Editorial: steps towards global flood risk modelling
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In 2002, when colleagues at HR Wallingford, Halcrow and my team (then at the University of Bristol) undertook the first National Flood Risk Assessment (NaFRA) for the Environment Agency (EA), it was a breathtaking exercise. The methodology was hot off the press from the EA's RASP (Risk Assessment for Strategic Planning) project. The Department of the Environment Food and Rural Affairs (DEFRA) had conducted a National Appraisal of Assets at Risk from Flooding and Coastal Erosion in 2001, but the analysis did not take account of the effect of flood defence systems. The EA had then taken the hugely significant step of developing a National Flood and Coastal Defence Database. On top of this novel dataset, we built a flood defence system reliability calculation that took into account the spatial dependence in water levels. We piloted in the Parrett catchment (somewhere that became infamous for flooding last winter) and then pressed the button on the whole of England and Wales. Less than a year later, we were running climate change scenarios and scenarios of socio-economic change as part of the government's Foresight Future Flooding Project, and before long Sir David King was taking the results to the US, Russia, India and China. The race was on for massively broad-scale flood risk analysis. Next stop the world. The Catastrophe (Cat) modellers were making huge steps too, armed with back rooms full of analysts who seldom saw the light of day. One of their team leaders describes the tactic as ‘shock and awe’, a poignant phrase at the time of the 2003 invasion of Iraq. In the Cat modeller's vernacular, this meant releasing a model of some part of the world nobody at the time thought you could model (China, Vietnam), and then fixing the bugs while everyone else recovered from the shock. It is a trick that seemed to work, with a captive market and nobody allowed to look under the bonnet. By 2006, the team at the European Joint Research Centre (JRC) at Ispra was using their LISFLOOD broad-scale hydrological and flood inundation model to develop the first European-scale flood risk estimates, driven by ensembles of regional climate model (RCM) scenarios. Even then, the NaFRA and Foresight results for England and Wales were the only national risk estimate available for comparison with the JRC's results for Europe. The JRC's work was just the start. The EU's WATCH project, which ran from 2007 to 2011, provided new gridded hydrological datasets for the 20th century and multi-model ensembles of RCM and hydrological models. The process of model intercomparison has now taken hold in the Inter-Sectoral Impact Model Intercomparison Project (ISI-MIP): the first assessment of changes in global flood hazard (reported by Rutger Dankers et al. in a paper that appeared online in Proceedings of the National Academy of Sciences in December 2013) compared nine global hydrology and land surface models, along with five climate models. That work modelled hydrology and flood hazard, but global population and economic datasets are also enabling global mapping of vulnerability. The framework published by Philip Ward and colleagues from VU University in Amsterdam in 2013 brought together flood hydrology with socio-economic vulnerability in the latest major step towards global flood risk analysis. A similar path has been followed on the coast, where successive developments of the DIVA model, by Jochen Hinkel and Robert Nicholls, have steadily improved understanding of the global risks of coastal flooding and sea level rise. In global risk analysis, scarcity of information about the location, protection standard and condition of flood defence systems (including dikes, flood control reservoirs, channel modifications and beaches) means that for the time being risk assessments are based upon assumptions, in all but the few places in the world where national flood protection databases exist. I gather that a global database of flood protection standards may be published before too long. Other crucial human adaptations to flood risk are not mapped globally. For example, we do not know about the extent and enforcement of floodplain zoning, nor do we have complete information about the coverage of flood warning systems, even though these are some of the most cost-effective adaptations to flood risk. There remains the fundamental problem of validating flood risk estimates. Risk is not an observable quantity, so risk estimates cannot be validated directly. One route to validation is to scrutinise each element in the calculation and assemble evidence to validate the risk estimate piece by piece. At an aggregate level, in the long run, and all other things being equal, the average of observed damages should approximate to the expected annual damage (EAD). But all other things are not equal. The baseline flood hazard and human vulnerability are moving. Nonetheless, we should expect observed damages and model estimates to be comparable, a challenge that has been laid down by the giant of flood hazard research, Edmund Penning-Rowsell. In his controversial paper, published this year in the Transactions of British Geographers, he interpreted more than 20 years of flood damage data to reach the conclusion that ‘NaFRA appears to overestimate the economic risk by between four and five-fold (i.e. at c.£1.1 bn p.a., as opposed to a central estimate here of £0.25 bn p.a.)’. That is an outcome that does not surprise me – the uncertainties in risk estimates should not be underestimated. Edmund's estimate falls outside the uncertainty range we quoted in the 2002 NaFRA analysis (£0.6–2.2 bn), but that range was based only on uncertainty bounds on the flood defence fragility curves and the depth-damage functions, neglecting other uncertainties. A serious issue is that the effect of water level and dike crest level uncertainties in well-protected locations is asymmetric, but hugely influential. A small upward error in the crest level or downward error in the water level will take an already low risk estimate close to zero, while an error in the opposite direction will yield a large contribution to EAD. There are many other subtle sources of uncertainty. The aim that DEFRA originally had, to use NaFRA to monitor the benefits of its investments in risk reduction, has proven, for the time being, to be hampered by data uncertainties and improvements in methodology (another shifting baseline). That does not, however, mean that models like NaFRA are without merit – on the contrary, NaFRA still provides the best means we have of comparing risks and targeting scarce resources. For sure, it provides a more efficient way of managing risk than waiting for floods to happen and throwing money at the problem after the event. Yet perhaps the most significant observation is that the comparison with a long and reasonably reliable time series of flood damages, in the way that Penning-Rowsell has done for England and Wales, is only feasible in relatively few parts of the world. The Dartmouth Flood Observatory and EM-DAT teams are doing a great job in recording floods and their impacts, but underreporting, especially of relatively small and frequent floods, is a great obstacle to reliable validation of risk estimates. With uncertainties so endemic and model-dependent, it is a relief that the modelling process is becoming more open. Exercises like ISI-MIP, and associated online databases, are reducing the barriers to entry for researchers. Even the black boxes of Cat modelling are opening up in the OASIS open architecture loss modelling framework. The journey is far from over, but the pace of change is remarkable. If we look at engineering hydraulics, or even hydrological science, I would challenge anyone to point to a really significant advance that has been made in the last decade. Yet, in flood modelling, a revolution has been taken place. It is, frankly, a relief to see the first steps we took in 2002 being superseded by better methodologies and datasets. Those methodologies have in a short time made it out of universities and into the offices of analysts and consultants who are now tooled up to do the large numbers of runs that risk analysis requires. Hugely exciting, the steps have been taken, in just a few years, to move right up to the global scale, which presumably is the end of the road. Now all we have to do is fill in the gaps.Keywords:
Futures studies
Flood risk management
Abstract. In recent years, flood management has shifted from protection against floods to managing the risks of floods. In Europe, this shift is reflected in the Flood risk directive of October 2007 (2007/60/EC; FRD). The FRD requires EU Member States to undertake a preliminary assessment of flood risks and, for areas with a significant flood risk, to prepare flood hazard and flood risk maps and flood risk management plans. The purpose of this paper is to introduce the FRD and discuss the challenges that the FRD poses to research. These challenges include the issue how to define and measure ''flood risk'', the selection of alternatives to be assessed, coping with uncertainty, risk communication, nurturing trust and promoting collaboration. These research challenges cannot be addressed properly within any single discipline and without involving the flood risk managers and other stakeholders. The paper therefore concludes that there is a large need for interdisciplinary and participatory research. This constitutes in fact the biggest research challenge.
Flood risk management
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Flood risk management
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Risk Governance
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<p>The flood events of 13-15 July 2021 in Germany brought the relevance of flood prevention acutely and once again to our attention. As the earth's atmosphere heats up, nature has more and more intense events in store for us, which push our flood protection and management measures to their limits and beyond. For planning purposes, but also in case of an event, it is therefore highly relevant to improve the communication of uncertainties and the assessment of their potential impact, e.g. in the climate or flood forecast, in a target group-oriented manner.</p><p>In Germany and in the European Union, the conditions for flood risk management have been improved since 2007 with the implementation of the European Flood Risk Management Directive (FRMD) and the amendments to the Federal Water Act. Many new instruments such as flood hazard and risk maps, building regulations or the category of flood emergence areas were introduced. For example, flood hazard and flood risk maps and corresponding management plans have been prepared on the basis of historical discharge data, water levels and hydrological and hydraulic modelling. However, recent examples have shown that the objective of the FRMD to reduce flood-related risks to human health, the environment, infrastructure and property has only been achieved to a limited extent.</p><p>In this paper we discuss why the developed maps and plans do not lead to a sufficient risk perception and why, in case of a flood event, it is often not clear what actions need to be taken when and by whom. For this, we want to highlight three aspects in particular:</p><p>1) Data: importance of using measured data and dealing with historical flood events, which are only comparable to a limited extent to today's and future conditions, which are shaped by the influences of climate change.</p><p>2) Actors: importance of involving different actors in the flood risk management planning process to strengthen risk perception and responsibility.</p><p>3) Communication: Importance of communicating uncertainties target group-specific and visualising uncertainties and their possible impacts context-specific.</p><p>For effective and sustainable flood risk management, we therefore believe that we are in need of a communication and dissemination strategy in order to contribute to a transparent description of the roles of the actors and their responsibilities. Consequently, the already developed tools (e.g. flood hazard /risk maps) should be supplemented by involving regional actors, uncertainty information and its effects should be classified and communicated to all decision-making levels in a way that is appropriate for the target group.</p>
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Knowledge on the different components of flood risk has much improved over the last decades, but research which fully takes into account not only the interactions between those components but also between different areas in a catchment or delta is still rare. Integrated analyses based on a complete system's approach at sufficiently large scale will improve our understanding of how flood risk systems with flood protection infrastructure in place behave under extreme conditions, it may help to develop sensible long-term strategies, and allows us to better prepare for flood events of all magnitudes. To illustrate the relevance of a hydrodynamic system's approach for flood risk management we analyse the effect of defence breaches on flood risks elsewhere along the lower Rhine River and discuss the use of this knowledge for flood risk management.
Flood risk management
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Flood risk assessment
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The traditional means of flood defence in the UK has been to either increase the capacity of the watercourse or to build barriers between the watercourse and property at risk. The latter approach was used in the city of Grand Forks, North Dakota, USA, but in 1997 the raised defences (levees) were overwhelmed by a flood event with an annual probability of less than 0·5% (i.e. greater than a 1-in-200-year event). Flood management is now superseding the narrower engineered solution of flood defence alone, and while flood management may well include raised defences in some areas, other approaches are also used, as the solution to the Grand Forks flood risk shows. Here large-scale removal of property from one part of the floodplain has prevented that area from incurring further damage and has also provided more 'space' for the river in flood times. Differences between the UK and US situations are highlighted, including the type of flood risk, the role of the city engineer, and lessons that can be learned from the Grand Forks example.
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Alexandria experienced heavy rainfall in October 2015 resulting in wide spread flooding, huge damages and seven deaths. This paper presents the analysis of the hydro- meteorological data to characterise the extremity of the event. The flood map of the city and its adjoining area prepared with LANDSAT-8 satellite images shows the extent of flooding. The analysis with the rainfall forecast from the ECMWF clearly demonstrated that the extreme event could have been predicted days ahead. It is proposed to implement Anticipatory Flood Management in Alexandria (AFMA), which will allow using the extreme rainfall forecast to start pumping out water from Lake Maryot and Airport Lake before the event starts. This will enable extra storage space to accommodate some of the flood water from subsequent rain. An analysis of the October flood showed that 50% of the flood water due to the heavy rainfall could have been stored in the lakes had the AFMA been implemented. The study shows that the existing data allows us to implement AFMA to reduce flood consequences and pave the way to critically decide upon additional mitigation infrastructure. The recommendation of this study is currently being implemented.
Flood stage
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Flood risk management
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Abstract. This paper looks into flood events of the past 500 years in the SW Netherlands, addressing the issue of what kind of flooding events have occurred and which ones have mainly natural causes and which ones are predominantly human induced. The flood events are classified into two major categories: (a) flood events that were caused during storm surges and (b) flood events which happened during warfare. From both categories a selection of flood events has been made. Each flood event is discussed in terms of time, location, extent of the flooded area and specific conditions. Among these conditions, specific weather circumstances and how long they lasted, the highest water levels reached and dike maintenance are discussed as far as flood events caused during storm surges are concerned. Flood events during warfare as both offensive and defensive strategies are relevant; the paper demonstrates that although the strategic flood events obviously were man-made, the natural feature, being the use of fresh water or sea water, of these events also played a major role. Flood events caused during storm surge may have an obvious natural cause, but the extent of the flooding and damage it caused was largely determined by man.
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