The Environment Agency in England is investing £2.5 billion with the aim of reducing flood risk to at least 300,000 homes by 2020/21. Several of the schemes being considered are on rivers that have experienced an upsurge of flooding over recent years. Decisions on whether to invest and how high to build are usually made on the basis of stationary methods of flood frequency analysis that assume the probability of flood flows is unchanging over time. Following successive severe floods in Cumbria, trend tests and non-stationary flood frequency analysis techniques have been applied. These allow parameters of the frequency distribution to change over time or with some other covariate. The resulting estimates of flow, for the present day, were up to 55% higher than the stationary estimates at river gauges in north-west England. The results have been incorporated into the scheme appraisal process. A national analysis indicates that there is evidence of upward trends in peak flows at nearly a quarter of river flow gauges across Great Britain. Many rivers show an abrupt increase in flood flows in the late 1990s. Trends tend to occur in upland areas but they are also seen on some rivers across south-east England.
K E Y W O R D Sclimate change, flood frequency estimation, non-stationary
There is increasing interest in past occurrences of flooding from intense rainfall, commonly referred to as "flash flooding," and the associated socioeconomic consequences. Historical information can help us to place recent events in context and to understand the effect of low frequency climate variability on changing flash flood frequencies. Previous studies have focussed on fluvial flooding to reconstruct the temporal and spatial patterns of past events. Here, we provide an online flood chronology for the north and south-west of England for flash floods, including both surface water and fluvial flooding, with coverage from~1700 to~2013 (http://cegfepsys.ncl.ac.uk/fc). The primary source of documentary material is local newspaper reports, which often give detailed descriptions of impacts. This provides a new resource to inform communities and first responders of flood risks, especially those from rapid rise in water level whose severity may be greater than those of accompanying peak flow. Examples are provided of historical flash floods that exemplify how the chronologies can help to place recent floods in the context of the preinstrumental record for: (a) more robust estimates of event return period, (b) identification of catchment or settlement susceptibility to flash flood events, and (c) characterisation of events in ungauged catchments. K E Y W O R D S extreme events, flash flood, flood chronologies, historical hydrology
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