One of the primary objectives of the frequency analysis of hydrologic data is to determine the recurrence interval of a hydrologic event of a given magnitude. The recurrence interval, which is the same as the return period, may also be defined as the average interval of time within which the magnitude of a hydrologic event will be equalled or exceeded once, on the average. Statistics is a branch of mathematics that studies the collection and organization of numerical data. Based on statistical analysis of data, inferences are made about larger data sets using the results of related smaller data sets. That part of mathematics used to predict the likelihood of the occurrence of a random even is probability and both of statistical and probability concept is frequently used in hydrologic analysis. Hydrologic frequency analysis is the approach of using probability and statistical analysis to estimate future frequencies (probabilities of hydrologic event occurring) based upon information contained in hydrologic records. Through the use of statistical methods, observed data is analyzed so as to provide not only a more accurate estimate of future frequencies than is indicated by the observed data, but also criteria for determining the reliability of frequency estimates.
The results of flood flow frequency analysis can be used for many engineering purposes such as for the design of dams, bridges, culverts, water supply systems, and flood control structures; to determine the economic value of flood control projects; to determine the effect of encroachments in the floodplain; to determine a reservoir stage for real estate acquisition and reservoir use purposes; for flood-plain zoning.
