A hydrogeomorphic assessment of twenty‐first century floods in the UK

Abstract

The occurrence of devastating floods in the British uplands during the first two decades of the twenty-first century poses two key questions: (1) are recent events unprecedented in terms of their frequency and magnitude; and (2) is climate and/or land-use change driving the apparent upturn in flooding? Conventional methods of analysing instrumental flow records cannot answer these questions because upland catchments are usually ungauged, and where records do exist they rarely provide more than 30-40 years of data. In this paper we analyse all lichen-dated upland flood records in the United Kingdom (UK) to establish the longer-term context and causes of recent severe flooding. Our new analysis of torrential sedimentary deposits shows that twenty-first century floods are not unprecedented in terms of both their frequency (they were more frequent before 1960) and magnitude (the biggest events occurred during the seventeenth-nineteenth centuries). However, in some areas recent floods have either equalled or exceeded the largest historical events. The majority of recent floods have been triggered by torrential summer downpours related to a marked negative phase of the summer North Atlantic Oscillation (NAO) between 2007 and 2012. It is of concern that historical data suggests there is far more capacity in the North Atlantic climate system to produce wetter and more prolonged flood-rich periods than hitherto experienced in the twenty-first century. Looking forwards, an increased likelihood of weather extremes due to climate change means that geomorphological based flood series extensions must be placed at the centre of flood risk assessment in the UK uplands and in similar areas worldwide. © 2016 John Wiley & Sons, Ltd

The influence of hydroclimatic variability on flood frequency in the Lower Rhine

“…This adds to the current debate on the relation between (anthropogenic) climate change and a possible increase in (and clustering of) extreme events in the last decades (e.g. Milly et al , ; Mudelsee et al , ; Foulds and Macklin, ).…”

Stormy geomorphology: geomorphic contributions in an age of climate extremes

“…For example, event‐scale flood and storm data extending back centuries (or millennia in some cases) are now available in a growing number of upland (e.g. Macklin and Rumsby, ; Foulds and Macklin, ) and lowland (River Severn, UK: Jones et al , ; River Rhine, Germany: Toonen et al , ) riverine, lacustrine (European Alps: Swierczynski et al ., ) and coastal environments (northwest Spain: Feal‐Pérez et al , ). Multi‐centennial length Holocene flood‐rich and flood‐poor periods have also been identified, and precisely dated, in Europe and North Africa (Benito et al , ), the American southwest (Harden et al , ) and on an interhemispheric basis (Macklin et al , ).…”

“…These are very rarely more than 100 years in length and typically span less than 50 years (Jones et al , ). Palaeoflood records by contrast do show both short‐ and long‐term trends in extreme flood events and, most importantly, reveal regional and local variability in river and coastal response and flooding to recent climate changes influenced by El Niño‐Southern Oscillation (ENSO, Maas et al , ), North Atlantic Oscillation (NAO, Feal‐Pérez et al , ; Benito et al , ; Foulds and Macklin, ) and Pacific Decadal Oscillation (PDO, Greenbaum et al , ). A single flood or storm can result in the complete transformation of river and coastal landscapes, which resets boundary conditions and strongly influences geomorphic evolution over multi‐decadal and longer periods (e.g.…”

“…They argue that these improvements would maintain the statistical credibility of its guidelines and improve the accuracy of risk and uncertainty assessments [although see Klemes () on “hydrological dilettantism”]. Geomorphological approaches on both contemporary and palaeo timecales can help supply this crucial missing information (Baker, ; Foulds and Macklin, ; Toonen et al , ).…”

Incorporating sedimentological data in UK flood frequency estimation

“…Documentary, epigraphic, and sedimentological evidence of past floods in the United Kingdom can be used to extend systematic gauged flood records by many hundreds, and in some cases even thousands of years (Foulds & Macklin, ; Macklin & Rumsby, ; Macklin, Rumsby, & Heap, ). Information on historical floods can be found from a variety of documentary sources and can be compiled for most locations in the United Kingdom (Kjeldsen, Macdonald, et al, ).…”