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

Hydrol. Earth Syst. Sci., 18, 2735–2772, 2014 www.hydrol-earth-syst-sci.net/18/2735/2014/ doi:10.5194/hess-18-2735-2014 © Author(s) 2014. CC Attribution 3.0 License. Understanding flood regime changes in Europe: a state-of-the-art assessment J. Hall1 , B. Arheimer2 , M. Borga3 , R. Brázdil4,5 , P. Claps6 , A. Kiss1 , T. R. Kjeldsen7 , J. Kriaučiūnienė8 , Z. W. Kundzewicz9,10 , M. Lang11 , M. C. Llasat12 , N. Macdonald13,14 , N. McIntyre15,* , L. Mediero16 , B. Merz17 , R. Merz18 , P. Molnar19 , A. Montanari20 , C. Neuhold21 , J. Parajka1 , R. A. P. Perdigão1 , L. Plavcová1,** , M. Rogger1 , J. L. Salinas1 , E. Sauquet11 , C. Schär22 , J. Szolgay23 , A. Viglione1 , and G. Blöschl1 1 Institute of Hydraulic Engineering and Water Resources Management, Vienna University of Technology, Vienna, Austria 2 Swedish Meteorological and Hydrological Institute, Norrköping, Sweden 3 Department of Land, Environment, Agriculture and Forestry, University of Padova, Padua, Italy 4 Institute of Geography, Masaryk University, Brno, Czech Republic 5 Global Change Research Centre, Academy of Sciences of the Czech Republic, Brno, Czech Republic 6 Department Environment, Land and Infrastructure Engineering (DIATI), Politecnico di Torino, Turin, Italy 7 Department of Architecture and Civil Engineering, University of Bath, Bath, UK 8 Laboratory of Hydrology, Lithuanian Energy Institute, Kaunas, Lithuania 9 Institute for Agricultural and Forest Environment, Polish Academy of Sciences, Poznan, Poland 10 Potsdam Institute for Climate Impact Research, Potsdam, Germany 11 Irstea, UR HHLY, Hydrology-Hydraulics Research Unit, Rue de la Doua, Villeurbanne Cedex, France 12 Department of Astronomy and Meteorology, University of Barcelona, Barcelona, Spain 13 Department of Geography and Planning, School of Environmental Sciences, University of Liverpool, Liverpool, UK 14 Institute of Risk and Uncertainty, University of Liverpool, Liverpool, UK 15 Department of Civil and Environmental Engineering, Imperial College London, London, UK 16 Department of Civil Engineering: Hydraulic and Energy, Technical University of Madrid, Madrid, Spain 17 Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Potsdam, Germany 18 Department for Catchment Hydrology, Helmholtz Centre for Environmental Research – UFZ, Halle, Germany 19 Institute of Environmental Engineering, ETH Zürich, Zurich, Switzerland 20 Department of Civil, Chemical, Environmental and Materials Engineering (DICAM), University of Bologna, Bologna, Italy 21 Federal Ministry of Agriculture, Forestry, Environment and Water Management, Division VII/5 – Flood Control Management, Vienna, Austria 22 Institute for Atmospheric and Climate Science, ETH Zürich, Zurich, Switzerland 23 Department of Land and Water Resources Management, Slovak University of Technology, Bratislava, Slovakia * now at: Centre for Water in the Minerals Industry, Sustainable Minerals Institute, The University of Queensland, Brisbane, Australia ** now at: Institute for Systematic Botany and Ecology, Ulm University, Ulm, Germany Correspondence to: J. Hall () Received: 2 December 2013 – Published in Hydrol. Earth Syst. Sci. Discuss.: 18 December 2013 Revised: 25 April 2014 – Accepted: 14 May 2014 – Published: 30 July 2014 Published by Copernicus Publications on behalf of the European Geosciences Union. 2736 J. Hall et al.: Understanding flood regime changes in Europe Abstract. 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 1.1 Floods are changing – are they? 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). Hydrol. Earth Syst. Sci., 18, 2735–2772, 2014 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 di (...truncated)