Reassessing flood frequency for the Sussex Ouse, Lewes: the inclusion of historical flood information since AD 1650

N Macdonald,I Prosdocimi,H Sangster,T R Kjeldsen

doi:10.5194/nhess-14-2817-2014

Abstract

Abstract. The application of historical flood information as a tool for augmenting instrumental flood data is increasingly recognised as a valuable tool. Most previous studies have focused on large catchments with historic settlements, this paper applies the approach to the smaller lowland system of the Sussex Ouse in southeast England. The reassessment of flood risk on the Sussex Ouse is pertinent in light of the severe flooding in October 2000 and heightened concerns of a perceived increase in flooding nationally. Systematic flood level readings from 1960 and accounts detailing past flood events within the catchment are compiled back to ca. 1750. This extended flood record provides an opportunity to reassess estimates of flood frequency over a timescale not normally possible within flood frequency analysis. This paper re-evaluates flood frequency at Lewes on the Sussex Ouse downstream of the confluence of the Sussex Ouse and River Uck. The paper considers the strengths and weaknesses in estimates resulting from contrasting methods of analysis and their corresponding data: (i) single site analysis of gauged annual maxima; (ii) combined analysis of systematic annual maxima augmented with historical peaks of estimated magnitude; (iii) combined analysis of systematic annual maxima augmented with historical peaks of estimated magnitude exceeding a known threshold, and (iv) sensitivity analysis including only the very largest historical flood events. Use of the historical information was found to yield much tighter confidence intervals of risk estimates, with uncertainty reduced by up to 40% for the 100-year return frequency event when historical information was added to the gauged data.

Highlights

IntroductionThe application of historical records in flood frequency analysis has expanded rapidly over the last couple of decades (Brázdil et al, 1999, 2012; Barriendos et al, 2003; England et al, 2003; Glaser and Stangl, 2003; Macdonald et al, 2006; McEwen and Werritty, 2007; Glaser et al, 2010) following several severely damaging floods since the early 1990s in the UK (Hannaford and Marsh, 2008) and mainland Europe (Kundzewicz et al, 1999; Szlávik, 2003; Ulbrich et al, 2003; Böhm and Wetzel, 2006; Bezzola and Hegg, 2007)
The application of historical records in flood frequency analysis has expanded rapidly over the last couple of decades (Brázdil et al, 1999, 2012; Barriendos et al, 2003; England et al, 2003; Glaser and Stangl, 2003; Macdonald et al, 2006; McEwen and Werritty, 2007; Glaser et al, 2010) following several severely damaging floods since the early 1990s in the UK (Hannaford and Marsh, 2008) and mainland Europe (Kundzewicz et al, 1999; Szlávik, 2003; Ulbrich et al, 2003; Böhm and Wetzel, 2006; Bezzola and Hegg, 2007). These extreme events have led to heightened demands for flood risk assessments that can incorporate a greater understanding of past extreme events and the methods and data used for producing them, with historical records providing an accessible and detailed account of pre-instrumental flood events (Macdonald, 2012)
The value of historical records is recognised in several countries, with recommendations for its use in flood risk assessment in Germany, Spain, the UK and USA among others, and it has become enshrined within European law (EU Floods Directive – 2007/60/EC)

Summary

Introduction

The application of historical records in flood frequency analysis has expanded rapidly over the last couple of decades (Brázdil et al, 1999, 2012; Barriendos et al, 2003; England et al, 2003; Glaser and Stangl, 2003; Macdonald et al, 2006; McEwen and Werritty, 2007; Glaser et al, 2010) following several severely damaging floods since the early 1990s in the UK (Hannaford and Marsh, 2008) and mainland Europe (Kundzewicz et al, 1999; Szlávik, 2003; Ulbrich et al, 2003; Böhm and Wetzel, 2006; Bezzola and Hegg, 2007). The event of October 2000 flooded over 10 000 properties and caused an estimated GBP 130 million in damages (Environment Agency, 2004), with a subsequent improvement in flood defences and development of a multiagency flood plan (Lewes District Council, 2010)

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Discussion

Conclusion