篇篇学之2：The Gravity Dams

There are three mainly types of dams: gravity dams,embankment dams and arch dams.(Considering these dams are ordinarily,we just take these for example.),Here we just describe the gravity dam.
NO1:Gravity dams
The gravity dam is a dam which sites the horizontal thrust of the water entirely by their own weight.Concrete gravity dams are typically used to block streams through narrow gorges.Because it is the weight holding the water back,Concrete gravity dams tend to use a large amount of concrete.so the cost of the gravity is expensive.
The gravity dam uses their weight to hold back the water in the reservior.It can be made of earth or rock fill or concrete.These dams can be very expensive because of how much material they use.The shape of the gravity dam resembles a triangle.This is because the triangular distribution of the water pressure.The deeper the water,the more horizontal pressure it exerts on the dam.So at the surface of the reservoir,the water is exerting no pressure and at the bottom of the reseroir,the water is exerting the maximum pressure.

Words:
gravity dam 重力坝 embankment dam 堤坝（土石坝） arch dam 拱坝 site 以**为定点 阻挡
horizontal 地平线/水平的 thrust 猛推（插） gorge 山谷 reservior 水库
resemble 像 triangle 三角形 distribution 分布