The chronology and the hydrometeorology of catastrophic floods on Dartmoor, South West England

Abstract

Abstract:Extreme floods are the most widespread and often the most fatal type of natural hazard experienced in Europe, particularly in upland and mountainous areas. These 'flash flood' type events are particularly dangerous because extreme rainfall totals in a short space of time can lead to very high flow velocities and little or no time for flood warning. Given the danger posed by extreme floods, there are concerns that catastrophic hydrometeorological events could become more frequent in a warming world. However, analysis of longer term flood frequency is often limited by the use of short instrumental flow records (last 30-40 years) that do not adequately cover alternating flood-rich and flood-poor periods over the last 2 to 3 centuries. In contrast, this research extends the upland flood series of South West England (Dartmoor) back to ca AD 1800 using lichenometry. Results show that the period 1820 to mid-1940s was characterized by widespread flooding, with particularly large and frequent events in the mid-to-late 19th and early 20th centuries. Since ca 1850 to 1900, there has been a general decline in flood magnitude that was particularly marked after the 1930s/mid-1940s. Local meteorological records show that: (1) historical flood-rich periods on Dartmoor were associated with high annual, seasonal and daily rainfall totals in the last quarter of the 19th century and between 1910 and 1946, related to sub-decadal variability of the North Atlantic Oscillation and receipt of cyclonic and southerly weather types over the southwest peninsula; and (2) the incidence of heavy daily rainfall declined notably after 1946, similar to sedimentary archives of flooding. The peak period of flooding on Dartmoor predates the beginning of gauged flow records, which has practical implications for understanding and managing flood risk on rivers that drain Dartmoor.

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

“…Decadal frequency of lichen‐dated boulder‐berms in upland areas of England and Wales. Where available (Dartmoor, Cambrian Mountains and central Brecon Beacons) lichen dating accuracy is indicated by ±2 σ error bars; for full details see Foulds et al (, ) and Matthews ().…”

“…These were followed by a flood‐rich period (especially summers) bracketed by the very wet years of 2007 and 2012 (Marsh, ; Parry et al, ). Macklin and Rumsby () commented that it was too early to say if the north Cornwall and North Yorkshire events were an indication of changing conditions in the British uplands; eight years later, our new geomorphological analysis shows a transition from a well‐documented multi‐decadal flood‐poor period in the late twentieth century (Macklin and Rumsby, ), to the beginnings of a flood‐rich phase since 2007 (Foulds et al, ; Figure ).…”

“…The 2008 flood was triggered by a torrential localized downpour of ~100 mm of rain in approximately one hour; a detailed account and photographs of this event are available online at UKweatherworld (). In contrast, the June 2012 flood in Mid Wales (Figure c) was of similar magnitude to historical events: on one river – the Leri – it was the largest flood since June 1935 (Foulds et al, , ).…”

Meta-analysis of Holocene fluvial sedimentary archives: A methodological primer

“…The overall tendency of the relative probability in this period (above or below the mean) can be meaningfully compared with earlier periods in the relative probability distribution but the fluctuations in relative probability within these three centuries are unlikely to provide a sound basis for subdividing the period into multiple episodes above and below the mean. Historical, instrumental and lichenometric-dated (Merrett and Macklin, 1999;Foulds et al, 2013) flood records may provide a more detailed and reliable indication of variations in flood occurrence during the past three centuries.…”

Flood hazard assessment for extreme flood events

“…This being the case, an extreme flood event can be simply characterized as a flood event with a small probability of occurrence, but with a significant impact on human society in terms of general damage, human casualties and overall social disruption. Taking into account all of these considerations, four types of flood events can be generally characterized as extreme flood events, including: (1) dam-break floods (Duffaut 2013;Bergman et al 2014;Raška and Emmer 2014), (2) storm surges (Chau et al 2013;Breilh et al 2014;Androulidakis et al 2015), (3) flash floods (Moussa and Bocquillon 2009;Martínez Ibarra 2012;Foulds et al 2014;Amengual et al 2015;Kvočka et al 2015), and (4) extreme/large river floods (Zhi-Yong et al 2013;Bruwier et al 2015;Herget et al 2015;Schröter et al 2015;Antico et al 2016). Among these types of extreme flood events, flash floods and large river floods are the most common and generally the most serious extreme events (Ashley and Ashley 2008;Di Baldassarre et al 2010), which pose the greatest flood risk to the general population.…”