Peat bog restoration: Implications of erosion and sediment transfer at Flow Moss, North Pennines

The impacts of peatland management strategies used to restore degraded bare peat flats have received little attention. This study aims to improve the understanding of geomorphological processes acting on an upland bare peat flat which is undergoing restoration at Flow Moss, North Pennines, UK. A sediment budget is constructed which provides a baseline framework for assessing the effectiveness of peatland restoration measures in reducing peat erosion rates. Erosion monitoring of aeolian and active slope processes was undertaken between October 2010 and July 2011 using a network of sediment traps and erosion pins installed across the 7 hectare site. Meteorological conditions were monitored using an Automatic Weather Station and local water table was recorded using a pressure transducer. This allowed relationships between weather patterns, hydrology and sediment transfer to be developed. Meteorological conditions are important in controlling the wind erosion of peat with the highest rates of erosion occurring when heavy rainfall (> 5 mm hr-1) was combined with high wind-speeds (> 18 m s-1). Windward facing traps collected up to 8 times the peat collected by leeward facing traps. Freeze-thaw weathering and surface desiccation are important in generating loose material on the surface for subsequent sediment transport. A two-phase model is proposed to explain wind splash erosion dynamics where weathered material is transported preferentially before the intact peat layer is eroded. Sediment transport across bare peat flats is very active (3.2 t a-1) but the eroding flats are disconnected from the ephemeral channel system. Moreover, the channel system contains pools where the majority of suspended peat is deposited. This leads to a low net overall sediment yield for the catchment of approximately 0.01 t a-1. The terrestrial carbon store (~2060 tonnes) at Flow Moss is relatively stable as, in the worst case scenario, it is losing 117 gC m-2 yr -1, amounting to just 0.4% of the total store. It is estimated that Flow Moss will become a carbon sink when 90% of the bare peat areas have been re-vegetated so it is therefore vital that the restoration measures are successful. Continued monitoring of sediment transfer will allow a full evaluation of the impact of the restoration measures in reducing erosion rates

Knickpoints in Martian channels indicate past ocean levels

On Mars, the presence of extensive networks of sinuous valleys and large channels provides evidence for a wetter and warmer environment where liquid water was more abundant than it is at present. We undertook an analysis of all major channel systems on Mars and detected sharp changes in elevation along the river long profiles associated with steep headwall theatre-like valleys and terraces left downstream by channel incision. These breaks in channel longitudinal slope, headwalls and terraces exhibit a striking resemblance with terrestrial fluvial features, commonly termed 'knickpoints'. On Earth, such knickpoints can be formed by more resistant bedrock or where changes in channel base-level have initiated erosion that migrates upstream (such as tectonic uplift or sea level change). We observed common elevations of Martian knickpoints in eleven separate channel systems draining into the Martian Northern lowlands. Numerical modeling showed that the common elevations of some of these knickpoints were not random. As the knickpoints are spread across the planet, we suggest that these Martian knickpoints were formed in response to a common base level or ocean level rather than local lithology. Thus, they potentially represent a record of past ocean levels and channel activity on Mars

Trials and tribulations in locating tree farmers and sites for research and extension activities

The major aim of ACIAR project ASEM 2003/052 is to improve financial returns to existing smallholder tree farms in Leyte through a number of extension activities. In order to identify sites suitable for extension activities, visits were made to some tree farms (either registered or not registered with DENR) in Leyte. For this purpose, the initial aim was to identify at least 30 tree farms representing a range of age classes, species, soil types, elevation and climate. In addition, tree farms should have an area of at least 0.25 ha. Various difficulties were encountered in fieldwork designed to locate these tree farms. The main reason was associated with inconsistencies in the database of registered tree farms compiled by Community Environment and Natural Resources Offices (CENROs) which included information concerning tree farm location, owner, species and plantation area. Specific difficulties encountered in finding sites included nonexistence of some registered tree farms, inability to interview some farm owners because they do not reside near their tree farm, some tree farms have a low stocking against what was listed with the CENRO. Despite these difficulties, 76 tree farms were found during October to December 2004. Seventy one tree farms were GPS referenced and 37 tree farm owners were interviewed

Beyond equilibrium: Re-evaluating physical modelling of fluvial systems to represent climate changes

© 2018 Elsevier B.V. The interactions between water, sediment and biology in fluvial systems are complex and driven by multiple forcing mechanisms across a range of spatial and temporal scales. In a changing climate, some meteorological drivers are expected to become more extreme with, for example, more prolonged droughts or more frequent flooding. Such environmental changes will potentially have significant consequences for the human populations and ecosystems that are dependent on riverscapes, but our understanding of fluvial system response to external drivers remains incomplete. As a consequence, many of the predictions of the effects of climate change have a large uncertainty that hampers effective management of fluvial environments. Amongst the array of methodological approaches available to scientists and engineers charged with improving that understanding, is physical modelling. Here, we review the role of physical modelling for understanding both biotic and abiotic processes and their interactions in fluvial systems. The approaches currently employed for scaling and representing fluvial processes in physical models are explored, from 1:1 experiments that reproduce processes at real-time or time scales of 10 −1 -10 0 years, to analogue models that compress spatial scales to simulate processes over time scales exceeding 10 2 –10 3 years. An important gap in existing capabilities identified in this study is the representation of fluvial systems over time scales relevant for managing the immediate impacts of global climatic change; 10 1 – 10 2 years, the representation of variable forcing (e.g. storms), and the representation of biological processes. Research to fill this knowledge gap is proposed, including examples of how the time scale of study in directly scaled models could be extended and the time scale of landscape models could be compressed in the future, through the use of lightweight sediments, and innovative approaches for representing vegetation and biostabilisation in fluvial environments at condensed time scales, such as small-scale vegetation, plastic plants and polymers. It is argued that by improving physical modelling capabilities and coupling physical and numerical models, it should be possible to improve understanding of the complex interactions and processes induced by variable forcing within fluvial systems over a broader range of time scales. This will enable policymakers and environmental managers to help reduce and mitigate the risks associated with the impacts of climate change in rivers

Erosion during extreme flood events dominates Holocene canyon evolution in northeast Iceland

Extreme flood events have the potential to cause catastrophic landscape change in short periods of time (10(0) to 10(3) h). However, their impacts are rarely considered in studies of long-term landscape evolution (>10(3) y), because the mechanisms of erosion during such floods are poorly constrained. Here we use topographic analysis and cosmogenic (3)He surface exposure dating of fluvially sculpted surfaces to determine the impact of extreme flood events within the Jökulsárgljúfur canyon (northeast Iceland) and to constrain the mechanisms of bedrock erosion during these events. Surface exposure ages allow identification of three periods of intense canyon cutting about 9 ka ago, 5 ka ago, and 2 ka ago during which multiple large knickpoints retreated large distances (>2 km). During these events, a threshold flow depth was exceeded, leading to the toppling and transportation of basalt lava columns. Despite continuing and comparatively large-scale (500 m(3)/s) discharge of sediment-rich glacial meltwater, there is no evidence for a transition to an abrasion-dominated erosion regime since the last erosive event because the vertical knickpoints have not diffused over time. We provide a model for the evolution of the Jökulsárgljúfur canyon through the reconstruction of the river profile and canyon morphology at different stages over the last 9 ka and highlight the dominant role played by extreme flood events in the shaping of this landscape during the Holocene

Constraining bedrock erosion during extreme flood events

The importance of high-magnitude, short-lived flood events in controlling the evolution of bedrock landscapes is not well understood. During such events, erosion processes can shift from one regime to another upon the passing of thresholds, resulting in abrupt landscape changes that can have a long lasting legacy on landscape morphology. Geomorphological mapping and topographic analysis document the evidence for, and impact of, extreme flood events within the Jökulsárgljúfur canyon (North-East Iceland). Surface exposure dating using cosmogenic 3He of fluvially sculpted bedrock surfaces determines the timing of the floods that eroded the canyon and helps constrain the mechanisms of bedrock erosion during these events. Once a threshold flow depth has been exceeded, the dominant erosion mechanism becomes the toppling and transportation of basalt lava columns and erosion occurs through the upstream migration of knickpoints. Surface exposure ages allow identification of three periods of rapid canyon cutting during erosive flood events about 9, 5 and 2 ka ago, when multiple active knickpoints retreated large distances (> 2 km), each leading to catastrophic landscape change within the canyon. A single flood event ~9 ka ago formed, and then abandoned, Ásbyrgi canyon, eroding 0.14 km3 of rock. Flood events ~5 and ~2 ka ago eroded the upper 5 km of the Jökulsárgljúfur canyon through the upstream migration of vertical knickpoints such as Selfoss, Dettifoss and Hafragilsfoss. Despite sustained high discharge of sediment-rich glacial meltwater (ranging from 100 to 500 m3 s-1); there is no evidence for a transition to an abrasion-dominated erosion regime since the last erosive flood: the vertical knickpoints have not diffused over time and there is no evidence of incision into the canyon floor. The erosive signature of the extreme events is maintained in this landscape due to the nature of the bedrock, the discharge of the river, large knickpoints and associated plunge pools. The influence of these controls on the dynamics of knickpoint migration and morphology are explored using an experimental study. The retreat rate of knickpoints is independent of both mean discharge, and temporal variability in the hydrograph. The dominant control on knickpoint retreat is the knickpoint form which is set by the ratio of channel flow depth to knickpoint height. Where the knickpoint height is five times greater than the flow depth, the knickpoints developed undercutting plunge pools, accelerating the removal of material from the knickpoint base and the overall retreat rate. Smaller knickpoints relative to the flow depth were more likely to diffuse from a vertical step into a steepened reach or completely as the knickpoint retreated up the channel. These experiments challenge the established assumption in models of landscape evolution that a simple relationship exists between knickpoint retreat and discharge/drainage area. In order to fully understand how bedrock channels, and thus landscapes, respond and recover to transient forcing, further detailed study of the mechanics of erosion processes at knickpoints is required

Supercritical River Terraces: Processes driving rapid landscape change following climatic variability

International audienceThe alternating cycle of strath planation and strath terrace abandonment due to variations in sediment supplyrelative to river transport capacity is a common feature in many mountainous environments, often driven by climaticvariability. However, our understanding of the mechanics of the processes that drive this cycle remains poorlyquantified. Here, we used an experimental and numerical study to identify the geomorphic and hydraulic controlsdriving the response of alluvial rivers to variable sediment supply, discharge and slope. The experimental channelsexhibit a newly identified multi-stage response: the narrowing of the channel and stripping of the alluvial coverin a downstream migrating incision wave followed by destabilisation of the bedrock surface and development ofself-formed knickpoints when the hydraulic conditions are supercritical. Headward erosion by knickpoints is themost efficient process of strath terrace abandonment, contributing the majority of the total vertical incision, even inthe absence of base-level fall or any vertical offset in channel elevation.We also demonstrate the possibility of selfformedknickpoints developing under supercritical flow conditions in driving the rapid response of fluvial systemsto external climatic perturbations in natural systems, highlighting a previously unrecognised process leading tostrath terrace abandonment. This has implications for the understanding of distributions of strath terrace ages, andhow landscapes respond to climatic or tectonic perturbations

Why Did You Leave Me? Identification of a Two-Phase Fluvial Incision Response to Climate, Land-Sse and Tectonic Changes Leading to Strath Terrace Formation

International audienceSurpassing thresholds between different erosion regimes following abrupt changes in external forcing have the potential to shift rivers dramatically from one characteristic state to another. The alternating cycle of strath planation and strath terrace abandonment due to variations in sediment supply relative to the transport capacity is a common feature in many mountainous environments, yet our understanding of the mechanics of the processes that drive this landscape change remains poorly quantified.Here, we present an experimental laboratory study that identifies the response of alluvial rivers to variable sediment supply, discharge and tilting, proxies for climatic, land-use and tectonic changes. The experimental channels exhibit a two-phase response: the narrowing of the channel and stripping of the alluvial cover in a downstream migrating incision wave followed by further focussing of the flow, destabilisation of the bed and development of knickpoints mid-channel under supercritical flow conditions, in the absence of any base level fall. Once formed, in the absence of any base level fall, the headward erosion by knickpoints is the most efficient process of strath terrace abandonment, with the majority of the total vertical incision occurring as the knickpoint migrates upstream. The resulting landscape state is characterised by a narrow deeply incised channel in strong contrast with the initial wide, laterally migrating system present before the perturbation was forced during the experiments.These experiments demonstrate the importance of self-formed knickpoints in driving the rapid response of fluvial systems to external perturbations, highlighting a previously unrecognised process leading to strath terrace abandonment. This has implications for the understanding of distributions of strath terrace ages derived from geochronological techniques, and how rivers will respond to future perturbations driven by climate or land-use changes