Propensity of rivers to extreme floods:climate-landscape controls and early detection (PREDICTED)

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The occurrence around the world of several major river floods in a short period of time, pictured by sharp upward deviations (called step changes) of the curves used to assess how large floods can be and how often they will occur (called flood frequency curves), recently led citizens, engineers and scientists to wonder whether floods have changed. This is an important issue for social endeavors and economic sectors which require trustworthy evaluations of the flooding hazard of river basins, such as urban planning, the management of water resources and the insurance industry. Alterations of the climate and loss of soil storage due to human activities (e.g. intensive agriculture and urbanization) are often claimed to trigger these phenomena. However, the normal functioning of river basins might also cause floods that appear as extreme because they were not yet recorded in the available data series.The project aims to ascertain whether the interactions between climate and landscape of river basins are able to produce extreme floods, also when no changes of the external drivers (e.g. rainfall) occur. Moreover, it aims to provide handy methods to assess this possibility based on features that can be characterized also in areas where extreme events have not been observed yet and long records of floods are not available. Methods to detect in an objective way the beginning of distinct rises of observed and modeled flood frequency curves (i.e. the position of step changes) will be developed. After verifying that step changes may be more than mere mathematical fakes due to the limited length of the available data, and that modifications of rainfall or land use are not necessarily required for having step changes, attributes of river basins linked to their appearance will be investigated. The focus will be placed on climatic and landscape features that have been suggested as relevant for streamflow generation by a recently proposed simplified description of the processes taking place within river basins. An index which summarizes how these features interact to produce (or not) step changes will be derived. The capability of this index to predict the position of the step change will be tested against observations in a large set of river basins from different geographical regions. The possibility to adopt the step change as a divide between normal and extreme floods will be evaluated as well. The ultimate objective of this project is the development of charts describing the reliability of observed time series for assessing flood hazard in river basins subject to different climatic and landscape settings, or the required length of monitoring to obtain trustworthy estimates of floods. Modifications to these indications of practical interest recommended as a consequence of climate change will be also provided by means of scenarios.