100-Year Return Period: Of What?

In water projects often it is said that the project is designed or protected at a level of x-year return period. But this x-year design (or protection) return period is often misunderstood. Part of the misunderstanding comes from inadequate appreciation of probability. The more serious problem is that the X-year return period of what is not clearly specified. For instance, urban storm sewers are designed based on X-year return period of rainfall, not of flood. On the other hand, spillways are designed for a flood of X-year return period. The protection level of the land behind a levee has a return period different from the 100-year return period of the flood stage, of the peak discharge used in the design of the levee. The objective of this paper is to point out these differences and their implications in water engineering design and management.