[Viet Nam] Comp-Flood: Compound flooding in coastal Viet Nam

Floods are among the most dangerous and costly natural hazards. Since 1980, floods have accounted for more than 200,000 fatalities globally and resulted in at least $1 trillion in economic losses. More than 50% of these deaths and a large proportion of the losses have occurred in densely populated low-lying deltas. Water related disasters are a major concern in deltas because they are located between the sea and major rivers, and hence are subject to flooding from the coastal zone and from rivers. Furthermore, deltas occupy large low-lying areas that are densely populated.

In deltas, flooding arises from three main sources: (i) storm tides (storm surges plus tides); but also, from heavy precipitation, either through (ii) increased river discharge (fluvial) and/or (iii) direct surface runoff (pluvial). To date the majority of flood risk assessments in deltas (and other environments) have considered these main causes of flooding separately, because of the lack of information on their inter-dependence, and because of the perceived difficulty in handling the necessary underpinning statistics (known as joint probability analysis methods). However, the adverse consequences of a flood in a delta can be greatly increased when the coastal, fluvial and surface flood sources occur concurrently, or in close succession, resulting in a disproportionately extreme event referred to as 'compound flooding'. Despite their high impact potential, compound events remain poorly understood. This is why the World Climate Research Program has identified compound flood events as an international research priority.

This project will bring together UK and Vietnamese expertise to map and characterise present and predict future flood risk, from coastal, fluvial, and surface sources and, uniquely, to assess the risk of compound flooding across the Mekong delta. Exposed to heavy monsoon rains that can cause both fluvial and pluvial flooding, and tropical cyclones that cause coastal flooding, the Mekong is one of the three most vulnerable deltas in the world. Our hypothesis is that previous flood assessments have underestimated the source drivers and hence the likelihood of flooding and associated risk, as compound events have not previously been considered.

We propose a new integrated approach, to make a step change in our understanding and prediction of the source mechanisms driving compound flood events in delta regions. We will assess the large-scale drivers of variability in storms and monsoon rainfall that impact Viet Nam and develop novel (for both the past/present and future) meteorological datasets needed to drive the coupled flood models of the Mekong delta and its catchment. This involves use of next-generation climate models, which can simulate both intense monsoon events and tropical cyclones, providing datasets that are sufficiently large for our statistical analysis of flood risk.

We will calculate the past/present and future likelihood of coastal and fluvial flooding across the delta, quantifying the occurrence of compound flooding events. For key hot spot areas, we will estimate areas of land inundated, numbers of people affected and how infrastructure and agriculture might be impacted, now and in the future. In particular we will examine low probability, high impact, events and quantify how compounding flood effects from multiple flood sources exacerbate impacts to coastal communities. Working in close partnership with our national, regional and provincial governmental project partners, we will consider management and planning options and provide guidance that will increase preparedness and resilience to future flood events.

Our new methods will enable us, for the first time, to fully assess and predict all the source variables associated with compound events in the Mekong delta (at present and in the future) and will result in a major advance in the way compound flooding is understood, quantified and managed.

This project will develop new understanding of compound flooding in the Mekong delta. Outputs of the project will strongly support disaster management, assist decision making and aid development planning at national, regional and local scales in Viet Nam. This will have a direct and immediate positive impact, which in turn will have significant economic benefits for development, livelihood sustainability and food security. The project's major impact will therefore be in the delivery of new capability in flood risk communication, management and planning. Our long-term goal is that the project will be able to contribute to delta-wide coordination of flood resilience practices, policy-making and regional actions in the face of evolving climate and environmental change. Although our focus here is on the Mekong delta, our results and policy driven impact will be relevant to other coastal regions in Viet Nam and other vulnerable deltas around the world.

The pathways to impact have been developed and will be delivered through strong engagement with key government institutions at national (Vietnam Disaster Management Authority), regional (our co-I's in the Southern Institute of Water Resources Research and the Hydro-meteorological Observatory Southern Region) and provincial level (Bac Lieu, Cà Mau, Dong Thap provinces). The letters of support from the stakeholder organisations shows the need and demand for the proposed research and highlights their commitment to end-user 'buy in' from the outset of the project.

The primary way our project will achieve impact is through direct, regular and two-way engagement with our project partners, and other end-users identified throughout the project. At a national scale we will engage with our project partner the Viet Nam Disaster Management Authority (DMA). Together with them, we will use the outputs from the project, to: determine types and pathways of storms that are likely to pose the most hazard to the delta; identify the most flood prone regions, and the most vulnerable communities within these areas; locate potential evacuation routes; identify key critical infrastructure that could be impacted, which would have knock on effects; and inform risk communication. The DMA partner with various NGOs and results from our study will be used to guide their plans regarding deployment of aid and other emergency resources to flood hit areas.

Working with the DMA, our co-I from the Hydro-meteorological Observatory Southern Region and colleagues at the Met Office, we will consider the potential for seasonal forecasting relating to flooding, in terms of the intensity of the monsoon and likelihood of an exceptional storm season.

An important area that the outcomes of our project will contribute to is in regard to planning of future development across the Mekong delta. In November 2017, the Prime Minister of Viet Nam released a new resolution, in which he outlined the government's ambitious development plans for the Mekong delta. The outcomes of our project can link a better appreciate of flooding to the development of the delta, ensuring future development is either flood resilient or situated in areas with lower flood risk. With our stakeholders we will also consider the impacts of our findings in relation to the Mekong Delta Plan.

At a local level we will engage with government staff from the Bac Lieu, Cà Mau and Dong Thap provinces. As outlined in their letter of support they have distinct challenges relating to flood risk. Throughout the project, we will engage with these organisations to understand the unique challenges they face in regarding to flooding.

Our impact strategy will deliver results to key governmental stakeholders who are best positioned to effectively influence policy and decision making. The two main ways in which we will engage with end-users are embedded with the main work plan via stakeholder workshops and a policy analysis report.