Showing papers in "Earth Surface Processes and Landforms in 2014"

Mitigating systematic error in topographic models derived from UAV and ground-based image networks

Abstract: High resolution digital elevation models (DEMs) are increasingly produced from photographs acquired with consumer cameras, both from the ground and from unmanned aerial vehicles (UAVs). However, although such DEMs may achieve centimetric detail, they can also display systematic broad-scale error that estricts their wider use. Such errors which, in typical UAV data are expressed as a vertical ‘doming’ of the surface, result from a combination of near-parallel imaging directions and inaccurate correction of radial lens distortion. Using simulations of multi-image networks with near-parallel viewing directions, we show that enabling camera self-calibration as part of the bundle adjustment process inherently leads to erroneous radial distortion estimates and associated DEM error. This effect is relevant whether a traditional photogrammetric or newer structure-from-motion (SfM) approach is used, but errors are expected to be more pronounced in SfM-based DEMs, for which use of control and check point measurements are typically more limited. Systematic DEM error can be significantly reduced by the additional capture and inclusion of oblique images in the image network; we provide practical flight plan solutions for fixed wing or rotor-based UAVs that, in the absence of control points, can reduce DEM error by up to two orders of magnitude. The magnitude of doming error shows a linear relationship with radial distortion and we show how characterisation of this relationship allows an improved distortion estimate and, hence, existing datasets to be optimally reprocessed. Although focussed on UAV surveying, our results are also relevant to ground-based image capture.

Plants as river system engineers

Abstract: I would like to acknowledge three research grants/contracts that are supporting my current research on this theme: Grant F/07 040/AP from the Leverhulme Trust; Grant NE/F014597/1 from the Natural Environment Research Council, UK, and the REFORM collaborative project funded by the European Union Seventh Framework Programme under grant agreement 282656.

The stream power river incision model: evidence, theory and beyond

Abstract: The stream power incision model (SPIM) is a cornerstone of quantitative geomorphology. It states that river incision rate is the product of drainage area and channel slope raised to the power exponents m and n, respectively. It is widely used to predict patterns of deformation from channel long profile inversion or to model knickpoint migration and landscape evolution. Numerous studies have attempted to test its applicability with mixed results prompting the question of its validity. This paper synthesizes these results, highlights the SPIM deficiencies, and offers new insights into the role of incision thresholds and channel width. By reviewing quantitative data on incising rivers, I first propose six sets of field evidence that any long-term incision model should be able to predict. This analysis highlights several inconsistencies of the standard SPIM. Next, I discuss the methods used to construct physics-based long-term incision laws. I demonstrate that all published incising river datasets away from knickpoints or knickzones are in a regime dominated by threshold effects requiring an explicit upscaling of flood stochasticity neglected in the standard SPIM and other incision models. Using threshold-stochastic simulations with dynamic width, I document the existence of composite transient dynamics where knickpoint propagation locally obeys a linear SPIM (n=1) while other part of the river obey a non-linear SPIM (n>1). The threshold-stochastic SPIM resolves some inconsistencies of the standard SPIM and matches steady-state field evidence when width is not sensitive to incision rate. However it fails to predict the scaling of slope with incision rate for cases where width decreases with incision rate. Recent proposed models of dynamic width cannot resolve these deficiencies. An explicit upscaling of sediment flux and threshold-stochastic effects combined with dynamic width should take us beyond the SPIM which is shown here to have a narrow range of validity.

Terrestrial laser scanning of rock slope instabilities

Abstract: This manuscript presents a review on the application of a remote sensing technique (terrestrial laser scanning, TLS) to a well-known topic (rock slope characterization and monitoring). Although the number of publications on the use of TLS in rock slope studies has rapidly increased in the last 5–10 years, little effort has been made to review the key developments, establish a code of best practice and unify future research approaches. The acquisition of dense 3D terrain information with high accuracy, high data acquisition speed and increasingly efficient post-processing workflows is helping to better quantify key parameters of rock slope instabilities across spatial and temporal scales ranging from cubic decimetres to millions of cubic metres and from hours to years, respectively. Key insights into the use of TLS in rock slope investigations include: (a) the capability of remotely obtaining the orientation of slope discontinuities, which constitutes a great step forward in rock mechanics; (b) the possibility to monitor rock slopes which allows not only the accurate quantification of rockfall rates across wide areas but also the spatio-temporal modelling of rock slope deformation with an unprecedented level of detail. Studying rock slopes using TLS presents a series of key challenges, from accounting for the fractal character of rock surface to detecting the precursory deformation that may help in the future prediction of rock failures. Further investigation on the development of new algorithms for point cloud filtering, segmentation, feature extraction, deformation tracking and change detection will significantly improve our understanding on how rock slopes behave and evolve. Perspectives include the use of new 3D sensing devices and the adaptation of techniques and methods recently developed in other disciplines as robotics and 3D computer-vision to rock slope instabilities research.

Geophysical constraints on deep weathering and water storage potential in the Southern Sierra Critical Zone Observatory

Abstract: The conversion of bedrock to regolith marks the inception of critical zone processes, but the factors that regulate it remain poorly understood. Although the thickness and degree of weathering of regolith are widely thought to be important regulators of the development of regolith and its water-storage potential, the functional relationships between regolith properties and the processes that generate it remain poorly documented. This is due in part to the fact that regolith is difficult to characterize by direct observations over the broad scales needed for process-based understanding of the critical zone. Here we use seismic refraction and resistivity imaging techniques to estimate variations in regolith thickness and porosity across a forested slope and swampy meadow in the Southern Sierra Critical Zone Observatory (SSCZO). Inferred seismic velocities and electrical resistivities image a weathering zone ranging in thickness from 10 to 35 m (average = 23 m) along one intensively studied transect. The inferred weathering zone consists of roughly equal thicknesses of saprolite (P-velocity < 2 km s−1) and moderately weathered bedrock (P-velocity = 2–4 km s−1). A minimum-porosity model assuming dry pore space shows porosities as high as 50% near the surface, decreasing to near zero at the base of weathered rock. Physical properties of saprolite samples from hand augering and push cores are consistent with our rock physics model when variations in pore saturation are taken into account. Our results indicate that saprolite is a crucial reservoir of water, potentially storing an average of 3 m3 m−2 of water along a forested slope in the headwaters of the SSCZO. When coupled with published erosion rates from cosmogenic nuclides, our geophysical estimates of weathering zone thickness imply regolith residence times on the order of 105 years. Thus, soils at the surface today may integrate weathering over glacial–interglacial fluctuations in climate. Copyright © 2013 John Wiley & Sons, Ltd.

Size, Shape and Spatial Arrangement of Mega-scale Glacial Lineations from a Large and Diverse Dataset

Abstract: Mega-scale glacial lineations (MSGLs) are a characteristic landform on ice stream beds. Solving the puzzle of their formation is key to understanding how ice interacts with its bed and how this, in turn, influences the dynamics of ice streams. However, a comprehensive and detailed characterization of this landform's size, shape and spatial arrangement, which might serve to test and refine formational theories, is largely lacking. This paper presents a detailed morphometric analysis and comparison of 4043 MSGLs from eight palaeo-ice stream settings: three offshore (Norway and Antarctica), four onshore (Canada), and one from under a modern ice stream in West Antarctica. The length of MSGLs is lower than previously suggested (mode 1000–2000 m; median 2892 m), and they initiate and terminate at various locations on an ice stream bed. Their spatial arrangement reveals a pattern that is characterized by an exceptional parallel conformity (80% of all mapped MSGLs have an azimuth within 5° from the mean values), and a fairly constant lateral spacing (mode 200–300 m; median 330 m), which we interpret as an indication that MSGLs are a spatially self-organized phenomenon. Results show that size, shape and spatial arrangement of MSGLs are consistent both within and also generally between different ice stream beds. We suggest this results from a common mechanism of formation, which is largely insensitive to local factors. Although the elongation of MSGLs (mode 6–8; median 12.2) is typically higher than features described as drumlins, these values and those of their width (mode 100–200 m; median 268 m) overlap, which suggests the two landforms are part of a morphological continuum and may share a similar origin. We compare their morphometry with explicit predictions made by the groove-ploughing and rilling instability theories of MSGL formation. Although the latter was most compatible, neither is fully supported by observations. © 2014 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.

Coupled numerical–analytical approach to landscape evolution modeling

Abstract: The Earth's topography is shaped by surface processes that operate on various scales. In particular, river processes control landscape dynamics over large length scales, whereas hillslope processes control the dynamics over smaller length scales. This scale separation challenges numerical treatments of landscape evolution that use space discretization. Large grid spacing cannot account for the dynamics of water divides that control drainage area competition, and erosion rate and slope distribution. Small grid spacing that properly accounts for divide dynamics is computationally inefficient when studying large domains. Here we propose a new approach for landscape evolution modeling that couples irregular grid-based numerical solutions for the large-scale fluvial dynamics and continuum-based analytical solutions for the small-scale fluvial and hillslope dynamics. The new approach is implemented in the landscape evolution model DAC (divide and capture). The geometrical and topological characteristics of DAC's landscapes show compatibility with those of natural landscapes. A comparative study shows that, even with large grid spacing, DAC predictions fit well an analytical solution for divide migration in the presence of horizontal advection of topography. In addition, DAC is used to study some outstanding problems in landscape evolution. (i) The time to steady-state is investigated and simulations show that steady-state requires much more time to achieve than predicted by fixed area calculations, due to divides migration and persistent reorganization of low-order streams. (ii) Large-scale stream captures in a strike-slip environment are studied and show a distinct pattern of erosion rates that can be used to identify recent capture events. (iii) Three tectono-climatic mechanisms that can lead to asymmetric mountains are studied. Each of the mechanisms produces a distinct morphology and erosion rate distribution. Application to the Southern Alps of New Zealand suggests that tectonic advection, precipitation gradients and non-uniform tectonic uplift act together to shape the first-order topography of this mountain range. Copyright © 2014 John Wiley & Sons, Ltd.

Hyperscale terrain modelling of braided rivers: fusing mobile terrestrial laser scanning and optical bathymetric mapping

Abstract: Quantifying the morphology of braided rivers is a key task for understanding braided river behaviour. In the last decade, developments in geomatics technologies and associated data processing methods have transformed the production of precise, reach-scale topographic datasets. Nevertheless, generating accurate Digital Elevation Models (DEMs) remains a demanding task, particularly in fluvial systems. This paper identifies a threefold set of challenges associated with surveying these dynamic landforms: complex relief, inundated shallow channels and high rates of sediment transport, and terms these challenges the ‘morphological’, ‘wetted channel’ and ‘mobility’ problems, respectively. In an attempt to confront these issues directly, this paper presents a novel survey methodology that combines mobile terrestrial laser scanning and non-metric aerial photography with data reduction and surface modelling techniques to render DEMs from the resulting very high resolution datasets. The approach is used to generate and model a precise, dense topographic dataset for a 2.5 km reach of the braided Rees River, New Zealand. Data were acquired rapidly between high flow events and incorporate over 5 x 109 raw survey observations with point densities of 1600 pts m-2 on exposed bar and channel surfaces. A detailed error analysis of the resulting sub-metre resolution is described to quantify DEM quality across the entire surface model. This reveals unparalleled low vertical errors for such a large and complex surface model; between 0.03 and 0.12 m in exposed and inundated areas of the model, respectively. Copyright © 2013 John Wiley & Sons, Ltd.

Bedload transport measurements with impact plate geophones: comparison of sensor calibration in different gravel‐bed streams

Abstract: Indirect bedload transport measurements have been made with the Swiss plate geophone system in five gravel-bed mountain streams. These geophone sensors record the motion of bedload particles transported over a steel plate mounted flush with the channel bed. To calibrate the geophone system, direct bedload transport measurements were undertaken simultaneously. At the Erlenbach in Switzerland, a moving-basket sampler was used. At the Fischbach and Ruetz streams in Austria, a Helley–Smith type bedload sampler provided the calibration measurements. A Bunte-type bedload trap was used at the Rofenache stream in Austria. At the Nahal Eshtemoa in Israel, Reid-type slot bedload samplers were used. To characterize the response of the geophone signal to bedload particles impacting on the plate, geophone summary values were calculated from the raw signal and stored at one second intervals. The number of impulses, i.e. the number of peaks above a pre-defined threshold value of the geophone output signal, correlated well with field measured gravel transport loads and was found to be a robust parameter. The relations of impulses to gravel transport loads were generally near-linear, but the steepness of the calibration relations differed from site to site. By comparing the calibration measurements from the different field sites and utilizing insights gained during preliminary flume experiments, it has been possible to identify the main factors that are responsible for site specific differences in the calibration coefficient. The analysis of these calibration measurements indicates that the geophone signal also contains some information about the grain size distribution of bedload. Copyright © 2013 John Wiley & Sons, Ltd.

Multichannel rivers : their definition and classification

Abstract: The etymology and historic usage of such terms as anabranch', anastamose' and braided' within river science are reviewed. Despite several decades of modern research to define river channel typologi ...

Post‐fire bedload sediment delivery across spatial scales in the interior western United States

Abstract: Post-fire sediment yields can be up to three orders of magnitude greater than sediment yields in unburned forests. Much of the research on post-fire erosion rates has been at small scales (100 m2 or less), and post-fire sediment delivery rates across spatial scales have not been quantified in detail. We developed relationships for post-fire bedload sediment delivery rates for spatial scales up to 117 ha using sediment yield data from six published studies and two recently established study sites. Sediment yields and sediment delivery ratios (SDRs; sediment delivered at the catchment scale divided by the sediment delivered from a plot nested within the catchment) were related to site factors including rainfall characteristics, area, length, and ground cover. Unit-area sediment yields significantly decreased with increasing area in five of the six sites. The annual SDRs ranged from 0.0089 to 1.15 and these were more closely related to the ratio of the plot lengths than the ratio of plot areas. The developed statistical relationships will help quantify post-fire sediment delivery rates across spatial scales in the interior western United States and develop process-based scaling relationships. Published in 2013. This article is a U.S. Government work and is in the public domain in the USA.

Bedload hysteresis in a glacier-fed mountain river

Abstract: Sediment transport during flood events often reveals hysteretic patterns because flow discharge can peak before (counterclockwise hysteresis) or after (clockwise hysteresis) the peak of bedload. Hysteresis in sediment transport has been used in the literature to infer the degree of sediment availability. Counterclockwise and clockwise hysteresis have been in fact interpreted as limited and unlimited sediment supply conditions, respectively. Hysteresis has been mainly explored for the case of suspended sediment transport, but it was rarely reported for bedload transport in mountain streams. This work focuses on the temporal variability of bedload transport in an alpine catchment (Saldur basin, 18.6 km2, Italian Alps) where bedload transport was monitored by means of an acoustic pipe sensor which detects the acoustic vibrations induced by particles hitting a 0.5m‐long steel pipe. Runoff dynamics are dominated by snowmelt in late spring/early summer, mostly by glacier melt in late summer/early autumn, and by a combination of the snow and glacier melt in mid‐summer. The results indicate that hysteretic patterns during daily discharge fluctuations are predominantly clockwise during the snowmelt period, likely due to the ready availability of unpacked sediments within the channel or through bank erosion in the lower part of the basin. On the contrary, counterclockwise hysteresis tend to be more frequent during late glacier melting period, possibly due to the time lag needed for sediment provided by the glacial and peri‐glacial area to be transported to the monitoring section. However, intense rainfall events occurring during the glacier melt period generated predominantly clockwise hysteresis, thus indicating the activation of different sediment sources. These results indicate that runoff generation processes play a crucial role on sediment supply and temporal availability in mountain streams.

Modeling the functional influence of vegetation type on streambank cohesion

Abstract: The important role of vegetation in adding cohesion and stabilizing streambanks has been widely recognized in several aspects of fluvial geomorphology, including stream restoration and studies of long-term channel change. Changes in planform between braided, meandering, and anabranching forms have been attributed to the impacts of vegetation on hydraulic roughness and bank stability. However, these studies focus either on flume studies where analog vegetation is used, or case studies featuring one species, which is commonly invasive. We present functional differences of bank-stabilizing root characteristics and added cohesion, with vegetation categorized as woody and non-woody and by the vegetation groups of trees, shrubs, graminoids, and forbs. We analyzed root morphology and tensile strength of 14 species common to riparian areas in the southern Rocky Mountains, in field sites along streambanks in the montane and subalpine zones of the Colorado Front Range. Using the vegetation root component (RipRoot) of a physically-based bank stability model (BSTEM), we estimated the added cohesion for various sediment textures with the addition of each of the 14 species. Significant differences exist between woody and non-woody vegetation and between the four vegetation categories with respect to the coefficient of the root tensile strength curve, lateral root extent, and maximum root diameter. Woody vegetation (trees and shrubs) have higher values of all three parameters than non-woody species. Tree roots add significantly more cohesion to streambanks than forb roots. Additionally, rhizomes may play an important role in determining the reach-scale effects of roots on bank stabilization. Differences in root characteristics and added cohesion among vegetation categories have several important implications, including determining the likelihood of planform change, developing guidelines for the use of bank-stabilizing vegetation, and linking the effect of vegetation to geomorphic structure that can benefit ecosystem functioning. Copyright © 2014 John Wiley & Sons, Ltd.

Impact of reach geometry on stream channel sensitivity to extreme floods

Abstract: Predicting spatial and temporal variations in bank erosion due to extreme floods presents a long-standing challenge in geomorphology. We develop two methodologies for rapid, regional-scale assessments of stream reaches susceptible to channel widening. The first proposes that channel widening occurs when unit stream power exceeds a critical threshold (300 W/m2). The second is motivated by the observation that widening often occurs at channel bends. We introduce a new metric, the bend stress parameter, which is proportional to the centripetal force exerted on a concave bank. We propose that high centripetal forces generate locally high bank shear forces and enhance channel bank erosion. We test both metrics using the geomorphic signature of Tropical Storm Irene (2011) on the White and the Saxtons Rivers, Vermont. Specifically, we test if reaches where significant channel widening occurred during Irene required one or both metrics to exceed threshold values. We observe two distinct styles of channel widening. Where unit stream power and bend stress parameter are high, widening is usually due to bank retreat. Elsewhere widening is usually due to the stripping of the upstream end of mid-channel islands. Excluding widening associated with the stripping of the heads of mid-channel islands, almost all the widening (> 98%) occurred along reaches identified as susceptible to widening. The combined metrics identify up to one-quarter of the reaches lacking susceptibility to channel widening. Copyright © 2014 John Wiley & Sons, Ltd.

Evaluation of trenching, ground penetrating radar (GPR) and electrical resistivity tomography (ERT) for sinkhole characterization

Abstract: This paper explores the suitability and advantages of combining the trenching technique with geophysical surveys [ground penetrating radar (GPR) and electrical resistivity tomography (ERT)] for sinkhole characterization in a mantled karst area. The approach is applied to two active sinkholes concealed by anthropogenic deposits and formed by contrasting subsidence mechanisms; collapse and sagging. The ERT section acquired across the collapse sinkhole images the clayey fill of the depression as an obvious low resistivity area, showing the approximate location of the sinkhole edges. Spatially dense GPR surveys provide information on the position of the boundaries of the concealed subsidence structures and their three-dimensional (3D) internal geometry, revealing the dominant subsidence mechanism. We illustrate the impact of several factors on the quality of the GPR data such as sinkhole size, nominal frequency of the antennas, antenna shielding, and the presence of backfilled excavations and above-surface objects. Trenches provided detailed information on the subsurface structure of the sinkhole, subsidence magnitude, partitioning of the strain, and the position of the sinkhole edges, especially when they are deep enough and excavated across the central sector and perpendicular to the boundaries. The stratigraphic and structural relationships observed in the trench were then used to infer the spatial evolution of the sinkholes (e.g. enlargement), their kinematic behavior (episodic versus progressive), and to differentiate discrete subsidence events and their associated magnitude. Numerical dates were used to estimate average subsidence rates and the recurrence of subsidence events. Such integrated data sets may be used as an objective basis to forecast the future behavior of potentially damaging sinkholes and to assess the associated hazard and risk. Copyright © 2013 John Wiley & Sons, Ltd.

The biogeomorphological life cycle of poplars during the fluvial biogeomorphological succession: a special focus on Populus nigra L.

Abstract: Riverine ecosystems are recurrently rejuvenated during destructive flood events and vegetation succession starts again. Poplars (i.e. species from Populus genera) respond to hydrogeomorphological constraints, but, in turn, also influence these processes. Thus, poplar development on bare mineral substrates is not exclusively a one-way vegetative process. Reciprocal interactions and adjustments between poplar species and sediment dynamics during their life cycle lead to the emergence of biogeomorphological entities within the fluvial corridor, such as vegetated islands, benches and floodplains. Based on a review of geomorphological, biological and ecological literature, we have identified and described the co-constructing processes between riparian poplars and their fluvial environment. We have explored the possibility that the modification of the hydrogeomorphological environment exerted, in particular, by the European black poplar (Populus nigra L.), increases its fitness and thus results in positive niche construction. We focus on the fundamental phases of dispersal, recruitment and establishment until sexual maturity of P. nigra by describing the hierarchy of interactions and the pattern of feedbacks between biotic and abiotic components. We explicitly relate the biological life cycle of P. nigra to the fluvial biogeomorphic succession model by referring to the ‘biogeomorphological life cycle’ of P. nigra. Finally, we propose new research perspectives based on this theoretical framework. Copyright © 2014 John Wiley & Sons, Ltd.

Effects of slope angle and aspect on plant cover and species richness in a humid Mediterranean badland

Abstract: Soil erosion is one of the most severe land degradation processes in the Mediterranean region. Although badlands occupy a relatively small fraction of the Mediterranean area, their erosion rates are very high. Many studies have investigated to what extent vegetation controls soil erosion rates. This study, however, deals with the impact of erosion on vegetation establishment. In semi-arid badlands of the Mediterranean, soil water availability constitutes the main limiting factor for vegetation development. As a consequence, south-facing slopes are typically less vegetated due to a very large water stress. However, these findings do not necessarily apply to humid badlands. The main objective of this paper is to determine the topographic thresholds for plant colonization in relation to slope aspect and to assess the spatial patterns of vegetation cover and species richness. We surveyed 179 plots on highly eroded badland slopes in the Central Pyrenees. We defined four aspect classes subdivided into slope angle classes. Colonization success was expressed in terms of vegetation cover and species richness. Slope angle thresholds for plant colonization were identified for each slope aspect class by means of binary logistic regressions. The results show that a critical slope angle exists below which plants colonize the badland slopes. Below this critical slope angle, plant cover and species richness increase with a decreasing slope angle. The largest critical slope angles in humid badlands are observed on south-facing slopes, which contrasts with the results obtained in semi-arid badlands. North-facing slopes however are characterized by a reduced overall vegetation cover and species richness, and lower topographic threshold values. The possible underlying processes responsible for this slope-aspect discrepancy in vegetation characteristics are discussed in terms of environmental variables that control regolith development, weathering and erosion processes. Moreover, possible restoration strategies through the use of vegetation in highly degraded environments are highlighted. Copyright © 2014 John Wiley & Sons, Ltd.

Two-dimensional modelling of large wood transport during flash floods

Abstract: Large woody material (LWM) transported by rivers may be entrapped at critical stream geometry configurations (e.g. bridges) and therefore dramatically increase the destructive power of floods. This was the case in a Spanish mountain river where a flood event with a high degree of LWM transport took place in 1997. The aim of this study was to simulate a bridge clogging process and reconstruct the wood deposit patterns, modelling individual pieces of wood moving with the water flow and interacting among them and with the bridge. A two-dimensional numerical model was developed to simulate the transport of LWM and its effect on hydrodynamics. Different scenarios for the wood transport rate allowed us to study the influence of inlet boundary conditions on bridge clogging. For the studied event, the scenario which best reproduced the bridge clogging effect and flood characteristics was one in which 60% of the total wood entered before the peak discharge. This dropped to 30% at the peak itself, and finally fell to 10% during the recession curve. In addition, the accumulation patterns of LWM along the reach were computed and compared with post-event field photographs, showing that the model succeeded in predicting the deposition patterns of wood and those areas prone to form wood jams. Copyright © 2013 John Wiley & Sons, Ltd.

Large wood budget and transport dynamics on a large river using radio telemetry

Abstract: Despite the abundance of large wood (LW) river studies there is still a lack of understanding of LW transport dynamics on large low gradient rivers. This study used 290 radio frequency identification tagged (RFID) LW and 54 metal (aluminum) tagged LW, to quantify the percent of in-channel LW that moves per year and what variables play a role in LW transport dynamics. Aluminum tags were installed and monitored on LW in-transit during the rising limb of a flood, the mean distance traveled by those pieces during the week was 13.3 river kilometers (km) with a maximum distance of 72 km. RFID tagged LW moved a mean of 11.9 km/yr with a maximum observed at 101.1 km/yr. Approximately 41% of LW low on the bank moves per year. The high rate of transport and distance traveled is likely due to the lack of interaction between LW floating in the channel and the channel boundaries, caused primarily by the width of the channel relative to length of the LW. Approximately 80% of the RFID tags moved past a fixed reader during the highest 20% of river stage per year. LW transport and logjam dynamics are complicated at high flows as pieces form temporary jams that continually expand and contract. Unlike most other studies, key members that create a logjam were defined more by stability than jam size or channel/hydrologic conditions. Finally, using an existing geomorphic database for the river, and data from this study, we were able to develop a comprehensive LW budget showing that 5% of the in-channel LW population turns over each year (input from mass wasting and fluvial erosion equals burial, decomposition, and export out of system) and another 16% of the population moving within the system. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.

Enhanced rock‐slope failure following ice‐sheet deglaciation: timing and causes

Abstract: This research was supported by NERC Cosmogenic Isotope Analysis Facility [Grant Number: 9046.0308]

TerEx Toolbox for semi-automated selection of fluvial terrace and floodplain features from lidar

Abstract: Terraces and floodplains are important indicators of near-channel sediment dynamics, serving as potential sediment sources and sinks. Increasing availability of high resolution topography data over large areas calls for development of semi-automated techniques for identification and measurement of these features. In this study we introduce a novel tool that accommodates user-defined parameters including, a local-relief threshold selected by a variable-size moving window, minimum area threshold, and maximum distance from the channel to identify and map discrete terrace and floodplain surfaces. Each of the parameters can easily be calibrated for a given watershed or reach. Subsequently, the tool automatically measures planform area, absolute elevation, and height relative to the local river channel for each terrace polygon. We validate the tool in two locations where terrace maps were previously developed via manual digitization from lidar and extensive field mapping campaigns. The tool is also tested on six different types of rivers to provide examples of starting selection parameters, and to test effectiveness of the tool across a wide range of landscapes. Generally, the tool provides a high quality draft map of terrace and floodplain surfaces across the wide range of environmental conditions for which it has been tested. We find that the tool functions best in catchments where the terraces are spatially extensive, with distinct differences between the terrace and floodplain. The most challenging environments for semi-automated terrace and floodplain mapping include steep catchments with dense riparian vegetation, and very small terraces (~10 m2 in areal extent). We then apply the tool to map terraces and floodplains in the Root River watershed, southeastern Minnesota and generate exceedance plots for terrace heights. These plots provide a first pass analysis to indicate the tributaries and reaches of the river where terraces constitute a significant source of sediment. Copyright © 2013 John Wiley & Sons, Ltd.

Quantifying the rate and depth dependence of bioturbation based on optically‐stimulated luminescence (OSL) dates and meteoric 10Be

Abstract: Both the rate and the vertical distribution of soil disturbance modify soil properties such as porosity, particle size, chemical composition and age structure; all of which play an important role in a soil's biogeochemical functioning Whereas rates of mixing have been previously quantified, the nature of bioturbation's depth dependence remains poorly constrained Here we constrain, for the first time, the relationship between mixing rate and depth in a bioturbated soil in northeast Queensland, Australia using a novel method combining OSL (optically-stimulated luminescence) ages and meteoric beryllium-10 (10Be) inventories We find that the best fit mixing rate decreases non-linearly with increasing soil depth in this soil and the characteristic length scale of 028 m over which the mixing coefficient decays is comparable to reported rooting depth coefficients In addition we show that estimates of surface mixing rates from OSL data are highly dependent on erosion rate and that erosion rate must be constrained if accurate mixing rates are to be quantified We calculate surface diffusion-like mixing coefficients of 18 × 10−4 and 21 × 10−4 m2 yr−1 for the studied soil for two different estimates of soil erosion © 2014 The Authors Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd

Ultra‐rapid topographic surveying for complex environments: the hand‐held mobile laser scanner (HMLS)

Abstract: Terrestrial laser scanning is the current technique of choice for acquiring high resolution topographic data at the site scale (i.e. over tens to hundreds of metres), for accurate volume measurements or process modelling. However, in regions of complex topography with multiple local horizons, restricted lines of sight significantly hinder use of such tripod-based instruments by requiring multiple setups to achieve full coverage of the area. We demonstrate a novel hand-held mobile laser scanning technique that offers particular promise for site-scale topographic surveys of complex environments. To carry out a survey, the hand-held mobile laser scanner (HMLS) is walked across a site, mapping around the surveyor continuously en route. We assess the accuracy of HMLS data by comparing survey results from an eroding coastal cliff site with those acquired by a state-of-the-art terrestrial laser scanner (TLS) and also with the results of a photo-survey, processed by structure from motion and multi-view stereo (SfM-MVS) algorithms. HMLS data are shown to have a root mean square (RMS) difference to the benchmark TLS data of 20 mm, not dissimilar to that of the SfM-MVS survey (18 mm). The efficiency of the HMLS system in complex terrain is demonstrated by acquiring topographic data covering ~780 m2 of salt-marsh gullies, with a mean point spacing of 4.4 cm, in approximately six minutes. We estimate that HMLS surveying of gullies is approximately 40 times faster than using a TLS and six times faster than using SfM-MVS. © 2013 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.

How do sediment yields from post‐wildfire debris‐laden flows depend on terrain slope, soil burn severity class, and drainage basin area? Insights from airborne‐LiDAR change detection

Abstract: We derived a high-resolution, spatially continuous map of erosion and deposition associated with the debris-laden flows triggered by the 2011 Las Conchas wildfire and subsequent rainstorms over a 197 km2 area in New Mexico, USA. This map was produced using airborne-LiDAR-derived bare-earth digital elevation models (DEMs) acquired approximately one year before and one year after the wildfire. Differencing of the pre-wildfire and post-wildfire-and-rainstorm bare-earth DEMs yielded a DEM-of-difference (DoD) map that quantifies the magnitude of ground-surface elevation changes due to erosion/deposition within each 1 m2 pixel. We applied a 0.3 m threshold filter to our DoD to remove changes that could have been due to artifacts and/or imperfect georeferencing. The 0.3 m value for the threshold filter was chosen based on the stated accuracy of the LiDAR as well as a comparison of areas of significant topographic change mapped in aerial photographs with those predicted using a range of candidate threshold values for the DoD filter. We developed an automated procedure that accepts the DoD map as input and computes, for every pixel in the DEM, the net sediment volume exported through each pixel by colluvial and/or fluvial processes using a digital routing algorithm. An analysis of the resulting sediment volume map for the Las Conchas fire demonstrates that sediment volume is proportional to upstream contributing area. After normalized by contributing area, the average sediment yield (defined as the sediment volume divided by the contributing area) increases as a power-law functions of the average terrain slope and soil burn severity class (SBSC) with exponents equal to approximately 1.5. Our analysis quantifies the relationships among sediment yield, average terrain slope, and average soil burn severity class at the watershed scale and should prove useful for predicting the geomorphic response of wildfire-affected drainage basins. Copyright © 2014 John Wiley & Sons, Ltd.

A comparison of wetness indices for the prediction of observed connected saturated areas under contrasting conditions

Abstract: For lack of other widely available spatial information, topography is often used to predict water fluxes and water quality in mesoscale watersheds. Such data have however proven to be misleading in many environments where large and flat valley bottoms and/or highly conducive soil covers determine water storage and water transport mechanisms. Also, the focus is generally on the prediction of saturation areas regardless of whether they are connected to the catchment hydrographic network or rather present in isolated topographic depressions. Here soil information was coupled with terrain data towards the targeted prediction of connected saturated areas. The focus was on the 30 km2 Girnock catchment (Cairngorm Mountains, northeast Scotland) and its 3 km2 sub-catchment, Bruntland Burn in which seven field surveys were done to capture actual maps of connected saturated areas in both dry and humid conditions. The 1 km2 resolution UK Hydrology of Soil Types (HOST) classification was used to extract relevant, spatially variable, soil parameters. Results show that connected saturated areas were fairly well predicted by wetness indices but only in wet conditions when they covered more than 30% of the whole catchment area. Geomorphic indices including information on terrain shape, steepness, aspect, soil texture and soil depth showed potential but generally performed poorly. Indices based on soil and topographic data did not have more predictive power than those based on topographic information only: this was attributed to the coarse resolution of the HOST classification. Nevertheless, analyses provided interesting insights into the scale-dependent water storage and transport mechanisms in both study catchments. Copyright © 2013 John Wiley & Sons, Ltd.

Inventorying slope movements in an Alpine environment using DInSAR

Abstract: Differential SAR Interferometry (DInSAR) is a technique that can be used to detect and characterize slope movements. It is investigated here as a tool for establishing a detailed overview of complex slope movements at a regional scale in an Alpine context. This paper gives specific recommendations to use and to understand DInSAR signals in mountainous areas located above the tree line, excluding glaciated areas. It proposes a systematic procedure based on accurate interpretations of interferometric signals from a large DInSAR dataset to locate and estimate the displacement rate of moving zones. The methodology was successfully applied in the Western Swiss Alps, where about 1500 moving objects were detected above the tree line using a large dataset of ERS and JERS interferograms dating from the 1990s. The DInSAR-detected movements had a displacement rate ranging from a few centimeters to several meters per year and were attributed to various types of mass wasting phenomena (rock glaciers, landslides, etc.). This kind of inventory derived from DInSAR can be used as a preliminary tool for natural hazard management and process understanding in mountain areas. As automatic data archiving and systematic acquisition of SAR data are ensured worldwide for most SAR sensors, a similar methodology can basically be applied in many other parts of the globe – also by using data from current SAR sensors – as long as a high resolution DEM is available. Copyright © 2014 John Wiley & Sons, Ltd.

Network concepts to describe channel importance and change in multichannel systems : Test results for the Jamuna River, Bangladesh

Abstract: Most of the largest rivers on Earth have multiple active channels connected at bifurcations and confluences. At present a method to describe a channel network pattern and changes in the network beyond the simplistic braiding index is unavailable. Our objectives are to test a network approach to understand the character, stability and evolution of a multi-channel river pattern under natural discharge conditions. We developed a semi-automatic method to derive a chain-like directional network from images that represent the multi-channel river and to connect individual network elements through time. The Jamuna River was taken as an example with a series of Landsat TM and ETM+ images taken at irregular intervals between 1999 and 2004. We quantified the overall importance of individual channels in the entire network using a centrality property. Centrality showed that three reaches can be distinguished along the Jamuna with a different network character: the middle reach has dominantly one important channel, while upstream and downstream there are about two important channels. Temporally, relatively few channels changed dramatically in both low-flow and high-flow periods despite the increase of braiding index during a flood. Based on the centrality we calculated a weighted braiding index that represents the number of important channels in the network, which is about two in the Jamuna River and which is larger immediately after floods. We conclude that the network measure centrality provides a novel characterization of river channel networks, highlighting properties and tendencies that have morphological significance. Copyright © 2013 John Wiley & Sons, Ltd.

Detection ranges and uncertainty of passive Radio Frequency Identification (RFID) transponders for sediment tracking in gravel rivers and coastal environments

Abstract: Since the earliest use of this technology, a growing number of researchers have employed passive Radio Frequency Identification (RFID) transponders to track sediment transport in gravel rivers and coastal environments. RFID transponders are advantageous because they are inexpensive, durable and use unique codes that allow sediment particle mobility and displacement to be assessed on a clast-by-clast basis. Despite these advantages, this technology is in need of a rigorous error and detection analysis. Many studies work with a precision of ~1 m, which is insufficient for some applications, and signal shadowing can occur due to clustering of tagged particles. Information on in-field performance is also incomplete with respect to burial and submergence, especially for different transponders and antennae combinations. The objectives of this study are to qualify and quantify the factors that influence the detection zone of RFID tracers including antenna type, transponder size, transponder orientation, burial depth, submergence and clustering. Results of this study show that the detection zone is complex in shape due to a set of lobes in the detection field and provide a better understanding of transponder detection shape for different RFID transponder/antenna combinations. This study highlights a strong influence of clustering and submergence, but no significant effect of burial. Finally we propose standard operating procedures for tagging and tracking in rivers and coastal environments. Copyright © 2014 John Wiley & Sons, Ltd.

Invasive crayfish as drivers of fine sediment dynamics in rivers: field and laboratory evidence

Abstract: The research was supported by a British Society for Geomorphology Research Grant (‘Field evidence for signal crayfish impacts on fine sediment dynamics in a lowland river’). Tom Moorhouse was funded by the Esmee Fairbairn Foundation.