Subject Matter Expert: Anthony O. Righellis, P.E.
Hydrologic analysis and results are statistically based. The term 'year event' is the common way to express the hydrologic magnitude of precipitation and runoff quantities, but it does not tell the entire story. What does it really mean?
Discussing drainage and flood issues with the general public and policymakers sometimes needs more than thinking just in terms of 'year event'. Its brevity in expression does not illustrate important considerations. One such is that the 100-year event has only a 63% chance of happening in 100 consecutive years yet can occur more than once in a single year.
Three other primary expressions are "probability," "service level," and "expected occurrence." There is a direct relationship between these terms and "year event." All are important to understand and doing so can help an engineer to better grasp situations and relate the impact of their work to others. This course relates the four terms with discussion, formulas, and examples.