Design flood estimation and its uncertainty under current climate in the Czech Republic

Design flood estimates are essential for the construction and maintenance of hydrological infrastructure. In practice, flood frequency distributions and their parameters are estimated using limited samples of observed data. However, both the estimation process and the data themselves are subject to considerable uncertainty. One of the key challenges in this context is the occurrence of extreme floods. For example, in September 2024, a flood ranking among the most significant hydrological events of the past several decades occurred in the Czech Republic. The Jeseník region was the most affected area. Despite the hydrological drought conditions preceding the event, the flood return period in this region was estimated to be as high as 500 years. An interesting fact is that this region experienced a second extreme flood within less than 30 years (since July 1997). This recurrence highlighted the need to verify existing design flood estimates and proceed with their revision. Frequency analysis of observed annual peak discharges indicates that the design flood with a 100-year return period may be underestimated at some gauging stations in the region by 10–20%. On the contrary, data from recent decades suggest an overestimation of less extreme desing floods with return periods of approximately 1 to 10 years. Consequently, an innovative approach to design flood estimation that incorporates both of these findings is currently being developed.