Flooding is the most damaging and costly hydrometeorological hazard affecting millions of people globally every year. In Viet Nam, low-lying coastal cities, particularly in river deltas, face increased flood risk and vulnerability due to rapid urban development and climate change. To reduce flood risk in urban areas, the recent decade has seen increased appreciation of the potential of Blue/Green Infrastructure (BGI), such as natural and man-made wetlands, vegetated river banks and restored floodplains, to reduce flood risk and provide additional benefits, such as controlling water pollutants, providing recreational opportunities, improving air quality and increasing resilience to other stressors, such as heat waves and noise pollution. However, despite the growing interest in BGI in a flood risk management context, assessments of the effectiveness and viability of such measures have in the past been mostly piecemeal, focusing on individual impacts of such measures (e.g. flood risk reduction, provision of urban green space). Such sectorial assessments cannot account for the potential trade-offs or complementarities between the multiple impacts of individual installations, let alone whole networks of BGI. Therefore, ValBGI seeks to develop a multidisciplinary, stakeholder-informed assessment framework for the effectiveness of BGI to reduce flood risk and improve urban natural capital. Thereby, the project examines the role of BGI in short- and long-term urban development, with application to the city of Can Tho, Viet Nam. This holistic framework integrates a number of disciplines and does not only follow an interlinked and multidisciplinary research agenda but also advances the academic state-of-the-art in the individual disciplines involved. This includes the engagement of key stakeholders in the research process to co-develop solutions and disseminate evidence to key decision-makers; innovative high-resolution modelling of flooding and BGI at the city-scale; and spatially explicit assessment and valuation of changes in the provision of ecosystem services enabling the quantification of the investment into urban natural capital effectuated by BGI. To achieve these objectives, the project comprises four work packages which function as interlinked components within the multidisciplinary assessment framework: (1) The operational backbone of ValBGI is a work package that establishes a stakeholder group to work alongside the research team from start to end. This component assesses (and when necessary stimulates) the awareness of alternative natural processes-based BGI options among local and regional urban planners and other key stakeholders. (2) A second work package reviews existing flood models for Can Tho and the Vietnamese Mekong Delta and develops a new high-resolution modelling system to assess the effectiveness of selected BGI measures in reducing flood risk. (3) In the third work package, changes in the provision of ecosystem services following the installation of BGI measures will be assessed and mapped. (4) In the fourth work package costs and benefits of BGI measures (in terms of flood risk reductions and improvements of urban natural capital) are quantified by means of valuation, and cost-benefit analyses of BGI scenarios are conducted. The continuous stakeholder engagement in ValBGI ensures awareness for and uptake of the research outcomes by decision-makers to maximise impact of the evidence produced. Furthermore, the holistic approach provides a better understanding of the potential trade-offs and complementarities between flood risk reduction and improvement of natural capital generated by BGI. Exploring all simultaneous impacts of BGI is beneficial in providing environmental managers and urban planners with a complete array of information for supporting planning decisions.

Flooding is the most damaging and costly hydrometeorological hazard affecting millions of people globally every year. In Viet Nam, low-lying coastal cities, particularly in river deltas, face increased flood risk and vulnerability due to rapid urban development and climate change. To reduce flood risk in urban areas, the recent decade has seen increased appreciation of the potential of Blue/Green Infrastructure (BGI), such as natural and man-made wetlands, vegetated river banks and restored floodplains, to reduce flood risk and provide additional benefits, such as controlling water pollutants, providing recreational opportunities, improving air quality and increasing resilience to other stressors, such as heat waves and noise pollution. However, despite the growing interest in BGI in a flood risk management context, assessments of the effectiveness and viability of such measures have in the past been mostly piecemeal, focusing on individual impacts of such measures (e.g. flood risk reduction, provision of urban green space). Such sectorial assessments cannot account for the potential trade-offs or complementarities between the multiple impacts of individual installations, let alone whole networks of BGI. Therefore, ValBGI seeks to develop a multidisciplinary, stakeholder-informed assessment framework for the effectiveness of BGI to reduce flood risk and improve urban natural capital. Thereby, the project examines the role of BGI in short- and long-term urban development, with application to the city of Can Tho, Viet Nam. This holistic framework integrates a number of disciplines and does not only follow an interlinked and multidisciplinary research agenda but also advances the academic state-of-the-art in the individual disciplines involved. This includes the engagement of key stakeholders in the research process to co-develop solutions and disseminate evidence to key decision-makers; innovative high-resolution modelling of flooding and BGI at the city-scale; and spatially explicit assessment and valuation of changes in the provision of ecosystem services enabling the quantification of the investment into urban natural capital effectuated by BGI. To achieve these objectives, the project comprises four work packages which function as interlinked components within the multidisciplinary assessment framework: (1) The operational backbone of ValBGI is a work package that establishes a stakeholder group to work alongside the research team from start to end. This component assesses (and when necessary stimulates) the awareness of alternative natural processes-based BGI options among local and regional urban planners and other key stakeholders. (2) A second work package reviews existing flood models for Can Tho and the Vietnamese Mekong Delta and develops a new high-resolution modelling system to assess the effectiveness of selected BGI measures in reducing flood risk. (3) In the third work package, changes in the provision of ecosystem services following the installation of BGI measures will be assessed and mapped. (4) In the fourth work package costs and benefits of BGI measures (in terms of flood risk reductions and improvements of urban natural capital) are quantified by means of valuation, and cost-benefit analyses of BGI scenarios are conducted. The continuous stakeholder engagement in ValBGI ensures awareness for and uptake of the research outcomes by decision-makers to maximise impact of the evidence produced. Furthermore, the holistic approach provides a better understanding of the potential trade-offs and complementarities between flood risk reduction and improvement of natural capital generated by BGI. Exploring all simultaneous impacts of BGI is beneficial in providing environmental managers and urban planners with a complete array of information for supporting planning decisions.

The world's major river deltas are facing a major sustainability crisis. This is because they are under threat from being 'drowned' by rising sea levels, with potentially severe consequences for the 500 million people who live and work there. At a qualitative level we have a relatively well developed understanding of the processes that are driving these rising sea levels. Changes in delta surface elevation occur when the summed rates of eustatic sea level rise and ground-surface subsidence are not balanced by gains in surface elevation, the latter being caused by the deposition of sediments supplied from river catchments upstream. Ongoing and major environmental changes are seemingly driving greater imbalances in these factors: eustatic sea levels are rising as a consequence of anthropogenic climate change while ground-surface subsidence, which occurs naturally in deltas as a result of sediment compaction, is in many cases being significantly accelerated by groundwater and/or hydrocarbon extraction. As a result, the only factor that could potentially offset these losses in delta surface elevation is sediment deposition on the delta surface. Unfortunately, many deltas are also being starved of their supply of river sediments as a result of anthropogenic activities, such as sand mining and damming, in the feeder catchments upstream. Estimating precise values of eustatic sea-level rise, sediment supply rate, surface deposition and ground-surface subsidence, is a significant challenge. In the near term the most significant factors in this balance are sediment deposition and subsidence (in the longer term eustatic changes will become relatively more significant). However, a particular issue in estimating sediment supply is that previous studies have focused on the sediment loads at the apices of deltas, with an almost complete absence of reliable data within the delta distributary channel network downstream of the apex. Moreover, the diversity of relevant disciplinary expertise involved in determining the other drivers contributing to relative sea-level rise has thus far conspired to inhibit the integrated synthesis that is really necessary to tackle the problem systematically. The world's third largest delta, the Mekong is SE Asia's rice basket and home to 20 million people, but it is being exposed to environmental risks as a result of rapid economic development, most notably through upstream damming and anthropogenic subsidence. The Mekong is therefore not only representative of many of the issues facing the world's deltas, but reliable data are urgently needed to help inform the sustainable management plans required to provide a safe operating space for the delta's inhabitants. In our NERC funded work we have developed new methods to estimate recent historical and future trends in the river sediments supplied to the apex of the delta. However, it is the flows of sediment within delta distributary networks, downstream of the delta apices, that are most critical in controlling local rates of delta surface deposition. In this proposal we will collaborate with Can Tho University and the Vietnamese Hydrological agency to access archived sediment transport measurements. Using novel methods developed in our existing work in the catchment upstream we will 'unlock' and translate these data into the very first estimates of sediment loads within and across the delta distributary network itself. Meanwhile, we will also work with other international groups who have been developing novel models to simulate rates of delta surface deposition (Potsdam) and ground-surface subsidence (Utrecht). Working together we will draw these data together to build the first integrated assessment of the factors driving near-term relative sea-level rise in a globally significant, iconic, delta, providing a template for similar analyses in other vulnerable deltas worldwide.

Layperson Summary (4000 characters max.) The world's major river deltas are hotspots of agricultural production that support rural livelihoods and feed much of the global population, but as 'climate change hot spots' deltas are facing a major sustainability crisis. Specifically, there are concerns that many deltas may in the coming decades be 'drowned' by rising sea levels as the oceans warm (up to 20% of land is projected to be lost in the major deltas of south and southeast Asia alone). The process of delta 'drowning' is a slow onset hazard where relative sea-level rise progressively exacerbates fluvial and coastal flood risk while simultaneously enhancing saline intrusion. However, progressive environmental change is punctuated by the occurrence of extreme weather events such as droughts or extreme rainfall and climate models project that these will occur more frequently. The co-occurrence of slow onset hazards with extreme events creates a 'perfect storm' that makes agriculture ever more challenging, but we have almost no insight into how slow onset changes interact with extreme events. A key question is the extent (much like a boxer 'softening up' her opponent with repeated body blows before landing the knockout punch) to which, in systems facing progressive reductions in resilience as a result of ongoing change, the additional burdens caused by occasional but damaging climatic extremes may cause a 'tipping point' to be crossed which makes it difficult for agricultural production to recover after severe episodes of drought or flooding. This is a critical issue because if we cannot correctly attribute the cause of major change we run the risk that 'solutions' will also be applied incorrectly. In this project we will develop a new model to examine how agricultural production and livelihoods are affected by combinations of progressive environmental change punctuated by extreme weather events. In particular we will focus on episodes of drought and flooding. Flooding is the most dangerous and costly of natural hazards, accounting for over 500,000 fatalities and economic losses of more than $1 trillion since 1980. Their low lying nature, alongside their location at the interface between coastal and fluvial environments means that deltas are disproportionately exposed to these risks. However, in the developing world, where agricultural production forms the mainstay of national economies and is central to livelihoods, drought can be a key driver of water and food (in)security, but we know significantly less about how droughts develop, persist and recover. We will further our understanding of the vulnerability of delta systems to extreme events by exploring how crop production and livelihoods are affected by the interplay between episodes of drought and flooding and ongoing environmental stress linked to upstream catchment management and climate change. Our project is focused on the world's third largest delta, the Mekong. The Mekong delta is SE Asia's rice basket and home to almost 20 million people, but it is exposed to severe environmental risks as a result of climate change and rapid economic development. We will collaborate with our Vietnamese partners, including in key government agencies, to bring UK expertise in (i) the modelling of droughts and floods; (ii) agricultural livelihoods; (iii) participatory stakeholder engagement processes and (iv) social-ecological systems dynamics to bear on this challenge. We will define policy relevant scenarios of future change and quantify the links between drought and flooding and agricultural livelihoods, delivering an integrated assessment of the factors driving changes to livelihoods and explore the effects that adaptations could make to help make the Mekong delta more resilient to climatic extremes. This will be done within a globally significant, iconic, delta, providing a template for similar analyses in other vulnerable deltas of the Global South.