Investigation of factors leading to extreme floods by space-time simulation of rainfall and runoff

Extreme floods are caused by special meteorological conditions matching critical space-time scales of flood generation processes in a catchment. Fortunately for most of the floods these conditions are not meet. However, it is hypothesized that for many events reasonable changes on the flood producing storms would lead to exceptional floods. In order to investigate which factors are most relevant for the maximisation potential of floods a simulation study has been carried out. Event based conditional space-time simulation of short-time step rainfall is applied. So, mainly the space-time patterns and the rainfall intensities are modified conditioned on observed rainfall data. The space-time rainfall is generated by sequential Gaussian simulation with and without considering temporal correlation and advection. The intensities are modified considering the observed saturation deficit. Many space-time rainfall realisations are produced and used as input for a rainfall-runoff model with varying initial conditions. This case study uses data from the Mulde river catchment in Germany and applies the methodology to a set of selected large flood events. The results will reveal how extreme the floods could have become and how much increased rainfall intensities, pattern modification or initial catchment conditions contribute each to the total maximisation potential of the floods.