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III distribution because the logarithm of zero is minus infinity. The method of combining probabilities recommended for determining frequency curves for records with zero flood years is described in Appendix 5. 6. Mixed Population--Flooding in some watersheds is created by differ- ent types of events. This results in flood frequency curves with abnormally large skew coefficients reflected by abnormal slope changes when plotted on logarithmic normal probability paper. In some situations the frequency curve of annual events can best be described by computing separate curves for each type of event. The curves are then combined. Two examples of combinations of different types of flood-producing events include: (1) rain with snowmelt, and (2) intense tropical storms with general cyclonic storms. Hydrologic factors and relationships operating during general winter rain flood are usually quite different from those operating during.spring snowmelt floods or during local summer cloudburst floods. One example of mixed population is in the Sierra Nevada region of California. Frequency studies there have been made separately for rain floods which occur principally during the months of,November through March, and for snowmelt floods, which occur during the months of April through July. Peak flows were segregated by cause--those predominately caused by snowmelt and those pre- dominately caused by rain. Another example is along the Atlantic and Gulf Coasts, where in some instances floods from hurricane and nonhurricane events have been separated, thereby improving frequency estimates. When it can be shown that there are two or more distinct and generally independent causes of floods it may be more reliable to segregate the flood data by cause, analyze each set separately, and then to combine the data sets using procedures such as described in (11). Separation by calendar periods in lieu of separation by events is not considered hydrologically reasonable unless the events in the separate periods are clearly caused by different hydrometeorologic conditions. The fitting procedures of this guide can be used to fit each flood series separately, with the exception that generalized skew coefficients cannot be used unless developed for specific type events being examined. If the flood events that are believed to comprise two or more populations cannot be identified and separated by an objective and hydrologically meaning- ful criterion, the record shall be treated as coming from one populationo \ , , 15