Coastal Flood Risk Management Research Articles

Coastal urban flood risk management: Challenges and opportunities − A systematic review

Generational mechanisms and spatio-temporal evolution patterns of Coastal urban flood risk involve complex interactions between climate change, sea level rise and human-induced factors, necessitating integrated adaptive flood management strategies to mitigate evolving vulnerabilities. This systematic review offers a thorough assessment of the challenges and strategic opportunities for sustainable adaptation in managing flood risk in Coastal urban areas. It integrates emerging innovative technologies and financial solutions to identify promising approaches to implement mitigation strategies and improve Coastal urban flood resilience. Enhancing governance and policy frameworks is crucial for the successful implementation of Coastal Urban Flood Risk Management (CUFRM) plans. An innovative participatory planning framework is developed to promote flood management practices which are socially inclusive and equitable. Funding for green infrastructure and nature-based solutions and the strategic use of public–private partnerships are effective methods for advancing sustainable flood risk management (FRM). The advancements in emerging technologies, such as artificial intelligence (AI), machine learning (ML), deep learning (DL), social media and digital twin technologies, provide dynamic and collaborative platforms for simulating flood scenarios and have potential to significantly improve CUFRM practices. In the end, a cross-country comparison of current practices in Australia, China, Netherlands, the UK and the USA reveals a diverse range of approaches and valuable insights derived from regional experiences. The review provides a comprehensive analysis for researchers, policymakers and practitioners aiming to improve flood resilience in coastal metropolitan regions by learning from effective UFRM approaches that enhance governance structures, infrastructure resilience and funding mechanisms.

Policy priorities to enable engaged and transformational adaptation on the coast: Learning from practitioner experiences in England

Coastal communities and their environments are facing unprecedented changes, with climate change driving rising global mean sea level, exacerbating extreme sea level events, and increasing hazards. Whilst adaptations to change have been central to coastal life for millennia, climate change brings a speed and intensity of change not previously experienced. Researchers are noting that adaptations are needed that are large scale and systemic with significant changes to lives and livelihoods – Transformational Adaptations – yet there is little evidence of this in practice, and there remains an operationalisation gap between ambitions and actions. This paper uses a qualitative case study method to assess how existing policy may enable and inhibit local stakeholder involvement in transformational adaptation in English coastal flood and erosion risk management. Through twenty interviews with coastal management stakeholders, the capacity for local coastal management stakeholders to initiate transformational adaptation and the perceived involvement of residents are analysed. The results indicate that transformational adaptation remains a distant aspiration in the English coastal management system, with local stakeholders possessing limited capacity to initiate it. The perceived role for residents in adaptation processes is often focused on their being recipients of adaptation interventions, and there are a range of barriers to their further involvement. The paper concludes that despite the theoretical interest in transformational adaptation, there is limited evidence its implementation in English coastal flood and erosion risk management, and there are multiple priority areas for policy development to support capacity for engaged transformational adaptation practices in coastal management contexts.

Challenges for compound coastal flood risk management in a warming climate: a case study of the Gulf Coast of the United States

Compound flooding (CF) events, driven by coincident/concurrent and mutually reinforcing factors such as heavy rainfall, storm surges, and river discharge, pose severe threats to coastal communities around the Globe. Moreover, the exacerbating influence of climate change and sea-level rise further amplifies these risks. This study delves into the complex and multifaceted issue of compound coastal flooding in two freshwater-influenced systems on the Gulf Coast of the United States – Southeast Texas and South Alabama. We first conduct a robust statistical analysis to evaluate the significance of non-stationarity, multi-dimensionality, and non-linearity of interactions among various drivers of CF. Second, to assess the extent to which current flood resilience policies and guidelines account for these characteristics of CF events, we perform a critical review of existing policy documents. The results of the statistical analysis reveal significant compounding and shifts in the statistics of flood drivers that emphasize the pressing need for a multi-mechanism, non-stationary approach to flood hazard assessment. We also found an evident lack of appropriate language/recommendation in policy documents of solid tools that systematically take non-stationarity, multi-dimensionality, and non-linearity of CF into account. By identifying the gaps between current policy measures and the detected complexities of CF, we seek to provide insights that can inform more effective flood resilience policies and design guidelines. Through this robust analysis, we aspire to bridge the divide between research and policy.

Is the coastal future green, grey or hybrid? Diverse perspectives on coastal flood risk management and adaptation in the UK

Abstract Climate change-induced sea level rise has exacerbated coastal change putting millions of people at risk from coastal hazards, such as flooding and coastal erosion. Nature-based solutions have been recognised as an opportunity to simultaneously address the coastal hazard risks and achieve biodiversity goals. While such solutions are included in climate adaptation strategies, “hard” engineered solutions are still often preferred by those implementing the schemes. We sought to explore the diverse perspectives on UK coastal flood risk management among interested and/or affected groups by utilising the Q-methodology. We identified five perspectives: (1) The Pro-Green Practitioners; (2) The Future-Planning Relocators; (3) The Case-by-Case Thinkers; (4) The Cautious Practitioners and (5) The Climate Change Concerned. All five perspectives strongly valued the co-benefits of nature-based solutions and their role in coastal risk reduction. None of the perspectives prioritised hard-engineered solutions as the primary flood protection strategy in the UK, though they recognised their role in protecting essential infrastructure. The main disagreements between perspectives were (1) on the need for relocation strategies, and (2) whether nature-based solutions could cause social inequalities. The Q-methodology does not identify how prevalent such perspectives are, thus further research is needed to assess the social acceptance of nature-based solutions.

The Coastal Genesis 2 research programme: Outputs, Outcomes and Impact

The long-term sediment demand of the Dutch coast is integral to the current Dutch Coastal Flood and Erosion Risk Management policy. The Coastal Genesis 2 research programme was initiated to address the sustainability of this policy under sea level rise by focusing on key uncertainties in the conceptual model of the sediment demand of the Dutch coast. The substantive scientific contributions of the Coastal Genesis 2 research programme are analysed in this paper by applying an output-outcome-impact framework. The direct outputs of the programme are categorised in terms of the knowledge types of a 5-element framework, namely measurement data, simulation models, system understanding, conceptual models, and policy and practice. The research outcomes arise from the interactions of these knowledge types. Our analysis of these outcomes highlights that synthesising new scientific insights into shared conceptual models is critical to achieving impact in policy and practice. In the Dutch situation, a new shared conceptual model of the long-term sediment demand enabled the development of four potential nourishment strategies aiming to meet the strategic goals of the Coastal Flood and Erosion Risk Management policy on a timescale up to 20 years. In 2021, the Minister of Infrastructure and Water Management officially articulated her intention to adopt the advised nourishment strategy from 2024 onwards. This represents a lasting impact of the Coastal Genesis 2 research programme in policy and practice. Further, the insight regarding the pivotal role of shared conceptual models as intermediary between science, policy and practice may prove useful in the design of future research programmes aiming to influence policy.

The complexity of the coastal zone: Definition of typologies in Portugal as a contribution to coastal disaster risk reduction and management

Mainland Portugal presents an extensive, diversified and complex coastal zone. This diversity gives rise to diverse geomorphologic features which support different densities of human occupancy and a range of land uses and activities. In this sense, the coastal zone becomes an area of great national strategic value, but has also into a multi-hazard zone. To understand this complexity, two statistical techniques, namely Cluster Analysis and Principal Component Analysis, were used to identify and differentiate, between the different coastal typologies, based on a set of variables that express the territorial complexity and the occurrences and impacts of coastal flooding. The chosen methodology enabled a diverse set of coastal typologies to be identified, which contributes towards differentiating local specific characteristics. The holistic and differentiating nature of the applied methodology and the results obtained allows it to contribute to the definition of mitigation strategies and the implementation of adaptation measures. The results enable us to define a set of typologies associated with Coastal Disaster Risk Reduction and Management which are an important contribution to comprehensive coastal flood risk assessment and management.

Morphological Analysis of a Nearshore Nourishment along the Atlantic Coast of New Jersey, USA

Nearshore nourishment is a common coastal flood risk management technique that can be constructed beneficially by using dredged sediment from navigation channels. A nearshore nourishment project was completed during the summer of 2021 in Harvey Cedars, NJ, USA, with 67,500 m3 of dredged sediment from Barnegat Inlet placed along approximately 450 m of beach in a depth of 3–4 m. In situ instruments were installed to monitor hydrodynamic conditions before and after dredged material placement, and nine topographic and bathymetric surveys were conducted to monitor nearshore morphological response to the nourishment. Shoreline location was extracted from satellite imagery using CoastSat software to compare historical trends to the shoreline response after construction. Seven months after construction, 40% of the nearshore nourishment was transported from the initial footprint and the centroid of the nourishment migrated towards shore and alongshore (north). The sheltering capacity of the nearshore berm appears to have captured an additional 58% of the placed volume from the longshore transport system and the beach width onshore of the placement increased by 10.9 m. Measured data, satellite imagery analysis, and rapid predictions all indicate that the nearshore nourishment at Harvey Cedars had a positive impact on the adjacent beach.

A typology of responsibility for coastal flood risk adaptation

The management of coastal flood risk is adapting to meet the challenges and increased risks posed by population change as well as by climate change, especially sea level rise. Protection is being targeted to areas where the benefits are highest, while elsewhere there is a shift towards more localized “living with floods” and “resilience” approaches. Such decentralized approaches to flood risk management (FRM) require a diverse range of stakeholder groups to be engaged as “flood risk citizens”. Engagement of households in FRM is central to this process. Despite significant research on stakeholder engagement in coastal and flood risk management, there is less focus on the nature of responsibility in coastal adaptation. There is no framework by which to assess the different types of responsibility in hazard management and adaptation, and little research on the implications of expecting these responsibilities of stakeholder groups. In this paper, we identify five types of responsibility that are embedded throughout the disaster risk reduction cycle of managing coastal flooding. We build this “typology of responsibility” on existing work on the evolution of stakeholder engagement and stakeholder responsibility relationships in risk management processes, and a dataset of institutional stakeholder interviews and households surveys conducted across three case studies in England, the United Kingdom, in 2018 and 2019. We analyze the interviews using thematic analysis to explore institutional stakeholder perceptions of responsibility in coastal FRM, and analyze the household survey through descriptive and inferential statistics. By developing the first disaster risk reduction focused typology of responsibility for coastal flooding, we provide researchers and decision-makers with a tool to guide their planning and allocation of responsibilities in risk management for floods and other climate-driven hazards.

Coastal Natural and Nature-Based Features: International Guidelines for Flood Risk Management

Natural and nature-based features (NNBF) have been used for more than 100 years as coastal protection infrastructure (e.g., beach nourishment projects). The application of NNBF has grown steadily in recent years with the goal of realizing both coastal engineering and environment and social co-benefits through projects that have the potential to adapt to the changing climate. Technical advancements in support of NNBF are increasingly the subject of peer-reviewed literature, and guidance has been published by numerous organizations to inform technical practice for specific types of nature-based solutions. The International Guidelines on Natural and Nature-Based Features for Flood Risk Management was recently published to provide a comprehensive guide that draws directly on the growing body of knowledge and practitioner experience from around the world to inform the process of conceptualizing, planning, designing, engineering, and operating NNBF. These Guidelines focus on the role of nature-based solutions and natural infrastructure (beaches, dunes, wetlands and plant systems, islands, reefs) as a part of coastal and riverine flood risk management. In addition to describing each of the NNBF types, their use, design, implementation, and maintenance, the guidelines describe general principles for employing NNBF, stakeholder engagement, monitoring, costs and benefits, and adaptive management. An overall systems approach is taken to planning and implementation of NNBF. The guidelines were developed to support decision-makers, project managers, and practitioners in conceptualizing, planning, designing, engineering, implementing, and maintaining sustainable systems for nature-based flood risk management. This paper summarizes key concepts and highlights challenges and areas of future research.

The ‘Research for Policy’ cycle in Dutch coastal flood risk management: The Coastal Genesis 2 research programme

The development of the Coastal Genesis 2 research programme and its role in contributing to Dutch coastal policy are described in the paper. The organisation of policy development related to coastal flood risk and erosion in The Netherlands is addressed, highlighting the division of responsibilities between the policy and operational directorates of the Ministry of Infrastructure and Water Management. A conceptual model of the long term sediment budget of the Dutch coast that underpins the current Coastal Flood and Erosion Risk Management policy is detailed. The role of the operational directorate Rijkswaterstaat in coordinating a ‘Research for Policy’ cycle as a means of generating new insights on the coastal system and ensuring their subsequent inclusion in a new/revised conceptual model, is highlighted. By detailing the new conceptual model of the long term sediment budget, the paper demonstrates how key uncertainties related to this model guided the determination of the research agenda for Coastal Genesis 2. The paper concludes by reflecting briefly on the outcomes of the research programme and the role of the ‘Research for Policy’ cycle in ensuring the sustainable future of the Dutch coast.

10 years and going strong? Coastal flood risk management in the wake of a major coastal event (the 2010 Xynthia storm, Charente Maritime, France)

2020 marked the 10th anniversary of the Xynthia storm that hit Western Europe at the end of February 2010. In France it triggered an unprecedented coastal flooding event, with most human and material damage concentrated on the Atlantic coast in the Vendée and Charente Maritime region. A range of reforms and measures followed to manage the risk of coastal flooding at the local and national levels.What conclusions can be drawn from these actions 10 years later? Did Xynthia mark a turning point in the doctrine of coastal flood risk management in France, in a similar way the 1953 storm and associated floods did for the Netherlands?To answer these questions we carried out a two-step analysis. First we compiled, classified and analysed all the recommendations and other “lessons learned” that were made public following the Xynthia storm. Second we looked into local risk management plans and strategies and conducted a series of semi-structured interviews in Charente Maritime to identify current flood risk mitigation practices. We analysed these in light of the identified recommendations.These analyses allow us both to take stock of the past ten years and to identify the acceleration effects attributable to storm Xynthia as well as the effects of lock-in and other elements hindering a more in-depth reform of coastal flood risk management in France.More importantly these analyses allow us to reflect on the process of stocktaking itself, its potential pitfalls and the need for the development of an analytical corpus. Learning to take stock of events that are considered “extraordinary” today is a necessary step to manage climate risks on the coast that are likely to become “ordinary” in the future.

Heterogeneity of values for coastal flood risk management with nature-based solutions

The purpose of this study was to understand preferences for different coastal flood protection measures and the factors that influence such preferences, to inform management and policy. The Discrete Choice Experiment applied to Wales residents revealed that there is willingness-to-pay for coastal flood protection, especially through Nature-based Solutions (NbS) including expanding saltmarsh area and increasing saltmarsh with high vegetation. The preference for NbS provides evidence for including specific coastal area targets in financial schemes initially aimed at other benefits from natural habitats, such as habitat creation for biodiversity. This joint action will maximise the benefits from NbS and ensure integrated and concerted efforts across, often disjointed, sectors. There were also high levels of heterogeneity for preferences in different groups of people. For instance, results support that direct flood experience and damage severity can give rise to behavioural intentions that support mitigation and adaptation measures. Findings also highlight how crucial environmental education and direct contact with the object of study are for securing support and buy-in for flood protection measures. This work is original in that it considers the different types of management for a habitat as NbS and the heterogeneity of preferences within a population. Results are significant in providing a basis for future NbS developments and in supporting flood risk policy and management.

Predictability of Coastal Extreme Wave Heights Based on a Nonstationary Hierarchical Bayesian Model: The Role of the Sea Surface Temperature

Kim, Y.-T.; Uranchimeg, S.; Park, M.H., and Kwon, H.-H., 2021. Predictability of coastal extreme wave heights based on a nonstationary hierarchical Bayesian model: The role of the sea surface temperature. In: Lee, J.L.; Suh, K.-S.; Lee, B.; Shin, S., and Lee, J. (eds.), Crisis and Integrated Management for Coastal and Marine Safety. Journal of Coastal Research, Special Issue No. 114, pp. 191–195. Coconut Creek (Florida), ISSN 0749-0208. The increasing frequency of extreme wave heights has been observed in many parts of the world due to recent global warming. Especially, the variability of extreme wave heights in the summer season has increased during the last three decades over the coast of South Korea that is partially attributable to the increase of the climate variability, leading to the changes in frequency and intensity of the extreme wave heights. Further, coastal areas are particularly vulnerable to the increased climate variability, and the associated risk can be exacerbated by the rapid coastal development over South Korea. Most of the extreme wave heights in South Korea are largely caused by a typhoon during the summer season. Recent studies revealed that the typhoon frequency and intensity in the Asian monsoon region are modulated by Sea Surface Temperature (SST) during the summer season from May to November. Traditional frequency analysis methods do not consider the year to year shifts in wave heights risk distributions that are attributed to changes in climate variability that affect the causal structure of coastal inundation risk. In this perspective, a climate-informed nonstationary frequency analysis model is proposed to predict the extreme wave heights and explore the role of the SST in a Bayesian regression framework. The parameters of the nonstationary frequency model are obtained using a Markov Chain Monte Carlo algorithm. The proposed model shows strong potential for predicting the extreme wave heights by linking large-scale climate patterns, providing motivation for developing dynamic coastal flood risk management strategies.

Long-Term Dynamic of Land Reclamation and Its Impact on Coastal Flooding: A Case Study in Xiamen, China

While land reclaimed from the sea meets the land demand for coastal development, it simultaneously causes socio-economic systems to be prone to coastal flooding induced by storm surges and sea-level rise. Current studies have seldom linked reclamation with coastal flood impact assessment, hindering the provision of accurate information to support coastal flood risk management and adaptation. This study, using Xiamen, China as a case study, incorporates the spatiotemporal dynamics of reclamation into a coastal flood impact model, in order to investigate the long-term influence of reclamation activities on coastal flood inundation and the consequent exposure of the population to coastal flooding. We find that rapid population growth, continual economic development and urbanization drive a substantial logarithmic increase in coastal reclamation. Historical and future expansions of seaward land reclamation are found to cause dramatic surges in the expected annual inundation (EAI) and the expected annual population (EAP) exposed to coastal flooding. In Xiamen, EAI is estimated to increase by 440.2% from 1947 to 2035, owing to continuing land reclamation. Consequently, the population living in the flooded area has also increased sharply: the EAP of total population is estimated to rise from 0.8% in 1947 to 4.7% in 2035, where reclamation contributes over 80% of this increase. Moreover, a future 10 cm sea-level rise in 2035 will lead to extra 5.73% and 8.15% increases in EAI and EAP, respectively, and is expected to cause massive permanent submersion in the new reclamation zone. Our findings emphasize an integration of hard structures and nature-based solutions for building resilient coasts.

Nature-Based Engineering: A Review on Reducing Coastal Flood Risk With Mangroves

Integration of mangroves in projects to reduce coastal flood risk is increasingly being recognised as a sustainable and cost-effective alternative. In addition to the construction of conventional hard flood protection infrastructure, mangroves not only contribute to attenuating flood events (functionality), they also recover in, and adapt to, a changing climate (persistence). The implementation of mangroves in flood risk reduction, however, remains complex. This is because the innate functionality and persistence of mangroves depend on a range of environmental conditions. Importantly, mangroves may collapse when environmental impacts or climatic changes exceed key system thresholds, bringing uncertainty into a situation where failure could endanger lives and livelihoods. The uncertainties in mangrove functionality and persistence can be dealt with by (1) improving insights in how ecological and physical processes affect mangrove functionality and persistence across scales, (2) advancing tools to accurately assess and predict mangrove functionality and persistence, and (3) adopting an adaptive management approach combined with appropriate engineering interventions to enhance mangrove functionality and persistence. Here, we review existing evidence, monitoring techniques and modelling approaches from the viewpoint of mangrove functionality and persistence. Inspired by existing guidelines for Nature-based Solutions (NbS) to reduce flood risk, we provide an operationalization for this new approach. In addition, we identify where further research efforts are required for the practical application of mangroves in coastal flood risk management. Key aspects in the variability and uncertainty of the functionality and persistence of mangroves are their failure and recovery mechanisms, which are greatly site- and storm-specific. We propose five characteristic damage regimes that result in increasing reductions of mangrove functionality as well as post-storm recovery periods. Further research on the quantification of these regimes and their thresholds is required for the successful integration of mangroves in coastal flood risk management. Ultimately, the key challenge is the development of adaptive management strategies to optimise long-term mangrove functionality and persistence, or their resilience. Such adaptive strategies should be informed by continued mangrove functionality and persistence assessments, based on continued monitoring and modelling of key mangrove thresholds, and supported through well-established guidelines.

A Comparative Study of the Effects of the 1872 Storm and Coastal Flood Risk Management in Denmark, Germany, and Sweden

From November 12th to 13th in 1872, an extreme coastal flood event occurred in the south Baltic Sea. An unusual combination of winds created a storm surge reaching up to 3.5 m above mean sea level, which is more than a meter higher than all other observations over the past 200 years. On the Danish, German, and Swedish coasts, about 300 people lost their lives. The consequences of the storm in Denmark and Germany were more severe than in Sweden, with significantly larger destruction and higher numbers of casualties. In Denmark and Germany, the 1872 storm has been more extensively documented and remembered and still influences local and regional risk awareness. A comparative study indicates that the collective memory of the 1872 storm is related to the background knowledge about floods, the damage extent, and the response to the storm. Flood marks and dikes help to remember the events. In general, coastal flood defence is to the largest degree implemented in the affected areas in Germany, followed by Denmark, and is almost absent in Sweden, corresponding to the extent of the collective memory of the 1872 storm. Within the affected countries, there is local variability of flood risk awareness associated with the collective memory of the storm. Also, the economic dependency on flood-prone areas and conflicting interests with the tourism industry have influence on flood protection decisions. The processes of climate change adaptation and implementation of the EU Floods Directive are slowly removing these differences in flood risk management approaches.

Managing coastal flood risk to residential properties in England: integrating spatial planning, engineering and insurance

Flooding is the most damaging natural hazard in England today. Coastal flood risk management aims to reduce the impacts of coastal flooding through adaptation measures including spatial planning, engineered hard and soft interventions, and insurance. Yet there are few reviews which collectively assess these measures. This paper aims to characterise and evaluate coastal flood risk management policy in England across planning, engineering and insurance approaches, focusing on their ability to manage risk to residential properties. An analysis of the literature and government reports reveals that together these management approaches address the different dimensions of flood risk. Nonetheless, the three approaches are legislated and regulated in relative isolation, and in their current formation have contrary implications for existing and future residential developments. There is also further scope to increase the resilience of planning, defence and insurance to social and environmental uncertainties in financing, governance and climate change. We recommend that future research and strategies in coastal flood risk management give greater consideration to multiple flood risk management approaches in conjunction, continuing to expand the integration between planning, engineering and insurance approaches.

The Legacy of Polders: Diagnosing Complex Flooding Processes and Adaptation Options in the Coastal Region of Bangladesh

Polders (enclosed coastal embankments) are often constructed to protect river and coastal floodplains in deltas. In Bangladesh, a total of 139 polders have been constructed between the 1960s and 1980s to safeguard agricultural lands from potential damage due to tidal floods, storm surge floods, and salinity intrusion. Whilst such flood control measures reduce the probability of flooding, they have had complex impacts on geomorphological and hydrological processes with consequence for human wellbeing. This legacy of interventions in the coastal system and subsequent human adaptations, now makes decision making even more challenging at a time of increasing environmental pressures. This thesis analyses historical floods in the coastal zone in Bangladesh, diagnosing whether the floods were attributable to monsoonal precipitation (pluvial flooding), high upstream river discharge into the tidal delta (fluvio-tidal flooding), or cyclone-induced storm surges. It quantifies polders’ effectiveness by modelling different forms of inundation to estimate what flooding might have been had the polders not been constructed. Considering the societal impacts of flooding, the effects of changing land-use and flood risk on poverty are quantified. The results answer the questions of (i) how the pattern of land use/land cover (LULC) change influences flood risk at present and continue to do so in the future; (ii) how LULC change and flood risk impact poverty spatially. The last section of the thesis focuses on estimating the impacts of potential Tidal River Management (TRM) on elevating the floodplains and reducing the annual probability of flooding. It identifies suitable TRM sites to model sediment deposition in those areas. The potential impact of TRM on flooding is investigated by comparing flood susceptibility ‘before and after’ the implementation of TRM. By analyzing complex coastal flood processes and adaptation options spatially and quantitatively, this thesis provides new evidence to the effectiveness of the past (polders), and potential future (TRM) adaptation options. It also presents a new approach to flood risk estimation, considering the societal impacts of flooding. The results of this thesis could be of importance both in research and practice for making scientifically informed decisions on coastal flood risk management, particularly in Bangladesh.

On the feasibility of managed retreat in the Wadden Sea of Schleswig-Holstein

Sea embankments along the mainland coastline in the Schleswig-Holstein Wadden Sea safeguard 130,000 people and 19 billion € of capital assets from flooding during storm surges. Due to climate change induced sea-level rise, these defences will become exposed to higher storm surges and adaptation becomes inevitable. As an alternative to strengthening, managed retreat is discussed in literature as a coastal risk management option for climate change adaptation. Based on an evaluation of examples from the Wadden Sea, this paper elaborates managed retreat as alternative climate change adaptation from a coastal risk management perspective. This paper concludes that, conform the principles of integrated coastal zone management, one precondition for successful implementation of managed retreat is local acceptance. With respect to natural resilience to sea-level rise, managed retreat may counteract the loss of Wadden Sea structures and habitats due to coastal squeeze. However, the effectiveness depends on the surface elevation in the opened polder. If large volumes of sediment are needed to restore a natural elevation, regional resilience against SLR-induced drowning may deteriorate. With respect to coastal flood risk management in Schleswig-Holstein, opened polders have no significance as flood retention rooms and managed retreat does not reduce the hazard of flooding in surrounding polders. Further, removal of primary embankments requires large efforts to secure flood safety in adjacent polders. Under certain conditions, removal or opening of embankments may constitute sustainable coastal risk management, e.g., if retreat leads to a shorter line of defence or if other win-win-situations like securing of NATURA 2000 coherence arise.

Optimization of multiple storm surge risk mitigation strategies for an island City On a Wedge

Managing coastal flood risks involves selecting a portfolio of different strategies. Analyzing this choice typically requires a model. State-of-the-art coastal risk models provide detailed regional information, but they can be difficult to implement, computationally challenging, and potentially inaccessible. Simple economic damage models are more accessible but may not incorporate important features and thus fail to model risks and trade offs with enough fidelity to support decision making. Here, we develop a new framework to analyze coastal flood risk management. The framework is computationally inexpensive yet incorporates common features of many coastal cities. We apply this framework to an idealized coastal city and assess and optimize two objectives using combinations of risk mitigation strategies against a wide range of future states of the world. We find that optimization using combinations of strategies allows for identification of Pareto optimal strategy combinations that outperform individual strategy options.