Gravity dam

A gravity dam is a dam constructed from concrete or stone masonry and designed to hold back water by primarily using the weight of the material alone to resist the horizontal pressure of water pushing against it. Gravity dams are designed so that each section of the dam is stable and independent of any other dam section. 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 to 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. Gravity dams generally require stiff rock foundations of high bearing strength (slightly weathered to fresh); although they have been built on soil foundations in rare cases. The bearing strength of the foundation limits the allowable position of the resultant which influences the overall stability. Also, the stiff nature of the gravity dam structure is unforgiving to differential foundation settlement; which can induce cracking of the dam structure. Gravity dams provide some advantages over embankment dams. The main advantage being that they can tolerate minor over-topping flows as the concrete is resistant to scouring. Large over-topping flows are still a problem, as they can scour the foundations if not accounted for in the design. A disadvantage of gravity dams is that due to their large footprint, they are susceptible to uplift pressures which act as a de-stabilising force. Uplift pressures (buoyancy) can be reduced by internal and foundation drainage systems which reduces the pressures.