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.