Flood frequency: expected and unexpected probabilities

Flood-frequency curves may be defined either with or without an ' expeced probability ' adustment; and the two curves differ in the way that they attempt to average the time-sampling uncertainties. A curve with no adustment is shown to estimate a median value of both discharge and frequency of occurrence, while an expected probability curve is shown to estimate a mean frequency of flood years. The attributes and constraints of the two types of curves for various uses are discussed.