Current European flood-rich period exceptional compared with past 500 years.

There are concerns that recent climate change is altering the frequency and magnitude of river floods in an unprecedented way1. Historical studies have identified flood-rich periods in the past half millennium in various regions of Europe2. However, because of the low temporal resolution of existing datasets and the relatively low number of series, it has remained unclear whether Europe is currently in a flood-rich period from a long-term perspective. Here we analyse how recent decades compare with the flood history of Europe, using a new database composed of more than 100 high-resolution (sub-annual) historical flood series based on documentary evidence covering all major regions of Europe. We show that the past three decades were among the most flood-rich periods in Europe in the past 500 years, and that this period differs from other flood-rich periods in terms of its extent, air temperatures and flood seasonality. We identified nine flood-rich periods and associated regions. Among the periods richest in floods are 1560-1580 (western and central Europe), 1760-1800 (most of Europe), 1840-1870 (western and southern Europe) and 1990-2016 (western and central Europe). In most parts of Europe, previous flood-rich periods occurred during cooler-than-usual phases, but the current flood-rich period has been much warmer. Flood seasonality is also more pronounced in the recent period. For example, during previous flood and interflood periods, 41 per cent and 42 per cent of central European floods occurred in summer, respectively, compared with 55 per cent of floods in the recent period. The exceptional nature of the present-day flood-rich period calls for process-based tools for flood-risk assessment that capture the physical mechanisms involved, and management strategies that can incorporate the recent changes in risk.

The extreme floods in the Ebro River basin since 1600 CE.

Reliable and complete knowledge of the historical floods is necessary for understanding the extreme hydrological dynamics of the rivers, their natural variability and anthropic changes. In this work we reconstruct the most important floods of the Ebro basin during the last 400 years in different areas of the basin. The analysis is based on four different areas: the Ebro River at Zaragoza, the Cinca River at Fraga, the Segre River at Lleida, and the Ebro River near its mouth at Tortosa. Based on a documentary research, we have first obtained relevant information about the initial conditions (rainfall duration and distribution, snow cover influence) and the maximum flood heights that allow to reconstruct the maximum peak flows by using hydraulic models and to calculate the subbasins contributions. The results show four main types of extreme floods: a) those affecting simultaneously all the subbasins with the highest peak discharges (Ebro at Tortosa in 1787: 0.15 m3 s-1 km-2); b) those originated at the western basin, upstream from Zaragoza, with an Atlantic origin, presenting moderate maximum peak flows, caused by persistent winter rainfall and where snowmelt significantly contributes to the flood; c) those originating at the central Pyrenean subbasins, with Mediterranean origin, occurring, with high peak discharges. These mainly occur during autumn as a consequence of rainfalls of different duration (between 3 days and 1 month), and without significant snow thawing and d) finally, less frequent but very intense flash floods events centered in the Lower Ebro area with low peak flows. In terms of frequency, two different periods can be distinguished: from 1600 until 1850, the frequency of events is low; since 1850 the frequency of events is clearly higher, due to an increase of the climatic variability during last stages of the Little Ice Age. From the 1960's reservoirs construction modifies discharges regime.