Reassessing flood frequency for the River Trent through the inclusion of historical flood information since ad 1320

Abstract

The estimation of return periods for floods likely to have significant societal impact is challenging unless suitably long records exist. Relatively few sites across the UK provide a continuous record of river level or discharge over 50 years, whilst records extending back to the nineteenth century are rare. This represents a significant problem in providing robust and reliable estimates of flood risk, as relatively short records often fail to include an adequate sample of large floods. The inclusion of historical flood levels/magnitudes prior to instrumental river flow recording presents a valuable opportunity to extend this dataset. This paper examines the value of using historical data (both documentary and epigraphic) to augment existing gauged records for the River Trent in Central England, as part of a multi-method approach to assessing flood risk. Single station and pooled methods are compared with flood risk estimates based on an augmented historical series (1795–2008) using the generalised logistic and generalised Pareto distributions. The value of using an even longer, but less reliable, extended historical series (1320–2008) is also examined. It is recommended that modelling flood risk for return periods >100 years should incorporate historical data, where available, and that a multi-method approach increases confidence in flood risk estimates.