Joint seasonal/annual flood frequency analysis

Flood frequency analysis, as commonly practiced, focuses on the estimation of return periods associated with annual maximum flood peaks of various magnitudes. In some applications, it is desirable to perform joint (i.e., simultaneous) flood frequency analyses on seasonal as well as annual bases. However, a problem one encounters in seasonal flood frequency analysis is that the consistency or interrelationship that must exist between the annual maximum and individual seasonal flood frequency distributions may not be preserved. The most important cause of inconsistencies is that one cannot arbitrarily specify the parametric forms of the annual and all of the seasonal distributions. A correct theoretical analysis of the joint frequency problem would require the use of a rather unusual and complicated distributional model. Since this is not practical, this paper presents two approximate but useful methods for joint frequency analysis using the log Pearson Type 3 distribution. It is shown via examples that the two methods can be applied to reasonably model annual and five seasonal flood distributions in the Tennessee Valley.