin. Usually gravity dams are built out of a strong material such as concrete or stone blocks, and are built into a triangular shape to provide the most support.

Earthquakes and Ecosystems
Gravity dams are built to withstand some of the strongest earthquakes. Even though, the foundation of gravity dams are built to support the weight of the dam and all the water, it is quite flexible in that it absorbs a large amount of energy and sends it in the earth's crust. It needs to be able to absorb the energy from an earthquake because, if the dam were to break, it would send a mass amount of water rushing down stream and destroying everything in its way. Earthquakes are the biggest danger to gravity dams and that is why, every year and after every major earthquake, they must be tested for cracks, durability, and strength. Although, gravity dams are expected to last anywhere 50-150 years, they need to be maintained and regularly replaced.

Another problem with gravity dams deal with ecosystems. Because the flow and amount of water changes when a dam is built, it generally has an impact on the area of the dam and everything afterwards. If water that normally flows two weeks out of the year in an area is now flowing constantly, new life is going to start living and growing there. Similarly, if you cut off water to somewhere that has water flow year round, things are going to start dying. Many environmentalist have problems with dams because of their effects on the environment. Though having more water supports more life, it also supports more predators which could result in an extinction of an entire species. This is why not a lot more dams have been built beside their expensive cost.

Effect on Society
The control of fresh water flow was essential to the settlement forming our first civilizations and substantially shaping societal evolution. Water is life; therefore, whoever controls water, has an immense amount of power. If you control water, you can grow more food. More food and a bountiful supply of water means more population growth. More population means more minds working together to advance society and knowledge of the world. Without water, life can not survive. The problem we face today, is overpopulation which means that one day, there will not be enough water to grow food and meet the needs of humans. Although, the Earth is mostly covered in water, most of the mainlands are not ,and not all of the water that is on the mainland can be used. One day, humans will need to engineer a way to control the flow of water much more efficiently.