Resilience assessment of an integrated green-grey-blue-control system for urban flood risk mitigation under changing climate and urbanization

The flooding system in urban catchments needs special consideration and targeted strategies for the prevention and mitigation of flood risks in urban areas as illustrated in the Pitt Review of the widespread urban flooding in Eng-land in 2007 (http://archive.cabinetoffice.gov.uk/pittreview/ thepittreview.html). In the past two decades, much research effort has been devoted to the assessment and analysis of flood vulnerability. The European Commission has been supporting this research since the early 1990s through its successive Framework Programmes for research and tech-nological development. Much of this has focused on large river catchments, coastal erosion and inundation, with only limited research considering the highly complex and dynamic urban context. As at the moment, many countries are embarking on their own urban flood risk management strategy – with greater or lesser effectiveness – there is a need to exchange best practices for flood risk management for these urban catchments at a European level. With this inten-tion, the European Cooperation in Science and Technology (EU COST) office launched the new action C22 – Urban Flood Management (COST-UFM) in 2005. More than 50 scientists and practitioners from 13 European nations coop-erated in this action with the objective to build upon the previous and current European research (such as FLOOD-site) and practice in urban flood risk, to highlight weaknesses of today’s knowledge and practice in urban flood manage-ment, to provide examples of best practice and to help support the European Union in providing a framework for future. The thematic structure of COST-UFM reflected the need for urban flood risk management to deal with the flood problem in a holistic way using resilience as a guiding prin-ciple. Resilience, as applied to the flooding system, is defined here in a very broad sense as the capacity of the whole system to absorb flood waves in annual variability and to reorganize while undergoing change in flood probability or severity. The absorptive capacity has three critical aspects: threshold/ resistive capacity (avoiding flood losses), coping capacity (alleviating flood losses) and recovery capacity (recovering from flood losses). The reorganizing capacity refers to the capacity of the system management (i.e. the individuals and groups acting to manage the system) to influence the threshold/resistive, coping and recovery capacity. In 2009, COST-UFM ended with an international conference. On behalf of the International Hydrological Program of UNESCO and EU COST politicians, policymakers and decision-makers, researchers and practitioners from all over the world attended the closing event of the COST action, the final conference: Road Map Towards a Flood Resilient Urban Environment. This conference was organized jointly by UNESCO-International Hydrological Programme and COST-UFM in the headquarters of UNESCO in Paris on November 26th and 27th, 2009, with the intention to combine the forces of global and European communities. Participants from 23 countries witnessed an event in which a representative selection of the world’s knowledge on this emerging topic of flood risk management was presented and discussed. About 100 papers were selected for presentation. These highlighted the recent advances towards flood-resilient cities. The Editorial Board of the Journal of Flood Risk Management (JFRM) has taken this opportunity to release a special edition of the journal in which a selection of papers from the conference provides information related to the four main topics of the conference. In Section A, three papers address the topic of ‘policy, decision making and the role of institutions’ in the devel-opment of flood-resilient cities. They focus on appropriate policies, regulations, institutions and actors to respond to the pressures and needs to adapt cities to climate change and socio-economic drivers. In Section B, three papers deal with the ‘impact assessment of climate change and anthropo-genic drivers’. These show new ideas of integrated modelling of local-scale climate change and the consequences for urban flood, inundation, vulnerability and damage, including probabilistic methods for the assessment of risk and hazard. Section C covers ‘Resilience Technology and non-structural measures – source, pathway and receptor control’. The selected papers focus on options for: a) flood-resilient-built environment; b) flood-resilient infrastructure; c) SUDS and conveyance systems for exceedance flows; d) management of pluvial, fluvial and coastal flooding; e) emergency responses during and post-flood. Section D addresses the ‘Strategy, communication and capacity building’ to support the transition process from traditional flood defence to flood risk management through empowering the public and others, sophisticated model-based decision support and new learning programmes to help build the capacity of the political, professional and public sectors. From a technical point of view, we have some ideas on how to design a flood-resilient city in which both flood probability and consequences are minimized. But we are still at the beginning in developing strategies and instruments to foster the transition towards implementation of these ideas and concepts in an efficient way. More focus on social and economic issues is required, enhancing the communication between and engagement of all stakeholders. For human societies to be resilient, they must factor in environmental change and long-term variability, whether driven by natural or human forces, rapid or slow. This means that urban com-munities have to deal with uncertainties and have to become ‘active learners’ to engage in new ideas and in order to take advantage of new knowledge as it evolves. New ‘action’ research is beginning to examine and inform how best to communicate the uncertainty of climate change and the consequences of different scenarios with stakeholders. Poli-ticians are sensitive to good examples, as they tend to adopt innovative ideas only if they are sure that these work and have short-term benefits. Learning and communication are regarded as the keys on our common ‘Road towards resilient Cities’. Hopefully, this special issue of the JFRM will be a stimulus for researchers and practitioners to take up this challenge and to take the opportunities now available. The numerous projects of the EU that helps Europe’s regions form partnerships to work together on common projects (INTERREG) and 7th Research Framework are an opportu-nity for major initiatives. But they need coordination through a global initiative such as a ′Flood Resilience Centre’, which should be delivered under the joint auspices of UNESCO and the EU. Hamburg, 21/02/2010

Over the past half-century, the risk of urban flooding in Dar es Salaam has increased due to changes in land cover coupled with climatic changes. This paper aimed to quantify the impacts of climate and land-cover changes on the magnitudes and frequencies of flood runoffs in urban Dar es Salaam, Tanzania. A calibrated and validated SWAT rainfall-runoff model was used to generate flood hydrographs for the period 1969–2050 using historical rainfall data and projected rainfall based on the CORDEX-Africa regional climate model. Results showed that climate change has a greater impact on change in peak flows than land-cover change when the two are treated separately in theory. It was observed that, in the past, the probability of occurrence of urban flooding in the study area was likely to be increased up to 1.5-fold by climate change relative to land-cover change. In the future, this figure is estimated to decrease to 1.1-fold. The coupled effects of climate and land-cover changes cause a much bigger impact on change in peak flows than any separate scenario; this scenario represents the actual scenario on the ground. From the combined effects of climate and land-cover changes, the magnitudes of mean peak flows were determined to increase between 34.4 and 58.6% in the future relative to the past. However, the change in peak flows from combined effects of climate and land-cover changes will decrease by 36.3% in the future relative to the past; owing to the lesser variations in climate and land-cover changes in the future compared with those of the past.

Towards flood risk reduction: Commonalities and differences between urban flood resilience and risk based on a case study in the Pearl River Delta

Despite traditional measures to prevent disasters, climate change and urbanization increase flood risk. Thus, flood resilience has attracted increased global concern. Understanding the commonalities and differences between flood resilience and risk is arguably important for flood risk reduction. However, these factors have been seldom reported in previous studies, and discussions on the role of flood resilience in flood risk analysis, assessment, and management are lacking. In this study, the association between flood resilience and risk is discussed using a case study in the Pearl River Delta. Flood resilience is quantified using a pressure-state-response (PSR) model, while flood risk is assessed based on the hazard-vulnerability framework and the extension catastrophe progression method. The implications of considering flood resilience in flood risk analysis, assessment, and management are proposed. The results suggest that the overall flood resilience (risk) in the study area is greater (lower) than that in the highly urbanized areas, and areas with low (high) flood resilience (risk) are mainly concentrated within the highly urbanized areas. Indices extracted from human society and highly related to human activities have the same attributes in both frameworks, while indices associated with climate and geography contribute to the two con- cepts differently. Flood resilience supplements the concept of flood risk, and can be incorporated into risk assessment as an index. Moreover, pre-disruption (post-disaster) measures should follow flood risk (resilience) assessment, and strategies that foster flood resilience should be included in flood risk management. This study provides references for flood resilience improvement and risk mitigation.

Managing flood risks in a changing climate

In the context of global climate change and intensifying human activities, effective management of urban ecosystem services (ESs) is crucial for achieving sustainable development goals. Urbanization-induced land use change and climate change are two primary drivers altering UESs. However, previous studies have predominantly focused on historical urban expansion and climate change effects on UESs, leaving future evolution scenarios and their relative impacts underexplored. This study investigates the Pearl River Delta urban agglomeration as a case study area, examining the relative impacts of land use and climate change on five UESs: water yield (WY), soil conservation (SC), water purification (WP), urban flood risk mitigation (FM), and urban cooling (UC). The results revealed that all five ecosystem services exhibited high values in the peri-urban areas and low values in the urban core of the Pearl River Delta. WY, FM, and UC showed an overall increasing trend, while SC and WP exhibited fluctuating trends from 1990 to 2050. Changes in SC were predominantly influenced by climate change, accounting for 76.24% of the total area, whereas WY, WP, FM, and UC services were primarily affected by land use change, accounting for over 67.16% of the area. In the future scenarios of 2035 and 2050, the impact of climate change on soil conservation varied the most across different Shared Socio-economic Pathway (SSP) scenarios. The impact of land use change on WY, WP, FM, and UC exhibited the most significant differences across urban areas, indicating a high sensitivity of urbanization-induced ecosystem services to human disturbances.

A probabilistic assessment of urban flood risk and impacts of future climate change

Impacts of combined land-use and climate change on streamflow in two nested catchments in the Southeastern United States

Urban flooding is an escalating threat driven by climate change, rapid urbanization, and altered land use patterns, resulting in increased impervious surfaces and disrupted hydrological cycles. Globally, flood risks have intensified due to rising temperatures and shifting precipitation patterns. Urban populations continue to grow, increasing the vulnerability and exposure of cities to flood hazards. In India, urban centers face frequent flood disasters with significant economic and social impacts. This study reviews recent advances in urban flood modeling, focusing on climatic and land-use influences, with applications of models such as PCSWMM, HEC-HMS, and coupled 1D-2D hydrodynamic approaches. Climate scenarios from IPCC’s latest reports and urban growth projections are integrated to assess future flood risks. Additionally, flood mitigation strategies, including Low Impact Development (LID) practices like detention ponds, permeable pavements, and green roofs, are evaluated for their effectiveness in reducing flood peaks and volumes. Case studies from Indian cities demonstrate the critical need for sustainable urban water management and adaptive infrastructure to enhance resilience against the increasing threat of urban floods induced by climatic and anthropogenic factors.

Climate change and urbanization intensify urban pluvial flooding, posing significant threats to human lives and infrastructure. This situation underscores the critical need for efficient and accurate predictive systems for disaster prevention and mitigation. Traditional flood simulation models, while precise, are often limited by their data-intensive requirements and substantial computational complexity. In contrast, deep learning (DL) models show their advantages by high efficiency and powerful capability in processing large-scale non-linear data, making them highly appropriate for modeling complex flood dynamics. Consequently, integrating DL with conventional urban flood models has emerged as a promising strategy to enhance the accuracy and efficiency of flood prediction systems. However, existing research predominantly focuses on inland flooding, with limited attention to the role of tidal levels in coastal cities, which can significantly impact the accuracy of urban flood simulations.To bridge the GAP, this study proposes an innovative hybrid DL approach that explores spatial and temporal data to improve the accuracy and efficiency of urban flood simulations, particularly in coastal areas. Simulation results from physics-based urban flood models are utilized to construct a comprehensive database for the DL model. Afterwards, patch-size and random sampling methods are employed to construct the sample dataset for training DL models. The convolutional neural network (CNN)-based data-driven urban pluvial flood model can simulate floods using topographic, rainfall, and tidal data, enabling the simulation of large urban areas within seconds. Incorporating diverse input data and advanced network architectures enhances model robustness and generalization across various scales and rainfall events. Fusion models that combine the strengths of DL and traditional hydrological models demonstrate improved prediction accuracy and computational efficiency by integrating tidal data and other environmental factors. Consequently, these hybrid models hold significant potential for integration into early warning systems and supporting decision-making processes in urban flood risk management.

Intensive urbanization and related increase of impervious surfaces, causes negative impacts on the hydrological cycle, amplifying the risk of urban floods. These impacts can get even worse due to potential climate change impacts. The urban areas of the Simeto River Valley (SRV), the largest river valley in Sicily (Italy), have been repeatedly hit by intense rainfall events in the last decades that lead to urban flooding, causing several damages and, in some instances, threats to population. In this paper, we present the results of a 10-question survey on climate change and risk perception in 11 municipalities of the SRV carried out within the activities of the LIFE project SimetoRES, which allowed to collect 1143 feedbacks from the residents. The survey investigated: (a) the level of worry about climate change in relation to extreme storms, (b) elements of urban flooding risk preparedness: the direct experience of the residents during heavy rain events, their trust in a civil protection regional alert system, and their knowledge of the correct behavior in case of flood, and (c) the willingness of citizens to implement sustainable drainage actions for climate change adaptation in their own municipality and real estates. The results show that more than 52% of citizens has inadequate knowledge of the correct behavior during flooding events and only 30% of them feel responsible for mitigation of flooding risk. There is a modest willingness by the population to support the construction of sustainable urban drainage infrastructures. A statistical cross-analysis of the answers to the different questions, based on contingency matrices and conditional frequencies, has shown that a greater worry about climate change has no significant impact either on the behavior of people in dangerous situations occurring during flooding events or on the willingness to support financially sustainable solutions. These results suggest that to build a higher worry about climate change and related urban flooding risk is not sufficient to have better preparedness, and that more direct educative actions are necessary in the area.

<p>Increased urbanization is causing evident negative consequences on the hydrological cycle. In particular, the increase of impervious surfaces is having a strong impact on the water cycle, amplifying the risk of urban floods. These impacts can get even worse for potential climate change impacts. The urban areas of the Simeto Valley, the largest river valley in Sicily (Italy), has been repeatedly hit by heavy rains in the last decades that caused urban flooding causing several problems and, in some instances, threats to population. The threats seem to derive also from a low awareness of the population on the correct behavior to have in potentially dangerous situations. Hence, it seems of key importance that residents develop and internalize a “culture of risk awareness”. The Life SimetoRES Project represents an opportunity to stimulate the development of a responsible and resilient community and at the implementation of best practices for storm water management. In the Simeto River Valley community has started in the recent decades to formally have an identity (for instance, by signing a River Agreement) and has already supported initiatives in the responsible and participatory co-management of the territory. Thus, this Valley represents an excellent context to investigate this problem and to understand the involvement of the citizens in solving climate change and urban floods. In order to maximize the effectiveness of the communication campaigns and the actions to safeguard the community, a study through a survey on the climate change and risk perception in 11 municipalities has been carried out, collecting 1143 answers. Starting from the current hydrogeological risk, quantified by the Flood Risk Management Plan, the goal was to identify the perception and the awareness of the citizens. A section of the questionnaire involved the direct experience of the residents during rain events, their relationship with the alert system and their knowledge of the correct behavior in case of flood. Finally, the survey investigated the willingness of citizens to implement adaptation actions in their own municipality and in their homes. The results show that over 52% of citizens is not aware of the real use of the infrastructures devised for urban drainage and only the 30% feels responsible about mitigation of flooding risk. Inaccurate weather warnings can endanger more inhabitants who don't trust the alert system. The results show that it is necessary to make incisive actions to educate people, especially in school age, on the correct behavior to take in case of urban flooding, and encourage citizens to acknowledge themselves as an active part of the mechanism of their own and community safety.</p>

Planning scale flood risk assessment and prediction in ultra-high density urban environments: The case of Hong Kong

Urbanization coupled with climate change is expected to put pressure on the urban stormwater network. To predict and mitigate the effects of these trends, accurate modeling of urban stormwater changes is required at scales and resolutions meaningful to stormwater management. Although numerous studies have analyzed the effect of climate change on urban flooding risk using the event-based approach, none have incorporated the continuous modelling approach to investigate the whole spectrum of changes in an urban catchment. This study analyzes seasonal changes in future urban hydrological behavior using a mini-ensemble of six state-of-the-art climate model projections and a calibrated hydrological-hydraulic Storm Water Management Model (SWMM). The modelling results show future changes in seasonal and monthly hydrological behavior. The notable winter warming is the major driver in the future snow processes, resulting in considerably less snow days and an increase in the flow events frequency during the winter months. The modelling results also suggest an increase in the annual maximum hourly flow in all seasons, with the clearest trend modelled in winter. Monthly average runoff during the cold period is modelled to increase, while no clear trends are detected for the rest of the year. There is a clear added benefit in using convection-permitting regional climate models throughout the year. Overall, the climate change mitigation and adaption strategies in urban catchments should focus more on the whole spectrum of changes rather than only on urban pluvial flooding risk. These findings call for a transition from traditional to a more advanced stormwater management.

Globally, flood events are considered the costliest natural hazard. Changes in precipitation patterns and large areas of impervious surfaces in urban environments are increasing the sensitivity of these systems to runoff production. At the same time, projected global sea-level rise may further increase the frequency of compound flooding due to simultaneous storm surge, sea-level rise and pluvial runoff that cause vast socio-economic and ecological impacts to coastal cities. In this context, over the last decade, the role of Nature-Based Solutions (NBS) has been recognised to support climate change adaptation by addressing ideas of multi-functionality, non-linearity and heterogeneity in urban design. Thus, increasing awareness about NBS benefits increases the willingness to accept these solutions. However, empirical evidence of NBS effectiveness at the urban catchment scale is still subject to debate. This study develops a spatial biophysical-economic framework that allows for the integrated assessment of NBS flood risk mitigation impacts, costs and benefits in the face of climate change, combining the Integrated Valuation of Ecosystem Services and Tradeoffs (InVEST) model, benefit transfer methods and Geographic Information System (GIS) tools. Specifically, the InVEST Urban Flood Risk Mitigation model was used to assess the biophysical impacts of NBS on urban pluvial flood risk, benefit-transfer methods were used to evaluate the economic implications of such solutions, and GIS was used to integrate and map biophysical impacts and economic implications. For the case of the coastal lagoon city of Aveiro (Portugal), NBS scenarios of green roofs and bioswales under current and future climate conditions were assessed. The main findings of this study show that green roofs scenarios would save 32 % of the flood damages to buildings and infrastructures every year, while bioswales help save only 0.1 %. Moreover, green roofs implementation provides larger benefits in the future climate scenario (representative concentration pathway – RCP – 4.5). The findings confirm the extent to which knowledge on NBS benefits and costs is partial and uncertain, thus requiring constant progress through biophysical-economic assessment to support an evolutive decision making process in climate adaptation planning.