Developing Innovative Flood Frequency Estimation for a Resilient nation (DIFFER)

洪水是人类历史上一直在与之作斗争的一种极其有害的自然灾害。今天，仍然不可能消除任何特定地区的洪灾风险，但可以做很多工作来更好地了解洪灾频率和规模之间的关系，并利用这些信息使人类住区更具弹性。缓解气候变化对极端事件的影响和制定有效的防洪计划，需要更好地了解频率-震级关系以及这种关系在未来可能发生的变化。拟议的项目通过开发洪水频率分析的高级统计模型来应对这一紧迫的科学和社会挑战，即研究在任何一年中预计超过给定规模的事件的频率。目前，这种分析通常基于系统测量的高峰流量的年度最大值(即任何一年有记录的最大事件)的系列。这种类型的分析被大样本的成熟理论结果证明是合理的，但由于只使用了给定年份的一个数据点，因此浪费了长期记录中可用的信息。另一种方法是基于更现代、更成熟的极端事件理论，是基于对河流流量记录中超过某一高阈值的数据点的分析，因此可以认为是极端的。在水文学中，这类数据的使用一直被忽视，部分原因是从连续流量记录中提取阈值超限的障碍，以及将分析结果转换为传统用于基础设施设计和维护的年度超限概率信息所需的不太直观的步骤。然而，正如《2016年洪水国家评估》所证明的那样，越来越多的人要求更先进的工具来获得更准确(即更少不确定性)的洪水概率估计，并评估由于气候变化和其他外部因素(例如土地利用变化)而导致的流量记录中是否存在变化。基于超出阈值分析的新统计方法的引入有可能成为洪水频率分析中的一步变化。以前的研究表明，这种类型的数据可以提供较少的变量估计，并有助于检测集水区变化对洪水产生过程的影响。拟议的研究项目将调查如何利用这类数据对全国各地洪水行为的变化和洪水频率的总体估计作出更可靠的估计。首先，将评估国家河流流量档案馆现有超出门槛记录的适用性，并确定高质量的测量记录。其次，将使用最高质量的记录，调查数据中存在的趋势，并仔细审查外部驱动因素(如降雨量或气候变异性指数)解释任何检测到的变化的潜在能力。在进行这一分析的同时，还将研究所检测到的阈值超标性质的潜在变化与目前估计年度超标概率的模型之间的相关性。最后，基于阈值超越特性的研究结果，将提出在全国范围内估计洪水频率的新方法，并与实践者和决策者讨论其可行性。虽然拟议的项目侧重于洪水频率估计方法，但预计其结果将与其他环境极端情况的分析相关，例如波高或阵风。

Flooding is a highly detrimental natural hazard with which man has struggled throughout history. Still today, it is not possible to eliminate the risk of flooding in any given area, but much can be done to gather a better understanding of the relationship between the frequency and magnitude of flooding and use this information to make human settlements more resilient. Mitigation of climate change impacts on extreme events and the development of effective flood protection schemes requires improved understanding of the frequency-magnitude relationship and of how this is likely to change in the future. The proposed project addresses this pressing scientific and societal challenge by developing advanced statistical models for flood frequency analysis, i.e. the study of the frequency with which events of given sizes are expected to be exceeded in any given year. At present, this analysis is typically based on series of the annual maximum (i.e. the largest recorded events for any given year) of systematically measured peak flow. This type of analysis is justified by well-established theoretical results for large samples, but is wasteful of the information available in long term records since only one data point for any given year is used. Another approach, which is based on more modern and well-established theory for extreme events, is based on the analysis of the data points in the river flow record which have exceeded a certain high threshold and can therefore be considered to be extreme. The use of this type of data has been neglected in hydrology, due partially to the hurdle of extracting the threshold exceedances from the continuous flow record and to the less intuitive steps needed to translate the outputs of the analysis to information on the annual exceedances probabilities traditionally used in the designing and maintaining of infrastructures. Nevertheless, as evidenced in the 2016 Flood National Review, there is a growing request for more advanced tools to obtain a more precise (i.e. less uncertain) estimation of flood probabilities and to assess the presence of changes in the flow records attributed to climate change and to other external factors (e.g. land-use changes).The introduction of new statistical methods based on the analysis of threshold exceedances has the potential to be a step-change in flood frequency analysis. Previous research has shown that this type of data can deliver less variable estimates and is useful for detecting the effect of changes in the catchment on the flood generating processes. The proposed research project will investigate how to make use of this type of data to make more reliable estimates of both changes in the behaviour of floods and in flood frequency estimation in general across the whole country. At first the suitability of the existing threshold exceedances records from the National River Flow Archive will be assessed and gauged records of high quality will be identified. Secondly, using the highest quality records, the presence of trends in the data will be investigated and the potential ability of external drivers such as rainfall or climatic variability indices to explain any detected changes will be scrutinised. In parallel to this analysis, the relevance of the potential detected changes in properties of the threshold exceedances in relation to the current models for the estimation of annual exceedance probabilities will also be studied. Finally, based on the findings on the properties of threshold exceedances, new methods for flood frequency estimation at a national scale will be proposed and their feasibility discussed with practitioners and decision makers.Although the proposed project is focussed on methods for flood frequency estimation, it is expected that its outcomes will be relevant for the analysis of other environmental extremes, for example wave heights or wind gusts.