Compound heavy rainfall and river flood events in small catchments

Heavy rainfall and flooding are extreme events with high hazard and risk potential for people and the environment. While there has been extensive research on the individual events of heavy rainfall and flooding, there is still a significant need for research into their combination. Analysing them separately may lead to an underestimation of the hazards and risks involved. Especially in small catchment areas or headwater catchments, a heavy rainfall event can cause flash flooding and a flood event in a river simultaneously. The relationship between the investigated area and its associated hydrological catchment area is crucial. In such areas, classic flood protection measures may not always be the most sensible option. Targeted measures and precautions must be taken, and knowledge of these events is of great importance.

Analyses are carried out to understand the relationship between combined heavy rainfall and flood events and the characteristics of the study area. For this purpose, we use precipitation radar data from the German Weather Service (RADKLIM, 5 min & 1x1 km) and discharge data from water gauges provided by the NRW State Office for Nature, Environment and Consumer Protection, as well as various water associations. The data is used to generate area averages of precipitation of various durations for the catchment areas under investigation. The compound events are analysed using value pairs, which are defined based on AMAX or threshold values. Statistical extreme value analyses were conducted on individual events using different extreme value distributions, including the metastatic and general extreme value distributions. These analyses were necessary to determine the probabilities of compound events such as heavy rainfall and flooding, with the aid of copula functions. The joint occurrence probabilities and correlation between precipitation and runoff events along a watercourse are also considered.

The results of these analyses provide insights into the integral consideration of floods and heavy rainfall, particularly in small catchment areas and during short heavy rainfall events. In addition, these methods can be used to define the joint probabilities of occurrence of these events. Furthermore, the investigations can not only contribute to a more precise assessment of the hazards and risks of compound events, but can also serve as a starting point for integral heavy rain and flood hazard maps. The maps derived from this can make a valuable contribution to the development of more precise and comprehensive risk management strategies.