Understanding Persistence to Avoid Underestimation of Collective Flood Risk

Human activities have a profound impact on climate and hydrological processes, contributing to changes in the frequency and severity of hydrological extremes and, consequently, growing socioeconomic vulnerability [1]. Rising sea levels, continuous urban development in low-lying coastal areas, and corresponding changes in flood risk have resulted in devastating flood impacts. Different Flood Risk Management (FRM) strategies have been adopted in various socioeconomic contexts and spatiotemporal scales, the most prevalent being structural protection. In recent years, numerous scholars have raised concerns about this approach, as studies have shown that increasing protection levels can increase socioeconomic vulnerabilities e.g., [2]. FRM strategies alter the dynamics of risk manifested in sociohydrological systems, which must be disentangled to avoid unintended consequences.In the “Cities and rising sea levels” project, scientists from different research disciplines, including hydrology, architecture, landscape architecture, and urban planning, collaborate to tackle these challenges. Combining multidisciplinary knowledge has been central to exploring the cross-sectoral processes involved in FRM. In the present study, we focused on (1) uncovering the cascading effects, including unintended consequences of FRM, as well as (2) highlighting the potentials for holistic assessments of FRM strategies.Our methods include the development of a Causal Loop Diagram (CLD) model describing critical sociohydrological processes of coastal cities operating at different spatial and temporal scales. We identified dynamic feedbacks between (1) flood risk, urban development and economic wealth, (2) flood risk, urban development and social equity, and (3) flood risk, trust in authorities, and institutional capacity, among others. . Based on the CLD, we analyzed key feedback mechanisms and their manifestation in theory and practice. Further, we explored the impacts of different FRM strategies on these feedback mechanisms to uncover differences in impacts on socioeconomic vulnerabilities and wider cross-sectoral impacts. The presentation will present and explore the conceptual model through semiquantitative analyses (Fuzzy Cognitive Maps (FCMs)) and spatiotemporal assessments using a specific case study. We aim at (1) getting case-specific insights into the dynamics produced by the local interplay of flooding events and socioeconomic processes influencing vulnerabilities, and (2) suggesting pathways for new integrated ways of FRM.

Globally, cities are rethinking their approaches to urban flood risk and water management in response to the changing hydro-climate, accelerating urbanisation, reductions in public green spaces, and growing recognition of the advantages of multifunctional solutions over piped-drainage systems in tackling social, economic and environmental challenges. In this context, blue–green infrastructure, which comprises a wide range of assets including green and blue roofs, bioswales, rain gardens, restored watercourses and reconnected floodplains, is becoming established as a key component of urban flood resilience and water security planning and design. A Blue–Green City moves beyond the implementation of integrated treatment trains of blue–green and grey assets, towards the goal of creating a more natural urban hydro-cycle capable of generating multiple benefits that are valued by citizens and communities. In Blue–Green Cities these benefits are distributed in ways that are socially equitable, and blue–green+grey systems are maintainable in ways that are economically and environmentally sustainable. This chapter introduces the Blue–Green Cities concept and presents an overview of research undertaken by the Blue–Green Cities research project. The Blue–Green Cities research consortium envisaged and investigated novel flood risk management strategies that could be delivered as part of wider, integrated planning, designed to achieve urban renewal and environmental enhancement, and identify and rigorously evaluate the multiple benefits of blue–green infrastructure. The consortium was centred on ‘stakeholder and community communications’, and employed cutting-edge research methods in the engineering, environmental and social sciences to advance knowledge and understanding, while ensuring that the outcomes of the consortium (in terms of deliverables) exceeded the sum of its parts.

Flood resilience is increasingly discussed in academia and practice as a complement to existing flood risk management approaches (Fekete, Hartmann, & Jüpner 2020). It is seen as a promising concept to deal with increasingly severe consequences of climate change in general, and with increasing flood risk in particular. The debate on flood resilience is linked to the paradigm shift from flood protection to risk management, which started in Europe after the major river flood events in 1993 and 1995 along the river Rhine (Hartmann, 2012), and has developed over the past decades—pushed by further major fluvial and pluvial flood events (Begum, Stive, & Hall, 2007; Hartmann & Juepner, 2014; Klijn, Samuels, & van Os, 2008; Patt & Jüpner, 2020). However, in flood risk management, the academic debate on resilience is only in its infancy (Jüpner et al., 2018; Vis, Klijn, Bruijn, & Buuren, 2003). Many different definitions exist (Disse, Johnson, Leandro, & Hartmann, 2020). Especially when it comes to specific implementation of resilience, the vagueness of the concept—which is sometimes described as one of its strengths (Baggio, Brown, & Hellebrandt, 2015; Brand & Jax, 2007)—can present a problem. How to design a resilient hydraulic infrastructure? This question has not only technical but also concerned with financial and legal aspects. This becomes more complicated when individual protection measures are put in a context of a larger system, such as a city or a region. When and how is a system such as a city or a region resilient, and how does one measure resilience? These questions are important, not only for construction but also for legal (such as liabilities), financial (budgetary), and political reasons. In other words, resilience is not (yet) a ready-to-use concept in flood risk management. Resilience challenges presumptions of traditional flood risk management in many ways. Resilience implies that more and different stakeholders and actors will be involved in flood risk management than before, such as landowners or spatial planners. Hitherto, flood risk management has been and still is mainly the domain of water management (Hartmann & Driessen, 2017), although the prevalence of the traditional civil engineering approach has been questioned before (van den Brink, 2009). This change is not positive for all actors—affected homeowners, but also city planners or mayors now must deal with questions that were previously outside of their realm of competence and responsibility. From a flood risk management perspective, many questions arise, such as: How does resilience add to and change the existing flood risk management system? How can resilience contribute to a more effective and efficient flood risk management approach? What are the specific advantages of integrating resilience in flood risk management? How can resilience be measured and quantified? Which parameters are most relevant? This special collection brings together contributions from different disciplines that address the specific challenges of implementing resilience in flood risk management. The special collection takes the European debate as a starting point. The European Floods Directive from 2007 (2007/EC/60) pushed the debate on flood risk management and resilience in Europe. This Directive established a legal frame on how countries in the European Union have to deal with flood risk for the first time (Hartmann & Jüpner, 2014). At the same time, the shift toward a risk-based approach raised questions such as "what can happen?" "what must not happen?" and "which security level can be realized at which costs?" Ultimately, flood risk management replaces the paradigm of complete protection against floods with the idea of managing the risk (Grünewald, 2005). This makes approaches to resilience and the reduction of damage potential (vulnerabilities)—not only for river floods but also for pluvial rainfall—important. The first three papers in this collection highlight the necessity and consequences of defining the concept of resilience. In their opinion paper, Fekete, Hartmann, and Jüpner (2020) point out that resilience is indeed a trend that neither can nor should be ignored in flood risk management, rather than it should be embraced—with a caveat: the concept is still not exactly defined and cannot be understood as a panacea for all shortcomings and challenges of flood risk management. So, is resilience merely a new buzzword? Dewulf et al. (2019) discuss the power and impact of defining resilience and therewith unravel the political-normative notion of flood resilience as a concept. This illustrates that, with flood resilience, flood risk management turns much more strongly towards social and political science than ever before. Discussing resilience and its definition is thus a crucial step toward successfully implementing it in flood risk management. In her advanced review, Rodina (2019) reveals different notions of resilience in different disciplines when taking on a broader view of the concept (i.e., not solely focusing on flood risks). She concludes that participation and stakeholder involvement are important components of resilience—a theme that is picked up in the next section of this special collection as well. So, though this article had not been part of the original set-up of the special collection, the conclusions are highly relevant for implementing flood resilience in flood risk management. Resilience requires more action by homeowners and citizens. All papers in this section of this special collection leave no doubt about that. Snel, Witte, Hartmann, and Geertman (2020), however, start with a very critical point of departure, namely that many publications on homeowner and citizen involvement do not seem to question if this involvement is necessary. The paper unravels the arguments that are prevalent in the academic debate for more involvement of homeowners in flood resilience, which supposedly comes along with a shift towards flood governance. Four types of main arguments are identified and discussed in the paper. Kuhlicke and the multidisciplinary team of authors writing on the behavioral turn in flood risk management (Kuhlicke et al., 2020) discuss in detail three underlying assumptions of increased involvement of citizens in flood risk management: the motivations of citizens, the effectiveness of measures, and capacities of citizens. In this vein, Rufat et al. (2020) discuss in their opinion paper need for a more nuanced debate on the link between flood risk perception and risk-mitigating behavior. Attems, Thaler, Genovese, and Fuchs (2020) then provide an overview of the measures that homeowners can take to enhance resilience by using property-level flood risk adaptation (PLFRA) measures. The paper confirms that such measures can indeed be an appropriate addition or complement to flood risk management. The third group of papers in this special collection addresses the issue of measuring and communicating resilience. The need for this comes not only from hydraulic engineering but also from the need to communicate about flood risk with homeowners and citizens, as they have to become more involved (as the earlier papers in this special collection clearly show). Pohl (2020) views resilience from a hydrological as well as a hydraulic engineering perspective and outlines what it means to include the concept in flood risk management. In particular, the contribution points out the difficulties of embedding resilience as a design criterion in hydraulic structures. Pohl concludes that—though the fuzziness of resilience is appreciated by some social scientists as a "boundary concept" (Baggio et al., 2015)—hydraulic engineering and hydrology inevitably require some quantifiable measurement of resilience. Fekete (2019) elaborates in his paper how cascading effects and critical infrastructure matter for flood resilience. It thereby shows how flood resilience with this perspective extends existing flood risk management approaches. Cascading effects of critical infrastructure can substantially affect the measurement of flood resilience. Schmitt and Scheid (2020) discuss the need for better risk communication and discuss approaches by focusing on pluvial floods. In particular, they show how a rainstorm severity index may supersede the communicative capacity of statistical rainfall parameters in the communication with nonexperts especially. In conclusion, the contributions in this special collection show how the concept of resilience is advancing flood risk management but also point out some crucial changes and challenges that flood resilience as a concept brings to the practical implementation of flood risk management. It seems inevitable that homeowners especially will play a much more fundamental role in flood resilience, which raises needs for measuring and communication of flood resilience. Along with this shift in flood risk management, the papers illustrate that flood resilience puts flood risk management in a hitherto unknown domain for many water managers, as flood resilience means embracing social and political science. Embedding flood resilience into flood risk management becomes a balancing act—on the one hand, resilience is a political and normative concept, but on the other hand, it needs to be implemented with concrete steps designed via civil engineering and hydrology. This tension is still unsolved and remains a dilemma in implementing flood resilience.

Since flood disasters still occur and even increase in frequency and severity, flood risk management must be reconsidered. This paper describes a new way of looking at flood risk management by applying a systems approach. This approach may result in flood risk management that is better suited to the socio-economic context in which this flood risk management occurs. The systems approach allows the definition of resilience and resistance strategies for flood risk management. Resistance strategies aim at flood prevention, while resilience strategies aim at minimising flood impacts and enhancing the recovery from those impacts. A resilience strategy is supposed to be able to better cope with uncertainties than a resistance strategy. To enable the evaluation of resilience and resistance strategies under different conditions the concepts of resilience and resistance must first be sufficiently understood. This paper discusses the meaning of resilience and resistance and applies the concepts to flood risk management systems. This discussion is exemplified by The Netherlands' flood risk management.

Climate change, urbanization, and inadequate infrastructure exacerbate urban flood risks, yet one critical factor remains largely overlooked: hazardous debris such as cars, construction materials, wood, plastic containers among others. In the Valencia 2024 flood alone, the Spanish Insurance Compensation Consortium reported about 144,000 vehicles damaged or destroyed, many of them mobilized by the flow, which demonstrates the scale of large-debris impacts during floods. Debris alters and intensifies flooding impacts by clogging drainage systems and streets, decreasing flow conveyance, and causing direct damage to infrastructure, lives, and ecosystems. Nevertheless, debris dynamics are largely absent from flood risk assessments and management strategies. This Commentary highlights the urgent need to integrate debris considerations into urban flood planning and emergency response. Using case studies from recent catastrophic floods, we illustrate how debris amplifies hazard. We explore emerging scientific insights into the influence of debris in different flood types (flash, fluvial, coastal, tsunamis), and discuss why current management strategies fail to incorporate this factor. A solution-oriented roadmap is possible and we propose an actionable strategy toward the integration of debris into flood risk management, contributing to adapting cities toward higher levels of safety and resilience.

European urban agglomerations face increasing flood risks due to urbanization and the effects of climate change. These risks are addressed at European, national and regional policy levels. A diversification and alignment of Flood Risk Management Strategies (FRMSs) can make vulnerable urban agglomerations more resilient to flooding, but this may require new Flood Risk Governance Arrangements (FRGAs) or changes in existing ones. While much technical knowledge on Flood Risk Management is available, scientific insights into the actual and/or necessary FRGAs so far are rather limited and fragmented. This article addresses this knowledge gap by presenting a research approach for assessing FRGAs. This approach allows for the integration of insights from policy scientists and legal scholars into one coherent framework that can be used to identify Flood Risk Management Strategies and analyse Flood Risk Governance Arrangements. In addition, approaches for explaining and evaluating (shifts in) FRGAs are introduced. The research approach is illustrated by referring to the rise of the Dutch risk-based approach called ‘multi-layered safety’ and more specifically its application in the city of Dordrecht. The article is concluded with an overview of potential next steps, including comparative analyses of FRGAs in different regions. Insights in these FRGAS are crucial to enable the identification of action perspectives for flood risk governance for actors at the level of the EU, its member states, regional authorities, and public-private partnerships.

A major consequence of climate change is the growing precipitation variability around many parts of the world leading to increased occurrences of floods. While there has been a rich literature on flood risk in the urban context, flood risk in the agricultural sector has been understudied. The goal of this study is to estimate the impact of flood risk on farmland values in a watershed with active agricultural activities. The novelty of the study is to use the spatial boundary discontinuities along floodplains as a way to control unobserved spatial heterogeneities in a regression analysis framework. Using parcel-level data from Lancaster County (Pennsylvania, USA), we show that cropland is more vulnerable to potential flood risk compared to noncrop farmland. Specifically, cropland parcels on average experience a 13% (or $3,895/acre in 2015 USD, with the 95% confidence interval being [$510, $7352]) value reduction if exposed to a 1% or higher annual chance of flooding. The flood risk impact on noncrop farmland is statistically insignificant. The difference between cropland and noncrop farmland in valuing the potential flood risk implies different flood preparedness and risk management strategies. Based on the empirical findings, policy implications for flood insurance, agricultural water management, land-use practice, and other flood mitigation strategies are explored.

Abstract. Public perception of flood risk and flood risk information is often overlooked when developing flood risk management plans. As scientists and the public at large perceive risk in very different ways, flood risk management strategies are known to have failed in the past due to this disconnect between authorities and the public. This paper uses a novel approach in exploring the role of public perception in developing flood risk communication strategies in Europe. Results are presented of extensive quantitative research of 1375 questionnaire responses from thirteen communities at risk across six European countries. The research forms part of two research projects funded under the 2nd ERA-Net CRUE Funding Initiative: URFlood and FREEMAN. Risk perception is conceptualised as a pillar of social resilience, representing an innovative approach to the issue. From this process recommendations are identified for improving flood risk management plans through public participation.

Economic resources to deal with natural hazards are usually limited. In the case of a highly affected territory in terms of floods, this means that they must be spent in an optimal way in order to reduce as much risk as possible. In this situation, flood risk management policies should always be supported by a quantitative scheme so that inequalities can be avoided. Cost–benefit analysis is a common applied tool, albeit it requires an accurate estimation of averted flood damages if their results are to be reliable. In fact, there are several sources of uncertainty in flood risk calculations that must be addressed before any conclusions may be drawn. In addition, other factors, such as social and environmental impacts, should be accounted for too within a multi-criteria analysis. Here we present a comprehensive approach of this sort that has been successfully developed in the Basque Country (northern Spain) in order to define the best possible flood risk management strategy. Starting with a macro-scale analysis to identify areas where a special attention must be paid, it makes use of detailed vulnerability data to select the best mitigation measures at a meso-scale level. This outstanding availability of information, together with a proper validation exercise, makes the methodology feasible and able to provide decision-makers with the necessary arguments to sustain their choices.

Despite an increasing number of people exposed to flood risks in Europe, flood risk perception remains low and effective flood risk management policies are rarely implemented. It becomes increasingly important to understand how local governments can design effective flood risk management policies to address flood risks. In this article, we study whether high flood exposure and flood risk perception correlate with the demand for a specific design of flood risk management policies. We take the ideal case of Switzerland and analyze flood risk management portfolios in 18 flood-prone municipalities along the Aare River. We introduce a novel combination of risk analysis and public policy data: we analyze correlations between recorded flood exposure data and survey data on flood risk perception and policy preferences for selected flood risk management measures. Our results indicate that local governments with high flood risk perception tend to prefer non-structural measures, such as spatial planning and ecological river restoration, to infrastructure measures. In contrast, flood exposure is neither linked to flood risk perception nor to policy preferences. We conclude that flood risk perception is key: it can decisively affect local governments’ preferences to implement specific diversified policy portfolios including more preventive or integrated flood risk management measures. These findings imply that local governments in flood-prone areas should invest in raising their population’s awareness capacity of flood risks and keep it high during periods without flooding.

Intuition suggests that a finite buffer limits the effect of traffic auto-correlations on the queue length. We investigate the extent to which finite buffers can, therefore, be expected to mitigate the effects of long-range dependence (LRD). With traffic sequences generated by the fractional autoregressive integrated moving average (f-ARIMA) models for LRD, and by AR models for short range dependence (SRD), we investigate the traffic performance for a range of finite buffers, both for single and multiplexed streams. For design, the aim is to 'match' a given LRD auto-correlation function with a suitable SRD function whose performance provides, for a wide range of traffic intensities and buffer sizes, a conservative bound on the performance associated with the LRD function. The results suggest that by suitably 'dominating' an LRD auto-correlation function by an SRD function, one can obtain conservative performance bounds for a realistic range of traffic intensities and buffer sizes (delays). Also, in several cases, a 'cross-over' phenomenon is observed for cell-loss probabilities as the buffer size increases, i.e., the loss probabilities are smaller for LRD than for SRD for small buffers, with the converse true for large buffers. This suggests that finite buffers, can in some cases, counteract the effects of LRD in traffic arrivals, and thus enable conservative designs to be based on Markovian traffic models.

Adaptive behaviour has become a crucial aspect in current flood risk management strategies across the globe, especially in response to potential consequences of flood hazards and facing challenges of climate change. There are several factors which influence the motivation to implement flood risk management strategies such as property-level flood risk adaptation (PLFRA) measures. This paper assesses and evaluates the role of risk communication, which is a vital and overarching driver or barrier in the successful implementation of PLFRA measures. We explored this issue through a bootstrapped Q-methodology with 20 residents in the urban area of Graz, Austria, who have been affected by flood events in the past. Additionally, semi-structured interviews concerning risk communication were conducted with the participants to understand the preferred risk communication modes. The results show that respondents have a high level of perceived self-efficacy (most have implemented PLFRA measures), that there is general distrust in public protection measures and that there is a high understanding of residual risk. Considering the communication modes preferred by a majority of respondents, face-to-face interaction with unbiased experts is more attractive than online applications. Additionally, citizens want to be engaged in decision-making processes concerning public protection measures in their area. This calls for participatory processes in flood risk management which involve mutual knowledge transfer and social learning.

Generating pulses and then converting them into flow are two main steps of daily streamflow generation. Three pulse generation models have been proposed on the basis of Markov chains for the purpose of generating daily intermittent streamflow time series in this study. The first one is based on two two-state Markov chains, whereas the second uses a three-state Markov chain. The third model uses harmonic analysis and fits Fourier series to the three-state Markov chain. Results for a daily intermittent streamflow data series show a good performance of the proposed models.

This paper deals with the tire dynamic characteristics, which enable to forecast the trend of vibration signals, obtained from the tire-road system. Theory of the long-range dependence and the finite-element method were employed in this paper. To forecast the resonance effects, we use different frequency modes to avoid the appearance of undesirable inherent frequencies. The vibration intensity of the tire is related to the attrition rate with long-range dependence, which is distinguished by the Hurst effect. The model data can be obtained by the tire vibration sequence from a common pavement. Consequently, we used the fractional auto-regressive integrated moving average approach to forecast the vibration trend and the tire spectrum characteristics simultaneously. The proposed method can be useful for the monitoring of driving performance, tire design, and driving comfort improvement.

The limited understanding of Natural Flood Management (NFM) performance, especially at large hydrological scales, is considered a critical barrier for the further funding and implementation of these nature-based solutions to the increasing international problem of flooding. The publications of the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report and Environment Agency’s National Flood and Coastal Erosion Risk Management Strategy (NFCERMS) for England have shown that extreme weather, including increased likelihood of high magnitude flood events, will occur and will require more novel management methods. This study focused on the ability of co-designed NFM measures to ameliorate downstream fluvial flooding by attenuating catchment response through a highly spatially distributed network of attenuating and roughening measures. Performance was characterised by the ability of NFM to attenuate flood peaks at different spatial scales across a large (187 km2) dendritic catchment, including the lowering of flood peaks and delaying the time-to-peak. Using a coupled modelling methodology and applying it to the upper Stour Valley, Warwickshire-Avon, UK, a rural response to the application of a set of NFM interventions was developed using the hydrodynamic model Flood Modeller Pro and XPSWMM ©. The method demonstrated a means of incorporating local knowledge in a realistic set of NFM schemes, tested to multiple flood risk scenarios (including climate change). Under frequent, smaller design storm events (e.g., Index Flood (QMED) and 3.3% AEP), flood peaks were lowered across all hydrological scales tested (5.8 km2 to 187 km2). As the design flood event severity increases, impact from upstream NFM attenuation on downstream peak response diminished significantly, especially at the largest hydrological scales. However, even at the largest hydrological scale, delays in time-to-peak were noted, increasing the ability of downstream communities to respond and enact flood preparation activities, thus increasing resilience to potential flooding events. While the benefits were limited to large flood events, the modelling indicated that NFM has the potential to reduce downstream flood risk. However, greater integration of observed data to improve model confidence and reduce uncertainty in modelled events is needed, especially the uncertainty associated with using single peaked design storm events from the Flood Estimation Handbook (FEH). This paper proposes a future Before–After Control–Impact (BACI) monitoring programme that could be integrated with models and applied across non-tidally influenced catchments seeking to empirically test the hydrological performance of in-situ NFM.