NSFGEO-NERC: CHANCE - Understanding compound flooding in the past, present and future for North Atlantic coastlines

洪水是所有自然灾害中最危险、最昂贵的。从1980年到2013年，全球洪水造成的损失超过1万亿美元，造成至少22万人死亡。其中50%以上的死亡和很大一部分损失发生在人口密集的低洼沿海地区，特别是沿海与河流交界的地区。因此，继续加深我们对洪水的了解是至关重要的。在沿海地区，洪水往往是由多种因素引起的。洪水可由以下因素共同发生：(1)风暴潮加上天文潮汐（风暴潮）及/或(2)本地或远处（涌浪）产生的海浪；但也来自强降水，要么通过(3)增加河流流量（河流）和/或(4)直接径流（河流）。迄今为止，大多数洪水风险评估都分别考虑了这四个主要的洪水驱动因素。然而，当海洋（风暴潮和波浪）、河流和/或洪源同时或连续发生时，沿海地区洪水的不利后果可能会大大加剧，这种情况被称为“复合洪水”，可能导致不成比例的极端事件。尽管复合事件具有很高的影响潜力，但人们对它们的了解仍然很少，这在很大程度上是因为缺乏关于驱动因素之间的相互依赖性的信息，这些因素在各地差别很大，而且人们认为分析这些相互依赖性所需的联合概率分析方法很困难。这就是为什么世界气候研究计划“极端气候大挑战”将气候复合事件确定为国际研究重点。最近的一个复合事件的例子是2017年与哈维飓风有关的事件。破纪录的降雨、河流流量和径流，加上温和但持续时间长的风暴潮，导致了休斯顿灾难性的洪水。这是美国历史上损失第二大的自然灾害。此外，人们认识到，如果不考虑复合洪水，休斯敦和其他地方的风险过去和现在都被大大低估了。在CHANCE中，我们将提供一种新的综合方法，将沿海地区洪水的四个主要来源驱动因素之间的所有空间和时间依赖性结合起来。这将使我们在理解和预测北大西洋盆地周围沿海地区复合洪水事件的源机制方面迈出一步。重点解决以下问题：复合事件在哪里发生（哪里不发生），哪些源变量的组合在不同的区域最重要？哪种天气类型有利于复合事件的发生，未来复合事件的频率是否会随着天气模式的变化而增加/减少？不同区域复合事件发生的可能性和空间程度如何？多个洪水来源的复合效应如何加剧对沿海社区的影响？我们将通过一系列方法创新（例如，新颖的依赖分析和最先进的天气类型方法，以及创造性的多元极值分析技术和先进的整体流体动力学建模）来实现这一目标，这些方法不仅与复合洪水的严重问题相关，而且还将转移到地球和环境科学中的其他级联危害，例如：热浪，干旱和丛林火灾；极端降雨、山体滑坡和悬崖坠落；地震和海啸；还有河流的排放和浊流。我们的新方法将使我们能够全面评估和预测与沿海地区（过去、现在和未来）复合洪水事件及其空间范围相关的所有源变量，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 the most dangerous and costly of all natural hazards. From 1980 to 2013, floods accounted for more than $1 trillion in losses and resulted in at least 220,000 fatalities globally. More than 50% of these deaths, and a large proportion of the losses, occurred in densely populated low-lying coastal regions, especially those at the coastal-river interface. Continuing to advance our understanding of flooding is therefore of utmost importance.In coastal regions, floods are often caused by multiple factors. Floods can arise through the joint occurrence of factors such as (1) storm surges plus astronomical tides (storm-tides) and/or (2) local or remotely (swell) generated waves; but also from heavy precipitation, either through (3) increased river discharge (fluvial) and/or (4) direct runoff (pluvial). Most flood risk assessments to date have considered these four main drivers of flooding separately. However, the adverse consequences of a flood in coastal regions can be greatly exacerbated when the oceanographic (storm-tides and waves), fluvial, and/or pluvial sources of flooding occur concurrently or in close succession, a condition known as 'compound flooding', which can result in disproportionately extreme events. Despite their high impact potential, compound events remain poorly understood, in large part because of the lack of information on the inter-dependence of the driving factors, which varies considerably from place to place, and the perceived difficulty of the joint probability analysis methods required to analyse these interdepencies. This is why the World Climate Research Program Grand Challenge on Extremes has identified climatic compound events as an international research priority.A recent example of a compound event is that associated with Hurricane Harvey in 2017. Record breaking rainfall, river discharge and runoff, combined with a moderate but long-lasting storm surge, resulted in disastrous flooding in Houston. It was the second costliest natural disaster in US history. Moreover, it is recognised that, by not considering compound flooding, the risk to Houston and elsewhere had been, and still is, greatly underestimated.In CHANCE we will deliver a new integrated approach, incorporating all the spatial and temporal dependencies between the four main source drivers of flooding in coastal regions. This will allow us to make a step change in our understanding and prediction of the source mechanisms driving compound flood events in coastal areas around the North Atlantic basin. We will address the following key questions:1. Where do (and where don't) compound events occur and which combinations of source-variables are most important in different regions?2. Which weather types favour the occurrence of compound events and will the frequency of compound events increase/decrease in the future as weather patterns change?3. What is the likelihood and spatial extent of compound events in different regions?4. How do compounding effects from multiple flood sources exacerbate impacts to coastal communities?We will do this through a series of methodological innovations (e.g., novel dependence analyses and state-of-the art weather typing approaches, along with inventive multivariate extreme value analysis techniques and advanced ensemble hydrodynamic modelling) that not only have relevance to the serious issue of compound flooding, but which will also be transferable to other cascading hazards in the earth and environmental sciences, such as: heat waves, drought and bush fires; extreme rainfall, landslides and cliff falls; earthquake and tsunami; and river discharge and turbidity currents. Our new methods will enable us to fully assess and predict all the source variables associated with compound flood events and their spatial extents in coastal regions (past, present and future) and will result in a major advance in the way compound flooding is understood, quantified and managed.