Understanding flood regime changes in Europe: a state-of-the-art assessment

There is growing concern that flooding is becoming more frequent and severe in Europe. A better understanding of flood regime changes and their drivers is therefore needed. The paper reviews the current knowledge on flood regime changes in European rivers that has traditionally been obtained through two alternative research approaches. The first approach is the data-based detection of changes in observed flood events. Current methods are reviewed together with their challenges and opportunities. For example, observation biases, the merging of different data sources and accounting for nonlinear drivers and responses. The second approach consists of modelled scenarios of future floods. Challenges and opportunities associated with flood change scenarios are discussed such as fully accounting for uncertainties in the modelling cascade and feedbacks. To make progress in flood change research, we suggest that a synthesis of these two approaches is needed. This can be achieved by focusing on long duration records and flood-rich and floodpoor periods rather than on short duration flood trends only, by formally attributing causes of observed flood changes, by validating scenarios against observed flood regime dynamics, and by developing low-dimensional models of flood changes and feedbacks. The paper finishes with a call for a joint European flood change research network. 1 Floods are changing – are they? 1.1 A need for understanding flood regime changes Europe has experienced a series of major floods in the past years: extreme floods in central Europe in August 2002 (e.g. Ulbrich et al., 2003) and in England in summer 2007 (e.g. Marsh, 2008), unprecedented flash flooding in western Italy in autumn 2011 (e.g. Amponsah et al., 2014), and more recently extreme floods in central Europe in June 2013 (e.g. Blöschl et al., 2013a). As these and many other recent floods have exceeded past recorded levels, there is a growing concern that flooding in Europe has become more frequent and severe together with an increasing apprehension that human actions may play a key role in driving these flood changes. There are strong physical arguments regarding a general intensification of the global hydrological cycle, with a general increase in the intensity and frequency of extreme climate events, which are likely to ultimately affect floods (IPCC, 2013). The public’s growing concern on anthropogenic influences and floods seems to be mainly caused by increasing flood damage and comprehensive media coverage of extreme events (Barredo, 2009). In fact, flood damage is rising throughout the world, mainly due to an increase in the value of the assets on floodplains, even though flood protection measures have also been strengthened (e.g. Pielke et al., 2008; Di Baldassarre et al., 2010; Seneviratne et al., 2012). However, not only has the value of assets on the floodplains changed but also the flood discharges may have changed concurrently. Hydrologists have been grappling with understanding and predicting floods since their science began. They are now exploring the question of whether floods are increasing and, if so, why. The difficulty lies in the erratic nature of floods, as one big flood event does not indicate an increasing trend in flooding. Future flood discharges and their exceedance probabilities, together with changes in the time of flood occurrence within the year, are the key variables needed in order to be able to prepare future flood management strategies. The flood discharges of a catchment, their exceedance probabilities and their seasonality are collectively termed “flood regime” in this paper. The purpose of this paper is to review the current understanding of flood regime changes of European rivers – in particular, whether changes have been observed in the past, the drivers of change, what changes are likely to be expected in the future and what methods are currently used to detect and analyse flood changes. Changes in coastal floods are not considered here as they are mainly linked to land subsidence and sea level rise rather than to river flooding. While dam break floods and urban floods may also be very important, this paper specifically focuses on river floods driven by rain and snowmelt (sometimes combined with ice jam), as these are the most common flood types in Europe. Changes in flood impacts such as vulnerability, exposure, perception or emergency management are also not considered as the paper specifically focuses on the changes in the river flood regime. 1.2 Potential drivers of flood regime changes In order to understand flood regime changes it is important to be clear about their physical causes. Merz et al. (2012) defined three groups of potential drivers of change (river channel engineering and hydraulic structures, land use change and climatic change), which are elaborated below and summarised in Table 1.