Evaluating the Return Period of a Notable Event

Having configured ReFH2 for modelling the observed event of interest and having obtained a satisfactory simulation the next step is to place the simulated flow in the context of event severity.

For an ungauged catchment, the FSR approach to assessing the return period of an individual flood event using the rainfall-runoff method consists the following steps (Houghton-Carr, 1999). This approach is the approach that is also recommended for ReFH2:

• construct the catchment flood frequency curve (FFC) using ReFH2; and
• compare the simulated peak flow of the observed event with the derived FFC and estimate the associated return period.

The rationale for using the FFC derived using ReFH rather than the FEH statistical method is that using the same method minimises any bias introduced by any fundamental differences in the results between the two methods for the catchment in question.