Risk, Reliability and Safety Factor

There are always some uncertainties associated with estimation of hydrological design values (such as design flood discharge and river stage during design flood). For example: Suppose a weir is designed for 75 years with an estimated flood of return period 125 years. Now, if the flood is more than estimated value in 75 years only, then weir may fail.

Thus, some terms are defined to deal with these uncertainties which are given as below:

• Risk, R

It is defined as probability of occurrence of an event at least once over a period on n successive years for given return period. Mathematically,

• Reliability, Re

Uncertainties may arise out of structural, constructional, operational and environmental causes as well as from non-technological consideration such as economic, sociological and political causes in a water resources development project.

• Saftey factor

A safety factor for a given hydrological parameter M is given as