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

洪水是最危险、最昂贵的自然灾害之一。自1980年以来，洪水已造成全球20多万人死亡，造成至少1万亿美元的经济损失。其中50%以上的死亡和大部分损失发生在人口稠密的低洼三角洲。与水有关的灾害是三角洲的一个主要问题，因为它们位于海洋和主要河流之间，因此容易受到来自沿海地区和河流的洪水的影响。此外，三角洲占据了人口密集的大片低洼地区。在三角洲，水浸主要有三个原因：(i)风暴潮（风暴潮加潮汐）；但是，从强降水中，要么通过（ii）增加河流流量（河流）和/或（iii）直接地表径流（河流）。迄今为止，三角洲（和其他环境）的大多数洪水风险评估都单独考虑了这些主要的洪水原因，因为缺乏关于它们之间相互依赖性的信息，并且因为在处理必要的基础统计数据（称为联合概率分析方法）方面存在明显的困难。然而，当沿海、河流和地面洪水源同时发生或连续发生时，三角洲洪水的不利后果可能会大大增加，从而导致被称为“复合洪水”的不成比例的极端事件。尽管它们具有很高的影响潜力，但人们对复合事件的了解仍然很少。这就是为什么世界气候研究计划将复合洪水事件确定为国际研究的重点。该项目将汇集英国和越南的专业知识，从沿海、河流和地表来源绘制和描述目前的洪水风险，并预测未来的洪水风险，并评估湄公河三角洲地区复合洪水的风险。湄公河是世界上最脆弱的三个三角洲之一，季风带来的强降雨会导致洪水泛滥，热带气旋也会导致沿海地区洪水泛滥。我们的假设是，以前的洪水评估低估了源驱动因素，因此洪水的可能性和相关风险，因为以前没有考虑复合事件。我们提出了一种新的综合方法，使我们对三角洲地区复合洪水事件源机制的理解和预测发生了一个台阶式的变化。我们将评估影响越南的风暴和季风降雨变率的大规模驱动因素，并开发新的（过去/现在和未来的）气象数据集，以驱动湄公河三角洲及其集水区的耦合洪水模型。这涉及到使用下一代气候模型，它可以模拟强烈的季风事件和热带气旋，为我们对洪水风险的统计分析提供足够大的数据集。我们将计算过去/现在和未来三角洲沿岸和河流洪水的可能性，量化复合洪水事件的发生。对于重点热点地区，我们将估计现在和将来被淹没的土地面积、受影响的人数以及基础设施和农业可能受到的影响。特别是，我们将研究低概率、高影响的事件，并量化来自多个洪水来源的复合洪水效应如何加剧对沿海社区的影响。我们将与国家、地区和省级政府项目合作伙伴密切合作，考虑管理和规划方案，并提供指导，以加强对未来洪水事件的准备和恢复能力。我们的新方法将使我们第一次能够全面评估和预测与湄公河三角洲（目前和未来）复合事件相关的所有源变量，并将在理解、量化和管理复合洪水的方式上取得重大进展。

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.