Return periods like the "T-year (100-year) event" describe an environmental event that has, on average, a 1-in-T (1% for the 100-year event) chance of occurring or being exceeded in any given year, at a given location. However, there is consensus that the frequency and magnitude of extremes such as floods are changing dynamically with shifts in climate (

Abstract. The Focused Rainfall Growth Extension (FORGEX) method produces rainfall growth curves focused on a subject site. Focusing allows the incorporation of rainfall extremes observed regionally while respecting local variations in growth rates. The starting point for the analysis is an extensive set of annual maximum rainfalls, with values at each gauged site standardized by the median. Following the philosophy of the earlier FORGE method, a strongly empirical approach is adopted. The rainfall growth curve is represented by linear segments on a Gumbel scale, and is fitted by a least-squares criterion. The selection of data points is intricate and includes both the traditional pooling of regional extremes and the incorporation of network maximum events. The latter comprise the largest events from successive hierarchical networks of gauges, focused on the site for which estimates are requires. Their treatment takes account of interdependence using the Dales and Reed model of spatial dependence in rainfall extremes.

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

Keith Beven was amongst the first to propose and demonstrate a combination of conceptual rainfall-runoff modelling and stochastically-generated rainfall data in what is known as the "continuous simulation" approach for flood frequency analysis. The motivations included the potential to establish better links with physical processes and to avoid restrictive assumptions inherent in existing methods applied in design flood studies. Subsequently attempts have been made to establish continuous simulation as a routine method for flood frequency analysis, particularly in the UK. The approach has not been adopted universally, but numerous studies have benefitted from applications of continuous simulation methods. This paper asks whether industry has yet realised the vision of the pioneering research by Beven and others. It reviews the generic methodology and illustrates applications of the original vision for a more physically-realistic approach to flood frequency analysis through a set of practical case studies, highlighting why continuous simulation was useful and appropriate in each case. The case studies illustrate how continuous simulation has helped to offer users of flood frequency analysis more confidence about model results by avoiding (or exposing) bad assumptions relating to catchment heterogeneity, inappropriateness of assumptions made in (UK) industry-standard design event flood estimation methods and the representation of engineered or natural dynamic controls on flood flows. By implementing the vision for physically-realistic analysis of flood frequency through continuous simulation, each of these examples illustrates how more relevant and improved information was provided for flood risk decision-making than would have been possible using standard methods. They further demonstrate that integrating engineered infrastructure into flood frequency analysis, and assessment of environmental change are also significant motivations for adopting the continuous simulation approach in practice.This article is protected by copyright. All rights reserved. IntroductionBuilding on advances in hydrological modelling made in the 1970s, in particular the physically-based TOPMODEL concepts (Beven and Kirkby, 1979), Keith Beven (1986, 1987 was amongst the first hydrologists to demonstrate what is now described as a "continuous simulation" (CS) approach for flood frequency analysis. This paper examines how the original vision to "provide more physically-based techniques for prediction of flood frequency characteristics" (Beven, 1987) has been realised, and what lessons can be learned from applications in practice.Probabilistic methods have underpinned the design and economic analysis of flood mitigation measures for over 50 years (Benson, 1968). In a flood frequency analysis, peak river flows are regarded as a random variable, Y, and the probability of the flow not exceeding a given where g Y (y) is a probability density function describing Y. The temporal scale is usually defined such that G Y (y) represents the annu...

Abstract. This paper illustrates the performance of the FORGEX method of rainfall growth estimation. Results are presented for three regions of the United Kingdom: the East Midlands, north-west England and south-west England. Focused rainfall growth curves are compared between regions and between different sites within each region. Typical growth curve shapes are discussed with reference to the climate of each region. Daily growth curves are derived from a large number of records of annual maximum rainfalls. A smaller number of hourly annual maximum series is available for estimation sub-daily rainfall growth curves. Rainfall growth rates are compared with the results of a widely used method. The present method allows more local and regional variation in growth rates. The new growth rates are higher for durations of 1 and 2 days in parts of south-west England, but lower for moderate return periods at some focal points in the north-west. In the East Midlands, the new 1-hour growth rates are considerably higher for long return periods. Confidence limits for growth rates are derived by bootstrapping. This is accomplished by fitting a large number of growth curves to resampled sets of rainfall data.