The term “spillway” means to pass the water flow from its source through the water ways in the water structures. Spillways facilitates the duties with controlling the water flow. It is also important for the operation and the stability of the dams. Spillway structures have significant cost in the construction of the dams. By determining their positions in the water structure, their statical and hydrological calculations are done. In the construction of the spillway, the selection of the material is very important. The most convenient solution is required in regarding to materials and hydraulics. For the safety of the structures of the dams, the spillways is supposed to pass the great amount of water floods through the water ways without causing any danger. The adequate planning, designing, building and safely operating of the dam is very important for the safety of the inhabitants who are living near the water ways. In this research, the basic information related to spillways which are safety structures of dams, are summarized and in the unloading of the spillways, the C coefficient is used and the effect the C coefficient change on the height of the body of the dams is examined.
Key Words: Spillway, Siphon Spillway, C Coefficient.