19 - Changes in Flood Risk: Retrospective and Prospective Approach

Details are given herein of the current main proposals for tidal energy provision from the Severn Estuary, in the UK, with particular emphasis being focused on the Severn Barrage project, as originally promoted by the Severn Tidal Power Group. In particular, emphasis has focused on assessing the potential hydro-environmental impacts and power outputs of a barrage across the estuary, with an unstructured grid, high resolution, model being developed and applied to the estuary to assess the implications of each of five shortlisted proposed schemes on the hydrodynamic, geomorphologic, flood risk and faecal indicator organism changes within the estuary. An outline is given of recent research on power refinements to the model to assess the options for power generation. The results show that the Severn Barrage has the potential to reduce the tidal currents in a highly dynamic estuary. This leads to the reduction of suspended sediment loads (particularly upstream of the barrage), an increase of light penetration within the water column and, potentially, an increase in the benthic bio-diversity and the level of aquatic life in the estuary. The results also show that the Severn Barrage will reduce markedly the risk of flooding upstream of the barrage and to a lesser extent downstream of the structure. In contrast the alternative options have far less impact on flood risk changes. In addition to the Severn Barrage some results are shown herein for a typical lagoon option, namely the Fleming Lagoon.

A geographic information system (GIS)‐based analysis tool, the Taihu Basin Risk Assessment System (TBRAS), is developed to perform scenario analysis of future flood risk changes in the Taihu Basin. The TBRAS integrates modules of climate modelling, hydrological and hydraulic modelling, dyke reliability analysis, socioeconomic analysis, and damage assessment. The results indicate that the TBRAS can provide a broad‐scale simulation of the flooding processes and damage evaluation for the entire Taihu Basin. Future climate change by 2050 in precipitation alone can approximately result in four or five times of risk increase compared with the 2005 baseline risk. Socioeconomic changes alone can increase flood risk by four to six times. The paper reveals that, without any proactive responses, the combination of socioeconomic change, sea‐level rise, and other climate changes can substantially increase the overall flood risks to 15–37 times of the present situation. We therefore conclude that both structural and non‐structural measures for reducing flood risks are required and provide a set of response options for managing and adapting future flood risk changes.

Tropical cyclones have a big impact on flood risk, and understanding how their activity interacts with population exposure under climate change is critical. Here we investigate spatiotemporal changes in flood risk using numerical models together with historical observations and future projections of tropical cyclone tracks. We find that tropical cyclone-related flood risk shifts from the Mekong Delta to the eastern lower Mekong Basin, driven by the interaction between tropical cyclones and population exposure. Historically, extreme precipitation from tropical cyclones increased flood risk in about 14% and decreased in 7% of the basin. Future tropical cyclones may increase flood risk in about 7% and reduce in nearly 18% of the basin. Moreover, population exposure growth has historically increased flood risk in 3% of the basin and is projected to result in a 1% increase. These findings highlight the complex interactions of tropical cyclone hazards and socioeconomic factors influencing flood risk.

Spatially-explicit assessment of flood risk caused by climate change in South Korea

There is a widely-held view that future climate change will increase flood risk in the UK. Extensive modelling studies have been undertaken at CEH to test this hypothesis. The development of continuous river flow simulation techniques for flood frequency estimation now provides a new methodology to predict climate change impacts on flood flows. This technique has been applied to a range of British catchments (work funded by the UK Department for the Environment, Food and Rural Affairs) to assess potential changes in flood risk and directly inform policy development for flood defence scheme appraisal. Using observed climate variables and river flows, the catchment models have been calibrated and validated for the present day. A wide range of climate change scenarios have then been applied to the driving climate data and river flow time series simulated indicative of future time horizons. The flood characteristics of the future have been compared with those of the current. There are many uncertainties associated with this type of climate change impact analysis, including the choice of which Global Climate Model (GCM) to use to derive scenarios of change and precisely how coarse-resolution climate changes (from GCMs) are “downscaled” to the catchment level, as well as hydrological modelling uncertainty. These uncertainties have been assessed, and their effects on the results are discussed. Despite the belief that flood flows will increase in the future, there is no clear message about the size, or even the direction of change in flood flow magnitudes in the UK. Changes in flood flows are catchment-specific, being driven by hydrological variability, such as geology, and the seasonal distribution of rainfall.

Flooding is a major natural hazard in many parts of the world, and its frequency and magnitude are projected to increase with global warming. With increased concern over ongoing climate change, more detailed and precise information about climate-change risks is required for formulating local-scale countermeasures. However, the impacts of biases in climate-model outputs on river-flood simulation have not been fully evaluated, and thus evaluation of future flood risks using hazard maps (high-resolution spatial distribution maps of inundation depths) has not been achieved. Therefore, this study examined methods for constructing future-flood-hazard maps and discussed their validity. Specifically, we compared the runoff-correction method that corrects for bias in general-circulation-model (GCM) runoff using the monthly climatology of reanalysis runoff with the lookup method, which uses the GCM simulation results without bias correction to calculate changes in the return period, and depends on the reanalysis simulation to determine absolute flood depths. The results imply that the runoff-correction method may produce significantly different hazard maps compared to those based on reanalysis of runoff data. We found that in some cases, bias correction did not perform as expected for extreme values associated with the hazard map, even under the historical climate, as the bias of extreme values differed from that of the mean value. we found that the direction of the change in future hazard (increase or decrease) obtained using the runoff-correction method relative to the reference reanalysis-based hazard map may be inconsistent with the changes projected by the GCMs in some cases. On the other hand, we confirmed that the lookup method can produce future-hazard maps that are consistent with the changes in flood risk projected by the GCMs, indicating the possibility of obtaining reasonable inundation-area distribution. The results imply that the lookup method is more suitable for future-flood hazard-map construction than the runoff-correction method. The lookup method also has the advantage of facilitating research on efficient construction of future-climate hazard maps, as it allows for improvement of the reanalysis hazard map through upgrading of the model and separate estimation of changes due to climate change. We discuss future changes in inundation areas and the affected population within the inundation area. Using the lookup method, the total population living in modeled inundation areas with flood magnitudes exceeding the 100-year return period under a future climate would be around 1.8 billion. In the assessment of future climate risks, we found that an affected population of around 0.5 billion may be missed if the historical hazard map is used as an alternative to constructing future-hazard maps and only frequency changes are considered. These results imply that in global flood-risk studies, future-hazard maps are important for proper estimation of climate-change risks, rather than assessing solely changes in the frequency of occurrence of a given flood intensity.

<strong class="journal-contentHeaderColor">Abstract.</strong> Flooding is a major natural hazard in many parts of the world, and its frequency and magnitude are projected to increase with global warming. With increased concern over ongoing climate change, more detailed and precise information about climate-change risks is required for formulating local-scale countermeasures. However, the impacts of biases in climate-model outputs on river-flood simulation have not been fully evaluated, and thus evaluation of future flood risks using hazard maps (high-resolution spatial distribution maps of inundation depths) has not been achieved. Therefore, this study examined methods for constructing future-flood-hazard maps and discussed their validity. Specifically, we compared the runoff-correction method that corrects for bias in general-circulation-model (GCM) runoff using the monthly climatology of reanalysis runoff with the lookup method, which uses the GCM simulation results without bias correction to calculate changes in the return period, and depends on the reanalysis simulation to determine absolute flood depths. The results imply that the runoff-correction method may produce significantly different hazard maps compared to those based on reanalysis of runoff data. We found that in some cases, bias correction did not perform as expected for extreme values associated with the hazard map, even under the historical climate, as the bias of extreme values differed from that of the mean value. we found that the direction of the change in future hazard (increase or decrease) obtained using the runoff-correction method relative to the reference reanalysis-based hazard map may be inconsistent with the changes projected by the GCMs in some cases. On the other hand, we confirmed that the lookup method can produce future-hazard maps that are consistent with the changes in flood risk projected by the GCMs, indicating the possibility of obtaining reasonable inundation-area distribution. The results imply that the lookup method is more suitable for future-flood hazard-map construction than the runoff-correction method. The lookup method also has the advantage of facilitating research on efficient construction of future-climate hazard maps, as it allows for improvement of the reanalysis hazard map through upgrading of the model and separate estimation of changes due to climate change. We discuss future changes in inundation areas and the affected population within the inundation area. Using the lookup method, the total population living in modeled inundation areas with flood magnitudes exceeding the 100-year return period under a future climate would be around 1.8 billion. In the assessment of future climate risks, we found that an affected population of around 0.5 billion may be missed if the historical hazard map is used as an alternative to constructing future-hazard maps and only frequency changes are considered. These results imply that in global flood-risk studies, future-hazard maps are important for proper estimation of climate-change risks, rather than assessing solely changes in the frequency of occurrence of a given flood intensity.

Damage due to floods has increased during the last few decades, and further increases are expected in several regions due to climate change and growing vulnerability. To address the projected increase in flood risk, a combination of structural and non-structural flood risk mitigation measures is considered as a promising adaptation strategy. Such a combination takes into account that flood defence systems may fail, and prepares for unexpected crisis situations via land-use planning and private damage reduction, e.g. via building precautionary measures, and disaster response. However, knowledge about damage-reducing measures is scarce and often fragmented since based on case studies. For instance, it is believed that private precautionary measures, like shielding with water shutters or building fortification, are especially effective in areas with frequent flood events and low flood water levels. However, some of these measures showed a significant damage-reducing effect also during the extreme flood event in 2002 in Germany. This review analyses potentials of land-use planning and private flood precautionary measures as components of adaptation strategies for global change. Focus is on their implementation, their damage-reducing effects and their potential contribution to address projected changes in flood risk, particularly in developed countries.

Chapter 1 - Introduction

Estuarine areas are currently at risk of compound flooding, the frequency and intensity of which is expected to increase with climate change. Even though efforts are made to adapt against single flood drivers using hard protection, potential subsequent changes in flood risk due to compound flooding are often overlooked in flood risk assessments. This is because risk assessments mostly focus on individual flood drivers and do not account for changes in risk from adaptation measures. We address this question and use hydrodynamic modeling to simulate compound flooding for two adaptation scenarios. We consider adaptation in terms of storm surge barriers, in two locations along the Trave estuary, namely Schlutup and Trave, at Lübeck, Germany. We assess the effectiveness of both storm surge barriers in reducing flooding by simulating individual‐driver, as well as low‐ and high‐magnitude compound flood scenarios. We find that while during low‐magnitude compound flooding both barriers reduce the overall flood extent by 25%–86%, high‐magnitude compound flooding leads to an increase of up to 100%, depending on the location of the barrier. Our results suggest that the river contribution is amplified by 52%–100% by the Schlutup Barrier. The Trave Barrier, however, only amplifies flood extents in the high‐magnitude compound flood scenarios. Our findings highlight the need to consider compound flooding in adaptation planning to avoid defense failure and unexpected increases in risk. However, as this study considers only two (low probability) extreme events, a more comprehensive approach is necessary to fully understand the overall impact on risk.

Spatial planning is increasingly being considered as an important mechanism in coping with flood risk due to climate change. One of the reasons for this is that engineering approaches are increasingly expensive and cannot provide complete certainty of protection against climate-related floods. The thesis examines whether and how spatial planning is used in urban areas to promote resilience to flood risk and climate change. In this study, planning is considered as the regulation of physical implementation as well as the process of policy-making that guides spatial development. This process mainly involves the interaction and collaboration between actors (both public and private). The notion of resilience is being used more and more in discussions of complex issues like the impact of climate-related flood risks on spatial development. The interpretations of resilience can vary significantly depending on the local context, the focus of spatial development and the interests of the actors involved in decision- making. The study proposes six characteristics of planning decision-making that can help to promote the resilience of cities. These comprise: (i) considering the current situation, (ii) examining trends and future threats, (iii) learning from previous experience, (iv) setting goals, (v) initiating actions, and (vi) involving the public. The importance of these characteristics over time for policy and practice is examined according to empirical evidence from detailed case study analysis. Six case studies are presented, four in Taiwan and two in the Netherlands. In all of the case studies, the issue of flood risk and spatial development is considered important by policy- makers, but the planning strategies used to tackle climate-related flood risks are often different, as are the experiences of flooding and governance arrangements. The information gathered is primarily based on interviews and the review of planning policies, government reports and research documents. Comparative analysis is a central focus of the study. The analysis has both a national and international perspective, comparing cases within Taiwan and between Taiwan and the Netherlands. The national comparison examines the way in which local planning governance is addressed in shaping decisions to deal with flood risks. This can vary among cases which share similar spatial development objectives and national institutional framework. The international comparison between Taiwan and the Netherlands examines the roles of planning to promote urban resilience in the context of flood risk and climate change. Three conclusions can be drawn. First, the interpretation of resilience is dependent on the views and interests of the actors involved. These change over time and can be seen in different episodes of policy-making. Second, the importance of the different assessment characteristics varies from one case to another. The interests of the leading actors, the interpretations of flood risks and the framework of local collaboration are all major factors that shape these differences. These factors are often associated with planning traditions and relatively stable in resistant to change. Third, when there is a collaborative framework for planning involving multiple actors, the result is a more comprehensive set of strategies in dealing with 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 &amp;#8220;Cities and rising sea levels&amp;#8221; 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.

Urban flooding is an increasingly important issue. Disaster statistics appear to show flood events are becoming more frequent, with medium-scale events increasing fastest. The impact of flooding is driven by a combination of natural and human-induced factors. As recent flood events in Pakistan, Brazil, Sri Lanka and Australia show, floods can occur in widespread locations and can sometimes overwhelm even the best prepared countries and cities. There are known and tested measures for urban flood risk management, typically classified as structural or engineered measures, and non-structural, management techniques. A combination of measures to form an integrated management approach is most likely to be successful in reducing flood risk. In the short term and for developing countries in particular, the factors affecting exposure and vulnerability are increasing at the fastest rate as urbanization puts more people and more assets at risk. In the longer term, however, climate scenarios are likely to be one of the most important drivers of future changes in flood risk. Due to the large uncertainties in projections of climate change, adaptation to the changing risk needs to be flexible to a wide range of future scenarios and to be able to cope with potentially large changes in sea level, rainfall intensity and snowmelt. Climate uncertainty and budgetary, institutional and practical constraints are likely to lead to a combining of structural and non-structural measures for urban flood risk management, and arguably, to a move away from what is sometimes an over-reliance on hard-engineered defenses and toward more adaptable and incremental non-structural solutions.

Flood insurance coverage can enhance financial resilience of households to changing flood risk caused by climate change. However, income inequalities imply that not all households can afford flood insurance. The uptake of flood insurance in voluntary markets may decline when flood risk increases as a result of climate change. This increase in flood risk may cause substantially higher risk-based insurance premiums, reduce the willingness to purchase flood insurance, and worsen problems with the unaffordability of coverage for low-income households. A socio-economic tipping-point can occur when the functioning of a formal flood insurance system is hampered by diminishing demand for coverage. In this study, we examine whether such a tipping-point can occur in Europe for current flood insurance systems under different trends in future flood risk caused by climate and socio-economic change. This analysis gives insights into regional inequalities concerning the ability to continue to use flood insurance as an instrument to adapt to changing flood risk. For this study, we adapt the “Dynamic Integrated Flood and Insurance” (DIFI) model by integrating new flood risk simulations in the model that enable examining impacts from various scenarios of climate and socio-economic change on flood insurance premiums and consumer demand. Our results show rising unaffordability and declining demand for flood insurance across scenarios towards 2080. Under a high climate change scenario, simulations show the occurrence of a socio-economic tipping-point in several regions, where insurance uptake almost disappears. A tipping-point and related inequalities in the ability to use flood insurance as an adaptation instrument can be mitigated by introducing reforms of flood insurance arrangements.

As the most productive ecosystems in the world, wetlands play a vital role in carbon cycling, climate change mitigation, socio-economic development, and natural disaster protection. The Yangtze River Basin (YRB) contains 40% of the national wetlands and the most frequent floods in China. In recent decades, the abundant wetland resources in the YRB have experienced substantial changes due to the climate change and human activities, significantly affecting the flood risk. Due to the lack of a long-term time series wetland dataset with comprehensive categories for the YRB, the effects of wetland variations on flood risk, as well as improved assessments of past and future flood risk incorporating wetland dynamics, remain underexplored. Moreover, wetland-related flood risk mitigation efforts in the YRB are less widely adopted. It is crucial to address these gaps for enhancing the sustainable wetland management and flood risk mitigation in the YRB. This thesis aims to achieve three primary objectives: 1) to establish a long-term time series wetland classification dataset with the comprehensive categories in the YRB and analyze driving forces of their variations; 2) to investigate the long-term wetland effects on the flood risk in the YRB based on an improved GIS-based multi-index flood risk assessment model incorporating wetlands input; and 3) to predict the future flood risk with wetland effects in the middle-lower YRB under climate change and socio-economic scenarios. The Long-term Wetland Classification Dataset for YRB (LTWCD_YRB) with nine wetland categories from 1984 to 2021 was created by using the Random Forest machine learning classifier on the Google Earth Engine platform with 30m resolution Landsat images. The LTWCD_YRB revealed that: 1) the total wetland area of the YRB in 2021 was larger than that in 1984, with a constant increase in human-made wetlands and fluctuating natural wetlands; 2) aquaculture ponds expanded the most by 4,987 km2, while inland marshes in the source region exhibited the most fluctuations; and 3) seasonal changes in wetlands were prominent in the Poyang Lake Basin, Dongting and Honghu Lake Basin, and YRB source region; and 4) human activities were found to be more dominant than natural driving forces in affecting wetlands. The LTWCD_YRB offers a consistent agreement of wetland area variations with the other satellite-based wetland datasets in the YRB. To investigate the long-term effects of wetland variations on flood risks in the YRB, this thesis developed an improved GIS-based multi-index flood risk assessment model incorporating the wetland input obtained from the LTWCD_YRB. The findings indicated that: 1) wetlands in the Taihu Lake Basin, Wanjiang Plain, Poyang Lake Basin, and Dongting and Honghu Lake Basin could mitigate flood risks, while wetlands in the Sichuan Basin have aggravated but limited impacts on flood risks; and 2) Precipitation in the Taihu Lake Basin and Poyang Lake Basin, runoff and vegetation cover in the Wanjiang Plain, GDP in the Taihu Lake Basin, population density in the Taihu lake Basin, Dongting and Honghu Lake Basin, and the Sichuan Basin are dominant flood risk indicators under wetland effects. The wetland-related suggestions to mitigate flood risks including maximizing stormwater storage capacity of wetlands and increasing vegetation coverage in urbanized and precipitated regions. The flood risk prediction of the middle-lower YRB applied the improved flood risk model to assess the flood risk from 2021 to 2100 under the Shared Socioeconomic Pathways (SSP) 2 - Representative Concentration Pathways (RCP) 4.5 and SSP5-RCP8.5 scenarios. The results indicated that: 1) the high and very high flood risk areas will totally cover 38% and 40% of the total study area under the SSP2-4.5 and SSP5-8.5 scenarios by 2100, respectively; 2) the overall flood risk of the MLYRB was predicted to become severer by 2100 under both scenarios; and 3) there would be a prominent northward expansion of the high and very high flood risk areas in Jiangxi, Hunan and the southern part of the Taihu Lake Basin in Jiangsu. In summary, this thesis provides the data support for the long-term wetland variations in the YRB, develops an improved flood risk model to investigate the long-term wetland effects on the flood risk and predicts the future flood risk incorporating wetland dynamics in the YRB. The efforts of thesis contribute to the sustainable wetland conservation and flood risk mitigation in the YRB, aligning with the United Nations Sustainable Development Goals.