The current version of HEC-SSP allows the user to compute mixed population curves. In this type of analysis, multiple frequency curves (that are representative of different causal mechanisms) can be combined using the probability of union concept to compute a frequency curve that reflects the occurrence of more than one type of flood in a year. In many watersheds, multiple types of flood-causing mechanisms are capable of generating large flows in a given year. The annual maximum flood for a given year is the largest of these flows. Generally, the annual maximum series (AMS) for a stream is analyzed without regard for the type of flood that caused each individual annual maximum; however, in many cases the AMS may be made of a mixture of floods of varying cause. A foundational assumption to estimating a population probability distribution from a limited sample is that the sample is representative of the population and made of independent and identically distributed (IID) observations. When multiple flood types create the AMS, the assumption of identical distribution can become tenuous, affecting the ability to make inferences about rare floods. Additionally, the behavior of the right-hand tail typically used in hydrologic studies may be controlled largely by a type of flood mechanism that is less dominant in the more-frequent part of the annual maximum flood frequency curve. Treating multiple flood types as separate, IID samples of floods of a particular cause allows for examination of the frequency of floods per type of flood. Once frequency curves are fit to each mechanism, they can be combined within a mixed population analysis to estimate the occurrence of more than one type of flood in a year.
Examples of this type of analysis can be found here:
