Hydrology Quiz #4

Define Annual Exceedance Probability

The probability that a given event, like flood, rainfall, or discharge, is met or exceeded in a given time period

Why do we assign hydrologic variables?

Hydrologic variables like rainfall and streamflow are assigned probabilities to quantify how often certain magnitudes of events are likely to occur. Because these natural processes vary randomly over time, assigning probabilities (or return periods) helps engineers design infrastructure that can safely handle specific levels of risk, such as a 25-year storm or a 100-year flood

How can this be performed for streamflow using observations?

For streamflow, this is done by analyzing historical observations of annual peak discharges, ranking them from largest to smallest, and assigning each a probability of exceedance using statistical formulas such as P=m/(n+1), where m is the rank and n is the number of years of record. The return period is then found as T=1/P. By fitting these data to a probability distribution, engineers can estimate the magnitude of floods corresponding to various recurrence intervals for design and risk assessment.

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Matt Olson