Showing papers in "Earth Surface Processes and Landforms in 2012"

Is climate change responsible for changing landslide activity in high mountains

Abstract: Climate change, manifested by an increase in mean, minimum, and maximum temperatures and by more intense rainstorms, is becoming more evident in many regions. An important consequence of these changes may be an increase in landslides in high mountains. More research, however, is necessary to detect changes in landslide magnitude and frequency related to contemporary climate, particularly in alpine regions hosting glaciers, permafrost, and snow. These regions not only are sensitive to changes in both temperature and precipitation, but are also areas in which landslides are ubiquitous even under a stable climate. We analyze a series of catastrophic slope failures that occurred in the mountains of Europe, the Americas, and the Caucasus since the end of the 1990s. We distinguish between rock and ice avalanches, debris flows from de-glaciated areas, and landslides that involve dynamic interactions with glacial and river processes. Analysis of these events indicates several important controls on slope stability in high mountains, including: the non-linear response of firn and ice to warming; three-dimensional warming of subsurface bedrock and its relation to site geology; de-glaciation accompanied by exposure of new sediment; and combined short-term effects of precipitation and temperature. Based on several case studies, we propose that the following mechanisms can significantly alter landslide magnitude and frequency, and thus hazard, under warming conditions: (1) positive feedbacks acting on mass movement processes that after an initial climatic stimulus may evolve independently of climate change; (2) threshold behavior and tipping points in geomorphic systems; (3) storage of sediment and ice involving important lag-time effects.

Sedimentary architecture of abandoned channel fills

Abstract: Channel fills are common elements of Holocene river systems and older fluvial sequences, but surprisingly little is known about formation and their sedimentary build-up. Abandoned channels result from channel shifting processes at various scales, including meander cutoff and channel-belt avulsion. Channel-fill sequences are of importance as containers of palaeoenvironmental proxy-records, can be used to reconstruct palaeochannel dynamics and derive palaeoflood records, and contain materials that allow dating the abandonment. Integrated knowledge on the dynamic nature (geometrical and physical insights) of channel abandonment and resultant sedimentary recording is a necessity for comparing and collating records from a series of abandoned channel fills. This paper intends to make channel-fill sedimentological sequences more useful recorders of channel abandonment processes and palaeofloods, for which improved understanding is needed of the internal build-up of channel fills. We review oxbow lake infilling along meandering rivers, and supplement this with highly detailed descriptions of two selected field examples of channel fills from the apex-region of the Netherlands' Rhine delta. From these examples it becomes clear that regional setting and type of abandonment result in different channel-fill end-members; oxbow cutoffs generally produce thick laminated clayey fills as the channel entrance is plugged rapidly, avulsion-abandoned channels are filled with coarse (proximal) deposits as a result of a maintained open river connection. Field examples of channel fills are integrated with knowledge on channel abandonment dynamics in meander cutoff and bifurcating river situations, including insights from recent numerical modelling. We propose a sedimentary-architecture descriptive scheme that distinguishes elements from two stages of channel-fill development; (i) the abandonment stage with initial proximal fill, and (ii) the subsequent fully abandoned palaeochannel that collects distal fill. Copyright © 2011 John Wiley & Sons, Ltd.

Bedload transport measurements at the Erlenbach stream with geophones and automated basket samplers

Abstract: In the Erlenbach stream, a pre-alpine steep channel in Switzerland, sediment transport has been monitored for more than 25 years. Near the confluence with the main valley river, stream flow is monitored and sediment is collected in a retention basin with a capacity of about 2000 m3. The basin is surveyed at regular intervals and after large flood events. In addition, sediment transport has been continuously monitored with piezoelectric bedload impact and geophone sensors since 1986. In 2008–2009, the measuring system in the Erlenbach stream was enhanced by installing an automatic system to obtain bedload samples. Movable metal baskets are mounted on a rail at the downstream wall of the large check dam above the retention basin, and they can be moved automatically into the flow to take bedload transport samples. The wire mesh of the baskets has a spacing of 10 mm to sample all sediment particles coarser than this size (which is about the limiting grain size detected by the geophones). The upgraded measuring system permits to obtain bedload samples over short sampling periods and to measure the grain size distribution of the transported material and its variation over time and with discharge. The analysis of calibration relationships for the geophone measuring system confirms findings from very similar measurements which were performed until 1999 with piezoelectric bedload impact sensors; there is a linear relationship between impulse counts and bedload mass passing over the sensors. Findings from flume experiments are used to discuss the most important factors which affect the calibration of the geophone signal. The bedload transport rates as measured by the moving baskets are among the highest measured in natural streams, with values of the order of several kilograms per meter per second. Copyright © 2012 John Wiley & Sons, Ltd.

Plant root traits affecting the resistance of soils to concentrated flow erosion

Abstract: The effect of plant species on erosion processes may be decisive for long-term soil protection in degraded ecosystems. The identification of functional effect traits that predict species ability for erosion control would be of great interest for ecological restoration purposes. Flume experiments were carried out to investigate the effect of the root systems of three species having contrasted ecological requirements from eroded marly lands of the French Southern Alps [i.e. Robinia pseudo acacia (tree), Pinus nigra austriaca (tree) and Achnatherum calamagrostis (grass)], on concentrated flow erosion rates. Ten functional traits, describing plant morphological and biomechanical features, were measured on each tested sample. Analyses were performed to identify traits that determine plant root effects on erosion control. Erosion rates were lowest for samples of Robinia pseudo acacia, intermediate in Achnatherum calamagrostis and highest in Pinus nigra austriaca. The three species also differed strongly in their traits. Principal components analysis showed that the erosion-reducing potential of plant species was negatively correlated to root diameter and positively correlated to the percentage of fine roots. The results highlighted the role of small flexible roots in root reinforcement processes, and suggested the importance of high root surface and higher tensile strength for soil stabilization. By combining flume experiment to plant functional traits measurements, we identified root system features influencing plant species performance for soil protection against concentrated flow erosion. Plant functional traits related to species efficiency for erosion control represent useful tools to improve the diagnosis of land vulnerability to erosion, plant community resistance and the prediction of ecosystem functioning after ecological restoration. Copyright © 2012 John Wiley & Sons, Ltd.

The beaver meadow complex revisited – the role of beavers in post-glacial floodplain development

Abstract: We evaluate the validity of the beaver-meadow complex hypothesis, used to explain the deposition of extensive fine sediment in broad, low-gradient valleys. Previous work establishes that beaver damming forms wet meadows with multi-thread channels and enhanced sediment storage, but the long-term geomorphic effects of beaver are unclear. We focus on two low-gradient broad valleys, Beaver Meadows and Moraine Park, in Rocky Mountain National Park (Colorado, USA). Both valleys experienced a dramatic decrease in beaver population in the past century and provide an ideal setting for determining whether contemporary geomorphic conditions and sedimentation are within the historical range of variability of valley bottom processes. We examine the geomorphic significance of beaver-pond sediment by determining the rates and types of sedimentation since the middle Holocene and the role of beaver in driving floodplain evolution through increased channel complexity and fine sediment deposition. Sediment analyses from cores and cutbanks indicate that 33–50% of the alluvial sediment in Beaver Meadows is ponded and 28–40% was deposited in-channel; in Moraine Park 32–41% is ponded sediment and 40–52% was deposited in-channel. Radiocarbon ages spanning 4300 years indicate long-term aggradation rates of ~0.05 cm yr-1. The observed highly variable short-term rates indicate temporal heterogeneity in aggradation, which in turn reflects spatial heterogeneity in processes at any point in time. Channel complexity increases directly downstream of beaver dams. The increased complexity forms a positive feedback for beaver-induced sedimentation; the multi-thread channel increases potential channel length for further damming, which increases the potential area occupied by beaver ponds and the volume of fine sediment trapped. Channel complexity decreased significantly as surveyed beaver population decreased. Beaver Meadows and Moraine Park represent settings where beaver substantially influence post-glacial floodplain aggradation. These findings underscore the importance of understanding the historical range of variability of valley bottom processes, and implications for environmental restoration. Copyright © 2011 John Wiley & Sons, Ltd.

Remote measurement of river morphology via fusion of LiDAR topography and spectrally based bathymetry

Abstract: This study developed and evaluated a hybrid approach to remote measurement of river morphology that combines LiDAR topography with spectrally based bathymetry. Comparison of filtered LiDAR point clouds with surveyed cross-sections indicated that subtle features on low-relief floodplains were accurately resolved by LiDAR but that submerged areas could not be detected due to strong absorption of near-infrared laser pulses by water. The reduced number of returns made the active channel evident in a LiDAR point density map. A second dataset suggested that pulse intensity also could be used to discriminate land from water via a threshold-based masking procedure. Fusion of LiDAR and optical data required accurate co-registration of images to the LiDAR, and we developed an object-oriented procedure for achieving this alignment. Information on flow depths was derived by correlating pixel values with field measurements of depth. Highly turbid conditions dictated a positive relation between green band radiance and flow depth and contributed to under-prediction of pool depths. Water surface elevations extracted from the LiDAR along the water's edge were used to produce a continuous water surface that preserved along-channel variations in slope. Subtracting local flow depths from this surface yielded estimates of the bed elevation that were then combined with LiDAR topography for exposed areas to create a composite representation of the riverine terrain. The accuracy of this terrain model was assessed via comparison with detailed field surveys. A map of elevation residuals showed that the greatest errors were associated with underestimation of pool depths and failure to capture cross-stream differences in water surface elevation. Nevertheless, fusion of LiDAR and passive optical image data provided an efficient means of characterizing river morphology that would not have been possible if either dataset had been used in isolation. Copyright © 2011 John Wiley & Sons, Ltd.

Testing bedload transport equations with consideration of time scales

Abstract: Bedload transport is known to be a highly fluctuating temporal phenomenon, even under constant (mean) flow conditions, as a consequence of stochasticity, bedform migration, grain sorting, hysteresis, or sediment supply limitation. Because bedload transport formulas usually refer to a single mean transport value for a given flow condition, one can expect that prediction accuracy (when compared to measurements) will depend on the amplitude and duration of fluctuations, which in turn depend on the time scale used for observations. This paper aims to identify how the time scale considered can affect bedload prediction. This was done by testing 16 common bedload transport formulas with four data sets corresponding to different measurement period durations: (i) highly fluctuating (quasi-)instantaneous field measurements; (ii) volumes accumulated at the event scale on two small alpine gravel-bed rivers, potentially affected by seasonal fluctuations; (iii) volumes accumulated at the interannual scale in a meandering gravel bed river, thought to be weakly subject to fluctuations; (iv) time-integrated flume measurements with nearly uniform sediments. The tests confirmed that the longer the measurement period, the better the precision of the formula's prediction interval. They also demonstrate several consequential limitations. Most threshold formulas are no longer valid when the flow condition is below two times the threshold condition for the largest elements' motion on the bed surface (considering D84). In such conditions, equations either predict zero transport, or largely overestimate the real transport, especially when D84 is high. There is a need for new sediment data collected with highly reliable techniques such as recording slot bedload samplers to further investigate this topic. Copyright © 2012 John Wiley & Sons, Ltd.

Bedload tracing in a high-sediment-load mountain stream

Abstract: This paper reports a radiofrequency identification (RFID) tracing experiment implemented in a high-sediment-load mountain stream typical of alpine gravel-bed torrents. The study site is the Bouinenc Torrent, a tributary to the Bleone River in southeast France that drains a 38·9-km² degraded catchment. In spring 2008, we deployed 451 tracers with b-axis ranging from 23 to 520 mm. Tracers were seeded along eight cross-sections located in the upstream part of the lowest 2·3 km of the stream. Three tracer inventories were implemented in July 2008, 2009 and 2010. Recovery rates calculated for mobile tracers declined from 78% in 2008 to 45% in 2009 and 25% in 2010. Observations of tracer displacement revealed very high sediment dispersion, with frontrunners having travelled more than 2 km only three months after their deployment. The declining recovery rate over time was interpreted as resulting from rapid dispersion rather than deep burial. We evaluated that 64% of the tracers deployed in the active channel were exported from the 2·3-km study reach three years after the onset of the tracing experiment. Travel distances were characterized by right-skewed and heavy-tailed distributions, correctly fitted by a power-law function. This supports the idea that in gravel-bed rivers with abundant sediment supply relative to transport capacity, bedload transport can be viewed as a superdiffusive sediment dispersion process. It is also shown that tracers initially deployed in the low-flow channel were characterized by a 15- to 30-fold increase of mobility compared to tracers deployed in gravel bars. Copyright © 2011 John Wiley & Sons, Ltd.

How natural are Alpine mountain rivers? Evidence from the Italian Alps

Abstract: Historical, human-induced channel adjustments in lowland gravel-bed rivers have been documented in several geographical contexts worldwide. In particular, it is now widely accepted that the vast majority of European rivers are far from any natural, reference state prior to anthropic disturbances, and a ‘complete’ restoration is hardly achievable. However, few investigations have addressed changes that have occurred in mountain rivers of the Alps, and these channels are commonly reckoned quite ‘natural’ by society. This paper intends to describe how human pressure on Italian Alpine basins has been quite relevant for several centuries – in terms of land-use variations, in-channel structures, timber transport (splash damming) and riparian vegetation management – such that nowadays ‘reference conditions’ cannot be found even in small mountain creeks. In addition, recent natural climatic variations (e.g. the Little Ice Age) are superimposed on human disturbances, thus defying the definition of any ‘equilibrium’ morphological conditions even under ‘human-free’ states. A summary of published as well as unpublished works on historical channel adjustments in rivers of the Italian Alps is presented in order to document the impacts deriving from human pressure at different basin scales and for different river morphologies, from steep confined streams to large unconfined rivers. General options for river management and restoration actions aiming to combine geomorphological functionality and flood hazard mitigation are discussed, in the light of the current European legislative context. Copyright © 2011 John Wiley & Sons, Ltd.

Towards constraining the magnitude of global agricultural sediment and soil organic carbon fluxes

Abstract: Reliable quantitative data on the extent and rates of soil erosion are needed to understand the global significance of soil-erosion induced carbon exchange and to underpin the development of science-based mitigation strategies, but large uncertainties remain. Existing estimates of agricultural soil and soil organic carbon (SOC) erosion are very divergent and span two orders of magnitude. The main objective of this study was to test the assumptions underlying existing assessments and to reduce the uncertainty associated with global estimates of agricultural soil and SOC erosion. We parameterized a simplified erosion model driven by coarse global databases using an empirical database that covers the conterminous USA. The good agreement between our model results and empirical estimates indicate that the approach presented here captures the essence of agricultural erosion at the scales of continents and that it may be used to predict the significance of erosion for the global carbon cycle and its impact on soil functions. We obtained a global soil erosion rate of 10.5Mg ha -1 y -1 for cropland and 1.7Mg ha -1 y -1 for pastures. This corresponds to SOC erosion rates of 193kg C ha -1 y -1 for cropland and 40.4kg C ha -1 y -1 for eroding pastures and results in a global flux of 20.5 (±10.3) Pg y -1 of soil and 403.5 (±201.8) Tg C y -1. Although it is difficult to accurately assess the uncertainty associated with our estimates of global agricultural erosion, mainly due to the lack of model testing in (sub-)tropical regions, our estimates are significantly lower than former assessments based on the extrapolation of plot experiments or global application of erosion models. Our approach has the potential to quantify the rate and spatial signature of the erosion-induced disturbance at continental and global scales: by linking our model with a global soil profile database, we estimated soil profile modifications induced by agriculture. This showed that erosion-induced changes in topsoil SOC content are significant at a global scale (an average SOC loss of 22% in 50years) and agricultural soils should therefore be considered as dynamic systems that can change rapidly. © 2011 John Wiley & Sons, Ltd.

Measuring gravel transport and dispersion in a mountain river using passive radio tracers

Abstract: Random walk models of fluvial sediment transport recognize that grains move intermittently, with short duration steps separated by rests that are comparatively long. These models are built upon the probability distributions of the step length and the resting time. Motivated by these models, tracer experiments have attempted to measure directly the steps and rests of sediment grains in natural streams. This paper describes results from a large tracer experiment designed to test stochastic transport models. We used passive integrated transponder (PIT) tags to label 893 coarse gravel clasts and placed them in Halfmoon Creek, a small alpine stream near Leadville, Colorado, USA. The PIT tags allow us to locate and identify tracers without picking them up or digging them out of the streambed. They also enable us to find a very high percentage of our rocks, 98% after three years and 96% after the fourth year. We use the annual tracer displacement to test two stochastic transport models, the Einstein–Hubbell–Sayre (EHS) model and the Yang–Sayre gamma-exponential model (GEM). We find that the GEM is a better fit to the observations, particularly for slower moving tracers and suggest that the strength of the GEM is that the gamma distribution of step lengths approximates a compound Poisson distribution. Published in 2012. This article is a US Government work and is in the public domain in the USA.

Beach erosion and recovery during consecutive storms at a steep‐sloping, meso‐tidal beach

Abstract: This study analyses beach morphological change during six consecutive storms acting on the meso-tidal Faro Beach (south Portugal) between 15 December 2009 and 7 January 2010. Morphological change of the sub-aerial beach profile was monitored through frequent topographic surveys across 11 transects. Measurements of the surf/swash zone dimensions, nearshore bar dynamics, and wave run-up were extracted from time averaged and timestack coastal images, and wave and tidal data were obtained from offshore stations. All the information combined suggests that during consecutive storm events, the antecedent morphological state can initially be the dominant controlling factor of beach response; while the hydrodynamic forcing, and especially the tide and surge levels, become more important during the later stages of a storm period. The dataset also reveals the dynamic nature of steep-sloping beaches, since sub-aerial beach volume reductions up to 30 m3/m were followed by intertidal area recovery (–2 < z < 3 m) with rates reaching ~10 m3/m. However, the observed cumulative dune erosion and profile pivoting imply that storms, even of regular intensity, can have a dramatic impact when they occur in groups. Nearshore bars seemed to respond to temporal scales more related to storm sequences than to individual events. The formation of a prominent crescentic offshore bar at ~200 m from the shoreline appeared to reverse the previous offshore migration trend of the inner bar, which was gradually shifted close to the seaward swash zone boundary. The partially understood nearshore bar processes appeared to be critical for storm wave attenuation in the surf zone; and were considered mainly responsible for the poor interpretation of the observed beach behaviour on the grounds of standard, non-dimensional, morphological parameters. Copyright © 2011 John Wiley & Sons, Ltd.

Hillslope response to tectonic forcing in threshold landscapes

Abstract: Hillslopes are thought to poorly record tectonic signals in threshold landscapes. Numerous previous studies of steep landscapes suggest that large changes in long-term erosion rate lead to little change in mean hillslope angle, measured at coarse resolution. New LiDAR-derived topography data enables a finer examination of threshold hillslopes. Here we quantify hillslope response to tectonic forcing in a threshold landscape. To do so, we use an extensive cosmogenic beryllium-10 (^(10)Be)-based dataset of catchment-averaged erosion rates combined with a 500 km^2 LiDAR-derived 1 m digital elevation model to exploit a gradient of tectonic forcing and topographic relief in the San Gabriel Mountains, California. We also calibrate a new method of quantifying rock exposure from LiDAR-derived slope measurements using high-resolution panoramic photographs. Two distinct trends in hillslope behavior emerge: below catchment-mean slopes of 30°, modal slopes increase with mean slopes, slope distribution skewness decreases with increasing mean slope, and bedrock exposure is limited; above mean slopes of 30°, our rock exposure index increases strongly with mean slope, and the prevalence of angle-of-repose debris wedges keeps modal slopes near 37°, resulting in a positive relationship between slope distribution skewness and mean slope. We find that both mean slopes and rock exposure increase with erosion rate up to 1 mm/a, in contrast to previous work based on coarser topographic data. We also find that as erosion rates increase, the extent of the fluvial network decreases, while colluvial channels extend downstream, keeping the total drainage density similar across the range. Our results reveal important textural details lost in 10 or 30 m resolution digital elevation models of steep landscapes, and highlight the need for process-based studies of threshold hillslopes and colluvial channels.

Wind direction and complex sediment transport response across a beach–dune system

Abstract: Evidence from a field study on wind flow and sediment transport across a beach–dune system under onshore and offshore conditions (including oblique approach angles) indicates that sediment transport response on the back-beach and stoss slope of the foredune can be exceedingly complex. The upper-air flow – measured by a sonic anemometer at the top of a 3·5 m tower located on the dune crest – is similar to regional wind records obtained from a nearby meteorological station, but quite different from the near-surface flow field measured locally across the beach–dune profile by sonic anemometers positioned 20 cm above the sand surface. Flow–form interaction at macro and micro scales leads to strong modulation of the near-surface wind vectors, including wind speed reductions (due to surface roughness drag and adverse pressure effects induced by the dune) and wind speed increases (due to flow compression toward the top of the dune) as well as pronounced topographic steering during oblique wind approach angles. A conceptual model is proposed, building on the ideas of Sweet and Kocurek (Sedimentology 37: 1023–1038, 1990), Walker and Nickling (Earth Surface Processes and Landforms 28: 111–1124, 2002), and Lynch et al. (Earth Surface Processes and Landforms 33: 991–1005, 2008, Geomorphology 105: 139–146, 2010), which shows how near-surface wind vectors are altered for four regional wind conditions: (a) onshore, detached; (b) onshore-oblique, attached and deflected; (c) offshore, detached; and (d) offshore-oblique, attached and deflected. High-frequency measurements of sediment transport intensity during these different events demonstrate that predictions of sediment flux using standard equations driven by regional wind statistics would by unreliable and misleading. It is recommended that field studies routinely implement experimental designs that treat the near-surface wind field as comprising true vector quantities (with speed and direction) in order that a more robust linkage between the regional (upper air) wind field and the sediment transport response across the beach–dune profile be established. Copyright © 2012 John Wiley & Sons, Ltd.

The coupling status of alluvial fans and debris cones: a review and synthesis

Abstract: Alluvial fans and debris cones link two zones of the fluvial system (e.g. hillslope gully systems to stream channels; mountain catchment sediment source areas to main river systems or to sedimentary basins) and therefore have important coupling or buffering roles. These roles may be both functional and preservational. The functional role includes debris-cone coupling, which controls sediment supply from hillslope gully systems to stream channels, influencing channel morphology. Coupling through larger alluvial fans, expressed by fanhead trenching, causes a distal shift in sedimentation zones, or when expressed by through-fan trenching, causes complete sediment by-pass. The preservational role stems from the fact that fans and cones are temporary sediment storage zones, and may preserve a record of source–area environmental change more sensitively than would sediments preserved further downsystem. Fan coupling mechanisms include distally-induced coupling (basal scour, ‘toe cutting’, marginal incision) and proximally-induced coupling (fanhead and midfan trenching). These mechanisms lead initially to partial coupling, either extending the immediate sediment source area to the stream system or shifting the focus of sedimentation distally. Complete coupling involves transmission of sediment from the feeder catchment through the fan environment into the downstream drainage or a sedimentary basin. The implications of coupling relate to downstream channel response, fan morphology, sedimentation patterns and vertical sedimentary sequences. Temporal and spatial scales of coupling are related, and with increasing scales the dominant controls shift from storm events to land cover to climatic and base-level change and ultimately to the relationships between tectonics and accommodation space. Finally, future research challenges are identified. Modern dating techniques and sophisticated analysis of remotely sensed data can greatly improve our understanding of fan dynamics, and should lead to better cross-scale integration between short-term process-based approaches and long-term sedimentological applications, while maintaining high quality field-based observations. Copyright © 2011 John Wiley & Sons, Ltd.

The interactions between vegetation and erosion: new directions for research at the interface of ecology and geomorphology

Abstract: Vegetation and processes of erosion and deposition are interactive. An objective of this paper is to review selected studies that emphasize the interdependencies. The reviews suggest new directions for research uniting ecology and geomorphology – the sub-discipline of biogeomorphology. The research, which recently has become vigorous, includes the sources, movement, and fates of fluvial loads of sediment, organic carbon, nutrients, contaminants, and woody debris to low-energy storage sites; the function of biota in causing soil evolution, stability, and sequestration of carbon; the development of new methods to characterize watersheds based on edaphic conditions; and the refinement of current empirical and conceptual models and dendrochronological techniques to measure landscape change. These well acknowledged topics and others less well anticipated ensure that biogeomorphology will remain vibrant. Published in 2011. This article is a US Government work and is in the publish domain in the USA.

Hydrogeomorphic processes and vegetation: disturbance, process histories, dependencies and interactions

Abstract: Riparian vegetation and hydrogeomorphic processes are intimately connected parts of upland catchment and fan environments. Trees, shrubs and grasses and hydrogeomorphic processes interact and depend on each other in complex ways on the hillslopes, channels and cone-shaped fans of torrential watersheds. While the presence and density of vegetation have a profound influence on hydrogeomorphic processes, the occurrence of the latter will also exert control on the presence, vitality, species, and age distribution of vegetation. This contribution aims at providing a review of foundational and more recent work on the dependencies and interactions between hydrogeomorphic processes and vegetation. In particular, we address the role of vegetation in the initiation of hydrogeomorphic processes and its impact on stream morphology as well as immediate and long-term effects of hydrogeomorphic disturbance on vegetation. Copyright © 2011 John Wiley & Sons, Ltd.

Use of ergodic reasoning to reconstruct the historical range of variability and evolutionary trajectory of rivers

Abstract: Applications of ergodic reasoning (or location for time substitution) aid efforts at environmental reconstruction and prediction, providing a useful tool to analyse and communicate stages of landscape evolution. Analysis of the historical range of behaviour and change that a river system has experienced can be used to interpret thresholds that have been breached, and underlying controls and/or triggers for adjustment and change. This information can be used to forecast future trajectories of adjustment and provide target conditions for management activities. This paper uses a case study from upper Wollombi Brook, New South Wales, Australia to demonstrate how ergodic reasoning can be used to assess river behaviour, change and responses to natural and human-disturbances. The ‘river evolution diagram’ developed by Brierley and Fryirs (Geomorphology and River Management: Applications of the River Styles Framework. Blackwell Publishing: Oxford, 2005) is presented as a means for depicting the range of behaviour and evolutionary variability of this river. These approaches can be readily applied in other systems. Implications for approaches to analysis of river evolution and management are outlined. Copyright © 2012 John Wiley & Sons, Ltd.

Implementation and validation of video monitoring for wood budgeting in a wandering piedmont river, the Ain River (France)

Abstract: The transport of wood in rivers during floods is an important process that underlies differences in habitat and morphology between water courses and regions. Quantitative data are needed to properly address management objectives and balance wood budgets. In this study we use a streamside video camera to detect wood passage and measure quasi-instantaneous rates of wood transport in the Ain River, France. The objectives are to verify the procedure, describe the relation between wood transport and discharge, and construct and validate a wood budget for the reach upstream of the camera. Verification of the procedure includes tests of detection frequency, wood velocity, and piece size. A log base two transformation is proposed to classify wood by piece length. It was found that a wood transport threshold occurs at approximately two thirds of the bankfull discharge. Wood transport follows a positive linear relation with discharge up to the bankfull discharge but is both more variable and less sensitive to discharge when the floodplain is inundated. Transport rates are approximately four times higher on the rising limb of the hydrograph than on the falling limb. Wood transport estimates from a three-stage rating curve are two to 10 times higher than those from a wood budget using local and aerial surveys of upstream dynamics. Future work should address uncertainties related to wood diameter measurements, sampling length and frequency, and antecedent floods. Copyright © 2012 John Wiley & Sons, Ltd.

Environmental change, hydromorphological reference conditions and the restoration of Polish Carpathian rivers

Abstract: In the twentieth century Polish Carpathian rivers were considerably modified by channelization and gravel mining, with significant detrimental effects to their ecological integrity, vertical stability of the streambeds and flood hazard to downstream river reaches. Restoration of the rivers is thus necessary to improve their ecological status and re-establish geomorphic dynamic equilibrium conditions. Various approaches to defining hydromorphological reference conditions, proposed to date in river restoration literature, have serious deficiencies. In particular, environmental changes that took place in the catchments of Carpathian rivers during the twentieth century invalidate the historical state of the rivers as reference for their restoration. This is illustrated by a change from bar-braided to island-braided channel pattern that occurred in the past century in unmanaged sections of the Czarny Dunajec in response to a reduction in flow and sediment dynamics of the river. We indicate that reference conditions should be defined as those which exist or would exist under present environmental conditions in the catchment but without human influence on the channel, riparian zone and floodplain of the river which is to be restored. This assumption was tested through the evaluation of hydromorphological river quality of the Czarny Dunajec according to the European Standard EN-14614. The evaluation confirmed a high-status hydromorphological quality in an unmanaged channel section, which can thus be used as a reference for restoration of impacted river sections. Copyright © 2012 John Wiley & Sons, Ltd.

The legacy of initial conditions in landscape evolution

Abstract: Landscapes subject to constant forcing tend to evolve toward equilibrium states in which individual landforms have similar characteristics. Yet, even in landscapes at or near equilibrium, there can be significant variability among individual landforms. Furthermore, sites subject to similar processes and conditions can have different mean landform characteristics. This variability is often ascribed to on-going transient evolution, or to heterogeneity in processes, material properties, forcing, or boundary conditions. Three surprising outcomes of landform evolution models suggest, however, that such variability could arise in equilibrium landscapes without any heterogeneity in the physical processes shaping the topography. First, homogeneous systems subjected to constant forcing can generate a heterogeneous distribution of equilibrium landforms. Second, even simple non-linear systems can have multiple stable equilibrium states. Third, evolving landscapes can exhibit path dependence and hysteresis. We show how these three mechanisms can produce variability in landforms that arises from the characteristics of the initial topographic surface rather than from heterogeneity in geomorphic processes. Numerical experiments on the formation of low-order fluvial valleys and transportational cyclic steps in erodible streambeds illustrate why it is important to consider the influence of initial conditions when comparing models with natural topography, estimating the uncertainty of model predictions, and studying how landscapes respond to disturbances. Copyright © 2011 John Wiley & Sons, Ltd.

Erosional power in the Swiss Alps: characterization of slope failure in the Illgraben

Abstract: Landslides and rockfalls are key geomorphic processes in mountain basins. Their quantification and characterization are critical for understanding the processes of slope failure and their contributions to erosion and landscape evolution. We used digital photogrammetry to produce a multi-temporal record of erosion (1963–2005) of a rock slope at the head of the Illgraben, a very active catchment prone to debris flows in Switzerland. Slope failures affect 70% of the study slope and erode the slope at an average rate of 0.39 ± 0.03 m yr¯¹. The analysis of individual slope failures yielded an inventory of ~2500 failures ranging over 6 orders of magnitude in volume, despite the small slope area and short study period. The slope failures form a characteristic magnitude–frequency distribution with a rollover and a power-law tail between ~200 m³ and 1.6 × 106 m³ with an exponent of 1.65. Slope failure volume scales with area as a power law with an exponent of 1.1. Both values are low for studies of bedrock landslides and rockfall and result from the highly fractured and weathered state of the quartzitic bedrock. Our data suggest that the magnitude–frequency distribution is the result of two separate slope failure processes. Type (1) failures are frequent, small slides and slumps within the weathered layer of highly fractured rock and loose sediment, and make up the rollover. Type (2) failures are less frequent and larger rockslides and rockfalls within the internal bedded and fractured slope along pre-determined potential failure surfaces, and make up the power-law tail. Rockslides and rockfalls of high magnitude and relatively low frequency make up 99% of the total failure volume and are thus responsible for the high erosion rate. They are also significant in the context of landscape evolution as they occur on slopes above 45° and limit the relief of the slope. Copyright © 2012 John Wiley & Sons, Ltd.

High-frequency sediment transport responses on a vegetated foredune

Abstract: Wind flow and sand transport intensity were measured on the seaward slope of a vegetated foredune during a 16 h storm using an array of sonic anemometers and Wenglor laser particle counters. The foredune had a compound seaward slope with a wave-cut scarp about 0.5 m high separating the upper vegetated portion from the lower dune ramp, which was bare of vegetation. Wind direction veered from obliquely offshore at the start of the event to obliquely onshore during the storm peak and finally to directly onshore during the final 2 h as wind speed dropped to below threshold. Sand transport was initially inhibited by a brief period of rain at the start of the event but as the surface dried and wind speed increased sand transport was initiated over the entire seaward slope. Transport intensity was quite variable both temporally and spatially on the upper slope as a result of fluctuating wind speed and direction, but overall magnitudes were similar over the whole length. Ten-minute average transport intensity correlates strongly with mean wind speed measured at the dune crest, and there is also strong correlation between instantaneous wind speed and transport intensity measured at the same locations when the data are smoothed with a 10 s running mean. Transport on the beach for onshore winds is decoupled from that on the seaward slope above the small scarp when the wind angle is highly oblique, but for wind angles <45° from shore perpendicular some sand is transported onto the lower slope. Copyright © 2012 John Wiley & Sons, Ltd.

Progressive failure of sheeted rock slopes: the 2009–2010 Rhombus Wall rock falls in Yosemite Valley, California, USA

Abstract: Progressive rock-fall failures in natural rock slopes are common in many environments, but often elude detailed quantitative documentation and analysis. Here we present high-resolution photography, video, and laser scanning data that document spatial and temporal patterns of a 15-month-long sequence of at least 14 rock falls from the Rhombus Wall, a sheeted granitic cliff in Yosemite Valley, California. The rock-fall sequence began on 26 August 2009 with a small failure at the tip of an overhanging rock slab. Several hours later, a series of five rock falls totaling 736 m3 progressed upward along a sheeting joint behind the overhanging slab. Over the next 3 weeks, audible cracking occurred on the Rhombus Wall, suggesting crack propagation, while visual monitoring revealed opening of a sheeting joint adjacent to the previous failure surface. On 14 September 2009 a 110 m3 slab detached along this sheeting joint. Additional rock falls between 30 August and 20 November 2010, totaling 187 m3, radiated outward from the initial failure area along cliff (sub)parallel sheeting joints. We suggest that these progressive failures might have been related to stress redistributions accompanying propagation of sheeting joints behind the cliff face. Mechanical analyses indicate that tensile stresses should occur perpendicular to the cliff face and open sheeting joints, and that sheeting joints should propagate parallel to a cliff face from areas of stress concentrations. The analyses also account for how sheeting joints can propagate to lengths many times greater than their depths behind cliff faces. We posit that as a region of failure spreads across a cliff face, stress concentrations along its margin will spread with it, promoting further crack propagation and rock falls. Published in 2012. This article is a US Government work and is in the public domain in the USA.

Through‐water terrestrial laser scanning of gravel beds at the patch scale

Abstract: Acquiring high resolution topographic data of natural gravel surfaces is technically demanding in locations where the bed is not exposed at low water stages. Often the most geomorphologically active surfaces are permanently submerged. Gravel beds are spatially variable and measurement of their detailed structure and particle sizes is essential for understanding the interaction of bed roughness with near-bed flow hydraulics, sediment entrainment, transport and deposition processes, as well as providing insights into the ecological responses to these processes. This paper presents patch-scale laboratory and field experiments to demonstrate that through-water terrestrial laser scanning (TLS) has the potential to provide high resolution digital elevation models of submerged gravel beds with enough detail to depict individual grains and small-scale forms. The resulting point cloud data requires correction for refraction before registration. Preliminary validation shows that patch-scale TLS through 200 mm of water introduces a mean error of less than 5 mm under ideal conditions. Point precision is not adversely affected by the water column. The resulting DEMs can be embedded seamlessly within larger sub-aerial reach-scale surveys and can be acquired alongside flow measurements to examine the effects of three-dimensional surface geometry on turbulent flow fields and their interaction with instream ecology dynamics. Copyright © 2011 John Wiley & Sons, Ltd.

Temporal variability of suspended sediment sources in an alpine catchment combining river/rainfall monitoring and sediment fingerprinting

Abstract: Influence of the rainfall regime on erosion and transfer of suspended sediment in a 905‐km$^2$ mountainous catchment of the southern French Alps was investigated by combining sediment monitoring, rainfall data, and sediment fingerprinting (based on geochemistry and radionuclide concentrations). Suspended sediment yields were monitored between October 2007 and December 2009 in four subcatchments (22–713 km$^2$. Automatic sediment sampling was triggered during floods to trace the sediment origin in the catchment. Sediment exports at the river catchment outlet (330 $\pm$ 100 t km$^{‐2}$ yr$^{‐1}$) were mainly driven (80%) by widespread rainfall events (long duration, low intensities). In contrast, heavy, local and short duration storms, generated high peak discharges and suspended sediment concentrations in small upstream torrents. However, these upstream floods had generally not the capacity to transfer the sediment down to the catchment outlet and the bulk of this fine sediment deposited along downstream sections of the river. This study also confirmed the important contribution of black marls (up to 70%) to sediment transported in rivers, although this substrate only occupies c . 10% of the total catchment surface. Sediment exports generated by local convective storms varied significantly at both intra‐ and inter‐flood scales, because of spatial heterogeneity of rainfall. However, black marls/marly limestones contribution remained systematically high. In contrast, widespread flood events that generate the bulk of annual sediment supply at the outlet were characterized by a more stable lithologic composition and by a larger contribution of limestones/marls, Quaternary deposits and conglomerates, which corroborates the results of a previous sediment fingerprinting study conducted on riverbed sediment.

Multitemporal ALSM change detection, sediment delivery, and process mapping at an active earthflow

Abstract: Remote mapping and measurement of surface processes at high spatial resolution is among the frontiers in Earth surface process research. Remote measurements that allow meter-scale mapping of landforms and quantification of landscape change can revolutionize the study of landscape evolution on human timescales. At Mill Gulch in northern California, USA, an active earthflow was surveyed in 2003 and 2007 by airborne laser swath mapping (ALSM), enabling meter-scale quantification of landscape change. We calculate four-year volumetric flux from the earthflow and compare it to long-term catchment average erosion rates from cosmogenic radionuclide inventories from adjacent watersheds. We also present detailed maps of changing features on the earthflow, from which we can derive velocity estimates and infer dominant process. These measurements rely on proper digital elevation model (DEM) generation and a simple surface-matching technique to align the multitemporal data in a manner that eliminates systematic error in either dataset. The mean surface elevation of the earthflow and an opposite slope that was directly influenced by the earthflow decreased 14 ± 1 mm/yr from 2003 to 2007. By making the conservative assumption that these features were the dominant contributor of sediment flux from the entire Mill Gulch drainage basin during this time interval, we calculate a minimum catchment-averaged erosion rate of 0·30 ± 0·02 mm/yr. Analysis of beryllium-10 (10Be) concentrations in fluvial sand from nearby Russian Gulch and the South Fork Gualala River provide catchment averaged erosion rates of 0·21 ± 0·04 and 0·23 ± 0·03 mm/yr respectively. From translated landscape features, we can infer surface velocities ranging from 0·5 m/yr in the wide upper ‘source’ portion of the flow to 5 m/yr in the narrow middle ‘transport’ portion of the flow. This study re-affirms the importance of mass wasting processes in the sediment budgets of uplifting weak lithologies. Copyright © 2011 John Wiley & Sons, Ltd.

Evaluating competing hypotheses for the origin and dynamics of river anastomosis

Abstract: Anastomosing rivers have multiple interconnected channels that enclose flood basins. Various theories potentially explain this pattern, including an increased discharge conveyance and sediment transport capacity of multiple channels, deltaic branching, avulsion forced by base-level rise, or a tendency to avulse due to upstream sediment overloading. The former two imply a stable anabranching channel pattern, whereas the latter two imply disequilibrium and evolution towards a single-channel pattern in the absence of avulsion. Our objective is to test these hypotheses on morphodynamic scenario modelling and data of a well-documented case study: the upper Columbia River. Proportions of channel and floodplain sediments along the river valley were derived from surface mapping. Initial and boundary conditions for the modelling were derived from field data. A 1D network model was built based on gradually varied flow equations, sediment transport prediction, mass conservation, transverse slope and spiral meander flow effects at the bifurcations. The number of channels and crevasse splays decreases in a downstream direction. Also, measured sediment transport is higher at the upstream boundary than downstream. These observations concur with bed sediment overloading from upstream, which can have caused channel aggradation above the surrounding floodplain and subsequent avulsion. The modelling also indicates that avulsion was likely caused by upstream overloading. In the model, multi-channel systems inevitably evolve towards single-channel systems within centuries. The reasons are that symmetric channel bifurcations are inherently unstable, while confluenced channels have relatively less friction than two parallel channels, so that more discharge is conveyed through the path with more confluences and less friction. Furthermore, the present longitudinal profile curvature of the valley could only be reproduced in the model by temporary overfeeding.

Predicting gully initiation: comparing data mining techniques, analytical hierarchy processes and the topographic threshold

Abstract: Predicting gully initiation at catchment scale was done previously by integrating a geographical information system (GIS) with physically based models, statistical procedures or with knowledge-based expert systems. However, the reliability and validity of applying these procedures are still questionable. In this work, a data mining (DM) procedure based on decision trees was applied to identify areas of gully initiation risk. Performance was compared with the analytic hierarchy process (AHP) expert system and with the commonly used topographic threshold (TT) technique. A spatial database was used to test the models, composed of a target variable (presence or absence of initial points) and ten independent environmental, climatic and human-induced variables. The following findings emerged: using the same input layers, DM provided better predictive ability of gully initiation points than the application of both AHP and TT. The main difference between DM and TT was the very high overestimation inherent in TT. In addition, the minimum slope observed for soil detachment was 2°, whereas in other studies it is 3°. This could be explained by soil resistance, which is substantially lower in agricultural fields, while most studies test unploughed soil. Finally, rainfall intensity events >62.2 mm h-1 (for a period of 30 min) were found to have a significant effect on gully initiation. Copyright © 2012 John Wiley & Sons, Ltd.

The role of rock surface hardness and internal moisture in tafoni development in sandstone

Abstract: The development of cavernous weathering features such as tafoni remains poorly understood. In particular, the roles played by internal moisture and case hardening remain unclear. In this study, Electric Resistivity Tomography (ERT) has been used to map moisture distribution within inner walls of tafoni developed in sandstone, and an Equotip device used to measure rock surface hardness as a proxy measure of the degree of weathering and case hardening. Seven large tafoni in the Golden Gate Highlands National Park (South Africa), varying in size and degree of development have been monitored. A dynamic relationship between surface hardness, degree of weathering and internal moisture regimes has been found. We propose a new conceptual model which illustrates the complex interaction between case hardening and internal moisture and suggests a new direction for cavernous weathering research. Copyright © 2011 John Wiley & Sons, Ltd.