ALBERT

Description: Past studies of hillslope evolution have typically assumed that soil creep processes are governed by a linear relationship between local hillslope angle and transport distance. The assumption of “linear diffusion” has fallen out of favor because, when coupled with an expression of mass continuity, it yields unrealistic hillslope profiles. As a consequence, a better understanding of the mechanics of sediment transport is needed. Here we report results from a series of flume experiments performed to investigate sediment transport by dry ravel, a common soil creep process in arid and semiarid environments. We find that, at gentle slopes, transport distances follow distributions characteristic of local transport. As gradients steepen, a fraction of the particles begins to exhibit nonlocal transport, and that fraction increases rapidly with slope. A stochastic discrete element model that couples an effective friction term with a shock term reproduces the results from the flume experiments, suggesting that it can be used to explore the nature of particle transport on rough surfaces. The model predicts that exponential distributions of transport distances on gentle slopes evolve into quasi-uniform distributions on steep slopes, and the transition occurs as slopes approach the angle of repose. Our results support previous findings that the angle of repose represents a threshold between friction and inertial regimes. In addition, we propose that the angle of repose represents a fuzzy boundary between local and nonlocal transport.

Description: Dating of gravel-capped strath terraces in basins adjacent to western U.S. Laramide Ranges is one approach to document the history of late Cenozoic fluvial exhumation. We use in situ 10Be measurements to date the broad surfaces adjacent to the eastern edge of the Rocky Mountains in Colorado, and compare these calculated ages with results from meteoric 10Be measurements. We analyze three sites near Boulder, Colorado (Gunbarrel Hill, Table Mountain, and Pioneer) that have been mapped as the oldest terrace surfaces with suggested ages ranging from 640 ka to the Plio-Pleistocene transition. Our in situ 10Be results reveal abandonment ages of 95 ± 129 ka at Table Mountain, 175 ± 27 ka at Pioneer, and ages of 251 ± 10 ka and 307 ± 15 ka at Gunbarrel Hill. All are far younger than previously thought. Inventories of meteoric 10Be support this interpretation, yielding ages that are comparable to Table Mountain and ∼20% lower than Pioneer in situ ages. We argue that lateral beveling by rivers dominated during protracted times of even moderate glacial climate, and that vertical incision rates of several mm/yr likely occurred during times of very low sediment supply during the few interglacials that were characterized by particularly warm climate conditions. In contrast to the traditional age chronology in the area, our ages suggest that the deep exhumation of the western edge the High Plains occurred relatively recently and at an unsteady pace.

Description: Simple mathematical models often allow an intuitive grasp of the function of physical systems. We develop a mathematical framework to investigate reactive or dissipative transport processes within karst conduits. Specifically, we note that for processes that occur within a characteristic timescale, advection along the conduit produces a characteristic process length scale. We calculate characteristic length scales for the propagation of thermal and electrical conductivity signals along karst conduits. These process lengths provide a quantitative connection between karst conduit geometry and the signals observed at a karst spring. We show that water input from the porous/fractured matrix is also characterized by a length scale and derive an approximation that accounts for the influence of matrix flow on the transmission of signals through the aquifer. The single conduit model is then extended to account for conduits with changing geometries and conduit flow networks, demonstrating how these concepts can be applied in more realistic conduit geometries. We introduce a recharge density function, ϕR, which determines the capability of an aquifer to damp a given signal, and cast previous explanations of spring variability within this framework. Process lengths are a general feature of karst conduits and surface streams, and we conclude with a discussion of other potential applications of this conceptual and mathematical framework.

Description: Recent studies suggest that orogens can achieve a topographic steady state whereby equilibrium is reached between tectonics and erosion. However, steady state topography may not be the norm in many orogens experiencing large changes in climate or tectonics, which can produce topographic transients. The quantification of transient topography over geologic timescales requires reconstructing paleotopography, but this has proven difficult in many cases. This study investigates the utility of bedrock thermochronometer data to reconstruct orogen paleotopography over million year timescales. Apatite (U-Th)/He and fission track ages are integrated with a thermokinematic model for a single-parameter inversion of paleotopography. An iterative scheme is used that minimizes the misfit between predicted and observed cooling ages to identify the range of paleotopographies that could produce observed ages within sample uncertainty. Two approaches are considered. First, synthetic 2-D topographies are used to test the robustness of the approach. The following topographic evolution scenarios are considered: (1) lateral ridge migration, (2) topographic relief change, and (3) valley widening and deepening from glaciation. Second, the method is applied in three dimensions to existing data from the Coast Mountains of British Columbia, Canada. Results from both applications of the model suggest that (1) paleotopographic reconstruction will typically underpredict the magnitude of topographic change, especially relief change; (2) paleotopography is most successfully reconstructed after lateral ridge migration in long-wavelength topographies; and (3) reconstructed paleotopography from the Coast Mountains, British Columbia, suggests that glacial erosion may have the potential to remove drainage divides and laterally shift topographic ridges and peaks.

Description: The mechanisms that control climate-dependent rockfall from permafrost mountain slopes are currently poorly understood. In this study, we present the results of an extensive rock slope monitoring campaign at the Matterhorn (Switzerland) with a wireless sensor network. A negative dependency of cleft expansion relative to temperature was observed at all clefts for the dominant part of the year. At many clefts this process is interrupted by a period with increased opening and shearing activity in the summer months. More specific, this period lasts from sustained melting within the cleft to the first freezing in autumn. Based on these empirical findings we identify two distinct process regimes governing the cleft motion observed. Combining current theories with laboratory evidence on rock slope movement and stability, we postulate that (1) the negative temperature-dependency is caused by thermomechanical forcing and is reinforced by cryogenic processes during the freezing period and, (2) the enhanced movement in summer originates from a hydro-thermally induced strength reduction in clefts containing perennial ice. It can be assumed that the irreversible part of the process described in (1) slowly modifies the geometric settings and cleft characteristics of permafrost rock slopes in the long term. The thawing related processes (2) can affect stability within hours or weeks. Such short-term stability minima may activate rock masses subject to the slow changes and lead to acceleration and failure.

Description: Iceberg calving is known to release substantial seismic energy, but little is known about the specific mechanisms that produce calving icequakes. At Yahtse Glacier, a tidewater glacier on the Gulf of Alaska, we draw upon a local network of seismometers and focus on 80 hours of concurrent, direct observation of the terminus to show that calving is the dominant source of seismicity. To elucidate seismogenic mechanisms, we synchronized video and seismograms to reveal that the majority of seismic energy is produced during iceberg interactions with the sea surface. Icequake peak amplitudes coincide with the emergence of high velocity jets of water and ice from the fjord after the complete submergence of falling icebergs below sea level. These icequakes have dominant frequencies between 1 and 3 Hz. Detachment of an iceberg from the terminus produces comparatively weak seismic waves at frequencies between 5 and 20 Hz. Our observations allow us to suggest that the most powerful sources of calving icequakes at Yahtse Glacier include iceberg-sea surface impact, deceleration under the influence of drag and buoyancy, and cavitation. Numerical simulations of seismogenesis during iceberg-sea surface interactions support our observational evidence. Our new understanding of iceberg-sea surface interactions allows us to reattribute the sources of calving seismicity identified in earlier studies and offer guidance for the future use of seismology in monitoring iceberg calving.

Description: We present a semi-distributed model that simulates suspended sediment export from a watershed in two stages: (1) delivery of sediments from hillslope and bank erosion into the river channel, and (2) propagation of the channel sediments through the river network toward the watershed outlet. The model conceptualizes a watershed as the collection of reaches, or representative elementary watersheds (REW), that are connected to each other through the river network, and each REW comprises a lumped representation of a hillslope and channel component. The flow of water along the stream network is modeled through coupled mass and momentum balance equations applied in all REWs and sediment transport within each REW is simulated through the sediment balance equations. Every reach receives sediment inputs from upstream REWs (if present) and from the erosion of adjacent hillslopes, banks and channel bed. We tested this model using 12 years (1982–1993) of high temporal resolution data from Goodwin Creek, a 21.3 km2 watershed in Mississippi, USA. The model yields good estimates of sediment export patterns at the watershed outlet, with Pearson correlation coefficient (R value) of 0.85, 0.87, and 0.95 at daily, monthly, and annual resolution, respectively. Furthermore, the model shows that the dynamics of sediment transport are controlled to a large extent by the differences in the behavior of coarse and fine sediment particles, temporary channel storage, and the spatial variability in climatic forcing. These processes have a bearing on the patterns of sediment delivery with increasing scale.

Description: Submarine gullies are the most common morphological features observed on Antarctic continental slopes. The processes forming these gullies, however, remain poorly constrained. In some areas, gully heads incise the continental shelf edge, and one hypothesis proposed is erosion by overflow of cold, dense water masses formed on the continental shelf. We examined new multibeam echo sounder bathymetric data from the Weddell Sea continental slope, the region that has the highest rate of cold, dense water overflow in Antarctica. Ice Shelf Water (ISW) cascades downslope with an average transport rate of 1.6 Sverdrups (Sv) in the southern Weddell Sea. Our new data show that within this region, ISW overflow does not deeply incise the shelf edge. The absence of gullies extending deeply into the glacial sediments at the shelf edge implies that cold, high salinity water overflow is unlikely to have caused the extensive shelf edge erosion observed on other parts of the Antarctic continental margin. Instead, the gullies observed in the southern Weddell Sea are relatively small and their characteristics indicative of small-scale slides, probably resulting from the rapid accumulation and subsequent failure of proglacial sediment during glacial maxima.

Description: Using numerical models, we evaluate hydrogeological regime changes in high-latitude river basins under conditions of ground surface warming. These models describe transient heat- and fluid flow coupled to the hydrogeological impacts of phase-changes from ice to liquid water. We consider an idealized unconsolidated sedimentary aquifer system in which groundwater flow is driven by topography, representing a series of small drainage basins in riverine terrain of relatively subdued topography. Various temporal and spatial surface temperature conditions are considered to control the initial permafrost distributions for the simulations. The simulated rates of increase in groundwater contribution to streamflow during and after permafrost thaw, are in the order of magnitude comparable to hydrogeological regime changes over the past decades as reported for several (sub-)Arctic rivers. The simulations further show that two distinct features of the subsurface response control the temporal evolution of base flow increase: (1) shifts in aquifer permeability architecture during permafrost degradation and (2) uptake of water into aquifer storage when sub-permafrost hydraulic heads rise. Model analysis shows that the latter process delays base flow increase by several decades to centuries. In order to evaluate the relative importance of both processes in natural systems, the current hydraulic regime of sub-permafrost aquifer systems as well as patterns of permafrost heterogeneity, taliks and their hydraulic connectivity are insufficiently known.

Description: Is the von Kármán constant affected by sediment suspension? The presence of suspended sediment in channels and fluvial streams has been known for decades to affect turbulence transfer mechanism in sediment-laden flows, and, therefore, the transport and fate of sediments that determine the bathymetry of natural water courses. This study explores the density stratification effects on the turbulent velocity profile and its impact on the transport of sediment. There is as yet no consensus in the scientific community on the effect of sediment suspension on the von Kármán parameter, κ. Two different theories based on the empirical log-wake velocity profile are currently under debate: One supports a universal value of κ = 0.41 and a strength of the wake, Π, that is affected by suspended sediment. The other suggests that both κ and Π could vary with suspended sediment. These different theories result in a conceptual problem regarding the effect of suspended sediment on κ, which has divided the research area. In this study, a new mixing length theory is proposed to describe theoretically the turbulent velocity profile. The analytical approach provides added insight defining κ as a turbulent parameter which varies with the distance to the bed in sediment-laden flows. The theory is compared with previous experimental data and simulations using a k-ε turbulence closure to the Reynolds averaged Navier Stokes equations model. The mixing length model indicates that the two contradictory theories incorporate the stratified flow effect into a different component of the log-wake law. The results of this work show that the log-wake fit with a reduced κ is the physically coherent approximation.

Description: Glacial earthquakes are anomalous earthquakes associated with large ice-loss events occurring at marine-terminating glaciers, primarily in Greenland. They are detectable teleseismically, and a proper understanding of the source mechanism may provide a remote-sensing tool to complement glaciological observations of these large outlet glaciers. We model teleseismic surface-wave waveforms to obtain locations and centroid–single-force source parameters for 121 glacial earthquakes occurring in Greenland during the period 2006–2010. We combine these results with those obtained by previous workers to analyze spatial and temporal trends in glacial-earthquake occurrence over the 18-year period from 1993–2010. We also examine earthquake occurrence at six individual glaciers, comparing the earthquake record to independently obtained observations of glacier change. Our findings confirm the inference that glacial-earthquake seismogenesis occurs through the capsize of large, newly calved icebergs. We find a close correspondence between episodes of glacier retreat, thinning, and acceleration and the timing of glacial earthquakes, and document the northward progression of glacial earthquakes on Greenland's west coast over the 18-year observing period. Our results also show that glacial earthquakes occur when the termini of the source glaciers are very close to the glacier grounding line, i.e., when the glaciers are grounded or nearly grounded.

Description: Beach surfaces containing shell materials represent one end-member of a range of environments in which armoring is the primary control on wind erosion. Unlike spheres and cylinders which have formed the basis of theoretical model formulation and much of the early work in wind tunnels, mollusc shells have complex and non-uniform shapes which vary with their orientation. Identification of shell perimeter, height and frontal area relative to the bed area (roughness density) is therefore a formidable task, but nonetheless is essential for modeling sediment entrainment from beach surfaces. A methodology is suggested in this paper for capturing and analyzing these geospatial data, in the context of a wind tunnel simulation designed to improve understanding of the geophysical processes involved in armoring. For deposits where non-erodible shells represent half of the volume of the parent material, the surface appears to be highly stable to wind erosion from the outset, although minor reworking of the intervening, erodible sediment does occur. In comparison, the shell coverage must increase to approximately 30% during wind erosion events in order for any given beach surface to stabilize, especially beach deposits with a low concentration of shells by volume. With suitable calibration, the Raupach shear stress partitioning model can be forced to perform well in predicting the threshold conditions for particle entrainment. However, this approach overlooks the pivotal involvement of particle impact and ricochet in the creation and sculpting of the armored bed. As a case in point, when the shells are removed from digital elevation models of armored beach surfaces formed in aeolian systems, the adjusted topography is not suggestive of the presence of coherent flow structures (e.g., horseshoe vortices and wedge shaped shelter areas) as assumed to exist in the stress partitioning approach for isolated flows. This would suggest that future work on the armoring of natural surfaces affected by wind erosion must allow for more complexity in the flow perturbation.

Description: The entrainment of sediments in rivers exhibits an intermittent behavior. Incipient motion should therefore be described as a random process requiring a stochastic predictive approach. The effect of near-bed turbulence on grain entrainment and the variation in bed particle stability due to local surface heterogeneity are included into a probabilistic framework. The intuitive evidence that hydrodynamic forces acting on the sediment bed and the resistance to motion of the bed particles are two mutually dependent aspects of a unique process is modeled by introducing a conditional independence hypothesis. Based on this concept, new insights into the stochastic aspects of incipient motion are obtained. For low ratios of the boundary shear stress to the critical shear stress, by including the mutual dependence of different processes the new theoretical development predicts up to 50% larger probabilities of grain removal from the bed surface compared to Grass' original formulation. This follows from the entrainment risk being not only dependent on the distributions of fluid forces and grain resistance, but also on the correlation that these distributions exhibit in relation to the geometrical configuration of the sediment bed. This complex interaction is neglected in existing probabilistic models of sediment transport. It is then demonstrated that such additional contribution explains better the influence of both flow turbulence and particle arrangement on key features of the overall grain entrainment process.

Description: Various methods have been proposed in the literature to predict the rainfall conditions that are likely to trigger landslides in a given area. Most of these methods, however, only consider the rainfall events that resulted in landslides and provide deterministic thresholds with a single possible output (landslide or no-landslide) for a given input (rainfall conditions). Such a deterministic view is not always suited to landslides. Slope stability, in fact, is not ruled by rainfall alone and failure conditions are commonly achieved with a combination of numerous relevant factors. When different outputs (landslide or no-landslide) can be obtained for the same input a probabilistic approach is preferable. In this work we propose a new method for evaluating rainfall thresholds based on Bayesian probability. The method is simple, statistically rigorous, and returns a value of landslide probability (from 0 to 1) for each combination of the selected rainfall variables. The proposed approach was applied to the Emilia-Romagna Region of Italy taking advantage of the historical landslide archive, which includes more than 4000 events for which the date of occurrence is known with daily accuracy. The results show that landsliding in the study area is strongly related to rainfall event parameters (duration, intensity, total rainfall) while antecedent rainfall seems to be less important. The distribution of landslide probability in the rainfall duration-intensity shows an abrupt increase at certain duration-intensity values which indicates a radical change of state of the system and suggests the existence of a real physical threshold.

Description: Fluvial sediment transport is caused by a complex interaction of interdependent grain and fluid processes many of which are stochastic in nature and cannot be adequately represented by deterministic equations. Random variable analysis has been used previously but limited data are available to describe the variability of grain resistance combined with particle arrangements, and thus validate such analysis. In this study low to medium bed load transport tests were carried out in a flume where sediment movement was monitored using a three-camera 3D PIV system. Simultaneous grain motion and flow velocity measurements were made on a plane located slightly above and parallel to the sediment bed. Detailed statistical velocity information was acquired to model the velocity distribution at the bed level. This was combined with the joint probabilistic distribution of particle exposures and grain resistance to motion, which were obtained from discrete particle modeling (DPM) simulations. DPM simulations were used to provide a stochastic mathematical description of the risk that a stationary particle is entrained by the flow. Predictions from the stochastic model equations replicated the observed pulsation in sediment transport. This demonstrates that it is possible to simulate sediment entrainment and transport at a high resolution by adequately modeling all the sub-processes. A number of flow patterns were identified that caused large fluctuations of the entrainment rate. These all exhibit high velocity flow structures, but they selectively cause the dislodgement of individual particles located at different positions. This selective behavior follows from the variability of the interaction between the near-bed flow and the particles having different exposure.

Description: Fluid microinclusions in stalagmites have provided samples of paleowaters present during the growth of the stalagmite, but only in microliter amounts. Genty et al. (2002) discovered much larger water-filled macroinclusions in some stalagmites. Using computerized tomographic (CT) X-ray-scanning and magnetic resonance imaging (MRI) we searched for such macroinclusions in 21 stalagmites from diverse localities in North and Central America and the Caribbean. We show that most stalagmites contained numerous mm to cm-sized internal cavities (macroholes). These do not penetrate the outer surfaces which in most cases are deceptively unblemished. Some stalagmites have up to 10% average internal porosity. Two types of macroholes are distinguishable: axial holes formed during growth due to slower calcite accumulation at the axial drip site; off-axis holes formed penecontemporaneously with growth in discrete layers; these cut previous growth laminae showing that they are post-depositional. Using MRI on uncut, apparently sealed specimens, we find that very few of these cavities contain significant quantities of water although they were clearly formed while the stalagmite was being continuously bathed by drip water. Presumably, the water has escaped post-depositionally, through micro fissures, extensive connected hole system, crystal boundaries or other defects.

Description: In an ice sheet, a preferred crystal orientation fabric affects deformation rates because ice crystals are strongly anisotropic: shear along the basal plane is significantly easier than shear perpendicular to the basal plane. The effect of fabric can be as important as temperature in defining deformation rates. Fabric is typically measured using analysis of thin sections under the microscope with co-polarized light. Due to the time-consuming and destructive nature of these measurements, however, it is difficult to capture the spatial variation in fabric necessary for evincing ice sheet flow patterns. Because an ice crystal is similarly elastically anisotropic, the speed of elastic waves through ice can be used as a proxy for quantify anisotropy. We use borehole sonic logging measurements and thin section data from Dome C, East Antarctica to define the relations between apparent fabric and borehole measured elastic speeds (compressional VP and vertically polarized shear VSV). These relations, valid for single maximum fabrics, allow in-situ, depth-continuous fabric estimates of unimodal fabric strength from borehole sonic logging. We describe the single maximum fabric using a1: the largest eigenvalue of the second-order orientation tensor. For ice at −16°C and a1 in the 0.7-1 range the relations are VP = 248 a13.7 + 3755 m s−1 and VSV = −210a17.3 + 1968 m s−1.

Description: Constraining the spatial variation of englacial radar attenuation is critical for accurate inference of the spatial variation of the englacial and basal properties of ice sheets from radar returned power. Here we evaluate attenuation models that account for spatial variations in ice temperature and chemistry and test them along the flowline that passes through the Vostok ice core site, Antarctica. The simplest model, often used but rarely valid, assumes a uniform attenuation rate everywhere along the flowline, so that total attenuation is proportional to ice thickness. The next simplest model uses spatially varying temperatures predicted by an ice-flow model and assumes uniform chemistry. Additional models account for spatially varying chemistry using englacial stratigraphy. We find that the roundtrip attenuation to the bed can easily differ by 10 dB or more between the uniform attenuation-rate model and models that account for variable ice temperature. Such differences are sufficient to confound the delineation of dry and wet beds. Also including spatial variations in chemistry produces smaller differences (

Description: A new physically based approach for calculating glacier ice thickness distribution and volume is presented and applied to all glaciers and ice caps worldwide. Combining glacier outlines of the globally complete Randolph Glacier Inventory with terrain elevation models (Shuttle Radar Topography Mission/Advanced Spaceborne Thermal Emission and Reflection Radiometer), we use a simple dynamic model to obtain spatially distributed thickness of individual glaciers by inverting their surface topography. Results are validated against a comprehensive set of thickness observations for 300 glaciers from most glacierized regions of the world. For all mountain glaciers and ice caps outside of the Antarctic and Greenland ice sheets we find a total ice volume of 170 × 103 ± 21 × 103 km3, or 0.43 ± 0.06 m of potential sea level rise.

Description: Ground penetrating radar (GPR) surveys of the 205 km2 Milne Ice Shelf conducted in 2008 and 2009 are compared with radio echo sounding (RES) data from 1981 to provide the first direct measurements of thinning for any northern Ellesmere Island ice shelf. Our results show an average thinning for the ice shelf as a whole of 8.1 ± 2.8 m, with a maximum of 〉30 m, over this 28-year period. Direct-line comparisons along a 7.5 km transect near the front of ice shelf indicate a mean thinning of 2.63 ± 2.47 m over the same period. Reductions in areal extent (29%, 82 ± 8.4 km2: 1950–2009) and volume (13%, 1.5 ± 0.73 km3 water equivalent (w.e.): 1981–2008/2009) indicate that the Milne Ice Shelf has been in a state of negative mass balance for at least the last 59 years. A comparison of mean annual specific mass balance measurements with the nearby Ward Hunt Ice Shelf (WHIS) suggests that basal melt is a key contributor to Milne Ice Shelf thinning. Glacier inflow to the ice shelf has also reduced markedly over the past 28 years. The transition of ice shelf ice to lake ice was the most important source of mass loss. A 28.5 km2 epishelf lake now exists on the landward side of the ice shelf. Given these recent changes, disintegration of the Milne Ice Shelf will almost certainly continue in the future.

Description: Climate may be the dominant factor affecting landscape evolution during the late Cenozoic, but models that connect climate and landscape evolution cannot be tested without precise ages of landforms. Zircon (U-Th)/He ages of clinker, metamorphosed rock formed by burning of underlying coal seams, provide constraints on the spatial and temporal patterns of Quaternary erosion in the Powder River basin of Wyoming and Montana. The age distribution of 86 sites shows two temporal patterns: (1) a bias toward younger ages because of erosion of older clinker and (2) periodic occurrence of coal fires likely corresponding with particular climatic regimes. Statistical t tests of the ages and spectral analyses of the age probability density function indicate that these episodes of frequent coal fires most likely correspond with times of high eccentricity in Earth's orbit, possibly driven by increased seasonality in the region causing increased erosion rates and coal exhumation. Correlation of ages with interglacial time periods is weaker. The correlations between climate and coal fires improve when only samples greater than 50 km from the front of the Bighorn Range, the site of the nearest alpine glaciation, are compared. Together, these results indicate that the interaction between upstream glaciation and downstream erosion is likely not the dominant control on Quaternary landscape evolution in the Powder River basin, particularly since 0.5 Ma. Instead, incision rates are likely controlled by the response of streams to climate shifts within the basin itself, possibly changes in local precipitation rates or frequency-magnitude distributions, with no discernable lag time between climate changes and landscape responses. Clinker ages are consistent with numerical models in which stream erosion is driven by fluctuations in stream power on thousand year timescales within the basins, possibly as a result of changing precipitation patterns, and is driven by regional rock uplift on million year timescales.

Description: At latitude 67°N, Penny Ice Cap on Baffin Island is the southernmost large ice cap in the Canadian Arctic, yet its past and recent evolution is poorly documented. Here we present a synthesis of climatological observations, mass balance measurements and proxy climate data from cores drilled on the ice cap over the past six decades (1953 to 2011). We find that starting in the 1980s, Penny Ice Cap entered a phase of enhanced melt rates related to rising summer and winter air temperatures across the eastern Arctic. Presently, 70 to 100% (volume) of the annual accumulation at the ice cap summit is in the form of refrozen meltwater. Recent surface melt rates are found to be comparable to those last experienced more than 3000 years ago. Enhanced surface melt, water percolation and refreezing have led to a downward transfer of latent heat that raised the subsurface firn temperature by 10°C (at 10 m depth) since the mid-1990s. This process may accelerate further mass loss of the ice cap by pre-conditioning the firn for the ensuing melt season. Recent warming in the Baffin region has been larger in winter but more regular in summer, and observations on Penny Ice Cap suggest that it was relatively uniform over the 2000-m altitude range of the ice cap. Our findings are consistent with trends in glacier mass loss in the Canadian High Arctic and regional sea-ice cover reduction, reinforcing the view that the Arctic appears to be reverting back to a thermal state not seen in millennia.

Description: Assessing output errors of ice flow models is a major challenge that needs to be addressed if we are to increase our confidence level in projections of mass balance in Antarctica and Greenland. Major inputs to ice flow models include geometry (ice thickness and surface elevation), constitutive laws and boundary conditions (geothermal flux, basal drag coefficient, surface temperature). These inputs can be either measured, in which case they carry errors due to instruments, or inferred using inverse methods (such as basal drag which is inverted using InSAR surface velocities) in which case they carry additional errors generated by the inversion process itself. In both cases, these input errors will result in uncertainties that propagate throughout a forward model, and that influence output diagnostics. In order to estimate the resulting error margins on diagnostics such as mass flux, we develop a new framework based on the Design Analysis Kit for Optimization and Terascale Applications (DAKOTA), which we interface to the Ice Sheet System Model (ISSM). We present results on the Pine Island Glacier, West Antarctica, for which we evaluate error margins of mass flux across the whole glacier, given currently known error margins on ice thickness, basal friction and ice hardness. Our results suggest errors in these inputs propagate linearly through the ice flow model, providing a way to 1) calibrate measurement requirements for field campaigns collecting data such as bedrock or surface topography 2) quantify uncertainties in projections of mass balance and 3) assess the sensitivity of model outputs to input parameters. This new error propagation model should help quantify confidence levels that we assign to model projections for the mass balance of Antarctica and Greenland, which will ultimately improve our projections of future sea level rise in a warming climate.

Description: Seismic attenuation α, or internal friction Q−1, in glacial ice is highly sensitive to temperature, particularly near the melting point. Here we detail a technique to estimate Q and apply it to active source seismic data from Jakobshavn Isbrae, Greenland. We compare our results to measured and modeled temperature profiles of the ice in the region. We find an excellent match, with differences between seismically estimated and modeled temperatures of less than 2°C. Mapping variations in seismic Q through glacial ice thus is shown to allow detailed estimation of englacial temperature profiles, which may be of special value in regions where in situ measurements are logistically difficult.

Description: In this study we investigate the long term evolutionary trend of tidal marshes and their possible tendency to approach equilibrium. We account of the dynamic interaction between the marsh and its adjacent environment, allowing for both variations of marsh elevation and displacement of the marsh boundary. We thus consider a 1-D configuration consisting of a tidal channel merging into a 1-D marsh. Starting from some initial configuration of the channel we model how a salt marsh forms at the landward end of the channel and determine the long term evolution of the channel - marsh configuration under different scenarios of relative sea level rise. Previously established results on the morphodynamic evolution of a tidal channel are extended accounting for the effects of vegetation and wind driven sediment resuspension in regions where tidal stresses are too weak to mobilize sediments. Results suggest that, for sufficiently low rates of relative sea level rise the marsh platform may be able to reach an equilibrium elevation, provided wind resuspension is able to maintain a sufficiently large sediment concentration close to the marsh boundary. This is in general agreement with recent results based on zero dimensional modeling. However, we find that the marsh boundary is unstable, as progradation or retreat is generally experienced depending on a delicate balance between tidal transport and transport driven by wind setup. In this sense, actual morphodynamic equilibrium is a rather exceptional and unstable state.

Description: Field measurements of sediment transport in gravel bed rivers often reveal hysteretic effects due to differences in sediment availability between the rising and falling limbs of a flood hydrograph. However, only a small number of flume studies have analyzed the dynamics of sediment transport during hydrographs. Three types of stepped hydrographs with contrasting durations and magnitudes are simulated here under sediment recirculation conditions. Bed load transport rate and grain size have been measured continuously. The dynamic behavior of the surface armor layer has been explored by analyzing digital photographs and laser scanner surveys of the bed surface taken during hydrographs. The results indicate that sediment transport during the falling limb was lower than during the rising limb in all of the three types of hydrographs. This reduction is more evident for the low-magnitude hydrographs. The grain size of the bed remained virtually constant throughout the hydrographs but the grain size of transported sediments exhibited a counterclockwise hysteresis. Also, a significant increase in the reference shear stress for sediment entrainment was measured during the falling limb of hydrographs. Additionally, a detailed analysis of partial transport dynamics of the bed surface sediments reveals a reduction in sediment mobility during the falling limb of the hydrographs. The difference in sediment entrainment and transport before and after the peak of the hydrographs appears to be caused by a change in the organization of the surface sediments. An analysis of detailed laser scan bed surveys reveals a phase of bed restructuring (lower vertical roughness, clast rearrangement) during the falling limb of hydrographs. Consequently, changes in the degree of organization and complexity of the bed surface are likely the cause of the reduced mobility of sediments, and thus of the reduced sediment transport rate, during the falling limb of the hydrographs.

Description: The origin of terra rossa, red or reddish-brown, clay-rich soils overlying high-purity carbonate substrates, has intrigued geologists and pedologists for decades. Terra rossa soils can form from accumulation of insoluble residues during dissolution of the host limestones, addition of volcanic ash, or addition of externally derived, long-range-transported (LRT) aeolian particles. We studied soils and paleosols on high-purity, carbonate aeolianites of Quaternary age on Bermuda, where terra rossa origins have been debated for more than a century. Potential soil parent materials on this island include sand-sized fragments of local volcanic bedrock, the LRT, fine-grained (

Description: Braided rivers are relatively simple to produce in the laboratory, whereas dynamic meandering rivers have not been sustained beyond initial bend formation. Meandering is theoretically explained by bend instability growing from planimetric perturbation, which convects downstream. In this study, we experimentally tested the importance of upstream perturbation and chute cutoff development in the evolution and dynamics of a meandering channel pattern. The initial straight channel had a transversely moving upstream inlet point and silt-sized silica flour was added to the sediment feed to allow floodplain formation. We obtained a dynamic meandering river with scroll bars. Bend growth was alternated by chute cutoffs that formed across the point bars. Meandering was maintained as one channel was disconnected by a plug bar. The curvature at the chute bifurcation transported sediment and build a new floodplain, while the other channel widens. At the end of the experiment, the fluvial plain exhibited a meandering channel, point bars, chutes and abandoned and partially filled channels with a slightly cohesive floodplain surface similar to natural meandering gravel bed rivers. We conclude that the necessary and sufficient conditions for dynamic meandering gravel bed river are a sustained dynamic upstream perturbation and floodplain formation.

Description: The net current (streaming) in a turbulent bottom boundary layer under waves above a flat bed, identified as potentially relevant for sediment transport, is mainly determined by two competing mechanisms: an onshore streaming resulting from the horizontal non-uniformity of the velocity field under progressive free surface waves, and an offshore streaming related to the nonlinearity of the waveshape. The latter actually contains two contributions: oscillatory velocities under nonlinear waves are characterized in terms of velocity-skewness and acceleration-skewness (with pure velocity-skewness under Stokes waves and acceleration-skewness under steep sawtooth waves), and both separately induce offshore streaming. This paper describes a 1DV Reynolds-averaged boundary layer model with k-ε turbulence closure that includes all these streaming processes. The model is validated against measured period-averaged and time-dependent velocities, from 4 different well-documented laboratory experiments with these processes in isolation and in combination. Subsequently, the model is applied in a numerical study on the waveshape and free surface effects on streaming. The results show how the dimensionless parameters kh (relative water depth) and A/kN (relative bed roughness) influence the (dimensionless) streaming velocity and shear stress and the balance between the mechanisms. For decreasing kh, the relative importance of waveshape streaming over progressive wave streaming increases, qualitatively consistent with earlier analytical modeling. Unlike earlier results, simulations for increased roughness (smaller A/kN) show a shift of the streaming profile in onshore direction for all kh. Finally, the results are parameterized and the possible implications of the streaming processes on sediment transport are shortly discussed.

Description: Analyses of mass and momentum exchange between a debris flow or avalanche and an underlying sediment layer aid interpretations and predictions of bed-sediment entrainment rates. A preliminary analysis assesses the behavior of a Coulomb slide block that entrains bed material as it descends a uniform slope. The analysis demonstrates that the block's momentum can grow unstably, even in the presence of limited entrainment efficiency. A more-detailed, depth-integrated continuum analysis of interacting, deformable bodies identifies mechanical controls on entrainment efficiency, and shows that entrainment rates satisfy a jump condition that involves shear-traction and velocity discontinuities at the flow-bed boundary. Explicit predictions of the entrainment rate E result from making reasonable assumptions about flow velocity profiles and boundary shear tractions. For Coulomb-friction tractions, predicted entrainment rates are sensitive to pore fluid pressures that develop in bed sediment as it is overridden. In the simplest scenario the bed sediment liquefies completely, and the entrainment-rate equation reduces to E = 2μ1gh1 cos θ(1 − λ1)/v¯1, where θ is the slope angle, μ1 is the flow's Coulomb friction coefficient, h1 is its thickness, λ1 is its degree of liquefaction, and v¯1 is its depth-averaged velocity. For values of λ1 ranging from 0.5 to 0.8, this equation predicts entrainment rates consistent with rates of 0.05 to 0.1 m/s measured in large-scale debris-flow experiments in which wet sediment beds liquefied almost completely. The propensity for bed liquefaction depends on several factors, including sediment porosity, permeability, and thickness, and rates of compression and shear deformation that occur when beds are overridden.

Description: We explore the role of plant matter accumulation in the sediment column in determining the response of fluvial-deltas to base-level rise and simple subsidence profiles. Making the assumption that delta building processes operate to preserve the geometry of the delta plain, we model organic sedimentation in terms of the plant matter accumulation and accommodation (space made for sediment deposition) rates. A spatial integration of the organic sedimentation, added to the known river sediment input, leads to a model of delta evolution that estimates the fraction of organic sediments preserved in the delta. The model predicts that the maximum organic fraction occurs when the organic matter accumulation rate matches the accommodation rate, a result consistent with field observations. The model also recovers the upper limit for coal accumulation previously reported in the coal literature. Further, when the model is extended to account for differences in plant matter accumulation between fresh and saline environments (i.e., methanogenesis versus sulfate reduction) we show that an abrupt shift in the location of the fresh-salt boundary can amplify the speed of shoreline retreat.

Description: The activity of inland aeolian dune fields is typically related to the external forcing imposed by climate: active (bare) dunes are associated with windy and/or arid settings, and inactive (vegetated) dunes are associated with humid and/or calm environments. When a climate shifts the dune field reacts; however, the behavior, rate, and potential impact of diverse dune geomorphologies on these transitions are poorly understood. Here, we use a numerical model to systematically investigate the influence of dune field geomorphology (dune height, organization and collisions) on the time a dune field takes to stabilize. To generate diverse initial un-vegetated dune field geomorphologies under unidirectional winds, we varied pre-stabilization growth time and initial sediment thickness (termed equivalent sediment thickness: EST). Following dune field development from a flat bed, we introduced vegetation (simulating a climate shift) and transport-vegetation feedbacks slowly stabilized the dune fields. Qualitatively, very young and immature dune fields stabilized quickly, whereas older dune fields took longer. Dune fields with greater EST stabilized quicker than those with less EST. Larger dunes stabilized quicker because of low celerity, which facilitated higher vegetation growth rates. Extended stabilization times were associated with the extension of parabolic dunes. Dune-dune collisions resulted in premature stabilization; the frequency of collisions was related to dune spacing. Quantitatively comparing the distribution of deposition rates in a dune field to the deposition tolerance of vegetation provides a promising predictor of relative stabilization time. Dune fields with deposition rates dominantly above the deposition tolerance of vegetation advanced unimpeded and prolonged stabilization as parabolic dunes. Paleoenvironmental reconstructions or predictions of dune field activity should not assume that dune activity directly translates to climate, considerable lags to stabilizing climate shifts may exist in unidirectional dune forms.

Description: We developed a multilayered physical snowpack model named Snow Metamorphism and Albedo Process (SMAP), which is intended to be incorporated into general circulation models for climate simulations. To simulate realistic physical states of snowpack, SMAP incorporates a state-of-the-art physically based snow albedo model, which calculates snow albedo and solar heating profile in snowpack considering effects of snow grain size and snow impurities explicitly. We evaluated the performance of SMAP with meteorological and snow impurities (black carbon and dust) input data measured at Sapporo, Japan during two winters: 2007–2008 and 2008–2009, and found SMAP successfully reproduced all observed variations of physical properties of snowpack for both winters. We have thus confirmed that SMAP is suitable for climate simulations. With SMAP, we also investigated the effects of snow impurities on snowmelt at Sapporo during the two winters. We found that snowpack durations at Sapporo were shortened by 19 days during the 2007–2008 winter and by 16 days during the 2008–2009 winter due to radiative forcings caused by snow impurities. The estimated radiative forcings due to snow impurities during the accumulation periods were 3.7 W/m2 (it corresponds to albedo reduction in 0.05) and 3.2 W/m2 (albedo reduction in 0.05) for the 2007–2008 and 2008–2009 winters, respectively. While during the ablation periods they were 25.9 W/m2 (albedo reduction in 0.18) and 21.0 W/m2 (albedo reduction in 0.17) for each winter, respectively.

Description: Models of sub-sea permafrost evolution vary significantly in employed physical assumptions regarding the paleo-geographic scenario, geological structure, thermal properties, initial temperature distribution, and geothermal heat flux. This work aims to review the underlying assumptions of these models as well as to incorporate recent findings, and hence develop an up-to-date model of the sub-sea permafrost dynamics at the Laptev Sea shelf. In particular, the sub-sea permafrost model developed here incorporates thermokarst and land-ocean interaction theory, and shows that the sediment salinity and a temperature-based parametrization of the unfrozen water content are critical factors influencing sub-sea permafrost dynamics. From the numerical calculations, we suggest development of open taliks may occur beneath submerged thaw lakes within a large area of the shelf.

Description: Anisotropy in englacial radar power was measured using 60-MHz and 179-MHz copolarized pulse-modulated radar at 19 sites in central West Antarctica. The study region is a 100 × 300 km2 area near the West Antarctic Ice Sheet Divide that separates ice flow toward the Ross and Amundsen Embayments. The frequency dependence of the returned power indicates that most of the radar data are affected by vertical variations in the crystal-orientation fabric (COF), though the 60-MHz data are more affected by acidity contrasts in the top 1000 m. Significant polarimetric variations occur at most sites, likely due to effects of the anisotropic COF patterns. More anisotropic variations occur at sites with significant horizontal strain, whereas more isotropic variations occur at sites where vertical compression dominates. Azimuthal shifts with depth of the principal axes of COF were found in shallow ice near the current flow divide and at greater depths over locations of rough bed. The former indicates that the divide has differentially migrated, resulting in a rotation of the principal COF axes. Nevertheless, the regionally consistent radar signatures suggest that the first-order ice properties in this area have remained constant and that no major changes in the strain configuration or ice topography have occurred for the past five to eight thousand years. We conclude that shallow polarimetric features can be related to the current strain configurations, and that englacial polarimetric features can help constrain current ice rheology and evolution of the ice topography.

Description: Glaciers have been principal erosional agents in many orogens throughout much of the recent geological past. A modern example is the St. Elias Mountains in southeastern Alaska; it is a highly convergent, complex orogen, which has been glaciated for much of its history. We examine the Seward-Malaspina Glacier system, which comprises two of the largest temperate glaciers in the world. We focus on the pattern of erosion within its narrow passage through the St. Elias Mountains, the Seward Throat. Measured glacier surface velocities and elevations provide constraints for a full-stress numerical flowband model that enables us to quantitatively determine the glacier thickness profile, which is not easily measured on temperate glaciers, and the basal characteristics relevant for erosion. These characteristics at the bed, namely the water pressure, normal and shear stresses, and sliding velocity, are then used to infer the spatial variation in erosion rates using several commonly invoked erosion laws. The calculations show that the geometry of the glacier basin exerts a far stronger control on the spatial variation of erosion rates than does the equilibrium line altitude, which is often assumed to be important in studies of glaciated orogens. The model provides a quantitative basis for understanding why erosion rates are highest around the Seward Throat, which is generally consistent with local and large-scale geological observations and thermochronologic evidence. Moreover, model results suggest how glacier characteristics could be used to infer zones of active or recent uplift in ice-mantled orogens.

Description: The hypothesis that the formation and dynamics of large scale shoreline sand waves can be explained by a feedback mechanism between waves and nearshore morphology under very oblique wave incidence is explored with a quasi 2D nonlinear morphodynamic model. Using constant wave conditions it is found that if the wave incidence angle at the depth of closure is larger than about 45° the rectilinear coastline becomes unstable and a shoreline sand wavefield develops from small random perturbations. Shoreline sand waves develop with wavelengths between 2 and 5 km, they migrate downdrift at about 0.5 km/yr and they reach amplitudes up to 120 m within 13 years. Larger wave obliquity, higher waves and shorter wave periods strengthen the shoreline instability. Cross-shore transport is essential for the instability and faster cross-shore dynamics leads to a faster growth of the sand waves. Simulations with variable wave incidence angles (alternating between 60° and 30°) show that a large proportion of high angle waves is required for spontaneous sand wave formation (at least 80%). Insight is provided into the physical mechanism behind high angle wave instability and the occurrence of a optimal length scale for sand wave growth. The generic model results are consistent with existing observations of shoreline sand waves, in particular with those along the southwest coast of Africa.

Description: Meltwater input often triggers a seismic response from glaciers and ice sheets. It is difficult, however, to measure melt production on glaciers directly, while subglacial water storage is not directly observable. Therefore, we document temporal changes in seismicity from a dry-based polar glacier (Taylor Glacier, Antarctica) during a melt season using a synthesis of seismic observation and melt modeling. We record icequakes using a dense six-receiver network of three-component geophones and compare this with melt input generated from a calibrated surface energy balance model. In the absence of modeled surface melt, we find that seismicity is well-described by a diurnal signal composed of microseismic events in lake and glacial ice. During melt events, the diurnal signal is suppressed and seismicity is instead characterized by large glacial icequakes. We perform network-based correlation and clustering analyses of seismic record sections and determine that 18% of melt-season icequakes are repetitive (multiplets). The epicentral locations for these multiplets suggest that they are triggered by meltwater produced near a brine seep known as Blood Falls. Our observations of the corresponding p-wave first motions are consistent with volumetric source mechanisms. We suggest that surface melt enables a persistent pathway through this cold ice to an englacial fracture system that is responsible for brine release episodes from the Blood Falls seep. The scalar moments for these events suggest that the volumetric increase at the source region can be explained by melt input.

Description: High resolution measurements of ice motion along a ∼120 km transect in a land-terminating section of the GrIS reveal short-term velocity variations (

Description: High relief and steep topography are thought to result in high erosion rates. In the Rwenzori Mountains of the Albert Rift, East Africa, where more than 3 km of relief have formed during uplift of the Rwenzori fault block, overall low denudation rates prevail. We measured in situ-derived cosmogenic denudation rates of 28.2 to 131 mm/kyr in mountainous catchments, and rates of 7.8 to 17.7 mm/kyr on the adjacent low-relief East African Plateau. These rates are roughly an order of magnitude lower than in other settings of similar relief. We present an extensive geomorphological analysis, and find that denudation rates are positively correlated with relief, hillslope gradient, and channel steepness, indicating that river incision controls erosional processes. In most upper headwater reaches above Quaternary ELA levels (〉4500 m a.s.l.), glacial imprinting, inherited from several older and recent minor glaciation stages, prevails. In regions below 4500 m a.s.l., however, mild climatic conditions impede frost shattering, favor dense vegetation, and minimize bare rock areas and associated mass wasting. We conclude that erosion of the Rwenzori Mountains is significantly slower than corresponding rates in other mountains of high relief, due to a combination of factors: extremely dense mountain cloud forest vegetation, high rock strength of gneiss and amphibolite lithologies, and low internal fracturing due to the extensional tectonic setting. This specific combination, unique to this extensional tropical setting, leads to unexpected low erosion rates that cannot outpace post-Pliocene ongoing rock uplift of the Rwenzori fault block.

Description: Estimation of ice sheet mass balance from satellite altimetry requires interpolation of point-scale elevation change (dH/dt) data over the area of interest. The largest dH/dt values occur over narrow, fast-flowing outlet glaciers, where data coverage of current satellite altimetry is poorest. In those areas, straightforward interpolation of data is unlikely to reflect the true patterns of dH/dt. Here, four interpolation methods are compared and evaluated over Jakobshavn Isbræ, an outlet glacier for which widespread airborne validation data are available from NASA's Airborne Topographic Mapper (ATM). The four methods are ordinary kriging (OK), kriging with external drift (KED), where the spatial pattern of surface velocity is used as a proxy for that of dH/dt, and their spatiotemporal equivalents (ST-OK and ST-KED). KED assumes a linear relationship between spatial gradients of velocity and dH/dt, which is confirmed for both negative (Pearson's correlation ρ  〈 −0.85) and, to a lesser degree, positive (ρ = 0.73) dH/dt values. When compared to ATM data, KED and ST-KED yield more realistic spatial patterns and higher thinning rates (over 20 m yr−1 as opposed to 7 m yr−1 for OK). Spatiotemporal kriging smooths inter-annual variability and improves interpolation in periods with sparse data coverage and we conclude, therefore, that ST-KED produces the best results. Using this method increases volume loss estimates from Jakobshavn Isbræ by up to 20% compared to those obtained by OK. The proposed interpolation method will improve ice sheet mass balance reconstructions from existing and past satellite altimeter data sets, with generally poor sampling of outlet glaciers.

Description: Antarctic ice shelves interact closely with the ocean cavities beneath them, with ice shelf geometry influencing ocean cavity circulation, and heat from the ocean driving changes in the ice shelves, as well as the grounded ice streams that feed them. We present a new coupled model of an ice stream-ice shelf-ocean system that is used to study this interaction. The model is capable of representing a moving grounding line and dynamically responding ocean circulation within the ice shelf cavity. Idealized experiments designed to investigate the response of the coupled system to instantaneous increases in ocean temperature show ice-ocean system responses on multiple timescales. Melt rates and ice shelf basal slopes near the grounding line adjust in 1–2 years, and downstream advection of the resulting ice shelf thinning takes place on decadal timescales. Retreat of the grounding line and adjustment of grounded ice takes place on a much longer timescale, and the system takes several centuries to reach a new steady state. During this slow retreat, and in the absence of either an upward-or downward-sloping bed or long-term trends in ocean heat content, the ice shelf and melt rates maintain a characteristic pattern relative to the grounding line.

Description: Sediment supply is widely held to be one of the primary controls on bar topography in alluvial channels, yet quantitative linkages between sediment supply and bar topography are not well developed. We explore the conditions under which alternate bars form and how they respond to the elimination of sediment supply in two linked laboratory experiments. The first set of experiments was conducted in a 28 m long, 0.86 m wide flume channel using a unimodal sand-gravel mix. The second set of experiments was conducted at field scale in a 55 m long, 2.74 m wide channel using a unimodal gravel mixture. In both experiments, alternate bars and patchy surface grain-size distributions developed under steady flow and sediment supply conditions. The cessation of the sediment supply induced a reduction in the surface grain-size heterogeneity and the bars were eliminated. In both flumes, mean boundary shear stress had declined, but were capable of moving sediments after the bars disappeared, albeit at relatively small rates compared to when the bars were present. In the smaller flume, the previously stationary bars migrated out of the flume and were not replaced with new bars. A nearly featureless bed formed with limited surface grain-size heterogeneity, a slightly coarsened surface and a slightly reduced slope. In the larger flume, the formation of alternate bars was induced by an imposed upstream flow constriction and as such, the bars did not migrate. Termination of sediment supply led to progressive erosion of bed topography and loss of the bars, coarsening of the bed surface, loss of bed texture patchiness and significant slope reduction. The original alternate bar topography redeveloped when the sediment supply was restored once sufficient deposition had occurred to reconstruct the original channel slope. This shows that the bar loss was reversible by establishing the previous conditions and highlights the importance of sediment supply for bar formation. The role of sediment supply in bar formation and stability is not often recognized in stream restoration. Our results suggest that the loss of sediment supply can significantly affect alternate bar topography and that considerable volumes of sediment may be needed restore channel bars.

Description: Pore fluid pressure variations play an important role in the motion of natural granular flows like debris and pyroclastic flows. Pore pressure in a defluidizing air-particle bed was investigated by means of experiments and numerical modeling. Experiments consisted of recording the defluidization process, measured as the decay of the basal pore fluid pressure in initially aerated granular mixtures. Mixtures were aerated to different degrees of fluidization by introducing a vertical air flux at the base of a granular column. The degree of fluidization was characterized by the parameter βo (pore fluid pressure/lithostatic pressure). Bed expansion occurred for βo 〉 0.8–0.9, with maximum expansions near 8% at βo ∼1. Pore pressure diffusion in our mixtures was modeled by a simple diffusion equation, taking into account a variable diffusion coefficient. When mixtures were expanded (βo 〉 0.8–0.9), continuous consolidation introduced nonlinearities in the diffusion coefficients, which retarded the decay of pore pressure. In contrast, for non-expanded mixtures, the diffusion coefficient remained constant (linear diffusion). Our results highlight that mixture compressibility can effectively reduce the pressure diffusion coefficient in initially expanded granular mixtures, thus increasing the duration of pressure diffusion. In our experiments, as well as for most self-consolidating natural granular mixtures, changes in permeability due to mixture consolidation appear to be negligible for the defluidizing process, as they are counteracted by changes in porosity and because the fluid behaves as incompressible, even when the fluid is air.

Description: A coupled ice stream-ice shelf-ocean cavity model is used to assess the sensitivity of the coupled system to far-field ocean temperatures, varying from 0.0 to 1.8°C, as well as sensitivity to the parameters controlling grounded ice flow. A response to warming is seen in grounding line retreat and grounded ice loss that cannot be inferred from the response of integrated melt rates alone. This is due to concentrated thinning at the ice shelf lateral margin, and to processes that contribute to this thinning. Parameters controlling the flow of grounded ice have a strong influence on the response to sub-ice shelf melting, but this influence is not seen until several years after an initial perturbation in temperatures. The simulated melt rates are on the order of that observed for Pine Island Glacier in the 1990s. However, retreat rates are much slower, possibly due to unrepresented bedrock features.

Description: The triggering of debris flows depends on a critical combination of available unconsolidated material and water supply. In periglacial environments, debris flows are generally triggered by liquefaction of loose material in a channel, or by progressive erosion during a large release of water. Here, we link an unusually dense and highly resolved database on periglacial debris flows with meteorological records dating back to AD 1864 to reconstruct ∼150 yr of rainstorms that triggered debris flows at high-elevation sites (source area elevations ranging from 2000 to 4545 m a.s.l.) in the Swiss Alps. Analysis is based on a tree ring-derived frequency series of debris flows from eight torrents, as well as on daily records from three meteorological stations and runoff data from four river gauging stations. Results show that the debris-flow season at these high-altitude sites now is much longer (May to October) than it used to be in the late nineteenth century when activity was limited to June–September. Debris flows early in the season are generally triggered by lower rainstorm totals (

Description: Changes in external forcing have traditionally been the main areas of interest in understanding sedimentary records, while in most stratigraphic interpretation, autogenic behavior has been thought of as a “noise” generator. This study aims to investigate autogenic processes in a fluvio-deltaic system under a range of discharge conditions and to show that autogenic processes generate distinct signatures rather than random noise. A matrix of nine different experiments is presented here to systematically evaluate the effects of sediment and water discharge variations on the timescale of fluvial autogenic processes. Temporary sediment storage regularly occurs by backfilling of sediment in the fluvio-deltaic channels, followed by a period of strong channelization that releases the stored sediment. These storage and release processes cycle along with changes in the fluvial slope and planform pattern of the flow. Here we propose that the autogenic behavior of deltaic progradation has a distinct timescale that is controlled by sediment and water discharges. An increase in sediment discharge primarily reduces the autogenic timescale as higher sediment supply fills the channels faster. In contrast, the high sediment discharge causes a morphologic feedback by increasing the magnitude of fluvial slope change between the storage and release events and increasing the size of the temporary sediment storage (termed “the fluvial envelope”). This works against the sediment discharge control as the autogenic timescale is increased. Increasing the water discharge increases the autogenic timescale by improving the fluvial organization toward a strongly channelized system. Changes in autogenic timescale due to variations in the sediment and water discharges are nonlinear for different sediment to water discharge ratios. As the ratio decreases, the fluvial system is better organized and the timescale is more linearly related to the change in sediment discharge. As the ratio increases, deltas develop poorly organized fluvial systems and the associated timescales converge even with different sediment discharges. The results presented here provide enhanced interpretation of sedimentary records by better decoupling of autogenic signatures from allogenic products developed across a wide range of discharge conditions.

Description: Hydrological response to earthquakes has long been observed, yet the mechanisms responsible still remain unclear and likely vary in space and time. This study explores the base flow response in small upland catchments of the Coastal Range of south-central Chile after the MW 8.8 Maule earthquake of 27 February 2010. An initial decline in streamflow followed by an increase of up to 400% of the discharge measured immediately before the earthquake occurred, and diurnal streamflow oscillations intensified after the earthquake. Neither response time, nor time to maximum streamflow discharge showed any relationship with catchment topography or size, suggesting non-uniform release of water across the catchments. The fast response, unaffected stream water temperatures and a simple diffusion model point to the sandy saprolite as the source of the excess water. Base flow recession analysis reveals no evidence for substantial enhancement of lateral hydraulic conductivity in the saprolite after the earthquake. Seismic energy density reached ∼170 J/m3 for the main shock and ∼0.9 J/m3 for the aftershock, exceeding the threshold for liquefaction by undrained consolidation only during the main shock. Although increased hydraulic gradient due to ground acceleration-triggered, undrained consolidation is consistent with empirical magnitude-distance relationships for liquefaction, the lack of independent evidence for liquefaction means that enhanced vertical permeability (probably in combination with co-seismic near-surface dilatancy) cannot be excluded as a potential mechanism. Undrained consolidation may have released additional water from the saturated saprolite into the overlying soil, temporarily reducing water transfer to the creeks but enlarging the cross-section of the saturated zone, which in turn enhanced streamflow after establishment of a new hydraulic equilibrium. The enlarged saturated zone facilitated water uptake by roots and intensified evapotranspiration.

Description: A morphodynamic model has been applied to explain the characteristics of transverse sandbars observed in the inner surf zone of open beaches. The model describes the feedback between waves, rollers, depth-averaged currents and bed evolution, so that self-organized processes can develop. The modeled bar characteristics, i.e. wavelength (30–70 m), crest orientation (up-current) and the e-folding growth time (about 12 hr) are in good agreement with those of observed transverse bars at Noordwijk beach, the Netherlands, but modeled migration speeds (tens of meters per day), turn out to be a factor 2 larger than those observed. The wavelength increases with the distance between the shoreline and the peak of the longshore current and the migration speed is correlated with the maximum longshore current. The model also explains why transverse bar formation at Noordwijk occurs for obliquely incident waves of intermediate heights. Realistic positive feedback leading to formation of up-current oriented bars like those observed is only obtained if a term related to the turbulence sediment resuspension created by the rollers is included in the transport formula. In that case, the depth-averaged sediment concentration decreases seaward across the inner surf zone, enhancing the convergence of sediment transport in the offshore directed flow perturbations that occur over the up-current bars. This offshore current deflection is mainly caused by frictional torques, but the roller radiation stresses also play an important role.

Description: Hillslope sediment transport models express the sediment flux at a point as a function of some topographic attributes of the system, such as slope, curvature, soil thickness, etc., at that point only (referred here as “local” transport models) or at an appropriately defined vicinity of that point (referred here as “nonlocal” transport models). Typically, topographic attributes are computed from digital elevation data (DEMs) and thus their estimates depend on the DEM resolution (1 m, 10 m, 90 m, etc.) rendering any sediment flux computation scale-dependent. Often calibration compensates for this scale-dependence resulting in effective parameterizations with limited physical meaning. In this paper, we demonstrate the scale-dependence of local nonlinear hillslope sediment flux models and derive a subgrid scale closure via upscaling. We parameterize the subgrid scale closure in terms of the low resolution, resolved topographic attributes of the landscape, thus allowing the reliable computation of a scale-independent sediment flux from low resolution digital elevation data. We also show that the accuracy of the derived subgrid scale closure model depends on the dimensionless erosion rate and the dimensionless relief of any given basin. Finally, we present theoretical arguments and demonstrate that the recently proposed nonlocal sediment flux models are scale-independent. These concepts are demonstrated via an application on a small basin (MR1) of the central Oregon Coast Range using high-resolution lidar topographic data.

Description: Emissions of salt dust from the shores of saline lakes significantly impact lake chemistry, air quality, transportation, human health, and climate. Quantitative methods for assessing these emissions, however, are still in the developmental stage. We investigate salt pathways from groundwater to dust using an approach that takes advantage of opportune conditions at a groundwater-fed, saline lake in the Nebraska Sand Hills region. The mass of salt in the lakeshore surface crust and soil was measured, as well as in the dust on the surrounding dune field. These data, together with information on the lake hydrology, show that dust emission is an important mechanism controlling lake salinity, even though a mere fraction of the salt crust is deflated each year under extant climatic conditions. Wind data collected at the lake site indicate high wind speeds capable of dust mobilization. Therefore, the physical and chemical bonding of salts in the crust is offered as the primary limiting factor for dust emission rates.

Description: The beds of steep streams are typically composed of relatively immobile boulders and more mobile patches of gravel and cobbles. Little is known about how variability in flow and sediment flux affect the area, thickness, composition, and grain mobility of sediment patches. To better understand patch dynamics, we measured flow, sediment transport, and bed properties in two steep channels. Patches close to the thalweg varied in area, thickness, and grain size, whereas those outside the thalweg did not. Local variations in transport of several orders of magnitude occurred, even on a patch with a spatially homogeneous grain size distribution. During moderate flow events, partial to selective transport dominated on the entire channel bed and all individual patches. Tracer particles moved freely between different patch classes (e.g., fine and coarse patches exchanged particles), and relatively fine sediment on all patch classes began motion at the same shear stress. Therefore, the selective transport observed for the entire bed was not a result of the preferential transport of only fine patches, but the high relative mobility of finer sediment on all patches. Our results suggest that local flow and sediment supply, and not spatial grain size variations, were the primary drivers of local bed load transport variability. The use of reach-averaged flow properties to understand local patch dynamics may not be applicable.

Description: Submarine permafrost degradation rates may be determined by a number of interacting processes, including rates of sea level rise and coastal erosion, sea bottom temperature and salinity regimes, geothermal heat flux and heat and mass diffusion within the sediment column. Observations of ice-bearing permafrost in shelf sediments are necessary in order to determine its spatial distribution and to quantify its degradation rate. We tested the use of direct current electrical resistivity to ice-bearing permafrost in Elson Lagoon northeast of Barrow, Alaska (Beaufort Sea). A sharp increase in electrical resistivity was observed in profiles collected perpendicular to and along the coastline and is interpreted to be the boundary between ice-free sediment and underlying ice-bearing submarine permafrost. The depth to the interpreted ice-bearing permafrost increases from

Description: Physically based models are valuable tools for exploring the detailed spatial and temporal responses of glaciers and ice sheets to climate forcing. However, while the last two decades have seen considerable progress in the development of increasingly sophisticated numerical descriptions, very little attention has been given to simulation uncertainty. In particular, glaciological models have traditionally been calibrated (or “tuned”) in order to identify a single set of parameters (e.g., snow density, surface albedo, temperature lapse rate) such that the model's behavior closely matches real-world observations. The present study disputes this classical approach by demonstrating that it is often difficult (if not impossible) to find a single “best” solution. Instead, multiple equally plausible parameter sets will usually exist. To address this limitation, we present a novel application of a calibration technique previously not used in glacial modeling – multiobjective optimization – designed to identify multiple optimal parameter sets that fit different characteristics of available observations, thereby enabling an assessment of the uncertainty associated with predictions. The strength and applicability of our approach is illustrated through the implementation of a surface mass balance model for two glaciers in Svalbard: Midre Lovénbreen and Kongsvegen. The model is forced using the ERA-40 reanalysis and calibrated against available mass balance measurements. The overall uncertainty range in modeled cumulative annual surface mass balance is −7.84 to −14.02 m w.e. for Midre Lovénbreen (over the period 1968–2001) and −0.91 to +9.80 m w.e. for Kongsvegen (over the period 1987–2001). The calibrated model is used to extend the mass balance records of the two glaciers back to the beginning of the ERA-40 reanalysis, giving a cumulative loss of loss of 13.2 ± 3.3 m w.e. for Midre Lovénbreen and a cumulative gain of 5.3 ± 3.4 m w.e. for Kongsvegen over the period 1958–2001. The multiobjective optimization results also indicate that current mass balance models may contain structural inadequacies relating to how the mass balance gradient is simulated.

Description: Erosion rates dictate the morphology of landscapes, and therefore quantifying them is a critical part of many geomorphic studies. Methods to directly measure erosion rates are expensive and time consuming, whereas topographic analysis facilitates prediction of erosion rates rapidly and over large spatial extents. If hillslope sediment flux is nonlinearly dependent on slope then the curvature of hilltops will be linearly proportional to erosion rates. In this contribution we develop new techniques to extract hilltop networks and sample their adjacent hillslopes in order to test the utility of hilltop curvature for estimating erosion rates using high-resolution (1 m) digital elevation data. Published and new cosmogenic radionuclide analyses in the Feather River basin, California, suggest that erosion rates vary by over an order of magnitude (10 to 250 mm kyr−1). Hilltop curvature increases with erosion rates, allowing calibration of the hillslope sediment transport coefficient, which controls the relationship between gradient and sediment flux. Having constraints on sediment transport efficiency allows estimation of erosion rates throughout the landscape by mapping the spatial distribution of hilltop curvature. Additionally, we show that hilltop curvature continues to increase with rising erosion rates after gradient-limited hillslopes have emerged. Hence hilltop curvature can potentially reflect higher erosion rates than can be predicted by hillslope gradient, providing soil production on hilltops can keep pace with erosion. Finally, hilltop curvature can be used to estimate erosion rates in landscapes undergoing a transient adjustment to changing boundary conditions if the response timescale of hillslopes is short relative to channels.

Description: Sliding glaciers and brittle ice failure generate seismic body and surface wave energy characteristic to the source mechanism. Here we analyze continuous seismic recordings from an array of nine short-period passive seismometers located on Bench Glacier, Alaska (USA) (61.033°N, 145.687°W). We focus on the arrival-time and amplitude information of the dominant Rayleigh wave phase. Over a 46-hour period we detect thousands of events using a cross-correlation based event identification method. Travel-time inversion of a subset of events (7% of the total) defines an active crevasse, propagating more than 200 meters in three hours. From the Rayleigh wave amplitudes, we estimate the amount of volumetric opening along the crevasse as well as an average bulk attenuation (Q¯ = 42) for the ice in this part of the glacier. With the remaining icequake signals we establish a diurnal periodicity in seismicity, indicating that surface run-off and subglacial water pressure changes likely control the triggering of these surface events. Furthermore, we find that these events are too weak (i.e., too noisy) to locate individually. However, stacking individual events increases the signal-to-noise ratio of the waveforms, implying that these periodic sources are effectively stationary during the recording period.

Description: The geomorphic literature contains many analytic solutions for the topographic evolution of gently sloping soil-mantled hillslopes responding to base level changes. Most of these solutions are limited to vertical base level changes and/or to simplified geometries, however. In this paper we present an analytic solution for the morphology of a valley and its adjacent hillslopes undergoing steady headward growth. The mathematics of this problem were first solved by Ivantsov (1947) in the context of heat flow near a parabolic solidification boundary. Here we test whether the Ivantsov solution provides an accurate first-order prediction of the morphology of valley heads and their adjacent hillslopes by comparing the model predictions to survey data from two study sites in southeastern Arizona. The model predicts that elevation contours of valley heads are parabolas and that topographic transects normal to contour lines are error functions. High-resolution Digital Elevation Models (DEMs) were constructed for the two study sites using Real-Time Kinematic Global Positioning System (RTK-GPS) measurements and a Terrestrial Laser Scanner (TLS). Our analyses show that the model reproduces the first-order morphology of headward-growing valleys and their adjacent hillslopes. We also show that by analyzing hillslope profiles at different distances from the valley head, the model framework can be used to infer likely changes in the valley head migration rate through time.

Description: Antarctic ice sheet surface melting can regionally influence ice shelf stability, mass balance, and glacier dynamics, in addition to modulating near-surface physical and chemical properties over wide areas. Here, we investigate variability in surface melting from 1999 to 2009 using radar backscatter time series from the SeaWinds scatterometer aboard the QuikSCAT satellite. These daily, continent-wide observations are explored in concert with in situ meteorological records to validate a threshold-based melt detection method. Radar backscatter decreases during melting are significantly correlated with in situ positive degree-days as well as meltwater production determined from energy balance modeling at Neumayer Station, East Antarctica. These results support the use of scatterometer data as a diagnostic indicator of melt intensity (i.e., the relative liquid water production during melting). Greater spatial and temporal melting detected relative to previous passive microwave-based studies is attributed to a higher sensitivity of the scatterometer instrument. Continental melt intensity variability can be explained in part by the dynamics of the Southern Annular Mode and the Southern Oscillation Index, and extreme melting events across the Ross Ice Shelf region may be associated with El Niño conditions. Furthermore, we find that the Antarctic Peninsula accounts for only 20% of Antarctic melt extent but greater than 50% of the total Antarctic melt intensity. Over most areas, annual melt duration and intensity are proportional. However, regional and localized distinctions exist where the melt intensity metric provides greater insight into melting dynamics than previously obtainable with other remote sensing techniques.

Description: River deltas and alluvial fans have channelization and deposition dynamics that are not entirely understood, but which dictate the evolution of landscapes of great social, economic, and ecologic value. Our lack of a process-based understanding of fan dynamics hampers our ability to construct accurate prediction and hazard models, leaving these regions vulnerable. Here we describe the growth of a series of experimental alluvial fans composed of a noncohesive grain mixture bimodal in size and density. We impose conditions that simulate a gravel/sand fan prograding into a static basin with constant water and sediment influx, and the resulting fans display realistic channelization and avulsion dynamics. We find that we can describe the dynamics of our fans in terms of a few processes: (1) an avulsion sequence with a timescale dictated by mass conservation between incoming flux and deposit volume; (2) a tendency for flow to reoccupy former channel paths; and (3) bistable slopes corresponding to separate entrainment and deposition conditions for grains. Several important observations related to these processes are: an avulsion timescale that increases with time and decreases with sediment feed rate; fan lobes that grow in a self-similar, quasi-radial pattern; and channel geometry that is adjusted to the threshold entrainment stress. We propose that the formation of well-defined channels in noncohesive fans is a transient phenomenon resulting from incision following avulsion, and can be directly described with dual transport thresholds. We present a fairly complete, process-based description of the mechanics of avulsion and its resulting timescale on our fans. Because the relevant dynamics depend only on threshold transport conditions and conservation of mass, we show how results may be directly applied to field-scale systems.

Description: Connectivity between fluvial and aeolian sedimentary systems plays an important role in the physical and biological environment of dryland regions. This study examines the coupling between fluvial sand deposits and aeolian dune fields in bedrock canyons of the arid to semiarid Colorado River corridor, southwestern USA. By quantifying significant differences between aeolian landscapes with and without modern fluvial sediment sources, this work demonstrates for the first time that the flow- and sediment-limiting effects of dam operations affect sedimentary processes and ecosystems in aeolian landscapes above the fluvial high water line. Dune fields decoupled from fluvial sand supply have more ground cover (biologic crust and vegetation) and less aeolian sand transport than do dune fields that remain coupled to modern fluvial sand supply. The proportion of active aeolian sand area also is substantially lower in a heavily regulated river reach (Marble–Grand Canyon, Arizona) than in a much less regulated reach with otherwise similar environmental conditions (Cataract Canyon, Utah). The interconnections shown here among river flow and sediment, aeolian sand transport, and biologic communities in aeolian dunes demonstrate a newly recognized means by which anthropogenic influence alters dryland environments. Because fluvial–aeolian coupling is common globally, it is likely that similar sediment-transport connectivity and interaction with upland ecosystems are important in other dryland regions to a greater degree than has been recognized previously.

Description: The rate at which transient knickpoints propagate through a landscape fundamentally controls the rate of geomorphic response to tectonic and climatic perturbation. Here we present knickpoint retreat rates upstream of active faults for 19 bedrock catchments in Turkey and 11 bedrock catchments in Italy where we have very good constraints on both the magnitude and timing of the tectonic perturbation and where climate histories are well documented. We show that the knickpoints have average retreat rates of between 0.2 and 2 mm/yr for catchments with drainage areas between 6 and 65 km2 and we test whether differences in rock mass strength and catchment size are sufficient to explain this range in retreat rates. Our analysis suggests that even accounting for these two variables, knickpoint propagation velocities differ markedly, and we show that channels crossing faults with higher throw rates have knickpoints that are retreating faster. The dependence of knickpoint retreat velocity on throw rate is at least as important as catchment drainage area. These results indicate, counterintuitively, that landscapes forced by large amplitude tectonic perturbations will have shorter response times than those perturbed by smaller amplitude changes. The link between the knickpoint propagation velocity and throw rate is largely (but not completely) explained by channel narrowing in areas of high uplift rate. Channel steepening upstream of the active faults may explain all of the residual dependency of knickpoint retreat rate on fault throw rate, but only if the slope exponent, n, in the standard stream power model is greater than 1.3. However, we cannot rule out a role for sediment supply in driving enhanced knickpoint retreat rate in addition to the well-documented channel narrowing effect. Finally, we find that mean knickpoints retreat rates in Turkey are only half of those in Italy, for catchments of equivalent size, crossing faults with similar throw rates. This difference in fluvial response time is accounted for by long-term differences in the ratio of precipitation to infiltration in the two areas over the last 1 My.

Description: Morainal banks are primary features at the margins of advancing and stable to quasi-stable temperate tidewater glaciers, yet their roles in glacier dynamics and terminus stability are poorly defined by submarine observations. Analysis of new and archival multibeam data and Landsat images of the advancing Hubbard Glacier, southeast Alaska, reveal that between 1978 and 2010 the ice face and morainal bank advanced together at an average rate of ∼34 m/yr, varying spatially and temporally between 14 and 80 m/yr. Morphological features including gullies and a boulder lag suggest cyclical deposition and gravitational erosion on the proximal slope of the morainal bank (15–18°), and possible ice pushing in an area without recent sedimentation. In contrast, the morainal bank of the nearby, quasi-stable (surging) Turner Glacier advanced steadily since 1978 by proximal sedimentation on the steep fjord wall below its hanging valley. Sedimentation in the deep (〉220 m) basin of Disenchantment Bay increased from 0.88 m/yr spanning 1978 to 1999, to 1.22 m/yr thereafter. This change appears to be a combined response to glacier advance and sediment dispersal farther down-fjord, and to an increase in sediment yield from other glacial and non-glacial sources. Analysis of Hubbard Glacier illustrates the direct correlation between movement of the terminus and morainal bank in advancing the grounding line of a marine-terminating glacier, and that morainal banks provide a fundamental stabilizing role for advance into a deep-water fjord, compensating for changes in water depth at the grounding line.

Description: Radiocarbon measurements at ice margin sites and blue ice areas can potentially be used for ice dating, ablation rate estimates and paleoclimatic reconstructions. Part of the measured signal comes from in situ cosmogenic 14C production in ice, and this component must be well understood before useful information can be extracted from 14C data. We combine cosmic ray scaling and production estimates with a two-dimensional ice flow line model to study cosmogenic 14C production at Taylor Glacier, Antarctica. We find (1) that 14C production through thermal neutron capture by nitrogen in air bubbles is negligible; (2) that including ice flow patterns caused by basal topography can lead to a surface 14C activity that differs by up to 25% from the activity calculated using an ablation-only approximation, which is used in all prior work; and (3) that at high ablation margin sites, solar modulation of the cosmic ray flux may change the strength of the dominant spallogenic production by up to 10%. As part of this effort we model two-dimensional ice flow along the central flow line of Taylor Glacier. We present two methods for parameterizing vertical strain rates, and assess which method is more reliable for Taylor Glacier. Finally, we present a sensitivity study from which we conclude that uncertainties in published cosmogenic production rates are the largest source of potential error. The results presented here can inform ongoing and future 14C and ice flow studies at ice margin sites, including important paleoclimatic applications such as the reconstruction of paleoatmospheric 14C content of methane.

Description: Direct measurement of the thickness of mountain glaciers is difficult over large areas, yet knowledge of the thickness is essential for calculating their volumes and future evolution. We develop a new method for estimating the ice thickness along glacier flow lines, using the “perfect-plasticity” rheological assumption that relates the thickness and surface slope to a yield stress. Previous studies have used this assumption with the shallow-ice approximation to estimate the ice thickness, but the standard approach neglects the effect of side drag on glacier stress balance. Our method addresses this shortcoming and extends the standard method by accounting for the side drag via the glacier width. Besides the assumed yield stress, the inputs for our method are the outline and surface topography of the glacier; surface velocity and mass balance data are unnecessary. We validated the extended method on five glaciers in northwest China where thickness data are available from radio echo soundings, finding that it can reproduce measured thicknesses with a mean absolute error of 11.8% (like the standard method). Moreover, for long glacier tongues confined to flow between parallel valley sides, this method is found to give more accurate thickness estimates than does the standard method, with a mean absolute error of as low as 5.3%. Sensitivity analysis shows that the estimated ice thickness depends strongly on yield stress and surface slope and less strongly on glacier width. Because this method is physically more realistic than the standard method and its inputs are easily derivable from remote-sensing observations, it has the potential to be used for processing large glacier data sets.

Description: Computational modeling of Earth system processes often requires simplifying assumptions of the real system. These necessary assumptions result in the definition of internal model parameters that can take a number of different values but must be explicitly defined for any one model simulation. The main issue with such an uncertain multidimensional input space is that many simulations are needed to adequately explore it. This study presents a generalized parameter screening experiment for use in future earth system modeling. This approach identifies model parameters that dominate uncertainty, therefore reducing to a manageable number the simulations required to explore the input space. The approach we adopt is relatively inexpensive to implement and can be applied at both the aggregate and disaggregate (e.g., regional) level. To demonstrate the potential of such a method, it is applied to a surface mass balance model of intermediate complexity over the Greenland ice sheet. All identified parameters were related to the surface melt parameterization, with albedo parameters being identified as the most important. Spatial distributions of the parameter sensitivities show that, in recent years, most parameter sensitivities are concentrated around the southwest and northern ice sheet margins. Simulations for the 21st century indicate an increase in sensitivity in these high melt areas especially in the northeast. Melt contributions from temperature and radiative effects are shown to be important on the order of parameters identified, and as a consequence, sensitivities are dependent on the present climate used for modeling surface mass balance.

Description: Recent studies have observed deviation from normal (Fickian) diffusion in sediment tracer dispersion that violates the assumption of statistical convergence to a Gaussian. Nikora et al. (2002) hypothesized that particle motion at short time scales is superdiffusive because of inertia, while long-time subdiffusion results from heavy-tailed rest durations between particle motions. Here we test this hypothesis with laboratory experiments that trace the motion of individual gravels under near-threshold intermittent bed load transport (0.027 〈 τ* 〈 0.087). Particle behavior consists of two independent states: a mobile phase, in which indeed we find superdiffusive behavior, and an immobile phase, in which gravels distrained from the fluid remain stationary for long durations. Correlated grain motion can account for some but not all of the superdiffusive behavior for the mobile phase; invoking heterogeneity of grain size provides a plausible explanation for the rest. Grains that become immobile appear to stay at rest until the bed scours down to an elevation that exposes them to the flow. The return time distribution for bed scour is similar to the distribution of rest durations, and both have power law tails. Results provide a physical basis for scaling regimes of anomalous dispersion and the time scales that separate these regimes.

Description: Acquiring spatially continuous ground-surface displacement fields from Terrestrial Laser Scanners (TLS) will allow better understanding of the physical processes governing landslide motion at detailed spatial and temporal scales. Problems arise, however, when estimating continuous displacement fields from TLS point-clouds because reflecting points from sequential scans of moving ground are not defined uniquely, thus repeat TLS surveys typically do not track individual reflectors. Here, we implemented the cross-correlation-based Particle Image Velocimetry (PIV) method to derive a surface deformation field using TLS point-cloud data. We estimated associated errors using the shape of the cross-correlation function and tested the method's performance with synthetic displacements applied to a TLS point cloud. We applied the method to the toe of the episodically active Cleveland Corral Landslide in northern California using TLS data acquired in June 2005–January 2007 and January–May 2010. Estimated displacements ranged from decimeters to several meters and they agreed well with independent measurements at better than 9% root mean squared (RMS) error. For each of the time periods, the method provided a smooth, nearly continuous displacement field that coincides with independently mapped boundaries of the slide and permits further kinematic and mechanical inference. For the 2010 data set, for instance, the PIV-derived displacement field identified a diffuse zone of displacement that preceded by over a month the development of a new lateral shear zone. Additionally, the upslope and downslope displacement gradients delineated by the dense PIV field elucidated the non-rigid behavior of the slide.

Description: Parts of the Alaska Range (Alaska, USA) stand in prominent exception to the “glacial buzzsaw hypothesis,” which postulates that terrain raised above the ELA is rapidly denuded by glaciers. In this paper, we discuss the role of a strong contrast in rock type in the development of this exceptional terrain. Much of the range is developed on pervasively fractured flysch, with local relief of 1000–1500 m, and mean summit elevations that are similar to modern snow line elevations. In contrast, Cretaceous and Tertiary plutons of relatively intact granite support the range's tallest mountains (including Mt. McKinley, or Denali, at 6194 m), with 2500–5000 m of local relief. The high granitic peaks protrude well above modern snow lines and support many large glaciers. We focus on the plutons of the Denali massif and the Kichatna Mountains, to the west. We use field observations, satellite photos, and digital elevation data to demonstrate how exhumation of these plutons affects glacier longitudinal profiles, the glacial drainage network, and the effectiveness of periglacial processes. In strong granite, steep, smooth valley walls are maintained by detachment of rock slabs along sheeting joints. These steep walls act as low-friction surfaces (“Teflon”), efficiently shedding snow. Simple scaling calculations show that this avalanching may greatly enhance the health of the modern glaciers. We conclude that, in places such as Denali, unusual combinations of rapid tectonic uplift and great rock strength have created the highest relief in North America by enhancing glacial erosion in the valleys while preserving the peaks.

Description: Data from large-scale debris-flow experiments are combined with modeling of particle-size segregation to explain the formation of lateral levees enriched in coarse grains. The experimental flows consisted of 10 m3 of water-saturated sand and gravel, which traveled ∼80 m down a steeply inclined flume before forming an elongated leveed deposit 10 m long on a nearly horizontal runout surface. We measured the surface velocity field and observed the sequence of deposition by seeding tracers onto the flow surface and tracking them in video footage. Levees formed by progressive downslope accretion approximately 3.5 m behind the flow front, which advanced steadily at ∼2 m s−1 during most of the runout. Segregation was measured by placing ∼600 coarse tracer pebbles on the bed, which, when entrained into the flow, segregated upwards at ∼6–7.5 cm s−1. When excavated from the deposit these were distributed in a horseshoe-shaped pattern that became increasingly elevated closer to the deposit termination. Although there was clear evidence for inverse grading during the flow, transect sampling revealed that the resulting leveed deposit was strongly graded laterally, with only weak vertical grading. We construct an empirical, three-dimensional velocity field resembling the experimental observations, and use this with a particle-size segregation model to predict the segregation and transport of material through the flow. We infer that coarse material segregates to the flow surface and is transported to the flow front by shear. Within the flow head, coarse material is overridden, then recirculates in spiral trajectories due to size-segregation, before being advected to the flow edges and deposited to form coarse-particle-enriched levees.

Description: Kolmogorov's classic theory for turbulence assumed an independence between velocity increments and the value for the velocity itself. However, recent work has called this assumption in to question, which has implications for the structure of atmospheric, oceanic and fluvial flows. Here we propose a conceptually simple analytical framework for studying velocity-intermittency coupling that is similar in essence to the popular quadrant analysis method for studying near-wall flows. However, we study the dominant (longitudinal) velocity component along with a measure of the roughness of the signal, given mathematically by its series of Hölder exponents. Thus, we permit a possible dependence between velocity and intermittency. We compare boundary layer data obtained in a wind tunnel to turbulent jets and wake flows. These flow classes all have distinct characteristics, which cause them to be readily distinguished using our technique and the results are robust to changes in flow Reynolds numbers. Classification of environmental flows is then possible based on their similarities to the idealized flow classes and we demonstrate this using laboratory data for flow in a parallel-channel confluence. Our results have clear implications for sediment transport in a range of geophysical applications as they suggest that the recently proposed impulse-based methods for studying bed load transport are particularly relevant in domains such as gravel bed river flows where the boundary layer is disrupted and wake interactions predominate.

Description: We use a nonlinear morphodynamic model to demonstrate that the presence of a single persistent offshore bathymetric anomaly strongly affects the formation, nonlinear evolution and saturation of surf zone rip channels. In the case of an offshore bump or trough and waves with oblique incidence, a rip channel shoreward of the anomaly is enforced by the more seaward alongshore variability in depth. The degree of rip channel enforcement is controlled by the strength of the rotational nature of surf zone rip current circulations, which is, in turn, driven by differential broken wave energy dissipation induced by wave refraction across the offshore bathymetric anomaly. The alongshore location of this forced rip channel is more stable with increasing offshore anomaly amplitude, decreasing offshore wave obliquity and decreasing bathymetric anomaly distance to the shore. Simulations show that rip channel behavior downdrift and updrift of the offshore perturbation are different. In our numerical experiments, downdrift rip channels have systematically larger alongshore scales, smaller alongshore migration rates and more erosive megacusps than those updrift. Rip channels therefore self-organize into patterns of different alongshore scales and migration rates as a result of an alongshore perturbation in the wave forcing enforced by wave refraction across an offshore bathymetric anomaly. These simulations are qualitatively corroborated by video observations of sandbar behavior during a down-state sequence at a site with a persistent offshore trough.

Description: In this, the first of a pair of papers which address the simulation and automated measurement of well-sorted natural granular material, a method is presented for simulation of two-phase (solid, void) assemblages of discrete non-cohesive particles. The purpose is to have a flexible, yet computationally and theoretically simple, suite of tools with well constrained and well known statistical properties, in order to simulate realistic granular material as a discrete element model with realistic size and shape distributions, for a variety of purposes. The stochastic modeling framework is based on three-dimensional tessellations with variable degrees of order in particle-packing arrangement. Examples of sediments with a variety of particle size distributions and spatial variability in grain size are presented. The relationship between particle shape and porosity conforms to published data. The immediate application is testing new algorithms for automated measurements of particle properties (mean and standard deviation of particle sizes, and apparent porosity) from images of natural sediment, as detailed in the second of this pair of papers. The model could also prove useful for simulating specific depositional structures found in natural sediments, the result of physical alterations to packing and grain fabric, using discrete particle flow models. While the principal focus here is on naturally occurring sediment and sedimentary rock, the methods presented might also be useful for simulations of similar granular or cellular material encountered in engineering, industrial and life sciences.

Description: The main objectives of the present study are to obtain improved models of hydrodynamic forces and torque on a particle sitting on a bed and to use these models for the investigation of incipient motion and resuspension of particles. The improved models for force and torque are obtained from numerical simulations of a particle sitting on a bed with a turbulent flow of logarithmic mean velocity profile approaching the particle. Since the mean turbulent velocity profile can depart from the logarithmic profile in case of macroscale rough beds or flow down steep slopes, we have also considered forces and torque on a particle due to both linear and uniform mean flow profiles. The computed drag and lift coefficients and the predicted critical shear stress for incipient particle motion and resuspension are compared against available experimental results. The improved force and torque models are also used to evaluate the effect of turbulent velocity fluctuations on the critical shear stress for incipient motion and resuspension. The present results are of direct relevance to cases where the particle is mostly exposed to the ambient flow. In cases where the particle protrusion is small and is submerged mostly within a pocket of other particles, the above formulation can be used with a redefined area of exposure and flow velocity seen by the particle. However, the drag, lift, and torque coefficients and the resisting forces will be influenced by the partial exposure and the details of pocket geometry, which require further investigation.

Description: Confluences with relatively low discharge and momentum flux ratios where a small steep tributary with a high supply of poorly sorted sediment joins a large, low-gradient main channel commonly occur in nature, but they have not yet been investigated. Measurements of the three-dimensional velocity field, turbulence, sediment transport, bed material grain size and morphology are reported in a laboratory setting that is representative of confluences on the Upper Rhone River, Switzerland. The difference between the low-flow depth in the steep tributary and the higher flow depth in the main channel creates a marked bed discordance in the tributary zone. Due to this bed discordance, the tributary flow penetrates into the main channel mainly in the upper part of the water column, whereas the main-channel flow is hardly hindered by the tributary in the lower part of the water column, giving rise to a two-layer flow structure in the confluence zone. In confluences with high supply of coarse sediment from the tributary, the development of a deposition bar downstream from the confluence reduces the flow area and causes flow acceleration that contributes to an increase in sediment transport capacity. The sediment supplied by the tributary is mainly sorted and transported on the face of the bar by the near-bed flow originating from the main channel. The sediment transport capacity is further increased by the three-dimensionality of the flow, which is characterized by maximum velocities occurring near the bed, and by a considerable increase in turbulent kinetic energy generated in the shear layer at the interface of the flows originating from the main channel and the tributary. A conceptual model is proposed for the hydro-morpho-sedimentary processes, and compared to existing conceptual models for confluences with different characteristics.

Description: We investigated seismic signals generated during a large-scale, multiple iceberg calving event that occurred at Jakobshavn Isbræ, Greenland, on 21 August 2009. The event was recorded by a high-rate time-lapse camera and five broadband seismic stations located within a few hundred kilometers of the terminus. During the event two full-glacier-thickness icebergs calved from the grounded (or nearly grounded) terminus and immediately capsized; the second iceberg to calve was two to three times smaller than the first. The individual calving and capsize events were well-correlated with the radiation of low-frequency seismic signals (

Description: Granular materials are composed of solid, discrete particles and exhibit mechanical properties that range from fluid to solid behavior. Some of the complexity exhibited by granular systems arises due to the long-range order that develops due to particle-particle contact. Inter-particle forces in granular materials often form a distributive network of filamentary force-accommodating chains (i.e., force chains), such that a fraction of the total number of particles accommodates the majority of the forces in the system. The force chain network inherent to a system composed of granular materials controls the macroscopic behavior of the granular material. Force transmission by these filamentary chains is focused (or localized) to the grain scale at boundaries such as the granular flow substrate. This investigation addresses the effects of force localization on the substrate by dynamic force chain processes and the implications for bed entrainment in dense, unconfined, two-dimensional, gravity-driven granular flows. Our experimental system employs photoelastic techniques and provides an avenue for quantitative force analysis via image processing and provides a data set that can be used validate discrete element modeling approaches. We show that force chains cause extreme bed-force localization in dynamic granular systems, and that these localized forces can propagate extensively into the substrate, even ahead of the flow front.

Description: We present analytical solutions for the steady state topographic profile of a soil-mantled hillslope retreating into a level plain in response to a horizontally migrating base level. This model applies to several scenarios that commonly arise in landscapes, including widening valleys, eroding channel banks, and retreating scarps. For a sediment transport law in which sediment flux is linearly proportional to the topographic slope, the steady state profile is exponential, with an e-folding length, L, proportional to the ratio of the sediment transport coefficient to the base level migration speed. For the case in which sediment flux increases nonlinearly with slope, the solution has a similar form that converges to the linear case as L increases. We use a numerical model to explore the effects of different base level geometries and find that the one-dimensional analytical solution is a close approximation for the hillslope profile above an advancing channel tip. We then compare the analytical model with hillslope profiles above the tips of a groundwater sapping channel network in the Florida Panhandle. The model agrees closely with hillslope profiles measured from airborne laser altimetry, and we use a predicted log linear relationship between topographic slope and horizontal distance to estimate L for the measured profiles. Mapping 1/L over channel tips throughout the landscape reveals that adjacent channel networks may be growing at different rates and that south facing slopes experience more efficient hillslope transport.

Description: Given that clay-rich landslides may become mobilized, leading to rapid mass movements (earthflows and debris flows), they pose critical problems in risk management worldwide. The most widely proposed mechanism leading to such flow-like movements is the increase in water pore pressure in the sliding mass, generating partial or complete liquefaction. This solid-to-liquid transition results in a dramatic reduction of mechanical rigidity in the liquefied zones, which could be detected by monitoring shear wave velocity variations. With this purpose in mind, the ambient seismic noise correlation technique has been applied to measure the variation in the seismic surface wave velocity in the Pont Bourquin landslide (Swiss Alps). This small but active composite earthslide-earthflow was equipped with continuously recording seismic sensors during spring and summer 2010. An earthslide of a few thousand cubic meters was triggered in mid-August 2010, after a rainy period. This article shows that the seismic velocity of the sliding material, measured from daily noise correlograms, decreased continuously and rapidly for several days prior to the catastrophic event. From a spectral analysis of the velocity decrease, it was possible to determine the location of the change at the base of the sliding layer. These results demonstrate that ambient seismic noise can be used to detect rigidity variations before failure and could potentially be used to predict landslides.

Description: Subglacial hydrology in East Antarctica is poorly understood, yet may be critical to the manner in which ice flows. Data from a new regional airborne geophysical survey (ICECAP) have transformed our understanding of the topography and glaciology associated with the 287,000 km2 Aurora Subglacial Basin in East Antarctica. Using these data, in conjunction with numerical ice sheet modeling, we present a suite of analyses that demonstrate the potential of the 1000 km-long basin as a route for subglacial water drainage from the ice sheet interior to the ice sheet margin. We present results from our analysis of basal topography, bed roughness and radar power reflectance and from our modeling of ice sheet flow and basal ice temperatures. Although no clear-cut subglacial lakes are found within the Aurora Basin itself, dozens of lake-like reflectors are observed that, in conjunction with other results reported here, support the hypothesis that the basin acts as a pathway allowing discharge from subglacial lakes near the Dome C ice divide to reach the coast via the Totten Glacier.

Description: Monitoring snow avalanches is necessary in order to better understand their triggering mechanisms and ultimately improve forecast performance. Seismic monitoring has been developed by several groups over the last 20 years and holds great potential to detect, locate, and characterize snow avalanches. During the 2009–2010 winter, a seismic antenna was installed in the French Alps close to the village of Saint-Christophe-en-Oisans (1700 m above sea level). The array of seven sensors operated during 50 days in October and November 2009 under snow-free conditions and during 40 days in January and February 2010 in presence of snow. It recorded different types of seismic events including snow avalanches, rockfalls, shots, and regional and local microearthquakes. Eighty avalanche signals were visually identified. Using a beam-forming method, we were able to locate snow avalanches on slopes of various orientations in a radius of about 3 km and track their propagation. The location technique allowed for the estimation of avalanches' front speed, which ranged between 12 and 32 m s−1. The method can also distinguish dry and wet snow avalanches. Durations of avalanches can be as long as 380 s because of the length of the slopes in the area. Seismic monitoring provides a catalog of avalanches with precise times, which can be used to analyze the impact of meteorological forcings on the avalanche triggering. Snowfall is found to be the dominant forcing of avalanche activity during this period, as revealed by the strongest correlation. For the period of study, our results suggest that the impact of precipitation on the snowpack instability lasts for about 6 days.

Description: In this, the second of a pair of papers on the structure of well-sorted natural granular material (sediment), new methods are described for automated measurements from images of sediment, of: 1) particle-size standard deviation (arithmetic sorting) with and without apparent void fraction; and 2) mean particle size in material with void fraction. A variety of simulations of granular material are used for testing purposes, in addition to images of natural sediment. Simulations are also used to establish that the effects on automated particle sizing of grains visible through the interstices of the grains at the very surface of a granular material continue to a depth of approximately 4 grain diameters and that this is independent of mean particle size. Ensemble root-mean squared error between observed and estimated arithmetic sorting coefficients for 262 images of natural silts, sands and gravels (drawn from 8 populations) is 31%, which reduces to 27% if adjusted for bias (slope correction between observed and estimated values). These methods allow non-intrusive and fully automated measurements of surfaces of unconsolidated granular material. With no tunable parameters or empirically derived coefficients, they should be broadly universal in appropriate applications. However, empirical corrections may need to be applied for the most accurate results. Finally, analytical formulas are derived for the one-step pore-particle transition probability matrix, estimated from the image's autocorrelogram, from which void fraction of a section of granular material can be estimated directly. This model gives excellent predictions of bulk void fraction yet imperfect predictions of pore-particle transitions.

Description: Sediment flux from rivers to oceans is the fundamental driver of fluvio-deltaic morphodynamics and continental margin sedimentation, yet sediment transport across the river-to-marine boundary is poorly understood. Coastal rivers typically are affected by backwater, a zone of spatially decelerating flow that is transitional between normal flow upstream and the offshore river plume. Flow deceleration in the backwater zone, as well as spreading of the offshore plume, should render rivers highly depositional near their mouths, leading to sedimentation and eventual elimination of the backwater zone at steady state. This reasoning is counter to observations of riverbed scour, erosional bed forms, and long-lived backwater zones near the mouths of some coastal rivers (e.g., Mississippi River, United States). To explain these observations, we present a quasi-2-D model of a coupled fluvial backwater and offshore river plume system and apply it to the Mississippi River. Results show that during high-discharge events the normal-flow depth can become larger than the water depth at the river mouth resulting in drawdown of the water surface, spatial acceleration of flow, and erosion of the riverbed. As proposed by Lane (1957), the transition to drawdown and erosion is ultimately forced by spreading of the offshore river plume. This points to the need to model coupled river and river plume systems with a dynamic backwater zone under a suite of discharges to accurately capture fluvio-deltaic morphodynamics and connectivity between fluvial sediment sources and marine depositional sinks.

Description: The “mass balance sensitivity” of a glacier provides a means for assessing its response to future warming and contribution to sea level rise. Many studies have concluded that the first-order control on mass balance sensitivity is climatic, where higher-precipitation (and less continental) glaciers are most sensitive while lower-precipitation (and more continental) glaciers are least sensitive. The Southern Alps in New Zealand experience a limited range of continentality (9–13 K) but strong gradients in precipitation (2.5–11 m a−1). Using an energy balance model applied on a regional scale we find that the central Southern Alps glaciers are very sensitive to temperature change (1.9 m w.e. a−1 K−1, with a range of −1.1 to −4.0 m w.e. a−1 K−1) and that an 82% increase in precipitation is required to offset a 1 K warming. Spatial variations in mass balance sensitivity cannot be simply explained as a function of precipitation. Topographic effects are important, and we find that debris cover reduces mass balance sensitivity. Mass balance amplitude, which takes into account debris cover, hypsometry and other topographic characteristics, is a better predictor of mass balance sensitivity than precipitation. The mass balance gradient is almost as good a predictor indicating that hypsometry is not a necessary component of sensitivity calculations. Estimating mass balance sensitivity as a function of mass balance gradient allows for parameterizations of mass balance sensitivity based on glacier inventory data. This provides a simple and robust way to assess glacier mass balance sensitivity on a global scale, which may refine future predictions of valley glacier melt and its contribution to sea level rise.

Description: Greater understanding of variations in firn densification is needed to distinguish between dynamic and melt-driven elevation changes on the Greenland ice sheet. This is especially true in Greenland's percolation zone, where firn density profiles are poorly documented because few ice cores are extracted in regions with surface melt. We used georadar to investigate firn density variations with depth along a ∼70 km transect through a portion of the accumulation area in western Greenland that partially melts. We estimated electromagnetic wave velocity by inverting reflection traveltimes picked from common midpoint gathers. We followed a procedure designed to find the simplest velocity versus depth model that describes the data within estimated uncertainty. On the basis of the velocities, we estimated 13 depth-density profiles of the upper 80 m using a petrophysical model based on the complex refractive index method equation. At the highest elevation site, our density profile is consistent with nearby core data acquired in the same year. Our profiles at the six highest elevation sites match an empirically based densification model for dry firn, indicating relatively minor amounts of water infiltration and densification by melt and refreeze in this higher region of the percolation zone. At the four lowest elevation sites our profiles reach ice densities at substantially shallower depths, implying considerable meltwater infiltration and ice layer development in this lower region of the percolation zone. The separation between these two regions is 8 km and spans 60 m of elevation, which suggests that the balance between dry-firn and melt-induced densification processes is sensitive to minor changes in melt.

Description: This study examines sensitivity of the layer-averaged equations of motion for turbidity currents to changes in the parameters that describe how mass, momentum, and turbulent kinetic energy (TKE) are transferred between the flow and its surroundings in a channelized environment. This analysis shows that one-dimensional flows traversing a constant slope are sensitive to small changes in the sediment and clear-water entrainment parameters that describe mass transfer relationships. Uncertainties within these proposed relationships were quantified by applying a Bayesian sampler to available laboratory data. Sampled entrainment parameter values show a strong preference to values differing from those previously determined. The uncertainties within the entrainment relationships were then propagated into the dynamics of a reference flow (described by flow velocity, sediment flux, and height) to assess the degree to which uncertainty within the mass transfer functions affects predictions of flow behavior down-slope. Given the range of sampled values, flow parameters were found difficult to predict beyond 200–500 meters. The entrainment relationships largely control the spatial and temporal patterns of erosion and deposition and thus the channel architectures. A wide range of entrainment parameters might be necessary to accurately predict the range of plausible flow dynamics and resulting channel morphologies.

Description: The late Neogene–Quaternary exhumation history of the European Alps is the subject of controversial findings and interpretations, with several thermochronological studies arguing for long-term steady state exhumation rates, while others have pointed to late Miocene–Pliocene exhumation pulses associated with tectonic and/or climatic changes. Here, we perform inverse thermal-kinematic modeling on dense thermochronological data sets combining apatite fission track (AFT) data from the literature and recently published apatite (U-Th-Sm)/He (AHe) data along the upper Rhône valley (Aar and Aiguilles Rouges massifs, Swiss Alps) in order to derive precise estimates on the denudation and relief history of this region. We then apply forward numerical modeling to interpret cooling paths quantified from apatite 4He/3He thermochronometry, in terms of denudation and relief-development scenarios. Our modeling results highlight the respective benefits of using AFT/AHe thermochronology data and 4He/3He thermochronometry for extracting quantitative denudation and relief information. Modeling results suggest a late Miocene exhumation pulse lasting until ∼8–10 Ma, consistent with recently proposed exhumation histories for other parts of the European Alps, followed by moderate (∼0.3–0.5 km Myr−1) denudation rates during the late Miocene/Pliocene. Both inverse modeling and 4He/3He data reveal that the late stage exhumation of the studied massifs can be explained by a significant increase (∼85–100%) in local topographic relief through efficient glacial valley carving. Modeling results quantitatively constrain Rhône valley carving to 1–1.5 km since ∼1 Ma. We postulate that recent relief development within this part of the Swiss Alps is climatically driven by the onset of major Alpine glaciations at the mid-Pleistocene climate transition.

Description: We describe the impact of three simultaneous earthquake-triggered rock avalanches on the dynamics of Black Rapids Glacier, Alaska, by using spaceborne radar imagery and numerical modeling. We determined the velocities of the glacier before and after landslide deposition in 2002 by using a combination of ERS-1/ERS-2 tandem, RADARSAT-1, and ALOS PALSAR synthetic aperture radar data. Ice velocity above the debris-covered area of the glacier increased up to 14% after the earthquake but then decreased 20% by 2005. Within the area of the debris sheets, mean glacier surface velocity increased 44% within 2 years of the landslides. At the downglacier end of the lowest landslide, where strong differential ablation produced a steep ice cliff, velocities increased by 109% over the same period. By 2007, ice velocity throughout the debris area had become more uniform, consistent with a constant ice flux resulting from drastically reduced ablation at the base of the debris. Without further analysis, we cannot prove that these changes resulted from the landslides, because Black Rapids Glacier displays large seasonal and interannual variations in velocity. However, a full Stokes numerical ice flow model of a simplified glacier geometry produced a reversal of the velocity gradient from compressional to extensional flow after 5 years, which supports our interpretation that the recent changes in the velocity field of the glacier are related to landslide-induced mass balance changes.

Description: We present laboratory experiments designed to quantify the stability and energy budget of buoyancy-driven iceberg capsize. Box-shaped icebergs were constructed out of low-density plastic, hydrostatically placed in an acrylic water tank containing freshwater of uniform density, and allowed (or forced, if necessary) to capsize. The maximum kinetic energy (translational plus rotational) of the icebergs was ∼15% of the total energy released during capsize, and radiated surface wave energy was ∼1% of the total energy released. The remaining energy was directly transferred into the water via hydrodynamic coupling, viscous drag, and turbulence. The dependence of iceberg capsize instability on iceberg aspect ratio implied by the tank experiments was found to closely agree with analytical predictions based on a simple, hydrostatic treatment of iceberg capsize. This analytical treatment, along with the high Reynolds numbers for the experiments (and considerably higher values for capsizing icebergs in nature), indicates that turbulence is an important mechanism of energy dissipation during iceberg capsize and can contribute a potentially important source of mixing in the stratified ocean proximal to marine ice margins.

Description: The measurement of temporal changes in active layer thickness (ALT) is crucial to monitoring permafrost degradation in the Arctic. We develop a retrieval algorithm to estimate long-term average ALT using thaw-season surface subsidence derived from spaceborne interferometric synthetic aperture radar (InSAR) measurements. Our algorithm uses a model of vertical distribution of water content within the active layer accounting for soil texture, organic matter, and moisture. We determine the 1992–2000 average ALT for an 80 × 100 km study area of continuous permafrost on the North Slope of Alaska near Prudhoe Bay. We obtain an ALT of 30–50 cm over moist tundra areas, and a larger ALT of 50–80 cm over wet tundra areas. Our estimated ALT values match in situ measurements at Circumpolar Active Layer Monitoring (CALM) sites within uncertainties. Our results demonstrate that InSAR can provide ALT estimates over large areas at high spatial resolution.

Description: The 16 October 1979 landslide at Nice airport, France, produced tsunami waves observed in several harbors, and the city of Antibes was significantly flooded. A new simulation of this event is presented in this paper to assess the sensitivity of the tsunami to the properties of the slide and to evaluate how local bathymetry and harbor resonance influence the effects of the tsunami at the coast. A study of slide viscosity and water incorporation is performed using a detailed numerical model with a single initial volume of about 107 m3. The propagation of the tsunami is then simulated with a multigrid finite difference code, using high-resolution bathymetry in the harbors, to a greater level of detail than previous studies. We show that while the slide parameters are of great importance to properly model the later stages of the slide, the initial volume and early dynamics of the slide are the relevant factors for determining the characteristics of the tsunami. Numerical results are compared with tide recordings in Nice and Villefranche, as well as a mapping of the flood. Computed maximum elevation maps are compared with witness reports, highlighting a focusing of the waves on Antibes, which explains its vulnerability. This kind of focusing pattern should be considered when assessing the tsunami hazard for coastal regions. Calculated resonant periods in Nice, Villefranche, Port Vauban, and La Salis are matched with components present in both observed and computed waveforms. Most of the observed waves in Nice and Villefranche can be attributed to resonance.

Description: Poorly understood processes controlling retention of meltwater in snow and firn have important implications for Greenland Ice Sheet's mass balance and flow dynamics. Here we present results from a 3 year (2007–2009) field campaign studying firn thermal profiles and density structure along an 85 km transect of the percolation zone of west Greenland. We installed one or two thermistor strings at 14 study sites, each string having 32 sensors spaced between 0 and 10 m depth. Data from our network of over 500 sensors were collected at 15–60 min intervals for 1–2 years, thereby recording the thermal signature of meltwater infiltration and refreezing during annual melt cycles. We document three types of heating of firn related to different mechanisms of meltwater motion and freezing, including heterogeneous breakthrough events, wetting front advance, and year-round heating from freezing of residual deep pore water. Vertically infiltrating meltwater commonly penetrates through cold firn accumulated over decades, even where ice layers are present at the previous summer surface and where ice layer thickness exceeds several decimeters. The offset between the mean annual air temperature and the 10 m firn temperature reveals the elevation dependency of meltwater retention along our transect. The firn is 〉 10°C warmer than the mean annual air temperature at the region where meltwater runoff initiates. During 2007–2009, runoff was limited to elevations lower than ∼1500 m with no sharp “runoff limit”; rather, the ratio of retention to runoff transitioned from all retention to all runoff across a ∼20 km wide zone.

Description: We investigate the role of landslide dams, spatial changes in lithology, and rock uplift on faults in the formation of knickzones on bedrock rivers. We focus our analysis in the southwestern Annapurna Range of the central Nepalese Himalaya where detailed geologic maps, topographic data, field observations, and aerial photographs are available. We identified knickzones in our study area from normalized river steepness indices (ksn values) extracted from river longitudinal profiles derived from a 25 m digital elevation model we interpolated from digitized topographic map contours. We compared the location of these knickzones with (1) lithologic contacts and faults from a detailed geologic map of the Modi Khola valley and (2) inferred ancient landslide dam features mapped from field observations and aerial photographs. The steepest location on the Modi Khola occurs near the same latitude as the steepest reach on the Mardi Khola located directly to the east, potentially highlighting a major topographic transition across the Annapurna. However, we find that landslide dams once blocked the flow of the Modi Khola, and damming followed by incision after landslide breaching can explain the location of these knickzones without the need for active faulting near the Main Central thrust. We also conclude that (1) knickzones do not correlate with any spatial changes in lithology and (2) knickzones generated by rock uplift on unmapped faults cannot be ruled out. We emphasize that disentangling the processes responsible for knickzone formation remains challenging even when high-resolution geologic and topographic data are available.

Description: We investigate the stability of marine ice sheets by coupling a gravitationally self-consistent sea level model valid for a self-gravitating, viscoelastically deforming Earth to a 1-D marine ice sheet-shelf model. The evolution of the coupled model is explored for a suite of simulations in which we vary the bed slope and the forcing that initiates retreat. We find that the sea level fall at the grounding line associated with a retreating ice sheet acts to slow the retreat; in simulations with shallow reversed bed slopes and/or small external forcing, the drop in sea level can be sufficient to halt the retreat. The rate of sea level change at the grounding line has an elastic component due to ongoing changes in ice sheet geometry, and a viscous component due to past ice and ocean load changes. When the ice sheet model is forced from steady state, on short timescales (

Description: This study examines bed load transport processes in a small gravel-bed river (Béard Creek, Québec) using three complementary methods: bed elevation changes between successive floods, bed activity surveys using tags inserted into the bed, and bed load transport rates from bed load traps. The analysis of 20 flood events capable of mobilizing bed material led to the identification of divergent results among the methods. In particular, bed elevation changes were not consistent with the bed activity surveys. In many cases, bed elevation changes were significant (1 to 2 times the D50) even if the bed surface had not been activated during the flood, leading to the identification of processes of bed dilation and contraction that occurred over 10% to 40% of the bed surface. These dynamics of the river bed prevent accurate derivation of bed load transport rates from topographic changes, especially for low magnitude floods. This paper discusses the mechanisms that could explain the dilation and contraction of particles within the bed and their implications in fluvial dynamics. Bed contraction seems to be the result of the winnowing of the fine sediments under very low gravel transport. Bed dilation seems to occur on patches of the bed at the threshold of motion where various processes such as fine sediment infiltration lead to the maintenance of a larger sediment framework volume. Both processes are also influenced by flood history and the initial local bed state and in turn may have a significant impact on sediment transport and morphological changes in gravel-bed rivers.

Description: The presence of sediment cover in bedrock rivers inhibits saltation-driven incision, and so accurate predictions of the relationship between bedrock exposure (Fe) and relative sediment flux (sediment supply rate over capacity sediment transport rate, Qs/Qt) are necessary to model incision and hence landscape evolution. Theoretical predictions of a linear or negative exponential form for this relationship are not consistent with laboratory data that instead demonstrate a range of different relationships. Here we use a cellular automaton (CA) model to establish how the relationship between Fe and Qs/Qt evolves from the dynamics of, and interactions between, individual sediment grains moving through a bedrock channel. The key model parameter is the probability of grain entrainment, which is altered as a function of the number of neighboring grains in order to reproduce the enhanced mobility of isolated grains on bedrock surfaces. For each model run, an equilibrium sediment cover is attained for a specified sediment input, enabling the relationship between Fe and Qs/Qt to be established. As well as both linear and exponential relationships, model runs reproduce other relationships observed in laboratory experiments. These other relationships require isolated grains to be more easily entrained than grains in sediment clusters, which is consistent with field observations of grain mobility. There is therefore a continuum of relationships between Fe and Qs/Qt; the relationship that is most applicable to a particular reach will depend on the role of channel slope, roughness and shear stress in controlling the entrainment of grains from bedrock and alluvial surfaces.

Description: How terrain, snow cover properties, and release conditions combine to determine avalanche speed and runout remains the central problem in avalanche science. Here we report on efforts to understand how surface roughness, snow properties, and internal mass fluxes control the generation of granular fluctuation energy within the basal shear layers of dense flowing snow avalanches, and the subsequent influence on avalanche speed and deposition patterns. For this purpose we augment the depth-averaged equations of motion to account for the generation of the kinetic energy associated with the particle fluctuations, and dissipation of this energy by collisional and frictional material interactions. Using high-resolution laser scans of the preevent snow cover and postevent deposits from two avalanches released at the Swiss Vallée de la Sionne observation station, we compare measured and calculated deposition heights. The model captures flow velocities and deposition heights without ad hoc adjustments of the constitutive parameters according to avalanche size. The model parameters are separated into a terrain and other pure material (snow) parameters. The investigations reveal how release conditions and snow entrainment influence the internal mass distribution, and control flow regime transitions between the fluidized regime (head) and plug regime (tail). The comparison between the measured and calculated velocities and deposition heights demonstrates why standard Voellmy-type submodels are suitable for many practical mitigation problems, but also why they are limited to cases where the calibrated parameters, already accounting for terrain, snow cover properties, avalanche size, and mass intake, are known.

Description: Particles transported as bed load within a specified streambed area possess at any instant a distribution of velocities. This distribution figures prominently in describing the rates of transport and dispersal of particles. High-speed imaging of sand particles transported as bed load over a planar bed reveals that the probability density functions of the streamwise and cross-stream particle velocities are exponential-like. For quasi-steady conditions the exponential-like density of streamwise velocities reflects a balance among three fluxes in momentum space: (1) an advection of streamwise momentum whose magnitude and sign vary with the momentum state; (2) a diffusion of momentum from higher to lower values of momentum density; and (3) a drift of momentum from regions in momentum space having high average rates of generation of kinetic energy toward regions having low rates of generation of kinetic energy. The probability density of cross-stream velocities similarly reflects a balance of fluxes of cross-stream momentum. Whereas the average net force acting on particles is zero under steady conditions, the mean, variance and asymmetry of the distribution of forces acting on particles vary with the momentum state of the particles. Numerical simulations of particle motions that are faithful to these statistical properties reproduce key empirical results, namely, the exponential-like velocity distribution and the nonlinear relation between hop distances and travel times. The simulations also illustrate how steady gradients in particle activity, the solid volume of particles in motion per unit streambed area, induce a diffusive flux as described in companion papers.