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和代价最高的。从1980年到2013年，洪水造成了超过1万亿美元的损失，导致全球至少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稠密的低洼沿海地区，特别是沿海-河流交界处。因此，继续提高我们对洪水的认识是至关重要的。在沿海地区，洪水往往是由多种因素引起的。洪水可通过以下因素共同发生：(1)风暴潮加上天文潮汐(风暴潮)和/或(2)本地或远程(涨潮)产生的海浪；但也可由强降水引起，例如(3)河流流量增加(河流)和/或(4)直接径流(暴雨)。到目前为止，大多数洪水风险评估都单独考虑了这四个导致洪水的主要因素。然而，当海洋(风暴潮和海浪)、河流和/或洪水的来源同时或紧密相继发生时，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相互依存关系的信息，这些相互依存关系因地而异，而且人们认为分析这些相互依存关系所需的联合概率分析方法存在困难。这就是为什么世界气候研究计划极端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年与飓风哈维有关的事件。破纪录的降雨、河流流量和径流，再加上温和但持续时间较长的风暴潮，导致休斯敦发生灾难性的洪水。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，对休斯顿和其他地方的风险过去和现在仍然被大大低估。我们可能会提出一种新的综合方法，将沿海地区洪水的四个主要来源之间的所有空间和时间相关性纳入其中。这将使我们能够逐步改变我们对推动北大西洋盆地周围沿海地区复合洪水事件的来源机制的理解和预测。我们将解决以下关键问题：1.复合事件在哪里(以及在哪里不发生)，以及不同地区哪些源变量的组合最重要？2.哪种天气类型更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？3.不同地区发生复合事件的可能性和空间范围有多大？4.来自多个洪水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，新的相关性分析和最先进的天气分型方法，以及创造性的多变量极值分析技术和先进的整体水动力学模型)，不仅与复合洪水这一严重问题相关，而且还将转移到地球和环境科学中的其他连锁灾害，如热浪、干旱和丛林火灾；极端降雨、山体滑坡和悬崖瀑布；地震和海啸；河流泄流和浑浊水流。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)的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方面取得重大进展。

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.