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

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. Copyright © 2015 John Wiley & Sons, Ltd.     

Spatial and temporal variability of flood seasonality in Wales

High‐magnitude floods across Europe within the last decade have resulted in the widespread reassessment of flood risk; this coupled with the introduction of the Water Framework Directive (2000) has increased the need for a detailed understanding of seasonal variability in flood magnitude and frequency. Mean day of flood (MDF) and flood seasonality were calculated for Wales using 30 years of gauged river‐flow records (1973–2002). Noticeable regional variations in timing and length of flood season are evident, with flooding occurring earlier in small catchments draining higher elevations in north and mid‐west Wales. Low‐altitude regions in West Wales exposed to westerly winds experience flooding during October–January, while large eastern draining catchments experience later flooding (January–February). In the northeast and mid‐east regions December–January months experience the greatest number of floods, while the southeast has a slightly longer flood season (December–February), with a noticeable increase in January floods. Patterns obtained from MDF data demonstrate their effectiveness and use in analysing regional patterns in flood seasonality, but catchment‐specific determinants, e.g. catchment wetness, size and precipitation regime are important factors in flood seasonality. Relatively strong correlations between precipitation and flood activity are evident in Wales, with a poorer relationship between flooding and weather types and the North Atlantic Oscillation (NAO). Copyright © 2010 John Wiley & Sons, Ltd.     

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

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. Copyright © 2013 John Wiley & Sons, Ltd.     

Using lake sediment archives to improve understanding of flood magnitude and frequency: Recent extreme flooding in northwest UK

We present the first quantitative reconstruction of palaeofloods using lake sediments for the UK and show that for a large catchment in NW England the cluster of devastating floods from 1990 to present is without precedent in this 558-year palaeo-record. Our approach augments conventional flood magnitude and frequency (FMF) analyses with continuous lake sedimentary data to provide a longer-term perspective on flood magnitude recurrence probabilities. The 2009 flood, the largest in >558 years, had a recurrence interval larger (1:2,200 year) than revealed by conventional flood estimation using shorter duration gauged single station records (1:1,700 year). Flood-rich periods are non-stationary in their correlation with climate indices, but the 1990-2018 cluster is associated with warmer Northern Hemisphere Temperatures and positive Atlantic Multidecadal Oscillation. Monitored records rarely capture the largest floods and our palaeoflood series shows, for this catchment, such omissions undermine evaluations of future risk. Our approach provides an exemplar of how to derive centennial palaeoflood reconstructions from lakes coupled well with their catchments around the world. © 2019 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.     

Forests and floods: Using field evidence to reconcile analysis methods

The extent to which forests, relative to shorter vegetation, mitigate flood peak discharges remains controversial and relatively poorly researched, with only a few significant field studies. Considering the effect purely of change of vegetation cover, peak flow magnitude comparisons for paired catchments have suggested that forests do not mitigate large floods, whereas flood frequency comparisons have shown that forests mitigate frequencies over all magnitudes of flood. This study investigates the apparent inconsistency using field-based evidence from four contrasting field programmes at scales of 0.34–3.1 km². Repeated patterns are identified that provide strong evidence of real effects with physical explanations. Magnitude and frequency comparisons are both relevant to the impact of forests on peak discharges but address different questions. Both can show a convergence of response between forested and grassland/logged states at the highest recorded flows but the associated return periods may be quite variable and are subject to estimation uncertainty. For low to moderate events, the forested catchments have a lower peak magnitude for a given frequency than the grassland/logged catchments. Depending on antecedent soil saturation, a given storm may nevertheless generate peak discharges of the same magnitude for both catchment states but these peaks will have different return periods. The effect purely of change in vegetation cover may be modified by additional forestry interventions, such as road networks and drainage ditches which, by effectively increasing the drainage density, may increase peak flows for all event magnitudes. For all the sites, forest cover substantially reduces annual runoff.     

Flooding and geomorphic impacts in a mountain torrent: Raise Beck,central Lake District,England

Raise Beck is a mountain torrent located in the central Lake District fells, northern England (drainage area of 1·27 km²). The torrent shows evidence of several major flood events, the most recent of which was in January 1995. This event caused a major channel avulsion at the fan apex diverting the main flood flow to the south, blocking the A591 trunk road and causing local flooding. The meteorological conditions associated with this event are described using local rainfall records and climatic data. Records show 164 mm of rainfall in the 24 hours preceding the flood. The peak flood discharge is reconstructed using palaeohydrological and rainfall–runoff methods, which provide discharge values of 27–74 m³ s^?1, and 4–6 m³ s^?1, respectively. The flood transported boulders with b‐axes up to 1400 mm. These results raise some important general questions about flood estimation in steep mountain catchments. The geomorphological impact of the event is evaluated by comparing aerial photographs from before and after the flood, along with direct field observations. Over the historical timescale the impact and occurrence of flooding is investigated using lichenometry, long‐term rainfall data, and documentary records. Two major historical floods events are identified in the middle of the nineteenth century. The deposits of the recent and historical flood events dominate the sedimentological evidence of flooding at Raise Beck, therefore the catchment is sensitive to high magnitude, low frequency events. Following the 1995 flood much of the lower catchment was channelized using rip‐rap bank protection, re‐establishing flow north towards Thirlmere. The likely success of this management strategy in containing future floods is considered, based on an analysis of channel capacities. It is concluded that the channelization scheme is only a short‐term solution, which would fail to contain the discharge of an event equivalent to the January 1995 flood. Copyright © 2002 John Wiley & Sons, Ltd.     

Climate-change driven increased flood magnitudes and frequency in the British uplands: Geomorphologically informed scientific underpinning for upland flood-risk management

Upland river systems in the UK are predicted to be prone to the effects of increased flood magnitudes and frequency, driven by climate change. It is clear from recent events that some headwater catchments can be very sensitive to large floods, activating the full sediment system, with implications for flood risk management further down the catchment. We provide a 15-year record of detailed morphological change on a 500-m reach of upland gravel-bed river, focusing upon the geomorphic response to an extreme event in 2007, and the recovery in the decade following. Through novel application of two-dimensional (2D) hydrodynamic modelling we evaluate the different energy states of pre- and post-flood morphologies of the river reach, exploring how energy state adjusts with recovery following the event. Following the 2007 flood, morphological adjustments resulted in changes to the shear stress population over the reach, resulting in higher shear stresses. Although the proportion of shear stresses in excess of those experienced using the pre-flood digital elevation model (DEM) varied over the recovery period, they remained substantially in excess of those experienced pre-2007, suggesting that there is still potential for enhanced bedload transport and morphological adjustment within the reach. Although volumetric change calculated from DEM differencing does indicate a reduction in erosion and deposition volumes in the decade following the flood, we argue that the system still has not fully recovered to the pre-flood state. We further argue that Thinhope Burn, and other similarly impacted catchments in upland environments, may not recover under the wet climatic phase currently being experienced. Hence systems like Thinhope Burn will continue to deliver large volumes of sediment further down river catchments, providing new challenges for flood risk management into the future.     

Flood-type trend analysis for alpine catchments

ABSTRACT

In many places, magnitudes and frequencies of floods are expected to increase due to climate change. To understand these changes better, trend analyses of historical data are helpful. However, traditional trend analyses do not address issues related to shifts in the relative contributions of rainfall versus snowmelt floods, or in the frequency of a particular flood type. We present a novel approach for quantifying such trends in time series of floods using a fuzzy decision tree for event classification and applied it to maximal annual and seasonal floods in 27 alpine catchments for the period 1980–2014. Trends in flood types were studied with Sen’s slope and double mass curves. Our results reveal a decreasing number of rain-on-snow and an increasing number of short rainfall events in all catchments, with flash floods increasing in smaller catchments. Overall, the results demonstrate the value of incorporating a fuzzy flood-type classification into flood trend analyses.     

Hydrologic response of upland catchments to wildfires

To which extent do wildfires affect runoff production, soil erosion and sediment transport in upland catchments? This transient effect is investigated here by combining data of long term precipitation, sediment yield and wildfire records with a fine resolution spatially distributed modeling approach to flow generation and surface erosion. The model accounts for changes in the structure and properties of soil and vegetation cover by combining the tube-flux approach to topographic watershed partition with a parsimonious parametrization of hydrologic processes. This model is used to predict hydrologic and sediment fluxes for nine small catchments in Saint Gabriel mountains of southern California under control (pre-fire) and altered (post-fire) conditions. Simulation runs using a 45 years record of hourly precipitation show the passage of fire to significantly modify catchment response to storms with a major effect on erosion and flood flows. The probability of occurrence of major floods in the post-fire season is shown to increase up to an order of magnitude under same precipitation conditions. Also, the expected anomaly of sediment yield can increase dramatically the desertification hazard in upland wildfire prone areas. One should further consider the role of firefloods produced by the combined occurrence of wildfires and storms as a fundamental source of non-stationarity in the assessment of hydrologic hazard.     

Interactions between sediment delivery,channel change,climate change and flood risk in a temperate upland environment

This paper uses numerical simulation of flood inundation based on a coupled one‐dimensional–two‐dimensional treatment to explore the impacts upon flood extent of both long‐term climate changes, predicted to the 2050s and 2080s, and short‐term river channel changes in response to sediment delivery, for a temperate upland gravel‐bed river. Results show that 16 months of measured in‐channel sedimentation in an upland gravel‐bed river cause about half of the increase in inundation extent that was simulated to arise from climate change. Consideration of the joint impacts of climate change and sedimentation emphasized the non‐linear nature of system response, and the possibly severe and synergistic effects that come from combined direct effects of climate change and sediment delivery. Such effects are likely to be exacerbated further as a result of the impacts of climate change upon coarse sediment delivery. In generic terms, these processes are commonly overlooked in flood risk mapping exercises and are likely to be important in any river system where there are high rates of sediment delivery and long‐term transfer of sediment to floodplain storage (i.e. alluviation involving active channel aggradation and migration). Similarly, attempts to reduce channel migration through river bank stabilization are likely to exacerbate this process as without bank erosion, channel capacity cannot be maintained. Finally, many flood risk mapping studies rely upon calibration based upon combining contemporary bed surveys with historical flood outlines, and this will lead to underestimation of the magnitude and frequency of floodplain inundation in an aggrading system for a flood of a given magnitude. Copyright © 2006 John Wiley & Sons, Ltd.     

Seasonality and magnitude of floods in Switzerland under future climate change

The flood seasonality of catchments in Switzerland is likely to change under climate change because of anticipated alterations of precipitation as well as snow accumulation and melt. Information on this change is crucial for flood protection policies, for example, or regional flood frequency analysis. We analysed projected changes in mean annual and maximum floods of a 22‐year period for 189 catchments in Switzerland and two scenario periods in the 21st century based on an ensemble of climate scenarios. The flood seasonality was analysed with directional statistics that allow assessing both changes in the mean date a flood occurs as well as changes in the strength of the seasonality. We found that the simulated change in flood seasonality is a function of the change in flow regime type. If snow accumulation and melt is important in a catchment during the control period, then the anticipated change in flood seasonality is most pronounced. Decreasing summer precipitation in the scenarios additionally affects the flood seasonality (mean date of flood occurrence) and leads to a decreasing strength of seasonality, that is a higher temporal variability in most cases. The magnitudes of mean annual floods and more clearly of maximum floods (in a 22‐year period) are expected to increase in the future because of changes in flood‐generating processes and scaled extreme precipitation. Southern alpine catchments show a different signal, though: the simulated mean annual floods decrease in the far future, that is at the end of the 21st century. Copyright © 2013 John Wiley & Sons, Ltd.     

Coupled modeling of rainfall-induced floods and sediment transport at the catchment scale

Rainfall-induced floods may trigger intense sediment transport on erodible catchments, especially on the Loess Plateau in China, which in turn modifies the floods. However, the role of sediment transport in modifying floods has to date remained poorly understood. Concurrently, traditional hydrodynamic models for rainfall-induced floods typically ignore sediment transport, which may lead to inaccurate results for highly erodible catchments. Here, a two-dimensional (2D) coupled shallow water hydro-sediment-morphodynamic (SHSM) model, based on the Finite Volume Method on unstructured meshes and parallel computing, is proposed and applied to simulate rainfall-induced floods in the Zhidan catchment on the Loess Plateau, Shaanxi Province, China. For six historical floods of return periods up to 2 years, the numerical results compare well with observations of discharge hydrographs at the catchment outlet. The computed runoff-sediment yield relation is quantitatively reasonable as compared with other catchments under similar geographical conditions. It is revealed that neglecting sediment transport leads to underestimation of peak discharge of the flood by 14%–45%, whilst its effect on the timing of the peak discharge varies for different flood events. For 18 design floods with return periods of 10–500 years, sediment transport may lead to higher peak discharge by around 9%–15%. The temporal pattern of concentrated rainfall in a short period may lead to a larger exponent value of the power function for the runoff-sediment yield relation. The current finding leads us to propose that incorporating sediment transport in rainfall-induced flood modeling is warranted. The SHSM model is applicable to flood and sediment modeling at the catchment scale in support of risk management and water and soil management.     

Simulation of the spatio‐temporal extent of groundwater flooding using statistical methods of hydrograph classification and lumped parameter models

This article presents the development of a relatively low cost and rapidly applicable methodology to simulate the spatio‐temporal occurrence of groundwater flooding in chalk catchments. In winter 2000/2001 extreme rainfall resulted in anomalously high groundwater levels and groundwater flooding in many chalk catchments of northern Europe and the southern United Kingdom. Groundwater flooding was extensive and prolonged, occurring in areas where it had not been recently observed and, in places, lasting for 6 months. In many of these catchments, the prediction of groundwater flooding is hindered by the lack of an appropriate tool, such as a distributed groundwater model, or the inability of models to simulate extremes adequately. A set of groundwater hydrographs is simulated using a simple lumped parameter groundwater model. The number of models required is minimized through the classification and grouping of groundwater level time‐series using principal component analysis and cluster analysis. One representative hydrograph is modelled then transposed to other observed hydrographs in the same group by the process of quantile mapping. Time‐variant groundwater level surfaces, generated using the discrete set of modelled hydrographs and river elevation data, are overlain on a digital terrain model to predict the spatial extent of groundwater flooding. The methodology is applied to the Pang and Lambourn catchments in southern England for which monthly groundwater level time‐series exist for 52 observation boreholes covering the period 1975–2004. The results are validated against observed groundwater flood extent data obtained from aerial surveys and field mapping. The method is shown to simulate the spatial and temporal occurrence of flooding during the 2000/2001 flood event accurately. British Geological Survey © NERC 2011. Hydrological Processes © 2011 John Wiley & Sons, Ltd.     

Spatial and seasonal patterns of flood change across Brazil

Brazil has some of the largest rivers in the world and has the second greatest flood loss potential among the emergent countries. Despite that, flood studies in this area are still scarce. In this paper, we used flood seasonality and trend analysis at the annual and seasonal scales in order to describe flood regimes and changes across the whole of Brazil in the period 1976–2015. We identified a strong seasonality of floods and a well-defined spatio-temporal pattern for flood occurrence. There are positive trends in the frequency and magnitude of floods in the North, South and parts of Southeast Brazil; and negative trends in the North-east and the remainder of Southeast Brazil. Trends in the magnitude (frequency) were predominant in the winter (summer). Overall, floods are becoming more frequent and intense in Brazilian regions characterized by wet conditions, and less frequent and intense in drier regions.     

To what extent have changes in channel capacity contributed to flood hazard trends in England and Wales?

The frequency of floods has been projected to increase across Europe in the coming decades due to extreme weather events. However, our understanding of how flood frequency is affected by geomorphic changes in river channel capacity remains limited. This paper seeks to quantify the influence of trends in channel capacity on flood hazards. Measuring and predicting the effect of geomorphic changes on freshwater flooding is essential to mitigate the potential effects of major floods through informed planning and response. Hydrometric records from 41 stream gauging stations were used to measure trends in the flood stage (i.e. water surface elevation) frequency above the 1% annual exceedance threshold. The hydrologic and geomorphic components of flood hazard were quantified separately to determine their contribution to the total trend in flood stage frequency. Trends in cross‐sectional flow area and mean flow velocity were also investigated at the same flood stage threshold. Results showed that a 10% decrease (or increase) in the channel capacity would result in an increase (or decrease) in the flood frequency of approximately 1.5 days per year on average across these 41 sites. Widespread increases in the flood hazard frequency were amplified through both hydrologic and geomorphic effects. These findings suggest that overlooking the potential influence of changing channel capacity on flooding may be hazardous. Better understanding and quantifying the influence of geomorphic trends on flood hazard will provide key insight for managers and engineers into the driving mechanisms of fluvial flooding over relatively short timescales. Copyright © 2016 John Wiley & Sons, Ltd.     

Characteristics of floods on the territory of Russia with regard to their natural and socioeconomic parameters

Parameters of 123 floods on the territory of the Russian Federation are analyzed, including the dates of young floods, numbers (frequency), duration, genetic type of floods, inundated land areas, total area of regions affected by flood, coordinates of their centers, number of population in these regions, total number of affected people, number of evacuated and killed people, number of buildings in inundation zone, and monetary damage. It is shown that material and human damages caused by rainfall floods and snowmelt floods in rivers are several orders of magnitude greater than the damages caused by the most common spring floods. Collation maps of main flood parameters have been prepared and analyzed; the areas of floods of main genetic types have been revealed and mapped and graphs of seasonal and long-term variability of flood parameters has been calculated.     

Channel and floodplain response to recent abrupt climate change: The tyne basin,Northern England

This paper examines the timing, nature and magnitude of river response in upland, piedmont and lowland reaches of the Tyne basin, northern England, to high-frequency (20–30 year) changes in climate and flood regime since 1700 AD. Over this period fluvial activity has been characterized by alternating phases of river-bed incision and stability coinciding with non-random, decadal-scale fluctuations in flood frequency and hydroclimate that appear to be linked to changes in large-scale upper atmospheric circulation patterns. Episodes of widespread channel bed incision (1760–1799, 1875–1894, 1955–1969) result from a higher frequency of large floods (> 20 year return period) and cool, wet climate under meridional circulation regimes. Phases of more moderate floods (5–20 year return period), corresponding to zonal circulation types (1820–1874, 1920–1954), are characterized by enhanced lateral reworking and sediment transfer in upper reaches of the catchment, and channel narrowing and infilling downstream. Rates of fluvial activity are reduced in intermediate periods (1800–1819, 1895–1919) with no dominant circulation regime associated with lower flood frequency and magnitude. The results of this study provide a valuable guide for forecasting probable drainage basin and channel response to future climate change.     

Changes of flood risk on the northern foothills of the Tatra Mountains

The present paper reviews selected outcomes of the FLORIST project devoted to flood risk in the region of the northern foothills of the Tatra Mountains in Poland and summarizes novel results. The project encompassed theoretical, field, and modeling work. It was focused around observation-based hydroclimatology; projections for the future; dendrogeomorphology; as well as influence of transport of large wood on fluvial processes. The project improved understanding and interpreting changes in high-flow frequency and magnitude as well as changes in flood risk in the region, related to the presence of large wood in mountain streams. A unique database on past episodes of intense precipitation and flooding was created, harnessing multiple sources. The project showed that the analysis of tree rings and wood logs can offer useful information, complementing and considerably enriching the knowledge of river floods in the region of northern foothills of the Tatra Mountains. Retrospective and scenario-defined modeling of selected past fluvial events in the region was also performed.     

How to improve attribution of changes in drought and flood impacts

For the development of sustainable, efficient risk management strategies for the hydrological extremes of droughts and floods, it is essential to understand the temporal changes of impacts, and their respective causes and interactions. In particular, little is known about changes in vulnerability and their influence on drought and flood impacts. We present a fictitious dialogue between two experts, one in droughts and the other in floods, showing that the main obstacles to scientific advancement in this area are both a lack of data and a lack of commonly accepted approaches. The drought and flood experts “discuss” available data and methods and we suggest a complementary approach. This approach consists of collecting a large number of single or multiple paired-event case studies from catchments around the world, undertaking detailed analyses of changes in impacts and drivers, and carrying out a comparative analysis. The advantages of this approach are that it allows detailed context- and location-specific assessments based on the paired-event analyses, and reveals general, transferable conclusions based on the comparative analysis of various case studies. Additionally, it is quite flexible in terms of data and can accommodate differences between floods and droughts.