Flood Risk Science and Management

Gareth Pender, Hazel Faulkner, Gareth Pender, Hazel Faulkner

• Publisher: Wiley-Blackwell
• Year: 2010
• Print ISBN: 9781405186575
• eBook ISBN: 9781444340761
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Environmental Management
• Index: Technology & Engineering

Approaches to avoid loss of life and limit disruption and damage from flooding have changed significantly in recent years. Worldwide, there has been a move from a strategy of flood defence to one of flood risk management. Flood risk management includes flood prevention using hard defences, where appropriate, but also requires that society learns to live with floods and that stakeholders living in flood prone areas develop coping strategies to increase their resilience to flood impacts when these occur. This change in approach represents a paradigm shift which stems from the realisation that continuing to strengthen and extend conventional flood defences is unsustainable economically, environmentally, and in terms of social equity. Flood risk management recognises that a sustainable approach must rest on integrated measures that reduce not only the probability of flooding, but also the consequences. This is essential as increases in the probability of inundation are inevitable in many areas of the world due to climate change, while socio-economic development will lead to spiralling increases in the consequences of flooding unless land use in floodplains is carefully planned.

Flood Risk Science and Management provides an extensive and comprehensive synthesis of current research in flood management; providing a multi-disciplinary reference text covering a wide range of flood management topics. Its targeted readership is the international research community (from research students through to senior staff) and flood management professionals, such as engineers, planners, government officials and those with flood management responsibility in the public sector. By using the concept of case study chapters, international coverage is given to the topic, ensuring a world-wide relevance.

Part 1

Introduction

Chapter 1

Setting the Scene for Flood Risk Management

Jim W. Hall and Edmund C. Penning-Rowsell

The Changing Context of Modern Flood Risk Management

A major shift in approaches to the management of flooding is now underway in many countries worldwide. This shift has been stimulated by severe floods, for example on the Oder (Odra; 1997), Yangtze (1998), Elbe (Labe; 2002), Rhône (2003), in New Orleans (2005), on the Danube (2006) and in the UK (2000, 2007 and 2009). Also important has been a recognition of the relentless upward global trend in vulnerability to flooding and hence losses (Munich Re Group 2007), as well as threats from the potential impacts of climate change on flood frequency. In this context this chapter examines the main characteristics of the emerging approach to flood risk management, as a prelude to the more detailed exploration of methods and models that follows in this volume.

Whilst recent floods have been a stimulus for changing flood risk management policy and practice in the UK (Johnson 2005; Penning-Rowsell 2006), the notion of an integrated risk-based approach to flood management is in fact well established (National Academy of Engineering 2000; National Research Council 2000; Sayers et al. 2002; Hall et al. 2003c). Methods for probabilistic risk analysis have been used for some years in the narrower context of flood defence engineering (CUR/TAW 1990; Vrijling 1993; USACE 1996; Goldman 1997). Indeed the notion of risk-based optimization of the costs and benefits of flood defence was laid out in van Dantzig's (1956) seminal analysis.

However, modern flood risk management no longer relies solely upon engineered flood defence structures, such as dikes, channel improvement works and barriers. It also considers a host of other measures that may be used to reduce the severity of flooding (e.g. land use changes in upstream catchments) or reduce the consequence of flooding when it does occur, by reducing either exposure (White and Richards 2007; Richards 2008) or vulnerability (Tapsell 2002). The criteria for the assessment of flood risk management options are now seldom solely economic (Penning-Rowsell et al. 2005; Johnson 2007a), but involve considerations of public safety (Jonkman and Penning-Rowsell 2008), equity (Johnson 2007b) and the environment (Green 2004). Furthermore, an increasing recognition of non-stationarity (Milly et al. 2008) means that flood risk management involves explicit consideration of the ways in which flood risk may change in future, due, for example, to climate change or the apparently inexorable process of floodplain development (Parker and Penning-Rowsell 2005). This leads to the notion of flood risk management being a continuous process of adaptive management rather than a ‘one-off’ activity (Hall et al. 2003c; Hutter and Schanze 2008).

The locus of power is also changing in many countries as governments seek more effective and efficient institutional arrangements. In the UK, as well as the devolved administrations in Wales and Scotland now taking somewhat different paths to those in England, some features of this new approach are now becoming embedded in flood risk management policy at the level of the European Union (EU), rather than just nationally. This is most notably the case with the European Directive on the Assessment and Management of Flood Risk, which entered into force on 26 November 2007. The Floods Directive (as it is commonly known) sets out a framework for delivering improved flood risk management in all 27 EU member states. The immediate impetus behind the new Directive lies in the significant flooding in central Europe in the preceding decade, which led to pressure on the European Commission to initiate action on flooding (Samuels 2008), but its gestation also coincided with rapidly evolving thinking about the management of flooding and flood risk.

The Directive therefore covers all sources of flooding (not just rivers, but coastal floods, urban and groundwater floods). It requires planning at a basin scale and has specific requirements for international basins; and in all cases, the potential impacts of climate change on the flood conditions need to be considered. By late 2011 preliminary flood risk assessments should be in place in all European river basins, and by late 2013 there will be flood risk maps in all areas with significant risk. Flood risk management plans are to be in place by late 2015; all these are important developments.

These wide-ranging developments in flood risk management in Europe are becoming increasingly linked with broader activity in river basin management, driven by the Water Framework Directive (WFD). This came into force in late 2000 and provides a basis for the management of the ecological status of water bodies, and it includes flood management although not as a primary objective. The links between the WFD and the Floods Directive are fully recognized in the Floods Directive with the requirement to use the same boundaries and administrative structures wherever possible.

The Floods Directive seeks a common European denominator, and hence sets a minimum framework for flood risk management, which is to be interpreted in the context of each of the member states where, in many cases, concepts of flood risk management have been developing for many years. Thus in the aftermath of the severe Rhine River flooding of 1993 and 1995, the Dutch government adopted a flood policy of 'more room for rivers' with an emphasis on establishing new storage and conveyance space. In the UK the Future Flooding project (Evans et al. 2004) stimulated the government's 'Making Space for Water' policy (Defra 2005). In France there has been a series of initiatives to emphasize risk management rather than flood management, through an emphasis on spatial planning (Pottier 2005). There has been corresponding progressive evolution of floodplain management in the USA (Interagency Floodplain Management Review Committee 1994; Galloway 2005; Kahan 2006).

Compelling as the promise of modern integrated flood risk management certainly is, it brings with it considerable complexity. The risk-based approach involves analysing the likely impacts of flooding under a very wide range of conditions and the effect of a wide range of mitigation measures. As the systems under consideration expand in scope and timescale, so too does the number of potential uncertainties and uncertain variables. There are many potential components to a portfolio of ‘hard’ and ‘soft’ flood risk management measures, and they can be implemented in many different sequences through time, so the decision space is potentially huge. Communicating risks and building the consensus that is necessary to engage effectively with stakeholders in flood risk management requires special aptitude for communication, facilitation and mediation (Faulkner et al. 2007).

Characteristics of Modern Flood Risk Management

It has long been recognized that ‘risk’ is a central consideration in providing appropriate flood protection and latterly in flood risk management. In the UK, the Waverley Report (Waverley Committee 1954) following the devastating east coast floods of 1953 recommended that flood defence standards should reflect the land use of the protected area, noting urban areas could expect higher levels of protection than sparsely populated rural areas (Johnson 2005).

However, the practical process of flood defence design, whilst having probabilistic content, was not fundamentally risk based, proceeding somewhat as follows:

1. establishing the appropriate standard for the defence (e.g. the ‘100-year return period’ river level), based on land use of the area protected, consistency and tradition;

2. estimating the design load, such as the water level or wave height with the specified return period;

3. designing (i.e. determining the primary physical characteristics such as crest level or revetment thickness) to withstand that load;

4. incorporating safety factors, such as a freeboard allowance, based on individual circumstances.

Meanwhile, as flood warning systems were progressively introduced and refined in the decades since the 1950s, the decision-making process was also essentially deterministic, based on comparing water level forecasts with levels that would trigger the need for and the dissemination of a warning.

Over the last two decades the limitations of such an approach in delivering efficient and sustainable flood risk management have become clear. Because ad hoc methods for decision-making have evolved in different ways in the various domains of flood risk management (flood warning, flood defence design, land use planning, urban drainage, etc.), they inhibit the integrated systems-based approach that is now promoted.

That systems approach is motivated by the recognition that there is no single universally effective response to flood risk (Proverbs 2008). Instead, portfolios of flood risk management measures – be they ‘hard’ structural measures such as construction of dikes, or ‘soft’ instruments such as land use planning and flood warning systems – are assembled in order to reduce risk in an efficient and sustainable way. The makeup of flood risk management portfolios is matched to the functioning and needs of particular localities and should be adapted as more knowledge is acquired and as systems change.

But there are institutional implications here. Implementing this approach involves the collective action of a range of different government authorities and stakeholders from outside government. This places an increasing emphasis upon effective communication and mechanisms to reach consensus. In this portfolio-based approach, risk estimates and assessments of changes in risk provide a vital common currency for comparing and choosing between alternatives that might contribute to flood risk reduction (Dawson et al. 2008).

The principles of flood risk assessment have become well established (CUR/TAW 1990; Vrijling 1993; USACE 1996; Goldman 1997) and are dealt with in more detail later in this volume. It is worth reviewing here how the risk-based approach addresses some of the main challenges of analysing flooding in systems (Sayers et al. 2002):

1. Loading is naturally variable: The loads such as rainfall and marine waves and surges on flood defence systems cannot be forecast beyond a few days into the future. For design purposes, loads have to be described in statistical terms. Extreme loads that may never have been observed in practice have to be accounted for in design and risk assessment. Extrapolating loads to these extremes is uncertain, particularly when based on limited historical data and in a changing climate.

2. Load and response combinations are important: The severity of flooding is usually a consequence of a combination of conditions. So, for example, overtopping or breach of a sea defence is usually a consequence of a combination of high waves and surge water levels, rather than either of these two effects in isolation. In complex river network systems the timing of rainfall and runoff at different locations in the catchment determines the severity of the flood peak. The severity of any resultant flooding will typically be governed by the number of defences breached or overtopped, as well as the vulnerability of the assets and preparedness of the people within the flood plain. Therefore, analysis of loads and system response is based on an understanding of the probability of combinations of random loading conditions and the system's responses, including the human dimension. Improved understanding of system behaviour has illustrated the importance of increasingly large combinations of variables.

3. Spatial interactions are important: River and coastal systems show a great deal of spatial interactivity. It is well recognized that construction of flood defences or urbanization of the catchment upstream may increase the water levels downstream in a severe flood event. Similarly, construction of coastal structures to trap sediment and improve the resistance of coasts to erosion and breaching in one area may deplete beaches down-drift (Dickson et al. 2007; Dawson 2009) and exacerbate erosion or flooding there, leading to economic damage or environmental harm. These interactions can be represented in system models, but engineering understanding of the relevant processes, particularly sedimentary processes over long timescales, is limited. Even where we have a detailed understanding of the physical processes, there may be fundamental limits to our ability to predict behaviour due to the chaotic nature of some of the relevant processes and loading.

4. Complex and uncertain responses must be accommodated: Models of catchment processes are known to be highly uncertain due to the complexity of the processes involved and the scarcity of measurements at appropriate scales (Beven 2006). The response of river, coast and man-made defences to loading is highly uncertain. The direct and indirect impacts of flooding depend upon unpredictable or perverse human behaviours for which relevant measurements are scarce (Egorova et al. 2008).

5. Flooding systems are dynamic over a range of timescales: Potential f...

Table of contents

• Cover
• Title Page
• Copyright
• Preface
• Contributors
• Foreword
• Acronyms/Glossary of terms
• Part 1: Introduction
• Part 2: Land Use and Flooding
• Part 3: Flood Forecasting and Warning
• Part 4: Flood Modelling and Mitigation
• Part 5: Systems Modelling and Uncertainty Handling
• Part 6: Policy and Planning
• Part 7: Case Studies
• Color Plates
• Index