Revisiting the design flood estimation practices under the dynamic unit hydrograph approach

The unit hydrograph (UH) has been a common tool used to represent the complicated processes of surface runoff routing. Key assumption is that the rainfall-runoff transformation is represented through a unit pulse response function of a linear system. The UH shape is mainly determined by the peak and base time, associated with the basin’s response time. However, it is known that the latter is significantly influenced by precipitation and should, thus, be regarded as variable. Consequently, the UH cannot be considered a characteristic basin property, but a dynamic element. In order to employ the concept of the dynamic UH, whose shape is adapted to excess rainfall intensity, an empirical synthetic UH is introduced, with parameters expressed as functions of the time of concentration, combined with the NRCS-CN method. The model’s validity is tested against observed events from basins located in Italy, Greece and Cyprus, and regional formulas are provided explaining the variability of the two parameters (base and peak time) across basins with different characteristics. Finally, a proposal for hydrological design for small basins is presented.