June 2005, published 15, doi: 10.1098/rsta.2005.1567363 2005 Phil. Trans. R. Soc. A

 Allan McRobie, Tom Spencer and Herman Gerritsen The Big Flood: North Sea storm surge  

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The Big Flood: North Sea storm surge

BY ALLAN MCROBIE1,2, TOM SPENCER

1,3AND HERMAN GERRITSEN

4

1Cambridge University Centre for Risk in The Built Environment,University of Cambridge, The Martin Centre, 6 Chaucer Road,

Cambridge CB2 2EB, UK(fam@eng.cam.ac.uk)

2Department of Engineering, University of Cambridge,Trumpington Street, Cambridge CB2 1PZ, UK

3Cambridge Coastal Research Unit, Department of Geography,University of Cambridge, Downing Place, Cambridge CB2 3EN, UK

4WL/Delft Hydraulics, PO Box 177, 2600 MH Delft-NL, The Netherlands

In the 50 years since the catastrophic southern North Sea storm surge of 31 January–1 February 1953, there have been technological advances in the engineering of floodprotection, increased understanding of physical processes in shallow seas and estuaries,and developments in the mathematical statistics of extreme events. This introductorypaper reviews how the scientific understanding of surge events, their impacts and thehuman responses to them is evolving on many fronts, often across disciplinaryboundaries. The question of how the long-term nature of the problem itself will beinfluenced by possible climate, land use and policy changes is addressed, along with theirassociated uncertainties.

Keywords: flood protection; ocean–atmosphere modelling; sea-level rise;extreme-event statistics; flood-risk management;

climate-change uncertainty analysis

On

Storm surges in the southern North Sea pose a complex, persistent and perhapsgrowing threat to the surrounding coastline of northwest Europe (e.g. Heaps1983; Lamb 1991; IPCC 2001). Only a small proportion of the general populaceof the United Kingdom has much appreciation of what a storm surge is, and thenature of the threat that the North Sea poses. Although more than one millionproperties are at risk from sea and tidal flooding (DEFRA 2004), correspondingto roughly 5% of the population (Jorissen et al. 2000), to most British people, aflood means something that comes down a river as a result of heavy rain ratherthan something which comes from the sea, and travels up river. In contrast, inthe Netherlands, the Dutch people have witnessed the gargantuan constructionof the Delta Project in recent decades. With two-thirds of the Netherlands at riskof flooding and 52% or some 8.5 million of the inhabitants actually living belowsea level, the battle against the North Sea is widely recognized as a safety issue ofcontinuing national importance (figure 1).

Phil. Trans. R. Soc. A (2005) 363, 1263–1270

doi:10.1098/rsta.2005.1567

Published online 15 June 2005

e contribution of 14 to a Theme ‘The Big Flood: North Sea storm surge’.

1263 q 2005 The Royal Society

Floodprone areaBelgiumDenmarkFranceGermanyNetherlandsUnited Kingdom

Datum : 29 juni 2000

0 50 100 150 200 Kilometer

Schaal (A4) 1 : 5 500.000

Ministerie van Verkeer en WaterstaatDirectoraat-Generaal RijkswaterstaatDienst Weg- en Waterbouwkunde

N

Figure 1. Flood-prone areas around (the southern part of) the North Sea (from Jorissen et al. 2000).For the UK, the largest flood-prone areas are found along and up-river of the Humber, the Washand the Thames, the Thames valley being the flood-prone area with by far the largest populationand capital investments at risk.

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Few in central London, be they commuters, tourists, politicians or generalusers of the London Underground, are conscious in their daily business that it isthe Thames Barrier that makes central London viable. Without the quietlyunseen protection of this engineering feat, the capital would be subject tofrequent minor inundations, and the possibility of a major and disastrousinundation would be dangerously high. Without the Thames Barrier, London’sexistence as a major world city would be too precarious and too risky. Like theDelta Project, the momentum for the development of the Thames Barrier aroseafter the last great inundation from the North Sea in 1953 (e.g. Rossiter 1954;Deltacommissie 1961; Pollard 1978; Summers 1978). The events of the evening of31 January and the morning of 1 February 1953, and the resulting aftermath, aredescribed in the first two papers of this issue, with Gerritsen (2005) and Baxter(2005) giving highly personal Dutch and British perspectives, respectively.

This issue arose from a meeting entitled ‘The Big Flood’, held at The RoyalSociety, London in spring 2003, 50 years after that event.1 Over the decades that

1 The meeting was organized by the Cambridge University Centre for Risk in the Built Environment(CURBE) and supported by FloodRiskNet. Sponsors were the British Geomorphological ResearchGroup, Halifax General Insurance Services Limited, the Risk Group, Risk Management Solutions andtheTyndallCentre forClimateResearch (ResearchTheme4 ‘Sustaining the coastal zone’).Aswell as toall contributors, we are grateful to Dr J. Brown (University of Amsterdam), Dr D. Goodman(Foundation for Science and Technology), Professor M. Hulme (Executive Director, Tyndall Centre),Dr I.Kelman (DeputyDirector, CURBE),Dr I.Moller (DeputyDirector, CambridgeCoastal ResearchUnit), Professor R. Spence (Director, CURBE) and Professor A.Watkinson (School of EnvironmentalSciences, University of East Anglia) for inputs into meeting planning and execution.

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have passed since the Big Flood itself, and even since the construction ofthe resulting flood protection, there have been technological advances, increasesin scientific understanding of the physical processes, and developments inmathematical statistics. Moreover, the physical setting is itself ever evolving.Landforms are changing, often in part the result of the changed processenvironment generated by human modification of both open coasts and estuaries(e.g. Clayton 1989; French 1997; Saeijs et al. 2004). The possibility of acceleratedeustatic sea-level rise or changes in storminess as a result of global environmentalchange were only partly anticipated when the great flood protection schemescommenced. Although the Rotterdam Barrier and Eastern Scheldt Barrier weredesigned for a lifetime of 200 years, accounting for 50 cm of sea-level rise, manyof the reinforcements and extensions to the line of flood defence put in place inthe UK after 1953 will approach the end of their design life over the next decade(Select Committee on Agriculture 1998). The potential impacts are notinsignificant: modelling for eastern England (Nicholls & Wilson 2002) suggeststhat under a ‘high’ climate change scenario (sea level rise of 71 cm by 2050 and107 cm by 2080, referenced to 1990) the 1 in 100 year defence standard could bereduced to 1 in 2–8 years by 2050, with many defences at or below the 1 in 1 yearstandard by 2080. All around the margins of the southern North Sea there arelikely to be consequences for coastal ecology and the population dynamics oflittoral communities (e.g. Austin et al. 2001). Furthermore, human populationpatterns have changed and further changes are planned. Thus, for example, theThames Gateway project, a major urban regeneration plan for East London andthe margins of the Thames estuary, envisages the construction of 120 000 newhomes, all in areas designated as high flood risk (ODPM 2003).

In the approach to managing flood risk and ensuring safety, significantgeographical, historical, social, cultural and political differences contribute to avariety of flood protection policies in the countries around the North Sea,especially with regard to the authorities involved and their responsibilities(Jorissen et al. 2000). For the United Kingdom, it has been estimated (Evanset al. 2004) that £22–£75 billion of new engineering works will be required by2080 to implement a portfolio of responses to managing river and coastal floodrisk in the UK, with an annual expenditure of £700 million to £1.1 billioncompared with approximately £500 million at the present time. While the policyframework is centralized, the decision-making is decentralized, often usingeconomic criteria and indicative standards. In the Netherlands, the safety levelagainst coastal flooding is defined in law as designing flood protection works forthe probability of one storm-surge event in 10 000 years for the provinces ofHolland, and 1 in 4000 years for Zeeland, Friesland and Groningen. For thegeneral population, these safety levels and how to maintain them are difficult toimagine as they considerably exceed the average human lifetime. The practicalapproach is simpler: every 5 years, the so-called (offshore) hydraulic boundaryconditions are nationally reviewed and adjusted, based on the latest insights intoNorth Sea processes, climate change, sea-level rise, and the understanding oftheir changes and interactions towards the coast. The state of the sea defences isassessed against these conditions and measures to ensure compliance areproposed. Based on national prioritization and government decisions, upgradeworks and new investments are executed in the following 5 year period, largelypaid for by the Treasury (MV & W 2003). One major upgrade for the period

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2002–2015 concerns improvements of dikes and dunes (estimated cost V715million), following new insights into the relevant wave conditions, notably waveperiod. Partly related is the renovation of block revetments for the sea dikes atan estimated cost of V1.1 billion (MV & W 2003). Over 70% of the regularmaintenance is paid for by the water boards via a system of local annualtaxation, ensuring the basic funds for continuous investment in safety (Jorissenet al. 2000).

The construction of the Thames Barrier did not, once and for all, solve thesafety problem. Even the more recently completed Delta Project is not immuneto subsequent changes in understanding, in geography, demography oratmosphere, leading to sometimes strongly controversial views (e.g. Saeijset al. 2004). The numerous changes and evolutions are such, in a climate ofconsiderable concern and focus on future flood management from a wide varietyof stakeholders (e.g. ABI 2003; Commission of the European Communities 2004;DEFRA 2004; English Nature 2004), that the whole issue of North Sea stormsurge needs to be reviewed. This issue attempts to bring together the work ofmany interested scientists, engineers, planners and insurers to assess the currentstate of knowledge on this subject.

Storm surge is a very broad, interdisciplinary field. It involves, in the firstinstance, meteorology and climatology and the study of the weather patternsthat are likely to lead to surge events. It involves hydrodynamics and the studyof how surges are created and how they develop, together with the study ofcoeval, massive and potentially damaging wind-generated waves. Both atmos-pheric and oceanographic fields have their long-term planning and modellingaspects with time-scales of decades, and both have their real-time operationalforms, on time-scales of hours ahead. The subject involves risk analysis andstatistics to assess the probability of threats from random and unpredictablefuture processes. It involves the engineering response, in terms of the design andplanning of flood protection works and understanding of the mechanics of theformation of breaches in embankments by wave impact. It involves social aspectsof vulnerability, disaster preparedness and urban planning; these issues need tobe properly appreciated and acted upon, not simply reduced to inappropriatequantitative measures, such as cost-benefit analysis. It involves financial aspects,both in the costs of protection and insurance levels for possible losses.

As a result, this issue is inevitably multidisciplinary. We have tried to organizethe papers into a rational denouement. The issue begins with the two scene-setting historical accounts of the 1953 event (Baxter 2005; Gerritsen 2005), eachwith important messages for current day planning and disaster preparedness ofemergency and health services. The volume then moves to the fundamentalclimate science, the oceanography, the hydrodynamics and the wave mechanicsof the ocean–atmosphere physics that underlies the phenomenon. Three papers,by Lowe (2005), Tsimplis et al. (2005) and Wolf & Flather (2005) cover thesefundamental aspects. Lowe, of the UK Meteorological Office, looks at the long-term climatological problem, and describes the results of the large numericalmodels that couple atmosphere and ocean dynamics. The specific aim is to assesshow global warming and other non-stationary processes may be changing themagnitude and the frequency of the threat over the coming century. The paper ofTsimplis et al. tackles the same problem, but from a wider geographical basis,describing the joint work of a number of UK institutions that have looked for

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correlations between broader ocean–atmosphere processes (such as the NorthAtlantic Oscillation) and the more local phenomena of increased frequency ofstorms liable to result in storm surges. The third paper, by Wolf & Flather, bothof the Proudman Oceanographic Laboratory, describes the state-of-the-art inhydrodynamic modelling of wind-induced waves, both looking back, to seewhether such models can reproduce the wave fields observed in the 1953 eventand forward, in terms of what 1953 would look like if it occurred in 2075.

The issue then stays with this longer-term perspective, but changes theemphasis somewhat, away from modelling and more towards the statisticalperspective. The paper by Van den Brink et al. (2005) of the Royal NetherlandsMeteorological Institute, describes how the results of ensembles of simulationsmay subsequently be evaluated by statistical techniques to assess the probabilityof extreme occurrences. The paper by Coles & Tawn (2005) describes howmodern statistical methods may be applied to that same problem. It provides anintroduction to elements of Bayesian inference, and its application to the analysisof extremes of spatio-temporal data.

Two subsequent papers, by Muir-Wood and co-authors, look in detail at theissue of engineering protection works and their reliability. The first (Muir-Woodet al. 2005) looks at catastrophe loss modelling, explaining methods by which riskto building stock on the coasts may be evaluated in specific scenarios. The second(Muir-Wood & Bateman 2005) focuses on the breaching of embankments bywaves and the subsequent inundation of the areas inland. This is a crucial anddifficult aspect of catastrophe loss modelling, yet a difficult one to assess. Thefinancial and insurance aspects are then covered in detail by Berz (2005), in thecontext of assessment of wider environmental risks.

There is then a switch towards the operational problem of real-timeforecasting and monitoring. The state-of-the-art in the Dutch operationaldefence procedures is described by Verlaan et al. (2005) of the National Institutefor Coastal and Marine Management in the Netherlands. The paper shares thesame underlying numerical ocean–atmosphere modelling techniques of the earlierpapers in the volume, but the time-scale is now one of hours. The topic is that ofoperational forecasting of what is about to happen in the next 36 h. It shows howassimilation of recent real-time data can significantly improve the forecasts forthe next 12–15 h, as field observations arrive during the build-up and passage of astorm surge.

The paper by Lavery (2005), of the UK Environment Agency, describes thespecific problems of urban planning in London and the Thames Gateway, and theflood risk management strategy being developed for the area with a view to thenext 100 years. The final paper is by Lord Hunt (2005) and reflects on the wholefield, from the underlying science and study of the physical processes, throughstatistical analysis and on to the policy implications.

There are omissions from the issue. There is no paper on the environmentaleffects that the construction of huge civil engineering defence works along coastsand across estuaries inevitably has on coastal processes and morphodynamics.Examples of such changes include the significant relocation and decrease of ebbtidal deltas owing to closures such as the Zuyderzee, the Lauwerszee and theScheldt estuary branches in the Netherlands (e.g. Kohsiek 1988; Kragtwijk et al.2004). The issue largely takes British and Dutch perspectives, and hasno contribution or comment on the issues affecting the Belgium, German

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and Danish coasts. There is little on the physics of wave impact on sea defences,or even on the detailed design of embankments, barriers and other protectionworks. One of the greatest omissions is the general silence on the thorny issue ofthe uncertainties in long-term extrapolation of ocean–atmosphere models. Theissue is touched upon by numerous papers, but it is still very complex to quantifyuncertainties or probabilities of possibly significant global sea-level rises or majorclimate change rearrangements (Webster et al. 2003; Murphy et al. 2004). Theuncertainties are large and often cumulative. They include those associated withscenarios of greenhouse gas emissions and socio-economic change, those linked tothe process of the mathematical modelling of environmental change and floodrisk, and those allied to the feedbacks between evolving flood risks and the waysin which society and the environment will react and adapt to near-future change.In the last of these factors, the greatest uncertainties appear to be associatedwith those environmental forcings which show the greatest inherent variability,including storminess (e.g. WASA Group 1998), and to which the coastal systemis most finely tuned, such as maximum windspeeds and dominant wavedirections. In all cases, the authors have limited themselves to scenario modellingwith some sensitivity study or extreme value analysis from ensemble runs atgiven scenarios. Despite these omissions, the editors hope that this collection ofpapers from leading exponents and practitioners in the many and various fields ofstorm-surge research will be a useful contribution, not least in helping to build amutually informative debate across the natural and social sciences (Turner 2001;Brown & Damery 2002). In practical terms, we hope these insights will helpprogress the understanding of the ongoing and evolving risks that the southernNorth Sea poses to those whose lives and livelihoods are dependent upon itswaters and its shores.

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