Extreme Flood Sediment Production and Export Controlled by Reach‐Scale Morphology

Rapid earth surface evolution is discrete in nature, with short-duration extreme events having a widespread impact on landscapes despite occurring relatively infrequently. Here, we exploit a unique opportunity to identify the broad, process-based, controls on sediment production and export during extreme rainfall-runoff events through a multi-catchment analysis. A 3 hr extreme rainfall event generated significantly different impacts across three catchments, ranging from (a) sediment export exceeding two orders of magnitude more than the typical long term average to (b) a minimal impact, with this variability primarily controlled by catchment steepness and the presence of reach-scale morphological transitions caused by postglacial landscape adjustment. In any catchment worldwide where populations are at risk, we highlight the importance of combining topographic analysis with detailed mapping of channel bed material (e.g., presence of transitions between process domains) and identification of sediment sources within morphological transition zones for accurately predicting the impact of extreme events

National-Scale Rainfall-Triggered Landslide Susceptibility and Exposure in Nepal

Nepal is one of the most landslide-prone countries in the world, with year-on-year impacts resulting in loss of life and imposing a chronic impediment to sustainable livelihoods. Living with landslides is a daily reality for an increasing number of people, so establishing the nature of landslide hazard and risk is essential. Here we develop a model of landslide susceptibility for Nepal and use this to generate a nationwide geographical profile of exposure to rainfall-triggered landslides. We model landslide susceptibility using a fuzzy overlay approach based on freely-available topographic data, trained on an inventory of mapped landslides, and combine this with high resolution population and building data to describe the spatial distribution of exposure to landslides. We find that whilst landslide susceptibility is highest in the High Himalaya, exposure is highest within the Middle Hills, but this is highly spatially variable and skewed to on average relatively low values. Around 4 × 106 Nepalis (∼15\% of the population) live in areas considered to be at moderate or higher degree of exposure to landsliding (>0.25 of the maximum), and critically this number is highly sensitive to even small variations in landslide susceptibility. Our results show a complex relationship between landslides and buildings, that implies wider complexity in the association between physical exposure to landslides and poverty. This analysis for the first time brings into focus the geography of the landslide exposure and risk case load in Nepal, and demonstrates limitations of assessing future risk based on limited records of previous events

Modelling post‐earthquake cascading hazards: Changing patterns of landslide runout following the 2015 Gorkha earthquake, Nepal

Coseismic landslides represent the first stage of a broader cascading sequence of geohazards associated with high-magnitude continental earthquakes, with the subsequent remobilisation of coseismic landslide debris posing a long-term post-seismic legacy in mountain regions. Here, we quantify the controls on the hazard posed by landslide remobilisation and debris runout, and compare the overlap between areas at risk of runout and the pattern of post-seismic landslides and debris flows that actually occurred. Focusing on the 2015 Mw 7.8 Gorkha earthquake in Nepal, we show that the extent of the area that could be affected by debris runout remained elevated above coseismic levels 4.5 years after the event. While 150 km2 (0.6% of the study area) was directly impacted by landslides in the earthquake, an additional 614 km2 (2.5%) was left at risk from debris runout, increasing to 777 km2 (3.2%) after the 2019 monsoon. We evaluate how this area evolved by comparing modelled predictions of runout from coseismic landslides to multi-temporal post-seismic landslide inventories, and find that 14% (85 km2) of the total modelled potential runout area experienced landslide activity within 4.5 years after the earthquake. This value increases to 32% when modelled runout probability is thresholded, equivalent to 10 km2 of realised runout from a remaining modelled area of 32 km2. Although the proportion of the modelled runout area from coseismic landslides that remains a hazard has decreased through time, the overall runout susceptibility for the study area remains high. This indicates that runout potential is changing both spatially and temporally as a result of changes to the landslide distribution after the earthquake. These findings are particularly important for understanding evolving patterns of cascading hazards following large earthquakes, which is crucial for guiding decision-making associated with post-seismic recovery and reconstruction

Preserving the Legacy of Historic Metal-Mining Industries in the Light of the Water Framework Directive and Future Environmental Change in mainland Britain: Challenges for the Heritage Community

Contemporary global metal mining is a source of environmental pollution, but in Britain it is our historic mining industry that has left a legacy of contamination in the landscape, around both the immediate mine sites as well as within the river valley floors that drain these orefields. It has been estimated that the levels of lead and zinc stored within some northern British river systems represent values comparable to present-day reserves of economically viable ore deposits and exposure to them can be detrimental to human health. Despite the prevalence and significance of these deposits, they have been neglected by the cultural heritage community in favour of more easily interpretable remains such as mine buildings, technologies of ore procurement and processing, and the final products of manufacture. This paper argues that in light of future climate change and legislation associated with the European Union Water Framework Directive, heritage managers and industrial archaeologists have to start investigating these deposits as part of their studies and to engage with the environmental science and geomorphological communities who are, at present, setting the agenda in terms of strategies for pollution mitigation and landscape remediation

Sediment source connectivity, torrent erosion and sediment delivery during a record-breaking UK flood

The significance of channel and hillslope sediment sources to sediment delivery during extreme floods in steep landscapes is difficult to evaluate due to the local nature of many mountain storms and lack of quantitative geomorphologic measurements. Using a rapid-appraisal sediment budget framework, which captured key evidence on sediment source contributions and; torrent erosion and deposition in the immediate aftermath of an extreme flood event, we present a unique record of detailed sediment balance calculations for multiple mountain catchments. This study focusses on record-breaking Storm Desmond (December 2015) which impacted the Helvellyn mountain massif in Northern England, UK. The storm was the largest in a 150 year local rainfall series and set a new UK 48-hour rainfall record of 405 mm (falling in 38 hours, 4-6 December). The steep mountain catchments studied here flank Thirlmere Reservoir (a major water supply asset, providing c. 11% of North-West England’s water) and range in size from 0.14 to 1.31 km2, have catchment slopes 0.34-0.67 m m-1 and channel slopes 0.4-0.75 m m-1. The torrents consist of steep, boulder-mantled / bedrock channels which are incised in to glacial and colluvium deposits. Lower slopes are forested and here a forest road network and major highway traverse the base of the main torrents. During the flood these roads intercepted sediment resulting in local deposition of debris cones / fans and extensive damage to built infrastructure. Geomorphological surveys enabled the volumes of erosion and deposition to be estimated and individual catchment sediment budgets to be constructed. Shallow hillslope landslides, local debris flows and extensive channel erosion (bank erosion and bed scour) supplied the majority of sediment. Channel erosion accounted for over 80% of the erosion and the net catchment sediment delivery was 60% with specific sediment yields of up to 2000 t km-2. Hillslope landslides were only weakly coupled to channels. Boulder size surveys, comparing the recent flood deposits with historic deposits (dated using lichenometry) show the 2015 flood was as least as large as any previous event in the area. Results provide an important baseline and modern analogue for understanding sediment delivery processes during extreme floods in temperate, steep mountain environments