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Foulds, S. A., Griffiths, H., Macklin, M., Brewer, P. (2014). Geomorphological records of extreme floods and their relationship to decadal-scale climate change. Geomorphology, 216, 193-207Extreme rainfall and flood events in steep upland catchments leave geomorphological traces of their occurrence in the form of boulder berms, debris cones, and alluvial fans. Constraining the age of these features is critical to understanding (i) landscape evolution in response to past, present, and future climate changes; and (ii) the magnitude?frequency of extreme, ungauged floods in small upland catchments. This research focuses on the Cambrian Mountains of Wales, UK, where lichenometric dating of geomorphological features and palaeohydrological reconstructions is combined with climatological data and documentary flood records. Our new data from Wales highlight a distinct flood-rich period between 1900 and 1960, similar to many other UK lichen-dated records. However, this study sheds new light on the underlying climatic controls on upland flooding in small catchments. Although floods can occur in any season, their timing is best explained by the Summer North Atlantic Oscillation (SNAO) and shifts between negative (wetter than average conditions with regular cyclonic flow and flooding) and positive phases (drier than average conditions with less frequent cyclonic flow and flooding), which vary from individual summers to decadal and multidecadal periods. Recent wet summer weather, flooding, and boulder-berm deposition in the UK (2007?2012) are related to a pronounced negative phase shift of the SNAO. There is also increasing evidence that recent summer weather extremes in the mid-latitudes may be related to Arctic amplification and rapid sea ice loss. If this is the case, continuing and future climate change is likely to mean that (i) unusual weather patterns become more frequent; and (ii) upland UK catchments will experience heightened flood risk and significant geomorphological changes.authorsversionPeer reviewe

River basins in Great Britain and Ireland have been characterized by periods of hillslope and valley floor instability during the Holocene, reflecting sensitivity to both climate change and anthropogenic disturbance. In contrast to climatic controls, which have been relatively well documented, human impacts on and interactions with river basins remain unclear. There is now, however, a growing impetus to elucidate more fully the impact of anthropogenic activity on sediment supply and runoff, given that land-use change is thought to have exacerbated recent flooding in the UK (eg, the ‘Millennium'floods of 2000). The aim of this paper is to critically review the significance of Holocene land use on hillslope and valley floor stability in Great Britain and Ireland. The most widely reported impacts of land-use change on geomorphic activity include hillslope erosion and gully development, valley floor alluviation, river channel incision and elevated water tables. In the majority of cases, however, causal relationships are difficult to establish, due primarily to inadequate dating control. Even where geomorphic instability can be linked to land-use change, it is apparent that eroded material is often stored as colluvium, which together with evidence of diachronus hillslope and valley floor instability, raises important questions and identifies uncertainties regarding the dynamics and extent of sediment transfer within river basins. Such uncertainty has important implications for understanding how river basins will behave in response to future environmental change.

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.

Major floods have increased in frequency in many parts of the world, and this is often attributed to anthropogenic climate change. Because of the short length of most gauged records (∼50 yr), it is unclear whether these events represent a short-term anomaly or a shift to a prolonged flood-rich period. In this paper, we use event-scale paleoflood records from upland and lowland floodplains to demonstrate the relationship between individual flood events, clusters of events in multiyear episodes, and multidecadal- to centennial-scale flood-rich phases. Catchment- and regional-scale data show that individual events and episodes generally fall within extended flood-rich phases controlled by climate. Furthermore, contrary to recent suggestions that environmental signals may be rendered incomplete in fluvial systems by autogenic processes, from a multidecadal (and longer) perspective it is clear that floodplain environments can register and preserve a useful multiscale hydromorphic signal of climate change.

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

Strong winds are a characteristic feature of UK upland areas. Despite this, understanding of aeolian processes in upland environments of the UK is limited. This paper presents direct measurements and observations of blanket peat erosion by wind action during a two week period of desiccation in the North Pennines, Northern England. A circular configuration of mass flux sediment samplers was used to collect peat eroded by wind action from 16 cardinal compass directions. Meteorological conditions (wind speed, wind direction, precipitation and temperature) were recorded by an automatic weather station set up adjacent to the site. Surface desiccation led to peat crust erosion and dust deflation. During short (≤ ≤ ≤ ≤ ≤1 hour) periods of precipitation, wind-driven rainfall also caused erosion. Typically, dust flux rates were up to two orders of magnitude lower than recorded during periods of sustained wet weather. Measurements demonstrate the hitherto unreported rapid switch in process regime between wind-driven rainfall and dry blow deflation in blanket peat environments. Dry blow processes of blanket peat erosion may become more important in UK upland areas if climate change promotes more frequent surface desiccation. Figure 3. Precipitation and temperature data recorded at Moss Flats using the automatic weather station (AWS). Desiccation processes were measured and observed between 13 and 27 May 2004 (indicated by the shaded box).desiccation conditions developed. Second, although rainfall totals and intensities were low during the dry period, average (0·14 m s −1 ) and maximum (0·34 m s −1 ) wind friction velocities were very similar to those recorded during periods of sustained wet weather. The friction velocity series (Figure 4) shows both diurnal peaks and discrete wind storms when the potential for wind erosion was at a maximum. Wind erosion of blanket peat during surface desiccation 485Erosion of the desiccated peat surface through crust detachment was directly observed at Moss Flats. Crust edges acted as the foci for wind action (Figure 1), which proceeded to detach platy peds through mechanical crust rupture. Crusts attached at the surface varied in width from ca. 40 to 250 mm. Once detached, entrained peds were observed moving by saltation and were found deposited in vegetation (Eriophorum) surrounding the eroding surface. Deposited crusts were typically smaller (ca. 20 -100 mm) than those attached at the surface, and varied in dry weight between 2 × 10 −4 and 4·4 × 10 −3 kg −1 , with a mean of 1·75 × 10 −3 kg. As well as direct observation of crust erosion, fine peat dust was observed blowing up to 1·87 m above the surface.

Physically and chemically distinctive late-Prehistoric and historical age alluvial deposits are characteristic of many upland and lowland river systems in the UK. Despite their widespread distribution, there have been few attempts to construct robust chronologies or to identify environmental factors that governed their formation. The Swale catchment in northern England is typical in this respect, with large areas of its valley floor covered by sedimentologically distinctive laminated sands and silts, enriched in organic material and Pb, and underlain by uncontaminated and structureless silts. Using 14C dating, chemostratigraphy, lichenometry and historical maps, a catchment-wide change in sedimentation style has been dated to between the mid 18th and early 19th centuries ad. Several causative factors were responsible for this change in sedimentation style and include the initiation of large-scale, intensive lead mining from the latter half of the 18th century onwards, embankment construction in the lowlands and historical peat erosion in the uplands. Transformation of the Swale floodplain also reflects longer-term land-use and climate change. In particular, deforestation of headwater tributaries by monastic grazing practices in the High Middle Ages (ad 1000–1300) led to a period of fine-grained sedimentation in upland catchments, as well as priming hillslopes for erosion and widespread channel network incision and increased fine sediment flux during the climatic downturn of the ‘Little Ice Age’. Sediment facies of a similar nature have been widely recorded in other northern English river catchments and represent a regional land use–climate signal characteristic of the Anthropocene. We introduce the term ‘agro-industrial alluvium’ to describe these types of deposit. They have similarities to post-settlement alluvium in North America and Australia, where historical land-use change had a similar impact on valley floor sedimentology.