Decadal changes of flood probabilities

Recently there have been a number of major floods in Europe and it seems as if they had increased in number and magnitude. These floods suggest that we may have entered a flood rich period, i.e., a period when floods are more frequent and large than usual. Novel methods and analyses are needed to ascertain whether this is actually the case, and if so why. Specific questions include whether the probabilities of the occurrence of extreme floods have changed in recent years, what are the drivers of such changes, and what is their impact on extreme floods in the future. In the first funding period, methods have been developed for identifying flood-rich and flood- poor periods based on observed flood data in Austria and Germany. Potential drivers of changing floods, such as climate change, land use change and river works, have been identified. The aim of the second funding period is to attribute flood changes to these drivers in a spatially distributed fashion. For example, in small agricultural catchments in the North of Germany, land use change is expected to be the most relevant driver while in the Austrian Alps climate processes may be more important. A formal attribution framework will be developed that accounts for spatially different runoff processes such as short convective rainstorms, long frontal precipitation and snow melt. As large floods are more relevant to society than smaller floods, we will explore how the flood changes and the role of their drivers vary with flood magnitudes. We will expand the flood data back to the sixteenth century by including historical information which will allow us to learn, e.g., whether floods were more frequent in cold or warm periods, which will support future flood predictions. A probabilistic flood change model will be developed that explicitly accounts for flood change mechanisms, such as soil compaction, loss of flood plain retention a nd a transition of snowfall to rain of extreme storms. The model will be used to explore how the drivers may affect future flood probabilities. The project breaks new scientific ground in at least three ways: (a) We will develop new methods for attributing flood changes to their drivers in heterogeneous regions that differ in terms of their flood generation processes. (b) We will explore how flood probabilit y changes and the role of their drivers vary with return period, i.e. if small and large floods have a similar behaviour or not. (c) We will develop a probabilistic flood -change model that explicitly captures change mechanisms and allows predictions of potential future flood changes and their uncertainties. The project, led by Prof. Günter Blöschl and Dr. Andrea Kiss, will improve the understanding of flood changes and their drivers, and will ultimately lead to improved flood estimation methods in both gauged and ungauged basins.

This project investigated the spatial variability of flood drivers, their dependence on flood return periods and changes in future flood probabilities. The idea of this project was to analyse how key statistical properties of flood series vary in space due to catchment size, climate and other controls in Europe. Precipitation-related variables are found to be the main controls on the spatial patterns of mean annual flood across most of Europe, except in regions where snowmelt contributes to flooding, where air temperature is more important. The variability of floods is instead controlled by an aridity index throughout Europe. Overall, the results suggest that climate variables best explain the spatial variability of the statistical properties of flood series and land surface characteristics, such as land use and soil type, are less decisive. A new statistical framework was developed to quantify the contribution of such climatic drivers to flood changes as a function of flood return period in a regional context. The framework was applied to European catchments and showed that in north-western Europe, extreme precipitation mainly contributes to increases in flood, while the contributions of antecedent soil moisture are of secondary importance. In southern Europe, both antecedent soil moisture and extreme precipitation contribute to flood changes and role of soil moisture depends on the return period. In eastern Europe, snowmelt is the most important reason for the decrease of both small and large floods. To investigate future flood probabilities, a new approach combining intensity-duration-frequency statistics of extreme precipitation and floods was proposed and applied to Austrian catchments. The approach consists of analysing elasticities, i.e. the percentage change in flood discharge for a 1% change in extreme rainfall. In wet catchments, elasticities tend to be unity, i.e. rainfall and flood frequency curves have similar steepness due to persistently high soil moisture. In dry catchments, the elasticities are much higher, implying steeper flood frequency curves than those of rainfall, which is interpreted in terms of more skewed distributions of event runoff coefficients. With increasing return period, the elasticities tend towards unity. While regional differences in elasticities can be attributed to both dominant regional rainfall mechanisms and regional catchment characteristics, our results suggest that catchment characteristics are the dominant controls. The overall results of the project provide new and comprehensive knowledge on the drivers of flood changes in Europe, their spatial variability and their dependence on the return period. Climatic drivers are found to control the spatial variability of the statistical properties of flood series and to drive flood changes at different return periods. Catchment characteristics, on the other hand, control the way in which catchments respond to changes in precipitation, which is relevant to the analysis of changes in future flood probabilities.