ALBERT

Description: No abstract is available for this article.

Description: Aeolian-derived soils are found throughout the world. Soil evolution processes in aeolian-dominated landscapes differ from processes in bedrock-weathering landscapes by a number of key aspects including the lack of: (1) soil-production depth-dependency; (2) surface armoring; and (3) grain size self-organization in the soil profile. We use here a soil evolution model (mARM5D) to study the differences between aeolian and bedrock-weathering dominated landscapes by analyzing soil evolution on a hillslope under various aeolian and bedrock soil supply settings subject to fluvial and diffusive sediment transport. The model simulates spatial and temporal variation in soil particle size distribution (PSD) and profile depth for each grid-cell on the landscape, as a function of physical weathering, aeolian deposition, and diffusive and fluvial sediment transport. Our results indicate that surface armoring plays a major role in soil evolution. Under bedrock-weathering dominated conditions, armoring reduces soil erosion and in conjunction with depth-dependent soil production, leads to steady-state soil grading and depth and a relatively uniform soil distribution. In contrast, aeolian-dominated landscapes tend to have considerable spatial variability in soil depth and PSD. Our results also indicate that in contrast with diffusive transport, which is assumed to be PSD-independent, fluvial sediment transport is strongly influenced by the soil-production mechanism (aeolian or bedrock-weathering). Based on the results presented here we propose that aeolian-dominated landscapes are more responsive to environmental changes (e.g. climatic and anthropogenic) compared with bedrock-weathering landscapes. We further propose that this sensitivity may help explain the patchy soil distribution that is often observed in aeolian-dominated regions.

Description: Supply and transport of sediment in catchments involve processes with fundamental consequences for river management, land use and the prediction of climate-driven sediment fluxes. In the present study we addressed spatial variability in the water routes through the surface and subsurface of a catchment and the suspended sediment discharge ( Q s ) over a mountain–piedmont system. We analyzed daily suspended sediment concentration ( C s ) and water discharge ( Q ) measurements at stations located in different topographic settings (mountain and piedmont) in the Biobío River basin (southern-central Andes, 37-39°S). In steep catchments, the Q vs. Q s relationship has a marked seasonal hysteresis. In the piedmont, Q s is proportional to Q , with no seasonal hysteresis. The contrast in the hysteresis pattern between catchments with different topographies is explained by differences in the routing of rainfall-derived water. In the piedmont, most of the rainfall is converted into surface runoff because the water table is near the surface. In the mountains, groundwater storage results in large seasonal variations in the proportion of Q that flows at the surface and transports sediment from the hillslopes, producing hysteresis. By separating the total Q into two components (direct discharge, Q d and baseflow, Q b ), we observed the response of Q s to the fraction of water that quickly leaves the catchment after a rainfall event ( Q d ). Similar results between the mountain and piedmont and the absence of hysteresis simplify the behavior of Q s into a linear relationship with Q d over the entire catchment and lead usto propose that sediment mobilization to the river along the Biobío catchment is primarily controlled by overland flow. Our findings highlight the importance of an adequate hydrological model for understanding the erosion and transport processes of a catchment, and which can be applied to other natural and modeled mountain–piedmont systems.

Description: [1]  Measurements of the loss or gain of ice mass from large ice sheets is presently achieved through satellite-based techniques such as GRACE (Gravity Recovery and Climate Experiment). The accuracy of these satellite-based measurements to changes in modern ice sheet mass depends on our knowledge of present day glacio-isostatic crustal uplift rates caused by past ice sheet changes. To improve models of glacio-isostatic rebound in East Antarctica we investigated ice histories along Rayner Glacier, Enderby Land, and a little explored sector of the ice sheet where GRACE data had suggested significant mass gain during the last decade. [2]  Observations from a recent glacial geomorphic reconnaissance coupled with cosmogenic nuclide dating indicate that in the lower part of the Rayner Glacier, Enderby Land, ice heights lowered by at least 300 m, and the calving margin retreated by at least 10 km in the early Holocene (~6 to 9 ka BP). The magnitude and timing of deglaciation is consistent with ice histories used to model the post-glacial rebound corrections for present-day GRACE mass trends. These observations strengthen the body of evidence that suggests ice mass gain in Enderby Land is presently partly offsetting mass loss in other parts of Antarctica.

Description: [1]  The tidally modulated, stick–slip events of Whillans Ice Stream in West Antarctica produce seismic energy from three locations near the grounding line. Using ice velocity records obtained by combining time series from co-located broadband seismometers and GPS receivers installed on the ice stream during the 2010–2011 and 2011–2012 austral summers, along with far-field seismic recordings of elastic waves, we locate regions of high rupture velocity and stress drop. These regions, which are analogous to “asperities” in traditional seismic fault studies, are areas of elevated friction at the base of the ice stream. Slip events consistently initiate at one of two locations: near the center of the ice stream, where events associated with the Ross Sea high tide originate, or a grounding-line spot, where events associated with the Ross Sea low tide initiate, as well as occasional high-tide events following a skipped low-tide event. The grounding-line site, but not the central site, produces Rayleigh waves observable up to 1000 km away, through fast expansion of the slip area. Grounding-line initiation events also show strong directivity in the downstream direction, indicating initial rupture propagation at 1.5 km/s, compared to an average of 0.150 km/s for the entire slip event. Following slip initiation, additional seismic energy is produced from two sources located near the grounding line: firstly, at the downstream end of Subglacial Lake Engelhardt and secondly, toward the farthest downstream extent of the ice stream. This evidence suggests that the stronger, higher friction material along the grounding line controls motion throughout the stick–slip region.

Description: [1]  We examine the relationship of channel steepness to incision rate from channels eroding into a previously tilted, planar, and progressively exhumed unconformity surface cut across erosionally resistant limestone bedrock. Key to this analysis is the calibrated substitution of down-slope and downstream distance for time of exposure of resistant rocks. Channel and unconformity slopes are measured from a suite of channels developed on resistant Paleozoic limestone exhumed by the removal of Cenozoic sediments from the Baybeiche Range bordering the Naryn basin in the western Tian Shan. The compiled data set, sampling five orders-of-magnitude of upstream drainage area (0.03 to 227 km 2 ), is used to derive the exponent, n , relating channel steepness to channel incision rate, and the ratio, K / V of the rate constant for channel incision of the resistant substrate, K , to the erosion rate, V , of the cover strata. We show that for a typical value of intrinsic concavity (slope-area exponent, θ  = 0.5), erosion rates that are proportional to specific stream power ( n  = 1) satisfy the data set. However, valley-width data suggest that the intrinsic concavity is higher ( θ  = 0.8) and that the channel-incision data can also be fit if erosion is proportional to basal shear stress ( n  = 2/3). Our results do not support values of n significantly greater than one. Using 36 Cl exposure age-dating of the unconformity surface, we independently demonstrate that theCenozoic cover strata have been progressively stripped downward from the unconformity surface at a vertical rate of 1 to 2 m/kyr. Using V  = 1 m/kyr, we constrain the rate constant, K , to between 6 ± 1 and 9 ± 2 × 10 − 4 kyr − 1 for incision of resistant limestone bedrock in this field setting.

Description: [1]  Low-lying barrier islands are ubiquitous features of the world's coastlines and the processes responsible for their formation, maintenance, and destruction are related to the evolution of smaller, superimposed features including sand dunes, beach berms, and sandbars. To varying degrees, the barrier island and its superimposed features interact with oceanographic forces (e.g., overwash) and exchange sediment with each other and other parts of the barrier island system. And these interactions are modulated by changes in storminess associated with changes in our climate. An opportunity to study these interactions resulted from the placement and subsequent evolution of a 2-m high sand berm constructed between June, 2010 and April, 2011 along the northern Chandeleur Islands, LA. The berm was monitored using satellite and aerial remote sensing and topographic and bathymetric surveys. Over the study period, from November 2010 through September 2011, the central portion of the berm was altered and ultimately destroyed by the passage of winter and tropical storms. We show that berm-length evolution is well predicted using a statistical model that was fit to the observations by estimating two parameters describing the rate of berm-length change. The model considers wave-driven runup, tides, and storm surge to evaluate the probability and duration of berm overwash. Specifically, computed overwash probabilities predict episodic berm erosion associated with major storm events when overwash is likely to occur. The episodic erosion is superimposed on a constant berm-length change rate that persists even when there is no overwash. Using the calibrated model, the analysis is extended to a 16-year time series of storm climatologies that includes both intra- and inter-annual variability of overwash events and potential variations in berm-length erosion. For a 2-m high feature on the Chandeleur Islands, overwash is expected to occur, on average, 4 days (96 hours) every year. Variability in interannual storminess produces as many as 10 or as few as 1 days of overwash conditions per year. The dependence of overwash frequency on feature elevation (e.g., the height of a berm, dune, or other superimposed feature) indicated that an increase in feature elevation from 2 m to 3.5 m above mean sea level would reduce the expected frequency of overwash events from 4 to 0.5 event-days per year. This approach can be applied to understanding barrier island and berm evolution at other locations using either (or both) past and future climates based on readily available observational or modeled data.

Description: [1]  To permit the tracking of turbulent flow structures in an Eulerian frame from single-point measurements, we make use of a generalization of conventional two-dimensional quadrant analysis to three-dimensional octants. We characterize flow structures using the sequences of these octants and show how significance may be attached to particular sequences using statistical mull models. We analyse an example experiment and show how a particular dominant flow structure can be identified from the conditional probability of octant sequences. The frequency of this structure corresponds to the dominant peak in the velocity spectra and exerts a high proportion of the total shear stress. We link this structure explicitly to the propensity for sediment entrainment, and show that greater insight into sediment entrainment can be obtained by disaggregating those octants that occur within the identified macro-turbulence structure from those that do not. Hence, this work goes beyond critiques of Reynolds stress approaches to bed-load entrainment that highlight the importance of outward interactions, to identifying and prioritizing the quadrants/octants that define particular flow structures.

Description: We present an inverse modelling approach to reconstruct annual accumulation patterns from ground-penetrating radar (GPR) data. A coupled surface energy balance - snow model simulates surface melt and the evolution of subsurface density, temperature and water content. The inverse problem consists of iteratively calibrating accumulation, serving as input for the model, by finding a match between modelled and observed radar travel times. The inverse method is applied to a 16-km GPR transect on Nordenskiöldbreen, Svalbard, yielding annual accumulation patterns for 2007–2012. Accumulation patterns with a mean of 0.75 m w.e. a −1 contain substantial spatial variability, with a mean annual standard deviation of 0.17 m w.e. a −1 , and show only partial consistency from year to year. In contrast to traditional methods, accounting for melt water percolation, refreezing and runoff facilitates accurate accumulation reconstruction in areas with substantial melt. Additionally, accounting for horizontal density variability along the transect is shown to reduce spatial variability in reconstructed accumulation, whereas incorporating irreducible water storage lowers accumulation estimates. Correlating accumulation to terrain characteristics in the dominant wind direction indicates a strong preference of snow deposition on leeward slopes, whereas weaker correlations are found with terrain curvature. Sensitivity experiments reveal a non-linear response of the mass balance to accumulation changes. The related negative impact of small-scale accumulation variability on the mean net mass balance is quantified, yielding a negligible impact in the accumulation zone and a negative impact of −0.09 m w.e. a −1 in the ablation area.

Description: Flow of glacial ice in the West Antarctic Sheet localizes in narrow bands of fast flowing ice streams bordered by ridges of nearly stagnant ice, but our understanding of the physical processes that generate this morphology is incomplete. Here, we study the thermal and mechanical properties of ice-stream margins, where flow transitions from rapid to stagnant over a few kilometers. Our goal is to explore under which conditions the intense shear deformation in the margin may lead to deformation-induced melting. We propose a 2D model that represents a cross-section through the ice-stream margin perpendicular to the downstream flow direction. We limit temperature to the melting point to estimate melt rates based on latent heat. Using rheology parameters as constrained by laboratory data and observations, we conclude that a zone of temperate ice is likely to form in active shear margins.

Description: The morphological evolution of shallow tidal systems strongly depends on gradients in transport that control sediment erosion and deposition. A spatially-refined quantitative description of suspended sediment patterns and dynamics is therefore a key requirement to address issues connected with dynamical trends, responses, and conservation of these systems. Here we use a combination of numerical models of sediment transport dynamics, high temporal resolution point observations, and high spatial resolution remote sensing data to overcome the intrinsic limitations of traditional monitoring approaches and to establish the robustness of numerical models in reproducing space-time suspended sediment concentration (SSC) patterns. The comparison of SSC distributions in the Venice Lagoon (Italy) computed with a numerical model with SSC retrievals from remote sensing data, allows us to define the ability of the model to properly describe spatial patterns and gradients in the SSC fields. The use of point observations similarly allows us to constrain the model temporally thus leading to a complete space-time evaluation of model abilities. Our results highlight the fundamental control exerted on sediment transport intensity and patterns by the sheltering effect associated with artificial and natural intertidal landforms. Furthermore, we show how the stabilizing effect of benthic vegetation is a main control of sediment dynamics at the system scale, confirming a notion previously established in the laboratory or at small field scales.

Description: We explore the behavior of a theoretical model of cross-shore headland relief caused by alongshore differences in lithology and rock strength on rocky coastlines. Results address the question of why some rocky coasts exhibit frequent headland and embayment sequences while others evolve to a flat, smooth, and sandy configuration. Main model predictions are that cross-shore headland amplitude is inversely proportional to beach sediment supply and the strength of wave energy convergence and divergence along the headland and bay, and proportional to the alongshore embayment length (or distance between headlands) and the difference between headland and bay rock strength. The coastline's initial physical properties (sea cliff height, composition, etc.) largely determine whether headlands will be persistent or transient landscape features. Model time scales over which the headland and bay reach steady-state amplitude, or disappear to a flat coastline, range from 120 to 175,000 years depending on how close the initial amplitude is to steady-state. In many cases, the coastline must evolve over several sea level highstands in order to reach equilibrium. A characteristic time scale (independent of initial conditions) shows that the coastline evolves most rapidly when: wave focusing is stronger; sea cliff rock is weaker or retreats faster in a given wave climate; the sea cliff retreat rate decreases rapidly as a function of beach width (i.e., the beach is very effective at dampening wave energy); and the coastline is sediment-rich. Comparisons to nature suggest that our model is qualitatively capturing general rocky coastline dynamics and that modeled headland amplitudes are consistent with observed amplitudes.

Description: We have developed an exploratory model of plan-view, millennial-scale headland and bay evolution on rocky coastlines. Cross-shore coastline relief, or amplitude, arises from alongshore differences in sea cliff lithology, where durable, erosion-resistant rocks protrude seaward as headlands and weaker rocks retreat landward as bays. The model is built around two concurrent negative feedbacks that control headland amplitude: 1) wave energy convergence and divergence at headlands and bays, respectively, that increases in intensity as cross-shore amplitude grows, and 2) the combined processes of beach sediment production by sea cliff erosion, distribution of sediment to bays by waves, and beach accumulation that buffers sea cliffs from wave attack and limits further sea cliff retreat. Paired with the coastline relief model is a numerical wave transformation model that explores how wave energy is distributed along an embayed coastline. The two models are linked through genetic programming, a machine learning technique that parses wave model results into a tractable input for the coastline model. Using a pool of 4,800 wave model simulations, genetic programming yields a function that relates breaking wave power density to cross-shore headland amplitude, offshore wave height, approach angle, and period. The goal of the coastline model is to make simple, but fundamental, scaling arguments on how different variables (such as sea cliff height and composition) affect the equilibrium cross-shore relief of headland and bays. The model's generality highlights the key feedbacks involved in coastline evolution and allows its equations (and model behaviors) to be easily modified by future users.

Description: We construct a simple morphodynamic model to investigate the long-term dynamic evolution of a coastal barrier system experiencing sea-level rise. Using a simplified barrier geometry, the model includes a dynamic shoreface profile that can be out of equilibrium and explicitly treats barrier sediment overwash as a flux. With barrier behavior primarily controlled by the maximum potential overwash flux and the rate of shoreface response, the modeled barrier system demonstrates four primary behaviors: height drowning, width drowning, constant landward retreat, and a periodic retreat. Height drowning occurs when overwash fluxes are insufficient to maintain the landward migration rate required to keep pace with sea-level rise. On the other hand, width drowning occurs when the shoreface response rate is insufficient to maintain the barrier geometry during overwash-driven landward migration. During periodic barrier retreat, the barrier experiences oscillating periods of rapid overwash followed by periods of relative stability as the shoreface resteepens. This periodic retreat, which occurs even with a constant sea-level rise rate, arises when overwash rates and shoreface response rates are large and of similar magnitude. We explore the occurrence of these behaviors across a wide range of parameter values and find that, in addition to the maximum overwash flux and the shoreface response rate, barrier response can be particularly sensitive to the sea-level rise rate and back-barrier lagoon slope. Overall, our findings contrast with previous research which has primarily associated complex barrier behavior with changes in external forcing such as sea-level rise rate, sediment supply, or backbarrier geometry.

Description: Motivated by convex-concave bedrock river profiles developed across a climate gradient on the wet side of the Kohala Peninsula of the Big Island of Hawai'i, we numerically model how rainfall gradients may influence longitudinal fluvial incision patterns. First, we model transient profile adjustment with two tectonic boundary conditions: subsidence and uplift. In this generalized analysis, we assume that rainfall gradients only influence incision by modifying the relation between upstream drainage area and local discharge. Using a detachment-limited model, downstream increases in rainfall lead to profile convexities during transient adjustment in both tectonic settings, and this is the opposite of the predicted increase in profile concavity that would develop in a steady-state uplifting profile. A transport-limited erosion model develops only concave channel profiles without clear signatures of the rainfall pattern. Second, we model development of convex-concave transient profiles and incision patterns on Kohala using a detachment-limited model. If rainfall gradients only influence incision through the local discharge, reasonable rainfall gradients can only develop channel convexities that are much smaller than those observed. Instead, we hypothesize that local bedrock erodibility increases with the degree of rainfall-dependent chemical weathering. When local erodibility is assumed to scale with local rainfall rate, the model can produce convex-concave profiles similar to those observed in Kohala. Our results suggest that changes in local bedrock erodibility due to local climate-dependent weathering may be an important mechanism by which climate influences landscape form and rates of evolution. This hypothesis requires further testing in this study area and beyond.

Description: Condit Dam on the White Salmon River, Washington, a 38-m-high dam impounding a large volume (1.8 million m 3 ) of fine-grained sediment (60% sand, 35% silt and clay, 5% gravel), was rapidly breached in October 2011. This unique dam decommissioning produced dramatic upstream and downstream geomorphic responses in the hours and weeks following breaching. Blasting a 5-m-wide hole into the base of the dam resulted in rapid reservoir drawdown, abruptly releasing ~1.6 million m 3 of reservoir water, exposing reservoir sediment to erosion, and triggering mass failures of the thickly accumulated reservoir sediment. Within 90 minutes of breaching, the reservoir's water and ~10% of its sediment had evacuated. At a gaging station 2.3 km downstream, flow increased briefly by 400 m 3  s -1 during passage of the initial pulse of released reservoir water, followed by a highly concentrated flow phase —up to 32% sediment by volume—as landslide-generated slurries from the reservoir moved downstream. This hyperconcentrated flow, analogous to those following volcanic eruptions or large landslides, draped the downstream river with predominantly fine sand. During the ensuing weeks, suspended-sediment concentration declined and sand and gravel bedload derived from continued reservoir erosion aggraded the channel by 〉 1 m at the gaging station, after which the river incised back to near its initial elevation at this site. Within 15 weeks after breaching over 1 million m 3 of suspended load is estimated to have passed the gaging station, consistent with estimates that 〉 60% of the reservoir's sediment had eroded. This dam removal highlights the influence of interactions among reservoir erosion processes, sediment composition, and style of decommissioning on rate of reservoir erosion and consequent downstream behavior of released sediment.

Description: Sediment supply from hillslopes to channels is an important control on basin functioning and evolution. However, current theoretical frameworks do not adequately consider processes of runoff-driven hillslope sediment supply, which affect river channels spatially and temporally. Mountainous dryland basins exhibit an important manifestation of these processes because their debris-mantled hillslopes produce coarse sediment and because rainfall is delivered as infrequent, high intensity, short duration rainstorms. This paper combines field measurements and modeling to explore runoff-driven coarse sediment supply from hillslopes to the channel, and assesses a range of plausible storms on the longitudinal patterns of sediment load and its caliber over a dryland basin reach. Our results show that modeled sediment load and its grain-size distribution are determined by the nonlinear interaction between rainfall characteristics and hillslope attributes, resulting in longitudinal fluctuations in sediment supply, the relative magnitude and location of which varies between storms. Results suggest that long hillslopes are most sensitive to rainfall and they exhibit large variations in supplied sediment load and grain size for different runoff characteristics. Short and steep hillslopes are less sensitive to rainfall variations as gradient effects dominate over the role of length in modulating runoff accumulation. Furthermore, the signal of the median fraction ( D 50 ) of modeled sediment supplied by the hillslope is preserved in the coarse fraction of the measured in-channel grain sizes ( D 90 ). Finally, we propose a simple index, which provides new insights into the effectiveness of different rainstorms in terms of the impact of hillslope sediment supply on the channel.

Description: Sediment transfer from rivers to the ocean is the fundamental driver of continental sedimentation with implications for carbon burial, land use dynamics, and unraveling global climate change and Earth history from sedimentary strata. Coastal rivers are dynamically coupled to their offshore plumes at the river mouth creating regions of non-uniform flow that can dictate patterns of erosion and deposition both onshore and offshore. However, there are limited experimental and modeling studies on sediment transport and morphodynamics of coupled river and river-plume systems and their response to multiple flood events. To address this knowledge gap, we developed a quasi-2D, morphodynamic numerical model and conducted exploratory flume experiments in a 7.5-m long flume where a 10-cm wide river channel was connected to a 76-cm wide “ocean basin.” Both the numerical model and the flume results demonstrate that (1) during low-discharge flows, backwater hydrodynamics cause spatial flow deceleration and deposition in the river channel and the offshore plume area, and (2) during high flows the water surface is drawn down to sea level, resulting in spatial flow acceleration and bed scour. During high-discharge flows, we also found that the offshore river plume self-channelized owing to both levee formation and bed scour. Our study suggests that coastal rivers may be in a perpetual state of morphodynamic adjustment and highlights the need to link rivers and river plumes under a suite of flow discharges to accurately predict fluvio-deltaic morphodynamics and connectivity between fluvial sediment sources and marine sinks.

Description: In the North American low arctic, increased retrogressive thaw slump frequency and headwall retreat rates have been linked with climate warming trends since the mid-20th century, but specific weather drivers of slump initiation timing are less clear. We examined relationships among retrogressive thaw slump initiation and annual air temperature, precipitation and snow cover using time series of satellite imagery and weather station data in northwest Alaska. Synthetic aperture RADAR and optical imagery were used to examine retrogressive thaw slump initiation between 1997 and 2010. Over 80% of the slump features examined in this study first appear within a 13 month span from late June 2004 to July 2005. Remote weather station data show that 2004 and 2005 are among several years exhibiting above average thawing indices and average summer temperatures between 1992 and 2011. However, 2004 is distinct from the rest of the record, with unusually warm temperatures primarily occurring early in the thaw season between April and early June, and including two intense precipitation events in May. Regional weather reported by the NOAA National Weather Service also reflects these local findings. Snowmelt timing in 2004 corresponded with warmer air temperatures and precipitation between April and May, exposing the ground surface more than two weeks earlier than average for 2001–2012 within the Noatak Basin. Future rates of thaw slump initiation may be linked with changing trends in the timing of weather, in addition to general climate warming.

Description: We have estimated recent river incision rates using the in situ-produced 36 Cl cosmogenic nuclide concentrations. The target site consists of a ~25 m high vertical profile along a polished river cliff located in Jurassic limestones in the Vésubie catchment area, in the southern French Alps. 36 Cl exposure ages of the sampled river polished surface range from 3 to 14 ka, i.e., after the Last Glacial Maximum. Our data suggest as a first approximation a linear age/height relationship and lead to a mean incision rate of 2.2 mm.a -1 over the last 14 ka. More precisely, incision rates are characterized by two peaks reaching ~2 and 4-5 mm.a -1 at 4-5 ka and 11-12 ka, respectively, separated by a period experiencing a lower incision rate (~1 mm.a -1 ). A chi-plot of the river longitudinal profile suggests that on the long term, the river is close to equilibrium conditions with a concavity index of 0.475. The evolution of the Vésubie River longitudinal profile over a time period of 2 Ma based on the stream power law of river incision was then modeled with varying erodibility coefficients and uplift rates ranging from 0.5 to 2 mm.a -1 . The best-fitting models yield erodibility coefficient values ranging from 2.5 to 9.0 x 10 -6  m -0.475 a -1 for the considered uplift rates. For long-term uplift rates lower than 2 mm.a -1 , an increase of the erodibility coefficient during the last 16 ka, with two peaks at 11-12 and 4-5 ka, is necessary to precisely match the observed incision rates and is interpreted as resulting from recent climatic changes. These variations do not strongly affect the general shape of the river profile and suggest that the measured short-term incision rate is dominated by a climatic signal, which does not preclude the possible role of tectonic uplift.

Description: A model of sand transport in water is produced by combining a turbulence-resolving large eddy simulation (LES) with a discrete element model (DEM) prescribing the motion of individual grains of medium sand. The momentum effect of each particle on the fluid is calculated at the LES cell containing the particle, and the fluid velocity and pressure, interpolated to each particle center, is used to derive fluid force on each particle in the DEM. Eleven numerical experiments are conducted of an initially flat bed of particles. The experiments span a range of motion, from essentially no motion to vigorous suspension. Hydraulic roughness is found to increase abruptly at the transition from bedload to suspended load transport. Suspended sediment extracts momentum from the flow and decreases the rate of shear. Whereas, slightly higher in the flow, vertical drag by suspended grains damps turbulence and increases the rate of shear. Vertical sediment diffusivity and effective particle settling velocity are much smaller than is commonly assumed in suspended sediment models. The bedload experiments suggest that saltation by itself is a poor model of bedload sand transport. In contrast to expectations from saltation models, the peak bedload flux occurs at essentially the same level as the bed, and grains move slowly in frequent contact with other grains. Higher and faster moving bedload grains that can be considered to be in saltation represent a smaller portion of the total flux. Entrainmentof bedload grains occurs in response to fluid penetration of the bed by high-vorticity turbulence structures embedded within broader high speed fluid regions referred to as a sweeps or high-speed wedges.

Description: Bathymetric measurements show that a deep, subtidal channel in San Pablo Bay, California, has consistently narrowed during the past 150 years. This raises general questions on the seasonal and intertidal morphodynamic processes acting at the subtidal channel-shoal interface. The current work addresses these questions using a process-based morphodynamic model (Delft3D). Model results reveal considerable morphodynamic activity during a tidal cycle. Deposition on the channel margin is largest during flooding of the shoals. Erosion rates (mainly occuring during ebb) remain relatively small, so that net accretion occurs on much of the channel margin. A remarkable finding is that locally generated wind waves are responsible for shoal extension and depositional channel narrowing. High SSC in the channel is a critical factor affecting channel narrowing. Wind waves suspend sediment on the shoals leading to high suspended sediment concentration (SSC) in the channel at ebb. Sensitivity analysis shows that wind direction even determines the location of channel margin accretion. Fluvial sediment supply is another cause of high SSC in the channel. Density currents, 3D circulation flows, sea level rise or varied sediment characteristics only have a limited effect on the erosion and sedimentation patterns. A 30 year forecast shows that deeper shoals and decreasing fluvial sediment supply lower SSC levels in the channel, limit channel margin accretion, and even lead to net channel margin erosion in some areas. Channel shape thus remains subject to dynamic processes related to local variations in sediment supply, albeit to a more limited extend than in earlier decades.

Description: ABSTRACT [1]  Bedload transport during storm events is both an agent of geomorphic change and a significant natural hazard in mountain regions. Thus predicting bedload transport is a central challenge in fluvial geomorphology and natural hazard risk assessment. Bedload transport during storm events depends on the width and depth of bed scour, as well as the transport distances of individual sediment grains. We traced individual gravels in two steep mountain streams, the Erlenbach (Switzerland) and Rio Cordon (Italy), using magnetic and radio frequency identification (RFID) tags, and measured their bedload transport rates using calibrated geophone bedload sensors in the Erlenbach and a bedload trap in the Rio Cordon. Tracer transport distances and bedload volumes exhibited approximate power-law scaling with both the peak stream power and the cumulative stream energy of individual hydrologic events. Bedload volumes scaled much more steeply with peak stream power and cumulative stream energy than tracer transport distances did, and bedload volumes scaled as roughly the third power of transport distances. These observations imply that large bedload transport events become large primarily by scouring the bed deeper and wider, and only secondarily by transporting the mobilized sediment farther. Using the sediment continuity equation, we can estimate the mean effective thickness of the actively transported layer, averaged over the entire channel width and the duration of individual flow events. This active layer thickness also followed approximate power-law scaling with peak stream power and cumulative stream energy, and ranged up to 0.57 m in the Erlenbach, broadly consistent with independent measurements.

Description: [1]  Bed load transport is a highly complex process. The probability density function (PDF) of particle velocities results from the local particle momentum variability in response to fluid drag and interactions with the bed. Starting from the forces exerted on a single particle under low transport rates (i.e. rolling and sliding regimes), we derive here the nonlinear stochastic Langevin equation (LE) to describe the dynamics of a single particle, accounting for both the deterministic and the stochastic components of such forces. Then, the Fokker-Planck equation (FPE), which describes the evolution of the PDF of the ensemble particle velocities, is derived from the LE. We show that the theoretical PDFs of both streamwise and cross-stream velocities obtained by solving the FPE under equilibrium conditions, have exponential-form (PDFs of both positive and negative velocities decay exponentially), consistent with the experimental data by Roseberry et al . [2012]. Moreover, we theoretically show how the exponential like PDF of an ensemble of particle velocities results from the forces exerted on a single particle. We also show that the simulated particle motions using the proposed Langevin model exhibit an emergent nonlinear relationship between hop distances and travel times (power law with exponent 5/3), in agreement with the experimental data, providing a statistical description of the particles’ random motion in the context of a stochastic transport process. Finally, our study emphasizes that the motion of individual particles, described by the LE, and the behavior of the ensemble, described by the FPE, are connected within a statistical mechanics framework.

Description: [1]  When a barchan dune migrates, the sediment trapped on its lee side is later mobilized when exposed on the stoss side. Then, sand grains may undergo many dune turnover cycles before their ejection along the horns, but the amount of time a sand grain contributes to the dune morphodynamics remains unknown. To estimate such a residence time, we analyze sediment particle motions in steady-state barchans by tracking individual cells of a 3D cellular automaton dune model. The overall sediment flux may be decomposed into advective and dispersive fluxes to estimate the relative contribution of the underlying physical processes to the barchan shape. The net lateral sediment transport from the center to the horns indicates that dispersion on the stoss slope is more efficient than the convergent sediment fluxes associated with avalanches on the lee slope. The combined effect of these two antagonistic dispersive processes restricts the lateral mixing of sediment particles in the central region of barchans. Then, for different flow strengths and dune sizes, we find that the mean residence time of sediment particles in barchans is equal to the surface of the central longitudinal dune slices divided by the input sand flux. We infer that this central slice contains most of the relevant information about barchan morphodynamics. Finally, we initiate a discussion about sediment transport and memory in the presence of bed forms using the advantages of the particle tracking technique.

Description: [1]  Ice-shelf fractures frequently terminate where they encounter suture zones, regions of material heterogeneity that form between meteoric inflows in ice shelves. This heterogeneity can consist of marine ice, meteoric ice with modified rheological properties or the presence of fractures. Here, we use radar observations on the Larsen C Ice Shelf, Antarctica to investigate i) the termination of a 25 km-long rift in the Churchill Peninsula suture zone, which was found to contain ~60 m of accreted marine ice, and ii) the along-flow evolution of a suture zone originating at Cole Peninsula. We determine a steady-state field of basal melting/freezing rates and apply it to a flowline model to delineate the along-flow evolution of layers within the ice shelf. The thickening surface wedge of locally accumulated meteoric ice, which likely has limited lateral variation in its mechanical properties, accounts for ~60% of the total ice thickness near the calving front. Thus, we infer that the lower ~40% of the ice column and the material heterogeneities present there are responsible for resisting fracture propagation and thereby delaying tabular calving events, as demonstrated in the 〉40 year time series leading up to the 2004/05 calving event for Larsen C. This likely represents a highly sensitive aspect of ice-shelf stability, as changes in the oceanic forcing may lead to the loss of this heterogeneity.

Description: [1]  Two-dimensional measurements of snowpack properties (stratigraphic layering, density, grain size and temperature) were used as inputs to the multi-layer Helsinki University of Technology (HUT) microwave emission model at a centimeter-scale horizontal resolution, across a 4.5 m transect of ground-based passive microwave radiometer footprints near Churchill, Manitoba, Canada. Snowpack stratigraphy was complex (between six and eight layers) with only three layers extending continuously throughout the length of the transect. Distributions of one-dimensional simulations, accurately representing complex stratigraphic layering, were evaluated using measured brightness temperatures. Large biases (36 to 68 K) between simulated and measured brightness temperatures were minimized (-0.5 to 0.6 K), within measurement accuracy, through application of grain scaling factors (2.6 to 5.3) at different combinations of frequencies, polarizations and model extinction coefficients. Grain scaling factors compensated for uncertainty relating optical SSA to HUT effective grain size inputs and quantified relative differences in scattering and absorption properties of various extinction coefficients. The HUT model required accurate representation of ice lenses, particularly at horizontal polarization, and large grain scaling factors highlighted the need to consider microstructure beyond the size of individual grains. As variability of extinction coefficients was strongly influenced by the proportion of large (hoar) grains in a vertical profile, it is important to consider simulations from distributions of one-dimensional profiles rather than single profiles, especially in sub-Arctic snowpacks where stratigraphic variability can be high. Model sensitivity experiments suggested the level of error in field measurements and the new methodological framework used to apply them in a snow emission model were satisfactory. Layer amalgamation showed a three-layer representation of snowpack stratigraphy reduced the bias of a one-layer representation by about 50%.

Description: [1]  Most currently, permafrost covered landscapes underwent fundamental shifts in the hydrogeological and the thermal regime as a result of deglaciation after the Last Glacial Maximum (LGM). The transient effects of heat and fluid flow associated with retreating ice sheets are important to consider for the present-day hydrogeology of these regions. In this paper, we use numerical models to consider the evolution of taliks underneath proglacial lakes during deglacation. In our models, the hydrological and thermal boundary conditions at the lake site are constraint by the hydrogeological impacts of ice sheet dynamics since the LGM. During the LGM, the ground surface was insulated from the air temperatures and as a result there was no permafrost underneath the wet-based ice. Subsequently, ice sheet retreat led to an exposure of a proglacial area to sub-zero air temperatures and the formation of permafrost. Where proglacial lakes form, inflow from deeper groundwater becomes focussed. In this scenario, subpermafrost groundwater flow is driven by a combination of direct subglacial recharge, or by elevated hydraulic heads preserved in that part of the aquifer. Advective heat flow can delay or prevent through taliks from freezing as function ofaquifer properties. The presence and evolution of through taliks in thick permafrost can create complex and transient hydrogeological phenomena.

Description: Due to environmental disturbances such as local human activity and global warming, melting of massive ground ice has resulted in thermokarst ponds, which are extensively distributed in the Qinghai-Tibet Plateau (QTP). Besides the global warming, the thermokarst pond, as a major heat source, speeds up the moisture change and degradation of its surrounding permafrost. To analyze the long-term coupled moisture-heat process near a representative non-penetrative thermokarst pond in a permafrost region, abundant temperature data over multiple years at different depths and horizontal distances from the center of the thermokarst pond have been collected at a field experimental station in QTP. A numerical model is built on the basis of soil moisture dynamics, heat transfer and physics of frozen soil. The thermokarst pond is taken for an example and the temperature and moisture processes of surrounding permafrost are simulated by this model and compared with measured temperature data. Our results show that if the rate of air temperature rise is 0.048 °C/y, which refers to a 2.4 °C temperature rise over 50 years, the thawing fronts underneath the thermokarst pond move downward at a linear rate of 0.18 m/y and the permafrost beneath the pond center would disappear after the year of 2281. Beyond that time, the impact range of the pond on the natural ground increases to about 50 m in horizontal direction. So a dish-shape thawing zone occurs around the thermokarst pond. Simultaneously, the moisture state is greatly changed in 2281 and becomes completely different from that in 2013. All of these would inevitably deteriorate the ecological and environmental system in QTP. This change could be a global environmental concern because of the importance of QTP, as the so-called “third pole” of the earth, in regulating the global climate.

Description: Since the collapse of the Dolomieu crater floor at Piton de la Fournaise Volcano (la Réunion) in 2007, hundreds of seismic signals generated by rockfalls have been recorded daily at the Observatoire Volcanologique du Piton de la Fournaise (OVPF). To study rockfall activity over a long period of time, automated methods are required to process the available continuous seismic records. We present a set of automated methods designed to identify, locate and estimate the volume of rockfalls from their seismic signals. The method used to automatically discriminate seismic signals generated by rockfalls from other common events recorded at OVPF is based on fuzzy sets and has a success rate of 92%. A kurtosis-based automated picking method makes it possible to precisely pick the onset time and the final time of the rockfall-generated seismic signals. We present methods to determine rockfall locations based on these accurate pickings and a surface-wave propagation model computed for each station using a Fast Marching Method. These methods have successfully located directly observed rockfalls with an accuracy of about 100 meters. They also make it possible to compute the seismic energy generated by rockfalls, which is then used to retrieve their volume. The methods developed were applied to a dataset of 12,422 rockfalls that occurred over a period extending from the collapse of the Dolomieu crater floor in April 2007 to the end of the UnderVolc project in May 2011 to identify the most hazardous areas of the Piton de la Fournaise volcano summit.

Description: We investigate the initiation and long-term evolution of tidal networks by comparing controlled laboratory experiments and their associated scaling laws with outputs from a numerical model. We conducted numerical experiments at both the experimental laboratory scale (ELS) and natural estuary scale (NES) and compared these simulations with experimental data and field observations. Sensitivity tests show that initial bathymetry, frictional parametrization, sediment transport and bed slope terms play an important role in determining the morphodynamic evolution and the final landscape. Consistent with experimental observations, the morphodynamic feedbacks between flow, sediment transport and bathymetry gradually lead the system to a less dynamic state, finally reaching a stable network configuration. In both the ELS and NES simulations, the initially planar lagoon with large inter-tidal areas is subject to erosion, indicating ebb-dominance. Based on quantitative analyses of the NES micro- and meso-tidal simulations (e.g., geometric characteristics and relationship between modified tidal prism and cross-sectional area), numerical simulations are consistent with laboratory experiments and show that both type of models provide a realistic, albeit simplified, representation of natural systems. The combination of laboratory and numerical experiments also allowed us to explore the possibility of reaching a long-term morphodynamic equilibrium. Both the physical and numerical modelsapproach a dynamic equilibrium characterized by negligible gradients in sediment fluxes. The equilibrium configuration appears to be consistent with traditional relationships linking tidal prism and cross-sectional area of the inlet. Finally, this contribution highlights the significance of complementary research between experimental and numerical modeling in investigating long-term morphodynamics of tidal networks.

Description: Many beaches have been built by an onshore supply of sand from the shoreface and future long-term coastal evolution critically depends on cross-shore sediment exchange between the upper and the lower shorefaces. Even so, cross-shore sediment supply remains poorly known in quantitative terms and this reduces confidence in predictions of long-term shoreline change. In this paper, field measurements of suspended sediment load and cross-shore transport on the lower shoreface are used to derive a model for sediment supply from the lower to the upper shoreface at large spatial and temporal scales. Data collection took place at five different field sites that exhibit a wide range of wave conditions and sediment characteristics. Data analysis shows that both suspended sediment load and cross-shore sediment transport scale with the grain-related mobility number which ranged up to ψ  ≈ 1000 in the measurements while the effect of orbital velocity skewness is more limited. A one-year long simulation of sediment transfers between the lower and the upper shorefaces on a natural beach compares well with transport rates estimated from long-term bar migration patterns and aeolian accretion on the same beach.

Description: There is a pressing need to understand how different delta morphologies arise because morphology determines a delta's ecologic structure, resilience to relative sea-level rise, and stratigraphic architecture. We use numerical modeling (Delft3D) to explain how deltaic processes and morphology are controlled by the incoming sediment properties. We conducted 36 experiments of river-dominated delta formation varying the following sediment properties of the incoming grain-size distribution: the median, standard deviation, skewness, and percent cohesive sediment, which is a function of the first three properties. Changing standard deviation and skewness produces minimal morphological variation, whereas an increase in dominant grain size ( D 84 ) and decrease in percent cohesive sediment produces a transition from elongate deltas with few channels to semi-circular deltas with many channels. This transition occurs because critical shear stresses for erosion and settling velocities of grains set the number of channel mouths and the dominant delta-building process. Together, the number of channel mouths and the dominant process – channel avulsion, mouth bar growth, or levee growth – set the delta morphology. Coarse-grained, non-cohesive deltas have many channels dominated by avulsion, creating semi-circular planforms with relatively smooth delta fronts. Intermediate-grained deltas have many channels dominated by mouth bar growth, creating semi-circular planforms with rugose delta fronts. Fine-grained, cohesive deltas have a few channels, the majority of which are dominated by levee growth, creating elongate planforms with smooth delta fronts. The process-based model presented here provides a previously lacking mechanistic understanding of the effects of sediment properties on delta channel network and planform morphology.

Description: Meander migration and planform evolution depend on the resistance to erosion of the floodplain materials. To date, research to quantify meandering river adjustment has largely focused on resistance to erosion properties that vary horizontally. This paper evaluates the combined effect of horizontal and vertical floodplain material heterogeneity on meander migration by simulating fluvial erosion and cantilever and planar bank mass failure processes responsible for bank retreat. The impact of streambank failures on meander migration is conceptualized in our RVR Meander model through a bank armoring factor associated with the dynamics of slump blocks produced by cantilever and planar failures. Simulation periods smaller than the time to cutoff areconsidered, such that all planform complexity is caused by bank erosion processes and floodplain heterogeneity and not by cutoff dynamics. Cantilever failure continuously affects meander migration, because it is primarily controlled by the fluvial erosion at the bank toe. Hence, it impacts migration rates and meander shapes through the horizontal and vertical distribution of erodibility of floodplain materials. Planar failures are more episodic. However, in floodplain areas characterized by less cohesive materials, they can affect meander evolution in a sustained way and produce preferential migration patterns. Model results show that besides the hydrodynamics, bed morphology and horizontal floodplain heterogeneity, floodplain stratigraphy can significantly affect meander evolution, both in terms of migration rates and planform shapes. Specifically, downstream meander migration can either increase or decrease with respect to the case of a homogeneous floodplain; lateral migration generally decreases as result of bank protection due to slump blocks; and the effect on bend skewness depends on the location and volumes of failed bank material caused by cantilever and planar failures along the bends, with possible achievement of downstream bend skewness under certain conditions. Therefore, floodplain stratigraphy must be accounted for when estimating meander migration within floodplains. Results also suggest that the generation of slump blocks due to mass failure mechanisms and their effect on rates of bank erosion must be included explicitly in models of meander migration: tweaking the bank erodibility parameters for fluvial erosion to indirectly account for those processes cannot sufficiently reproduce their effects on meander migration patterns, because slump block generation depends on river planform configuration, direction of migration, and location of the river bends with respect to the floodplain material patches.

Description: Gullies are dynamic fluvial features that can be the primary driver for landscape dissection and sediment production in many settings. This research exploits a well-constrained field area near West Bijou Creek, Colorado, USA in order to develop a natural experiment in which we explore gully headcut erosion rates, the controls on gully headcut height, and the morphology of gully longitudinal profiles. Analysis of headcut retreat using aerial photography and LiDAR imagery indicates that headcutretreat rates correlate with the square root of drainage area, approximately. We investigate how a drainage area control on headcut retreat translates into the longitudinal profile morphology over time using a simple numerical. The model combines fluvial erosion, deposition, and headcut retreat to identify the necessary and sufficient conditions needed to reproduce longitudinal profiles observed in the field. Field profiles are typically concave-upward, predominantly aggradational channel profiles with retreating headcuts whose height varies with catchment position. Systematic variation of environmental parameters in the model, showed that the most successful model was achieved when highly-resistant vegetation is applied throughout the channel,excluding a bare soil zone downstream of the headcut. This model scenario maintained an abrupt headcut over hundreds of model years, and produced a realistic longitudinal profile that aggrades downstream of the headcut over time. The vegetation pattern used in the best model fit is observed at the field site, where easily erodible, sparsely vegetated soil downstream of the headcut grades into a more resistant grassy channel downstream.

Description: Traditional methods of deriving temporal variability of Antarctic ice-shelf elevation from satellite altimetry use a fixed (“Eulerian”) reference frame, where the measured changes include advection of ice thickness gradients between measurement epochs. We present a new method which removes advection effects by using an independent velocity field to compare elevations in a moving (“Lagrangian”) reference frame. Applying the technique to ICESat laser altimetry for the period 2003-2009 over the two largest Antarctic ice shelves, Ross and Filchner-Ronne, we show that the Lagrangian approach reduces the variability of derived elevation changes by about 50% compared to the Eulerian approach, and reveals clearer spatial patterns of elevation change. The method simplifies the process of estimating basal mass budget from the residual of all other processes that contribute to ice-shelf elevation changes. We use field data and ICESat measurements over ice rises and the grounded ice sheet to account for surface accumulation and changes in firn air content, and remove the effect of ice-flow divergence using surface velocity and ice thickness data. The results show highest basal melt rates (〉5 m a -1 ) near the deep grounding lines of major ice streams, but smaller melt rates (〈5 m a -1 ) near the ice-shelf fronts are equally important to total meltwater production since they occur over larger areas. Integrating over the ice-shelf areas, we obtain basal mass budgets of -50 ± 64 Gt a -1 for Ross and -124 ± 66 Gt a -1 for Filchner-Ronne, with changes in firn air content as the largest error source.

Description: Modeling of dust emissions from the surface remains complex, especially in semi-arid regions where vegetation must be accounted for because of its potentially important protective effect. Protection is directly linked to the fraction of the soil surface covered by vegetation, but it is also driven by the interaction of vegetation elements with the wind field. The sensitivity of simulated dust emissions to various drag partition schemes – mainly those proposed by Raupach (1992), Marticorena and Bergametti (1995) and Okin (2008) - is evaluated for a typical Sahelian rangeland, covered by a seasonal grass layer, over a complete vegetation cycle. The application of these schemes requires a fine characterization of the vegetation cover; field measurements from an ecological survey are used to derive the geometric dimensions of the grass patches. Models are run with meteorological forcing from automatic weather stations. As a result, the impacts of soil moisture and grass cover are estimated over April to September. Soil moisture inhibits dust emissions by 27% in mass. The different drag partition schemes exhibit distinctive limitations, mostly due to the properties of the Sahelian grass cover, which is composed of large and low patches of short grass, with a strong seasonal dynamics. However, the drag partition schemes result in remarkably coherent estimations of dust emissions. When soil moisture is taken into account, vegetation reduces the total vertical mass fluxes by 6 to 26% of the emissions of a bare soil, depending on the drag partition scheme.

Description: The downwind margin of White Sands dune field is an abrupt transition from mobile aeolian dunes to a dune-free vegetated surface. This margin is also relatively stable; over the past sixty years it has migrated several times more slowly than the slowest dunes within the dune field, resulting in a zone of dune coalescence, aggradation, and, along most of the margin, development of a dune complex (i.e. dunes superimposed on draas). Repeat terrestrial-laser-scanning (TLS) surveys conducted over a three-month period demonstrate that sediment fluxes within the dune complex decrease on approach to the margin. Computational fluid dynamics (CFD) modeling indicates that this decrease is due, in part, to a decrease in mean turbulent bed shear stress on the lee side of the dune complex as a result of flow-line divergence or sheltering of the lee-side dunes by the stoss side of the dune complex. Conservation of mass demands that this decrease in bed shear stress cause aggradation. We speculate that aggradation on the lee side of the dune complex further enhances the sheltering effect in a positive feedback, contributing to the growth and/or maintenance of the dune complex and a relatively abrupt and stable dune-field margin. Our model and data add to a growing body of evidence that aeolian dune-field patterns are influenced by feedbacks that occur at scales larger than individual dunes.

Description: Breaching is a type of retrogressive submarine slope failure associated with pore pressure drops in both space and time, and this drop strengthens the failing deposit. Breaching is characterized by a near-vertical failure surface that retreats with a relatively constant velocity, on the order of a millimeter per second. Breaching is controlled by interactions between shear-dilation generated pore-pressure drops and pore pressure dissipation through intergranular fluid flow. Lab measurements show that shear dilation in a deposit increases with increasing effective stress ratio between the major principal effective stress and the minor principal effective stress as well as decreasing confining stress. We present a two-dimensional numerical modelthat indicates how effective stress ratio and confining stress produces spatially varying dilation, affecting the mechanics of breaching. Experimental results show that dilation in a breaching deposit increases with proximity to the failure surface. As a result, the maximum magnitude of pore pressure drop is very close to the failure surface. The numerical model confirms that the sediment release is dominated by pore pressure dissipation through intergranular fluid flow in the horizontal direction.This allows the erosion rate to be treated as a constant in the vertical direction. Numerical model results also show that because dilation decreases with increasing vertical depth, the deposit becomes less stable with depth, suggesting a potential upper limit for the thickness of the deposit undergoing breaching.

Description: Submarine landslides are often characterized by a basal surface of rupture parallel to the stratigraphy, in which downslope movement is initiated. However, little is known about the sedimentology and physical properties of the sediments within these surfaces. In this study, we present a multi-proxy analysis of the sediments collected from a giant piston core penetrating a shallow submarine mass transport deposit, in combination with high-resolution seismo-acoustic data to identify and characterise the basal glide plane and the weaker sediments in which movement was initiated. The initial phase of instability consists of a single fracture that formed due to the downslope movement of a mostly intact slab of sediments. The 16 m long core, comprising mostly undisturbed massive and laminated IRD-rich clay penetrated this slab. The base of the slab is characterised by a high-amplitude semi-continuous reflection visible on the sub-bottom profiler data at about 12.5 m depth, interpreted to originate from the glide plane on top of a plumite deposit. This plumite has dilative behaviour with pore pressure decrease with increasing shear strain and high undrained shear strength. Movement probably started within contouritic sediments immediately above the glide plane, characterised by higher sensitivities and higher water contents. The occurrence of the mass movements documented in this study are likely affected by the presence of a submarine landslide complex directly downslope. The slide scar of this landslide complex promoted retrogressive movement further upslope and progressive spreading of strain softening along the slide base and in the slide mass. Numerical models (infinite slope, BING, and retrogressive slope models) illustrate that the present-day continental slope is essentially stable and allow reconstruction of the failure processes when initiated by an external trigger.

Description: It is well known that pebble diameter systematically decreases downstream in rivers. The contribution of abrasion is uncertain, in part because: (1) diameter is insufficient to characterize pebble mass loss due to abrasion; and (2) abrasion rates measured in laboratory experiments cannot be easily extrapolated to the field. A recent geometric theory describes abrasion as a curvature-dependent process that produces a two-phase evolution: in Phase I, initially blocky pebbles round to smooth, convex shapes with little reduction in axis dimensions; then, in Phase II, smooth, convex pebbles slowly reduce their axis dimensions. Here we provide strong evidence that two-phase abrasion occurs in a natural setting, by examining downstream evolution of shape and size of thousands of pebbles over ~10 km in a tropical montane stream. The geometric theory is verified in this river system using a variety of manual and image-based shape parameters, providing a generalizable method for quantifying the significance of abrasion. Phase I occurs over ~1 kilometer, in upstream bedrock reaches where abrasion is dominant and sediment storage is limited. In downstream alluvial reaches, where Phase II occurs, we observe the expected exponential decline in pebble diameter. Using a discretized abrasion model (the so called “box equations”) with deposition, we deduce that abrasion removes more than 1/3 of the mass of a pebble, but that size-selective sorting dominates downstream changes in pebble diameter. Overall, abrasion is the dominant process in the downstream diminution of pebble mass (but not diameter) in the studied river, with important implications for pebble mobility and the production of fine sediments.

Description: Groundwater level changes of up to 0.716 m, and temperature changes of up to 0.0708 °C, have been observed in the Mile well, Yunnan Province, China, in response to earthquakes with a seismic energy density exceeding 1 × 10 -3  J⋅m -3 from 2004 to 2012. Sustained water level changes, however, only occurred in earthquakes with a surface wave magnitude M s ≥8.0. Groundwater temperature and permeability changes also only occurred following such large earthquakes. This indicates that larger earthquakes (with M s ≥8.0) are more effective at triggering hydrologic responses than smaller earthquakes. The amplitudes of changes in groundwater level and groundwater temperature were positively correlated, indicating that temperature changes are possibly caused by advection-diffusion and mixing following groundwater inflows into the well. We used a tidal response as a proxy to investigate aquifer properties, showing that the changes in groundwater level and temperature correlate well with phase shift and tidal factor. Based on the aquifer parameters calculated from the tidal response using the M 2 wave, we found that amplitudes of inferred changes in transmissivity and storage coefficient have a positive relationship with the groundwater level and temperature changes. The scale of transmissivity change was much larger than that of the storage coefficient. We interpret the co-seismic changes in groundwater level and temperature in the Mile well to be due to changes of permeability in the aquifer due to unclogging of fractures induced by seismic waves.

Description: Height changes of the ice surface above subglacial Lake Vostok, East Antarctica, reflect the integral effect of different processes within the subglacial environment and the ice sheet. Repeated GNSS (Global Navigation Satellite Systems) observations on 56 surface markers in the Lake Vostok region spanning eleven years and continuous GNSS observations at Vostok station over five years are used to determine the vertical firn particle movement. Vertical marker velocities are derived with an accuracy of 1 cm/yr or better. Repeated measurements of surface height profiles around Vostok station using kinematic GNSS observations on a snow mobile allow the quantification of surface height changes at 308 crossover points. The height change rate was determined at 1 ± 5 mm/yr, thus indicating a stable ice surface height over the last decade. It is concluded that both the local mass balance of the ice and the lake level of the entire lake have been stable throughout the observation period. The continuous GNSS observations demonstrate that the particle heights vary linearly with time. Non-linear height changes do not exceed ±1 cm at Vostok station and constrain the magnitude of spatio-temporal lake-level variations. ICESat laser altimetry data confirm that the amplitude of the surface deformations over the lake are restricted to a few centimeters. Assuming the ice sheet to be in steady state over the entire lake, estimates for the surface accumulation, on basal accretion/melt rates and on flux divergence are derived.

Description: The size of a shallow landslide is a fundamental control on both its hazard and geomorphic importance. Existing models are either unable to predict landslide size or are computationally intensive such that they cannot practically be applied across landscapes. We derive a model appropriate for natural slopes that is capable of predicting shallow landslide size but simple enough to be applied over entire watersheds. It accounts for lateral resistance by representing the forces acting on each margin of potential landslides using earth pressure theory, and by representing root reinforcement as an exponential function of soil depth. We test our model's ability to predict failure of an observed landslide where the relevant parameters are well constrained by field data. The model predicts failure for the observed scar geometry and finds that larger or smaller conformal shapes are more stable. Numerical experiments demonstrate that friction on the boundaries of a potential landslide increases considerably the magnitude of lateral reinforcement, relative to that due to root cohesion alone. We find that there is a critical depth in both cohesive and cohesionless soils, resulting in a minimum size for failure, which is consistent with observed size frequency distributions. Furthermore, the differential resistance on the boundaries of a potential landslide is responsible for a critical landslide shape which is longer than it is wide, consistent with observed aspect ratios. Finally, our results show that minimum size increases as approximately the square of failure surface depth, consistent with observed landslide depth-area data.

Description: Sediment tracers moving as bedload can exhibit anomalous dispersion behavior deviating from Fickian diffusion. The presence of heavy-tailed resting time distributions and thin-tailed step length distributions motivate adoption of fractional-derivative models (FDMs) to describe sediment dispersion, but these models require many parameters that are difficult to quantify. Here, we propose a considerably simplified FDM for anomalous transport of uniformly sized grains along straight channels, the subordinated advection equation (SAE), which is based on the concept of time subordination. Unlike previous FDM models with time index γ between 0 and 1, our SAE model adopts a value of γ between 1 and 2. This γ describes random velocities deviating significantly from the mean velocity and models both long resting periods and relatively fast displacements. We show that the model quantifies the dynamics of four bedload transport experiments recorded in the literature. In addition to γ , SAE model parameters–velocity and capacity coefficient–are related to the mean and variance of particle velocities, respectively. Successful application of the SAE model also implies a universal probability density for the heavy-tailed waiting time distribution (with Dong Chen, Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, China. finite mean) and a relatively lighter tailed step length distribution for uniform bedload transport from local to regional scales.

Description: Toward elucidating how a wavy porous sand bed perturbs a turbulent flow above its surface, we record pressure within a permeable material resembling the region just below desert ripples, contrasting these delicate measurements with earlier studies on similar impermeable surfaces. We run separate tests in a wind tunnel on two sinusoidal porous ripples with aspect ratio of half crest-to-trough amplitude to wavelength of 3% and 6%. For the smaller ratio, pore pressure is a function of streamwise distance with a single delayed harmonic decaying exponentially with depth and proportional to wind speed squared. The resulting pressure on the porous surface is nearly identical to that on a similar impermeable wave. Pore pressure variations at thelarger aspect ratio are greater and more complicated. Consistent with the regime map of [39], the flow separates, creating a depression at crests. Unlike flows on impermeable waves, the porous rippled bed diffuses the depression upstream, reduces surface pressure gradients, and gives rise to a slip velocity, thus affecting the turbulent boundary layer. Pressure gradients within the porous material also generate body forces rising with wind speed squared and ripple aspect ratio, partially counteracting gravity around crests, thereby facilitating the onset of erosion, particularly on ripples of high aspect ratio armored with large surface grains. By establishing how pore pressure gradients scale with ripple aspect ratio and wind speed, our measurements quantify the internal seepage flow that draws dust and humidity beneath the porous surface.

Description: In this, the second of a pair of papers on the statistical signatures of riverbed sediment in high-frequency acoustic backscatter, spatially explicit maps of the stochastic geometries (length- and amplitude-scales) of backscatter are related to patches of riverbed surfaces composed of known sediment types, as determined by geo-referenced underwater video observations. Statistics of backscatter magnitudes alone are found to be poor discriminators between sediment types. However, the variance of the power spectrum, and the intercept and slope from a power-law spectral form (termed the spectral strength and exponent, respectively) successfully discriminate between sediment types. A decision-tree approach was able to classify spatially heterogeneous patches of homogeneous sands, gravels (and sand-gravel mixtures), and cobbles/boulders with 95, 88, and 91% accuracy, respectively. Application to sites outside the calibration, and surveys made at calibration sites at different times, were plausible based on observations from underwater video. Analysis of decision trees built with different training data sets suggested that the spectral exponent was consistently the most important variable in the classification. In the absence of theory concerning how spatially variable sediment surfaces scatter high-frequency sound, the primary advantage of this data-driven approach to classify bed sediment over alternatives is that spectral methods have well understood properties and make no assumptions about the distributional form of the fluctuating component of backscatter over small spatial scales.

Description: Bed-sediment classification using high-frequency hydro-acoustic instruments is challenging when sediments are spatially heterogeneous, which is often the case in rivers. The use of acoustic backscatter to classify sediments is an attractive alternative to analysis of topography because it is potentially sensitive to grain-scale roughness. Here, a new method is presented which uses high-frequency acoustic backscatter from multibeam sonar to classify heterogeneous riverbed sediments by type (sand, gravel,rock) continuously in space and at small spatial resolution. In this, the first of a pair of papers that examine the scattering signatures from a heterogeneous riverbed, methods are presented to construct spatially explicit maps of spectral properties from geo-referenced point clouds of geometrically and radiometrically corrected echoes. Backscatter power spectra are computed to produce scale and amplitude metrics that collectively characterize the length scales of stochastic measures of riverbed scattering, termed ‘stochastic geometries’. Backscatter aggregated over small spatial scales have spectra that obey a power-law. This apparently self-affine behavior could instead arise from morphological- and grain-scale roughnesses over multiple overlapping scales, or riverbed scattering being transitional between Rayleigh and geometric regimes. Relationships exist between stochastic geometries of backscatter and areas of rough and smooth sediments. However, no one parameter can uniquely characterize a particular substrate, nor definitively separate the relative contributions of roughness and acoustic impedance (hardness). Combinations of spectral quantities do, however, have the potential to delineate riverbed sediment patchiness, in a data-driven approach comparing backscatter with bed-sediment observations (which is the subject of part two of this manuscript).

Description: Patagonian icefields are losing volume, and their loss is due partly to rapid changes in their outlet glaciers that terminate in lakes or the ocean. Despite this key influence from outlet glaciers, relatively few of these calving glaciers have had high-frequency measurements on their frontal variations and ice speed changes. We describe here recent frontal variations and ice speed changes of all 28 major calving glaciers in the Southern Patagonia Icefield (SPI), including ice speed maps covering approximately half of the entire icefield. The analysis is based on satellite data from 1984 to 2011. Over this period, only the two termini of Glaciar Pío XI advanced. Of the remaining glacial fronts, 12 changed less than ±0.5 km, but 17 retreated at least 0.5 km. In the latter group, three (Glaciar Jorge Montt, HPS12, and Upsala) retreated over 6 km. Averaged over all 31 glacial fronts of the calving glaciers, the front positions retreated 1.56 km (median: 0.71 km). Along the centerline within 20-km of the front, the ice speeds up to 5900 ± 200 m a −1 . Except for regions showing large acceleration or deceleration, the mean speed over the measured area decreased by 30 m a −1 from 1984 to 2011. The three most rapidly retreating glaciers showed much larger acceleration near the calving front, suggesting that ice dynamics drives their rapid retreat. Thus, we see retreat as a long-term trend for the calving glaciers in the SPI, with behavior that implies a dynamically controlled rapid recession that may explain the recently reported volume change of the SPI.

Description: We describe the morphology and internal structure of Mullins and Friedman Glaciers, two cold-based, debris-covered alpine glaciers that occur in neighboring valleys in the McMurdo Dry Valleys, Antarctica. Both glaciers are overlain by a single, dry supraglacial debris layer (8-75 cm thick); each mantling debris layer is marked with near-identical patterns of arcuate ridges and steps, as well as corresponding changes in bulk grain size, meter-scale surface topography, and thermal-contraction crack polygons. Results from 24 km of ground penetrating radar data show that the ice within the uppermost 1-2 km of Mullins and Friedman Glaciers is essentially free of englacial debris (〈1% by volume), but thereafter is interspersed with bands of englacial debris (each 〈3 m thick and dipping up glacier) that intersect the ice surface at all major surface ridges and steps. The similarity in number and pattern of englacial debris bands and corresponding surface ridges and steps across both glaciers, along with model results and observations that call for negligible basal entrainment, is best explained by episodic environmental change at valley headwalls. Our working hypothesis is that layers of englacial debris originate as supraglacial lags that form in ice-accumulation areas during times of reduced net ice accumulation; following renewed net ice accumulation, these lags are subsequently buried by snow and ice, flow englacially, and intersect the ice surface to impart distinctive changes in the texture of supraglacial debris and topographic relief. The implication is that the englacial structure and surface morphology of these cold-based, debris-covered glaciers preserves a consistent record of climate and environmental change.

Description: Tephras preserved in lake sediments are commonly used to synchronize sedimentary archives of climate and environmental change, and to correlate them with terrestrial environments. They also provide opportunities to reconstruct volcanic explosive activity, e.g., eruption frequency and tephra dispersal. Although sedimentary processes may affect the record of tephras in lakes, lake sediments are generally considered as one of the best archives of tephra stratigraphy. The 2011–2012 eruption of Cordón Caulle volcano (Chile, 40°S) offered an ideal opportunity to study the processes affecting tephra deposition in lakes. Although the prevailing westerlies transported the erupted pyroclastic material away from nearby Puyehue Lake, the tephra was identified within this relatively large lake with a thickness ranging from 1 to 〉10 cm. This is in contrast with smaller lakes, where tephra thickness was in agreement with ash fall distribution maps. Geomorphological observations and sedimentological analyses provide evidence that the tephra deposited in Puyehue Lake entirely consists of material reworked from the upper watershed, transported by rivers, and distributed by lake currents according to particle size and density. Our results have important implications for tephrochronology and volcanology. They suggest that (1) lakes do not act as passive tephra traps; (2) lakes with large watersheds record more eruptions than smaller lakes, which only register direct ash falls, affecting conclusions regarding the recurrence of volcanic eruptions; (3) using lakes with large watersheds for isopach mapping systematically leads to an overestimation of erupted tephra volumes. Smaller lakes with limited drainage basins are generally better suited for volcanological studies.

Description: This paper describes the relationship between the statistics of bedload transport flux and the time scale over which it is sampled. A stochastic formulation is developed for the probability distribution function of bedload transport flux, based on the Ancey et al. [2008] theory. An analytical solution for the variance of bedload transport flux over differing sampling time scales is presented. The solution demonstrates that the time-scale dependence of the variance of bedload transport flux reduces to a three-regime relation demarcated by an intermittency time scale ( t I ) and a memory time scale ( t c ). As the sampling time scale increases, this variance passes through an intermittent stage (〈〈 t I ), an invariant stage ( t I  〈  t  〈  t c ) and a memoryless stage (〉 〉  t c ). We propose a dimensionless number (Ra) to represent the relative strength of fluctuation, which provides a common ground for comparison of fluctuation strength among different experiments, as well as different sampling time scales for each experiment. Our analysis indicates that correlated motion and the discrete nature of bedload particles are responsible for this three-regime behavior. We use the data from three experiments with high temporal resolution of bedload transport flux to validate the proposed three-regime behavior. The theoretical solution for the variance agrees well with all three sets of experimental data. Our findings contribute to the understanding of the observed fluctuations of bedload transport flux over mono/multiple-size grain beds, to the characterization of an inherent connection between short-term measurements and long-term statistics, and to the design of appropriate sampling strategies for bedload transport flux.

Description: We address complications in the coupling of a dynamic ice sheet model (ISM) and forcing from an Earth System Model (ESM), which arise because of the unknown ISM initial conditions. Unless explicitly accounted for during ISM initialization, the ice sheet is far from thermomechanical equilibrium with the surface mass balance forcing from the ESM. Upon coupling to ESM forcing, the result is a shock and unphysical and undesirable transients in ice geometry and other state variables. Under the assumption of thermomechanical equilibrium, we present an approach for finding ISM initial conditions—characterized by optimization of the basal sliding coefficient and basal topography fields—that balance a best fit to surface velocity and basal topography observations against the minimization of unphysical transients when coupling to surface mass balance forcing. A quasi-Newton method is used to solve the resulting large-scale, PDE-constrained optimization problem, where the cost function gradients with respect to the parameter fields are computed using adjoints. After studying properties of our approach on a synthetic test problem, we apply the method towards obtaining optimal initial conditions for a model of the Greenland ice sheet. Our results show that, in the presence of uncertainties in the basal topography, ice thickness should also be treated as an optimization variable. While the focus here is on the coupling between an ISM and ESM-derived surface mass balance, the method is easily extended to include optimal coupling to forcing from an ocean model through submarine melt rates.

Description: Remote sensing, regional ground temperature and ground ice observations, and numerical simulation were used to investigate the size, distribution and activity of ice wedges in fine-grained mineral and organic soils across the forest-tundra transition in uplands east of the Mackenzie Delta. In the northernmost dwarf-shrub tundra, ice-wedge polygons cover up to 40% of the ground surface, with the wedges commonly exceeding 3 m in width. The largest ice wedges are in peatlands where thermal contraction cracking occurs more frequently than in nearby hummocky terrain with fine-grained soils. There are fewer ice wedges, rarely exceeding 2 m in width, in uplands to the south and none have been found in mineral soils of the tall-shrub tundra, although active ice-wedges are found there throughout peatlands. In the spruce forest zone, small, relict ice wedges are restricted to peatlands. At tundra sites, winter temperatures at the top of permafrost are lower in organic than mineral soils because of the shallow permafrost table, occurrence of phase change at 0 °C, and the relatively high thermal conductivity of icy peat. Due to these factors and the high coefficient of thermal contraction of frozen saturated peat, ice-wedge cracking and growth is more common in peatlands than in mineral soil. However, the high latent heat content of saturated organic active-layer soils may inhibit freezeback, particularly where thick snow accumulates, making the permafrost and the ice wedges in spruce forest polygonal peatlands susceptible to degradation following alteration of drainage or climate warming.

Description: Previous studies suggest that the seismic noise induced by rivers may be used to infer river transport properties and previous theoretical work showed that bedload sediment flux can be inverted from seismic data. However, the lack of a theoretical framework relating water flow to seismic noise prevents these studies from providing accurate bedload fluxes and quantitative information on flow processes. Here, we propose a forward model of seismic noise caused by turbulent flow. In agreement with previous observations, modeled turbulent-flow-induced noise operates at lower frequencies than bedload-induced noise. Moreover, the differences in the spectral signatures of turbulent-flow-induced and bedload-induced forces at the river bed are significantenough that these two processes can be characterized independently using seismic records acquired at various distances from the river. In cases with isolated turbulent flow noise, we suggest that river bed stress can be inverted. Finally, we validate our model by comparing predictions to previously reported observation. We show that our model captures the spectral peak located around 6–7 Hz and previously attributed to water flow at Hance Rapids in the Colorado River (USA); we also show that turbulent flow causes a significant part of the seismic noise recorded at the Trisuli River in Nepal, which reveals that the hysteresis curve previously reported there does not solely include bedload, but is also largely influenced by turbulent-flow-induced noise. We expect the framework presented here to be useful to invert realistic bedload fluxes by enabling the removal of the turbulent flow contribution from seismic data.

Description: Sand ripples formed by waves have a uniform wavelength while at equilibrium, and develop defects while adjusting to changes in the flow. These patterns arise from the interaction of the flow with the bed topography, but the specific mechanisms have not been fully explained. We use numerical flow models and laboratory wave tank experiments to explore the origins of these patterns. The wavelength of “orbital” wave ripples ( λ ) is directly proportional to the oscillating flow's orbital diameter ( d ), with many experimental and field studies finding λ / d  ≈ 0.65. We demonstrate a coupling that selects this ratio: the maximum length of the flow separation zone downstream of a ripple crest equals λ when λ / d  ≈ 0.65. We show that this condition maximizes the growth rate of ripples. Ripples adjusting to changed flow conditions develop defects that break the bed's symmetry. When d is shortened sufficiently, two new incipient crests appear in every trough, but only one grows into a full-sized crest. Experiments have shown that the same side (right or left) wins in every trough. We find that this occurs because incipient secondary crests slow the flow and encourage the growth of crests on the next flank. Experiments have also shown that when d is lengthened, ripple crests become increasingly sinuous and eventually break up. We find that this occurs because crests migrate preferentially towards the nearest adjacent crest, amplifying any initial sinuosity. Our results reveal the mechanisms that form common wave ripple patterns and highlight interactions among unsteady flows, sediment transport, and bed topography.

Description: Rock-ice avalanche events are among the most hazardous natural disasters in the last century. In contrast to rock avalanches, the solid phase (ice) can transform to fluid during the course of the rock-ice-avalanche and fundamentally alter mechanical processes. A real two-phase debris flow model could better address the dynamic interaction of solid (rock and ice) and fluid (water, snow, slurry and fine particles) than presently used single-phase Voellmy- or Coulomb-type models. We present a two-phase model capable of performing dynamic strength weakening due to internal fluidisation and basal lubrication and internal mass and momentum exchanges between the phases. Effective basal and internal friction angles are variable and correspond to evolving effective solid volume fraction, friction factors, volume fraction of the ice, true friction coefficients, and lubrication and fluidisation factors. Benchmark numerical simulations demonstrate that the two-phase model can explain dynamically changing frictional properties of rock-ice avalanches that occur internally and along the flow path. The interphase mass and momentum exchanges are capable of demonstrating the mechanics of frontal surge-head and multiple other surges in the debris body. This is an observed phenomenon in a real two-phase debris flow, but newly simulated here by applying the two-phase mass flow model [ Pudasaini , 2012]. Mass and momentum exchanges between the phases and the associated internal and basal strength weakening control the exceptional long run-out distances, provide a more realistic simulation especially during the critical initial and propagation stages of avalanche and explain the exceptionally high and dynamically changing mobility of rock-ice-avalanches.

Description: Over the past two decades Iceland's glaciers have been undergoing a phase of accelerated retreat set against a backdrop of warmer summers and milder winters. This paper demonstrates how the dynamics of a steep outlet glacier in maritime SE Iceland have changed as it adjusts to recent significant changes in mass-balance. Geomorphological evidence from Falljökull, a high mass-turnover temperate glacier, clearly shows that between 1990 and 2004 the ice-front was undergoing active retreat resulting in seasonal oscillations of its margin. However, in 2004–2006 this glacier crossed an important dynamic threshold and effectively reduced its active length by abandoning its lower reaches to passive retreat processes. A combination of ice-surface structural measurements with radar, LiDAR and differential GNSS data are used to show that the upper active section of Falljökull is still flowing forward, but has become detached from, and is being thrust over, its stagnant lower section. The reduction in the active length of Falljökull over the last several years has allowed it to rapidly re-equilibrate to regional snowline rise in SE Iceland over the past two decades. It is possible that other steep, mountain glaciers around the world may respond in a similar way to significant changes in their mass-balance, rapidly adjusting their active length in response to recent atmospheric warming.

Description: Observations indicate that the grounding line position of West Antarctica is sensitive to both forced and unforced ice stream variability. This study endeavors to characterize and understand unforced ice stream variability and associated grounding line migration. We employ a flowline ice stream model with an undrained plastic bed, lateral shear stresses and a stretched grid refined in the grounding zone. This model exhibits parameter space structure and hysteresis behavior similar to simpler ice stream models. Low prescribed temperature at the ice surface or weak geothermal heating produce thermal oscillations between active and stagnant phases. As in previous spatially-resolved ice flow models, thermal activation propagates as an “activation wave”. This model's fine resolution of the grounding zone allows for accurate simulations of transient, unforced grounding line migration. Upstream of the grounding zone, horizontal grid spacing of 1 km is required to accurately resolve activation waves. Activation waves induce the grounding line to migrate over 100 km at a rate that can exceed 1 km/yr. This is followed during the active phase by retreat, which then continues for the duration of the stagnant phase. Grounding line retreat is the result of a negative mass balance near the grounding line, but is not necessarily associated with negative mass balance for the entire ice stream in our simulations of internal variability. The novel approach and experiments described in this study showthat there can be large excursions in grounding line position in the absence of either external forcing or retrograde slopes.

Description: The evaluation of avalanche release conditions constitutes a great challenge for risk assessment in mountainous areas. The spatial variability of snowpack properties has an important impact on snow slope stability and thus on avalanche formation since it strongly influences failure initiation and crack propagation in weak snow layers. Hence, the determination of the link between these spatial variations and slope stability is very important, in particular for avalanche public forecasting. In this study, astatistical-mechanical model of the slab-weak layer (WL) system relying on stochastic finite element simulations is used to investigate snowpack stability and avalanche release probability for spontaneously releasing avalanches. This model accounts, inparticular, for the spatial variations of WL shear strength and stress redistribution by elasticity of the slab. We show how avalanche release probability can be computed from release depth distributions which allow us to study the influence of WL spatial variations and slab properties on slope stability. The importance of smoothing effects by slab elasticity is verified and the crucial impact of spatial variation characteristics on the so-called knock-down effect on slope stability is revisited using this model. Finally, critical length values are computed from the simulations as a function of the various model parameters and are compared to field data obtained with propagation saw tests.

Description: Ice shelves play a major role in buttressing ice-sheet flow into the ocean, hence the importance of accurate numerical modeling of their stress regime. Commonly used ice flow models assume a continuous medium and are therefore complicated by the presence of rupture features (crevasses, rifts and faults) that significantly affect the overall flow patterns. Here, we apply contact mechanics and penalty methods to develop a new ice-shelf flow model that captures the impact of rifts and faults on the rheology and stress distribution of ice shelves. The model achieves a best-fit solution to satellite observations of ice-shelf velocities to infer: 1) a spatial distribution of contact and friction points along detected faults and rifts, 2) a more realistic spatial pattern of ice-shelf rheology and 3) a better representation of the stress balance in the immediate vicinity of faults and rifts. Thus, applying the model to the Brunt/Stancomb-Wills Ice Shelf, Antarctica, we quantify the state of friction inside faults and the opening rates of rifts, and obtain an ice-shelf rheology that remains relatively constant everywhere else on the ice shelf. We further demonstrate that better stress representation has widespread application in examining aspects affecting ice-shelf structure and dynamics including the extent of ice mélange in rifts and the change in fracture configurations. All are major applications for better insight into the important question of ice-shelf stability.

Description: Alluvial river channels often exhibit a relatively abrupt transition from gravel to sand-bedded conditions. The phenomenon is well-documented but few prior studies have analyzed the spatial variability through reaches where transitions occur. The downstream fining pattern observed in the Fraser River is cited as a classic example of an abrupt gravel-sand transition in a large alluvial channel. However, important questions regarding the exact location of the transition, its sedimentology and morphology, and what controls its location remain unanswered. Here, we present observations of the downstream change in bed material grain-size, river bed topography and channel hydraulics through the reach within which the transition occurs. These observations indicate that the gravel-sand transition is characterized by a terminating gravel wedge, but there are patches of gravel downstream of the wedge forming a diffuse extension. We show that there is a dramatic decrease in shear stress at the downstream end of the wedge and a consequent cessation of general gravel mobility. We argue that the patches of gravel observed beyond the wedge are the result of enhanced mobility of fine gravel over a sand bed. We also find that sand in suspension declines rapidly at the downstream end of the wedge, suggesting that sand is delivered to the bed, completing the sedimentary conditions for a gravel-sand transition. We propose that the break in river slope associated with the transition is a consequential feature of the transition.

Description: Plants and animals affect stream morphodynamics across a range of scales, yet including biological traits of organisms in geomorphic process models remains a fundamental challenge. For example, laboratory experiments have shown that silk nets built by caddisfly larvae ( Trichoptera :Hydropsychidae) can increase the shear stress required to initiate bed motion by more than a factor of two. The contributions of specific biological traits are not well understood, however. Here we develop a theoretical model for the effects of insect nets on the threshold of sediment motion, τ * crit , that accounts for the mechanical properties, geometry, and vertical distribution of insect silk, as well as interactions between insect species. To parameterize the model, we measure the tensile strength, diameter, and number of silk threads in nets built by two common species of caddisfly, Arctopsyche californica and Ceratopsyche oslari . We compare model predictions with new measurements of τ * crit in experiments where we varied grain size and caddisfly species composition. The model is consistent with experimental results for single species, which show that the increase in τ * crit above the abiotic control peaks at 40-70% for 10–22 mm sediments and declines with increasing grain size. For the polyculture experiments, however, the model under-predicts the measured increase in τ * crit when two caddisfly species are present in sediments of larger grain sizes. Overall, the model helps explain why the presence of caddisfly silk can substantially increase the forces needed to initiate sediment motion in gravel-bedded streams, and also illustrates the challenge of parameterizing the behavior of multiple, interacting species in a physical model.

Description: Analysis of the thermal and mechanical response of high altitude glaciers to climate change is crucial to assess future glacier hazards associated with thermal regime changes. This paper presents a new fully thermo-mechanically coupled transient thermal regime model including enthalpy transport, firn densification, full-Stokes porous flow, free surface evolution, strain heating, surface meltwater percolation and refreezing. The model is forced by daily air temperature data and can therefore be used to perform prognostic simulations for different future climate scenarios. The set of equations is solved using the finite element ice sheet/ice flow model Elmer/Ice. This model is applied to the Col du Dôme glacier (Mont Blanc area, 4250 m a.s.l., France) where a comprehensive dataset is available. The results show that the model is capable of reproducing observed density and velocity fields as well as borehole temperature evolution. The strong spatial variability of englacial temperature change observed at Col du Dôme is well reproduced. This spatial variability is mainly a result of the variability of the slope aspect of the glacier surface and snow accumulation. Results support the use of this model to study the influence of climate change on cold accumulation zones, in particular to estimate where and under what conditions glaciers will become temperate in the future.

Description: A major control on bedrock incision is the interaction between alluvial cover and erosive mobile grains. The extent of alluvial cover is typically predicted as a function of relative sediment flux (sediment supply rate over bedload transport capacity, q bs / q bc ), yet little is known about how the bed roughness affects the alluvial cover. We performed field experiments with various flow discharges, sediment supply rates, grain sizes and bed surface topographies. We then developed physically-based models for estimating the threshold of sediment movement and the extent of alluvial cover, so as to include the effect of roughness change. The results for the threshold of sediment movement and the extent of alluvial cover obtained from our models show reasonable agreement with the results of the field experiments. We explored the sensitivity of the models to variations in sediment supply and bedrock relative roughness (bedrock hydraulic-roughness height over grain size, k sb / d ). The results suggest that: 1) a larger relative roughness yields a greater dimensionless critical shear stress required for initial sediment motion; 2) at a given sediment supply rate, the extent of alluvial cover is larger when the relative roughness is larger; 3) when the sediment supply rate and the relative roughness are small, throughput bedload moves over (and can abrade) a purely bedrock channel with no alluvial cover; and 4) the critical value of sediment supply rate below which throughput bedload transport occurs increases with decreasing relative roughness. The experimental results and analysis provide a framework for treating the a) incisional morphodynamics of purely bedrock rivers by throughput bedload with no alluvial cover, b) incisional/alluvial morphodynamics of mixed bedrock-alluvial rivers, and c) purely alluvial morphodynamics, as well as the transition between these states.

Description: Numerous research efforts have been devoted to understanding estuarine morphodynamics under tidal forcing. However, the impact of river discharge on estuarine morphodynamics is insufficiently examined. Inspired by the Yangtze Estuary, this work explores the morphodynamic impact of river discharge in a 560-km long tidal basin based on a 1D model (Delft3D). The model considers total load sediment transport and employs a morphodynamic updating scheme to achieve long-term morphodynamic evolution. We analyze the role of Stokes drift, tidal asymmetry and river discharge in generating tidal residual sediment transport. Model results suggest that morphodynamic equilibrium is approached within millennia by vanishing spatial gradients of tidal residual sediment transport. We find that the interaction between ebb-directed Stokes return flow/river flow with tides is an important mechanism that flushes river-supplied sediment seaward. Increasing river discharge does not induce continuously eroded or accreted equilibrium bed profiles because of the balance between riverine sediment supply and sediment flushing to the sea. An intermediate threshold river discharge can be defined which leads to a deepest equilibrium bed profile. As a result, the shape (concavity or convexity) of the equilibrium bed profiles will adapt with the magnitude of river discharge. Overall, this study reveals the significant role of river discharge in controlling estuarine morphodynamics by supplying sediment and reinforcing ebb-directed residual sediment transport.

Description: Understanding granular mass flow is a basic step in the prediction and control of natural or man-made disasters related to avalanches on the Earth. Savage and Hutter [1989] pioneered the mathematical modeling of these geophysical flows introducing Saint-Venant-type mass and momentum depth-averaged hydrostatic equations using the continuum mechanics approach. However, Denlinger and Iverson [2004] found that vertical accelerations in granular mass flows are of the same order as the gravity acceleration, requiring the consideration of non-hydrostatic modeling of granular mass flows. Although free surface water flow simulations based on non-hydrostatic depth-averaged models are commonly used since the works of Boussinesq [1872, 1877], they have not yet been applied to the modeling of debris flow. Can granular mass flow be described by Boussinesq-type gravity waves ? This is a fundamental question to which an answer is required, given the potential to expand the successfull Boussinesq-type water theory to granular flow over 3D terrain. This issue is explored in this work by generalizing the basic Boussinesq-type theory used in civil and coastal engineering for more than a century to an arbitrary granular mass flow using the continuum mechanics approach. Using simple test cases it is demonstrated that the above question can be answered in the affirmative way, thereby opening a new framework for the physical and mathematical modeling of granular mass flow in geophysics, whereby the effect of vertical motion is mathematically included without the need of ad-hoc assumptions.

Description: Partial alluvial cover in bedrock channels influences downcutting rates and mountain river morphodynamics. Flume experiments have demonstrated that a wide range of stable cover fractions are possible for a given ratio of sediment supply to transport capacity. Existing cover models impose different unique relationships between cover fraction and reach-scale transport variables (e.g., slope, discharge, sediment supply, grain size). Individually, these models cannot predict the range of cover behaviors observed experimentally, suggesting that additional variables may be important. I propose a 1-D model in which bedrock surface roughness is a control on partial alluvial cover. In the model, surface roughness affects both shear stresses and thresholds of grain motion, and therefore sediment transport capacity. The roughness of the combined bedrock-alluvial bed surface varies with the cover fraction. Model trends are reasonably consistent with previous experiments over ranges of roughness, slope, and shear stress. When bedrock roughness is much greater than grain size, cover increases approximately linearly with the ratio of sediment supply to transport capacity. When bedrock roughness is much less than the sediment diameter, the model tends to predict abrupt shifts between complete bedrock exposure and alluvial cover. This runaway alluviation occurs for model solutions that are unstable to perturbations in supply, which is the case when transport capacity decreases as alluvial cover increases. In addition, a model variant in which sediment supply is controlled by the amount of sediment on the bed suggests that a linear cover relation may be sufficient for modeling sediment flux-dependent erosion at landscape scales.

Description: This study numerically investigates the effects of variations in inflow conditions and planform geometry on large-scale coherent flow structures and bed friction velocities at a stream confluence with natural bathymetry and concordant bed morphology. Several numerical experiments are conducted in which either the Kelvin-Helmholtz mode or the wake mode dominates within the mixing interface (MI) between the two confluent streams as the junction angle and alignments of the tributaries are altered. In Kelvin Helmholtz mode, the MI contains mostly co-rotating vortices driven by the mean transverse shear across the MI, while in wake mode the MI contains counter-rotating vortices forming by the interaction of the separated shear layers on the two sides of a zone of stagnant fluid near the junction corner. A large angle between the two incoming streams is not necessary for the development of strongly coherent streamwise-oriented vortical (SOV) cells in the immediate vicinity of the MI. Results show that such SOV cells can develop and produce high bed friction velocities even for cases with a low angle between the two tributaries and for cases where the downstream channel is approximately aligned with the axes of the two tributaries (low-curvature cases). SOV cells tend not to develop only when the incoming streams are parallel and aligned with the downstream channel (junction angle of zero), and the incoming flows produce a strong Kelvin Helmholtz mode. Under such conditions, quasi-2D MI vortices play the primary role in mixing and the production of high bed shear velocities. Simulations with and without natural bed morphology/local bankline irregularities indicate that planform geometry and inflow conditions primarily govern the development of coherent flow structures, but that bathymetric and bankline effects can locally modify details of these structures.

Description: The interaction of the subsiding limb of the Hadley circulation and the easterly North Pacific Trade Winds establishes a persistent thermal inversion at about 2000 m above sea level in the subtropical Pacific. The inversion restricts convective rainfall to the lower elevations of the windward flank of the island of Hawaii, creating an order-of-magnitude vertical rainfall gradient, as well as high inter-basin variability in precipitation. In the high-rainfall zone, streams are incised 10s to 100 s of meters below the surface of the volcanic shield. We use a digital elevation model and 1-D numerical modeling to assess whether deep incision on the flank of Mauna Kea is tied to the elevation of the trade wind inversion. The 83 channels examined can be well-fit in aggregate by all models that account for differences in precipitation between basins; specifically, the maximum depth of incision in each drainage is a power function of precipitation-weighted drainage area with an exponent of ~ ½. Individual longitudinal stream profiles are generally better-fit by models that acknowledge both along-channel precipitation variability and the subsidence of the island through that gradient, but this relationship is not clearly demonstrable for many channels. We argue that island subsidence through the dry-to-wet precipitation gradient has resulted in less cumulative discharge in the lower reaches of the drainages over their lifetime, relative to a non-subsiding case. This reduces differential erosion between the wet and dry reaches by 20-30% over 300 ky, making the longitudinal profiles less sensitive to the very strong climate gradient.

Description: Mineral dust in the atmosphere has implications for Earth's radiation budget, biogeochemical cycles, hydrological cycles, human health, and visibility. Currently, the simulated vertical mass flux of dust differs greatly among the existing dust models. While most of the models utilize an erodibility factor to characterize dust sources, this factor is assumed to be static, without sufficient characterization of the highly heterogeneous and dynamic nature of dust-source regions. We present a high-resolution land cover map of the Middle East and North Africa (MENA) in which the terrain is classified by visually examining satellite images obtained from Google Earth Professional and ESRI Basemap. We show that the correlation between surface wind speed and MODIS deep blue aerosol optical depth (AOD) can be used as a proxy for erodibility, which satisfactorily represents the spatiotemporal distribution of soil-derived dust sources. This method also identifies agricultural dust sources, and eliminates the satellite-observed dust component that arises from long-range transport, pollution, and biomass burning. The erodible land cover of the MENA region is grouped into 9 categories as: (1) bedrock: with sediment, (2) sand deposit, (3) sand deposit: on bedrock, (4) sand deposit: stabilized, (5) agricultural and urban area, (6) fluvial system, (7) stony surface, (8) playa/sabkha, and (9) savanna/grassland. Our results indicate that erodibility is linked to the land cover type and has regional variation. An improved land cover map, which explicitly accounts for sediment supply, availability, and transport capacity, may be necessary to represent the highly dynamic nature of dust sources in climate models.

Description: The main objective of this study is to extract from reflectance spectra, the geophysical properties of mudflat sediments such as water content and grain size. As mentioned in the literature, difficulties remain in separating the respective contributions of grain size and water on the reflectance continuum. This paper deals with the evaluation of a new methodological approach, the Spectral Derivative - Modified Gaussian Model (SD-MGM) for establishing the relationship between the spectral features and the geophysical properties of sediments. The SD-MGM enables the deconvolution of spectra into two main components, (1) Gaussian curves for the absorption bands, and (2) a straight line in the wavenumber domain for the portion of the spectrum that represents continuum. While the retrieved Gaussian features are known to be reliable indicators of the composition, it is shown that the retrieved continuum can be used as a novel approach for determining grain size and water content. Based on regression analyses between the SD-MGM spectral output parameters and the geophysical properties, a quantitative relationship between water content and the way in which the shape of the water band depth at 0.97 µm and 2.8 µm changes has been found during dehydration. It is shown that it is possible to separate three water types present in the sediment structure: saturated, free and adsorbed waters with high coefficients of determination (r 2 ) of 0.97, 0.98 and 0.94 respectively. The continuum is also revealed to be a useful water content indicator because it is less affected by atmospheric effects.

Description: No abstract is available for this article.

Description: We evaluate how the growth and interaction of active normal faults in the Sperchios Basin and Northern Gulf of Evia, Greece is recorded by the landscape. We demonstrate that patterns in footwall relief along the faults reflect fault segmentation and we show that in this study area fault throw is two to three times the maximum footwall relief. Rivers crossing the faults typically have two knickpoints, which are unrelated to lithology. However, their heights, measured from the active fault trace, vary systematically. The height of the upper set of knickpoints scales linearly with the footwall relief of the faults and is typically 〉 85 % of the maximum relief. The height of the lower set of knickpoints also scales with footwall relief, but the heights are consistently lower. The existence of two sets of knickpoints suggests that the rivers have been perturbed by two changes in tectonic rates during faulting. We interpret the upper knickpoints to represent the initiation and growth of fault-generated topography, while the lower set of knickpoints reflects a throw rate increase due to fault linkage. Estimates of throw rate enhancement factor derived from fault interaction theory suggest the faults increased their rate by a factor ≥ 3 when they linked. This constraint, combined with the distribution of knickpoint heights, allows us to estimate throw rate and linkage time for the faults. The Sperchios Fault has a maximum throw rate of 1.5 – 2.0 mm/yr, while the Coastal Fault has a maximum throw rate of 0.8 – 1.2 mm/yr.

Description: In order to understand ice sheet response to climate change, it is critical to examine errors associated with ice flow model boundary conditions and forcing. It is also important to understand how these errors propagate through numerical ice sheet models and contribute to uncertainty in model output. Using established uncertainty quantification methods within the Ice Sheet System Model (ISSM), we investigate the sensitivity of ice flow within the Northeast Greenland Ice Stream (NEGIS) to key fields, including ice viscosity and basal drag, and compare them with model sensitivity to climate forcing. In addition, we examine how errors in model input manifest as mass flux uncertainties during a forward simulation of the NEGIS from 1989-2010. Overall, we find that mass flux is most uncertain in the main outlets: Nioghalvfjerdsbræ and Zachariæ Isstrøm, and that mass flux is most sensitive to basal drag, though errors associated with basal drag are poorly constrained and difficult to quantify. Given our knowledge of errors associated with the thermal properties of ice, we estimate that in the ablation area, the effects of cryo-hydrologic warming contribute over 4 times more mass flux uncertainty that do errors in geothermal heat flux. We find that NEGIS total ice discharge is associated with a 0.7 Gt/yr (2.6%) uncertainty due to errors in geothermal heat flux and a 3.3 Gt/yr (11.6%) uncertainty due to the added effects of cryo-hydrologic warming. In comparison, errors in surface mass balance contribute 4.5 Gt/yr to NEGIS total discharge uncertainty.

Description: Channel form and sediment transport are closely linked in alluvial rivers, and as such the development of a conceptual framework for the downstream controls on particle mobility and likely deposition sites has immense value in terms of the way we understand and predictively model rivers. Despite the development of conceptual models which frame flood-scale particle transport distance (termed path length ) as a function of channel bar locations, an understanding of the controls on such path lengths in braided rivers remains especially elusive, in large part due to the difficulty in explicitly linking morphology and particle transport distances in the field. Here we utilize a series of laboratory flume experiments to link path length distances with channel morphology. Our morphologic characterization is based on ultra-high-resolution digital elevation models and bar classifications derived from structure-from-motion topography, while we simultaneously capture particle path lengths using fluorescent tracer particles over the course of five physical model simulations. Our findings underscore the importance of channel bars in acting as deposition sites for particles in transport; 81% of recovered tracers were found in association with compound, point, lateral, or diagonal bars. Bar heads (29%) and bar margins (41%) were the most common bar-related deposition surfaces for recovered tracers. Peaks in particle deposition frequency corresponding to channel bars were often noted on path-length distributions from tracer data; most tracers were deposited in areas that had experienced shallow (∆z = 0.002 m) deposition. Average path length distance (2.5 m) was closely related to average confluence-diffluence spacing (2.3 m) across all runs. The transferability of this understanding to braided streams has important implications for the development of simplified morphodynamic models which seek to predict braided channel evolution across multi-flood timescales.

Description: New experimental data on bedform initiation under unidirectional, oscillatory, and combined flows are presented to gain quantitative insight into bedform genesis from artificially-generated defects on a flat sediment-laden bed. Planform changes revealed in time-lapse photography, allowed the study of the evolution of the downstream and upstream edges of defects from their initial geometric center. Based on this temporal data set and flow velocity profiles, it was observed that combined flow bedforms share the same bedform initiation processes as unidirectional and oscillatory flows, which are reflected directly in the generation of similar geometric patterns regardless of the hydrodynamic conditions. The development of these bed defects is strongly coupled to the direction and magnitude of the shear stress applied to the bed throughout the wave cycle. Under current-dominated combined flow conditions, no defect propagation occurred in the upstream direction, despite the presence of flow reversal. In addition, spectral analysis of the evolution of the downstream and upstream edges of the defects demonstrated that: (i) periodicity in the defect growth pattern scales with the wave period for pure oscillatory flows, (ii) wave-dominated combined flows possess a period-driven peak with respect to defect growth in experiments with relatively short wave periods, but this peak is absent in experiments possessing relatively long wave periods (T 〉15 s), and (iii) current-dominated combined flows and pure unidirectional flows do not display a period-driven peak in defect growth. These results suggest that the occurrence of long-period combined flows bedforms may be under-represented in the sedimentary record.

Description: No abstract is available for this article.

Description: [1]  Sandy shorelines exposed to very oblique wave incidence can be unstable and develop self-organized shoreline sand waves. Different types of models predict the formation of these sand waves with an initially dominant alongshore wavelength in the range 1-10 km, which is quite common in nature. Here we investigate the physical reasons for such wavelength selection with the use of a linear stability model. The existence of a minimum wavelength for sand wave growth is explained by an interplay of three physical effects: a) largest relative (to the local shoreline) wave angle at the downdrift flank of the sand wave, b) wave energy concentration at the updrift flank due to less refractive energy dispersion and c) wave energy concentration slightly downdrift of the crest due to refractive focusing. For small wavelengths, effects (a) and (c) dominate and cause decay, while for larger wavelengths, effect (b) becomes dominant and causes growth. However, the alongshore gradients in sediment transport decrease for increasing wavelength, making the growth rate diminish. There is therefore a growth rate maximum giving a dominant wavelength, L M . In contrast with previous studies, we show that L M scales with λ 0 / β ( λ 0 is the wavelength of the offshore waves and β is the mean shoreface slope, from shore to the wave base), an estimate of the order of magnitude of the distance waves travel to undergo appreciable transformation. Our model investigations show that the proportionality constant between L M and λ 0 / β is typically in the range 0.1-0.4, depending mainly on the wave incidence angle.

Description: [1]  New remote sensing technologies and improved computer performance now allow numerical flow modeling over large stream domains. However, there has been limited testing of whether channel topography can be remotely mapped with accuracy necessary for such modeling. We assessed the ability of the Experimental Advanced Airborne Research Lidar (EAARL), to support a multi-dimensional fluid dynamics model of a small mountain stream. Random point elevation errors were introduced into the lidar point cloud and predictions of water surface elevation, velocity, bed shear stress and bed mobility were compared to those made without the point errors. We also compared flow model predictions using the lidar bathymetry with those made using a total station channel field survey. Lidar errors caused 〈 1 cm changes in the modeled water surface elevations. Effects of the point errors on other flow characteristics varied with both the magnitude of error and the local spatial density of lidar data. Shear stress errors were greatest where flow was naturally shallow and fast and lidar errors caused the greatest changes in flow cross-sectional area. The majority of the stress errors were less than ± 5 Pa. At near bankfull flow, the predicted mobility state of the median grain size changed over ≤ 1.3% of the model domain as a result of lidar elevation errors and ≤ 3% changed mobility in the comparison of lidar and ground-surveyed topography. In this riverscape, results suggest an airborne bathymetric lidar can map channel topography with sufficient accuracy to support a numerical flow model.

Description: [1]  Abrasion by bed load is an important erosional mechanism of fluvial incision into bedrock. [36] introduced a saltation-abrasion model in which the erosion rate is linearly dependent on the kinetic energy transfer due to the vertical velocity of impacting grains. The model describes erosion of an “approximately planar” stream bed. However, the beds of most bedrock-floored channels exhibit topographic variations that yield deviations from a planar surface, which we refer to as “bed topography”. Observations show that this bed topography strongly affects erosion. Here, the saltation-abrasion model is extended for the case of a non-planar bed. A several-cm high bump, transverse to the flow direction, is repeated every 50 cm. The kinetic energy of grain impacts is calculated in two ways, based on (1) the impact velocity normal to bed topography; and (2) the vertical impact velocity. By comparing the latter case with the original planar model, it is possible to isolate the effects of topography on the interception of saltation trajectories, and thus on the impact rate of saltating grains. Incorporating bed topography into the saltation abrasion model changes the simulated erosion in three ways. First, erosion with bed topography is 10 to 1000 times faster, depending upon transport stage and grain size. Enhanced erosion results from both the interception of saltation trajectories by topography and the increased kinetic energy transfer associated with high angle impacts on the stoss side of bumps. Second, erosion increases monotonically with transport stage, whereas maximum erosion occurs at low to intermediate transport stage with a planar bed. Third, erosion decreases monotonically with grain size, whereas maximal erosion occurs with intermediate-sized grains for the planar bed. Although the model is highly simplified, the results show that bed topography should be considered when simulating erosion by abrasion in bedrock channels.

Description: [1]  We develop a simple model to describe vertical profiles of velocity and suspended sediment concentration in submarine channels. We consider a conservative turbidity current flowing in a confined channel under steady and uniform flow conditions. The turbulence closure for density stratification is adapted from the model of [20]. Solutions are obtained for both straight and constant curvature channels. In the latter case, in order to evaluate the secondary flow induced by curvature, we take advantage of the fact that the ratio of flow depth to radius of curvature is typically small in the field, which leads to a solution of the governing equations through an appropriate asymptotic expansion. Comparison of results on longitudinal velocity profiles in straight channels with experimental and field observations shows an excellent agreement and allows for the prediction of concentration profiles and suspended sediment size from known bed slope, current thickness, and normalized velocity at the turbidity current-ambient water interface. The model is able to capture the presence of multiple circulation cells on the vertical and the sense of rotation of the near-bottom secondary flow that can be either reversed (directed outward) or normal oriented (directed inward) with respect to the classical fluvial orientation. The parameters controlling the orientation of secondary flow in submarine channels are identified, and the implications on the bed morphology are discussed. The potential use and future developments of the present approach are also discussed.

Description: [1]  We present a nonlinear asymptotic theory of fully developed flow and bed topography in a wide channel of constant curvature to describe finite-amplitude perturbations of bottom topography, subject to an inerodible bedrock layer. The flow field is evaluated at the leading order of approximation as a slowly varying sequence of locally uniform flows, slightly perturbed by a weak curvature-induced secondary flow. Using the constraint of constant fluid discharge and sediment flux, we calculate an analytical solution for the cross-sectional profile of flow depth and bed topography, and we determine the average slope in the bend necessary to transport the sediment supplied from a straight, alluvial, upstream reach. Both fully-alluvial bends and bends with partial bedrock exposure are shown to require a larger average slope than a straight upstream reach; the relative slope increase is much larger for mixed bedrock-alluvial bends. Curvature and sediment supply are shown to have a strong effect on the characteristics of the point bars in mixed bedrock-alluvial channels. Higher curvature bends produce bars of larger amplitude and more bedrock exposure through the cross section, and increasing the sediment supply leads to taller and wider point bars. Differences in the relative roughness of sediment and bedrock have a smaller, secondary effect on point bar characteristics. Our analytical approach can potentially be extended to the case of arbitrary, yet slowly-varying, curvature, and should ultimately lead to an improved understanding of the formation of meanders in bedrock channels.

Description: Understanding the controls on the amount of surface meltwater that refreezes, rather than becoming runoff, over polar ice masses is necessary for modelling their surface mass balance and ultimately for predicting their future contributions to global sea level change. We present a modified version of a physically-based model that includes an energy balance routine and explicit calculation of near-surface meltwater refreezing capacity, to simulate the evolution of near-surface density and temperature profiles across Devon Ice Cap in arctic Canada. Uniquely, our model is initiated and calibrated using high spatial resolution measurements of snow and firn densities across almost the entire elevation range of the ice cap for the summer of 2004 and subsequently validated with the same type of measurements obtained during the very different meteorological conditions of summer 2006. The model captures the spatial variability across the transect in bulk snowpack properties although it slightly underestimates the flow of meltwater into the firn of previous years. The percentage of meltwater that becomes runoff is similar in both years, however, the spatial pattern of this melt-runoff relationship is different in the two years. The model is found to be insensitive to variation in the depth of impermeable layers within the firn, but is very sensitive to variation in air temperature, since the refreezing capacity of firn decreases with increasing temperature. We highlight that the sensitivity of the ice cap's surface mass balance to air temperature is itself dependent on air temperature.

Description: The surface structure of static armor layers generated from water-worked gravel bed channels was investigated with primary focus on the influence of sand content and flow rate. Flume experiments were conducted in which four sediment mixtures with sand contents between 1% and 38% were armored under one of three different flow rates. First and second order statistical analysis was applied to digital elevation models of unarmored, armored, and clustered bed surface areas to identify changes in surface structure. Results were combined with data from previous research to create an extended dataset of armored bed surfaces. Water-worked, unarmored bed surfaces established under a dynamic equilibrium flow rate impacted the topographic variability and structure of the armored beds. Surface complexity decreased with armor formation as surface grains preferentially aligned with the flow direction. The bed surface became smoother and, where sediment mixture sand content was constant, there was greater smoothing of the surface during higher armoring flows as grains re-arranged more easily. As bulk sand content increased, statistical analyses of the expanded data set showed beds with very little sand content developed static armor layers that remained rough and had greater topographic variability than armor layers from sediments with higher sand contents. The bulk sediment sand content exerted a stronger influence over the change in surface roughness and structure upon armoring than that of the flow rate during armor formation. When combined with knowledge of the local flow regime, the sand content may aid in predictions related to armored bed surface structure.

Description: Stick-slip behavior is a distinguishing characteristic of the flow of Whillans Ice Stream (Siple Coast, Antarctica). Distinct from stick-slip on northern hemisphere glaciers, which is generally attributed to supraglacial melt, the behavior is thought be be controlled by basal processes and by tidally-induced stress. However, the connection between stick-slip behavior and flow of the ice stream on long time scales, if any, is not clear. To address this question we develop a new ice flow model capable of reproducing stick-slip cycles similar to ones observed on the Whillans Ice Plain. The model treats ice as a viscoelastic material and emulates the weakening and healing that are suggested to take place at the ice-till interface. The model results suggest the long-term ice stream flow that controls ice discharge to surrounding oceans is somewhat insensitive to certain aspects of stick-slip behavior, such as velocity magnitude during the slip phase and factors that regulate it (e.g., elastic modulus). Furthermore, it is found that factors controlling purely viscous flow, such as temperature, influence stick-slip contribution to long-term flow in much the same way. Additionally, we show that viscous ice deformation, traditionally disregarded in analysis of stick-slip behavior, has a strong effect on the timing of slip events, and therefore should not be ignored in efforts to deduce bed properties from stick-slip observations.

Description: No abstract is available for this article.

Description: Dunes dominate the bed of sand rivers and are of central importance in predicting flow roughness and water levels. The present study has focused on the details of flow and sediment dynamics along migrating sand dunes in equilibrium. Using a recently developed acoustic system (ACVP: Acoustic Concentration and Velocity Profiler), new insights are obtained in the behavior of the bed and the suspended load transport along mobile dunes. Our data have illustrated that, due to the presence of a dense sediment layer close to the bed and migrating secondary bedforms over the stoss side of the dune towards the dune crest, the near-bed flow and sediment processes are significantly different from the near-bed flow and sediment dynamics measured over fixed dunes. It was observed that the shape of the total sediment transport distribution along dunes is mainly dominated by the bed load transport although the bed load and the suspended load transport are of the same order of magnitude. This means that it was especially the bed load transport that is responsible for the continuous erosion and deposition of sediment along the migrating dunes. Whereas the bed load is entirely captured in the dune with zero transport at the flow reattachment point, a significant part of the suspended load is advected to the downstream dune depending on the flow conditions. For the two flow conditions measured, the bypass fraction was about 10% for flow with a Froude number ( Fr ) of 0.41 and 27% for flow with Froude number of 0.51. This means that respectively 90% (for the Fr = 0.41 flow) and 73% (for the Fr = 0.51 flow) of the total sediment load that arrived at the dune crests contributed to the migration of the dunes.

Description: No abstract is available for this article.

Description: Glaciers respond to climate variations and leave geomorphic evidence that represents an important terrestrial paleoclimate record. However, the accuracy of paleoclimate reconstructions from glacial geology is limited by the challenge of representing mountain meteorology in numerical models. Precipitation is usually treated in a simple manner and yet represents difficult-to-characterize variables such as amount, distribution and phase. Furthermore, precipitation distributions during a glacial probably differed from present-day interglacial patterns. We applied two models to investigate glacier sensitivity to temperature and precipitation in the eastern Southern Alps of New Zealand. A 2-D model was used to quantify variations in the length of the reconstructed glaciers resulting from plausible precipitation distributions compared to variations in length resulting from change in mean annual air temperature and precipitation amount. A 1-D model was used to quantify variations in length resulting from interannual climate variability. Assuming that present-day interglacial values represent precipitation distributions during the last glacial, a range of plausible present-day precipitation distributions resulted in uncertainty in the Last Glacial Maximum length of the Pukaki Glacier of 17.1 km (24%) and the Rakaia Glacier of 9.3 km (25%), corresponding to a 0.5 °C difference in temperature. Smaller changes in glacier length resulted from a 50% decrease in precipitation amount from present-day values (–14% and –18%), and from a 50% increase in precipitation amount (5% and 9%). Our results demonstrate that precipitation distribution can produce considerable variation in simulated glacier extents, and that reconstructions of paleoglaciers should include this uncertainty.

Description: The Arctic climate is changing, inducing accelerating retreat of ice-rich permafrost coastal bluffs. Along Alaska's Beaufort Sea coast, erosion rates have increased roughly 3-fold from 6.8 to 19 m yr − 1 since 1955 while the sea-ice-free season has increased roughly 2-fold from 45 to 100 days since 1979. We develop a numerical model of bluff retreat to assess the relative roles of the length of sea-ice-free season, sea level, water temperature, nearshore wave field, and permafrost temperature incontrolling erosion rates in this setting. The model captures the processes of erosion observed in short term monitoring experiments along the Beaufort Sea coast, including evolution of melt-notches, topple of ice-wedge-bounded blocks, and degradation of these blocks. Model results agree with time-lapse imagery of bluff evolution and time series of ocean-based instrumentation. Erosion is highly episodic with 40% of erosion is accomplished during less than 5% of the sea-ice-free season. Among the formulations of the submarine erosion rate we assessed, we advocate those that employ both water temperature and nearshore wavefield. As high water levels are a prerequisite for erosion, any future changes that increase the frequency with which water levels exceed the base of the bluffs will increase rates of coastal erosion. The certain increases in sea level and potential changes in storminess will both contribute to this effect. As water temperature also influences erosion rates, any further expansion of the sea-ice-free season into the mid-summer period of greatest insolation is likely to result in an additional increase in coastal retreat rates.

Description: We have detected over 150,000 small (M 〈 1) low frequency (~1-5 Hz) repeating earthquakes over the past decade at Mount Rainier volcano, most of which were previously undetected. They are located high (〉3000 m) on the glacier-covered edifice and occur primarily in week- to month-long swarms composed of simultaneous distinct families of events. Each family contains up to thousands of earthquakes repeating at regular intervals as often as every few minutes. Mixed polarity first motions, a linear relationship between recurrence interval and event size, and strong correlation between swarm activity and snowfall suggest the source is stick-slip basal sliding of glaciers. The sudden added weight of snow during winter storms triggers a temporary change from smooth aseismic sliding to seismic stick-slip sliding in locations where basal conditions are favorable to frictional instability. Coda wave interferometry shows that source locations migrate over time at glacial speeds, starting out fast and slowing down over time, indicating a sudden increase in sliding velocity triggers the transition to stick-slip sliding. We propose a hypothesis that this increase is caused by the redistribution of basal fluids rather than direct loading because of a 1-2 day lag between snow loading and earthquake activity. This behavior is specific to winter months because it requires the inefficient drainage of a distributed subglacial drainage system. Identification of the source of these frequent signals offers a view of basal glacier processes, discriminates against alarming volcanic noises, documents short-term effects of weather on the cryosphere, and has implications for repeating earthquakes in general.

Description: Bedrock river valleys are fundamental components of many landscapes, and their morphologies—from slot canyons with incised meanders to wide valleys with strath terraces—may record environmental history. Several formation mechanisms for particular valley types have been proposed that involve changes in climatic and tectonic forcing, but the uniqueness of valley evolution pathways and the long-term stability of valley morphology under constant forcing are unknown, and are not predicted in existing numerical models for vertically incising rivers. Because rivers often migrate more rapidly through alluvium than through bedrock, we explore the hypothesis that the distribution of bank materials strongly influences river meandering kinematics and can explain the diversity of bedrock river valley morphology. Simulations using a numerical model of river meandering with vector-based bank-material tracking indicate that channel lateral erosion rate in sediment and bedrock, vertical erosion rate, and initial alluvial-belt width explain first-order differences in bedrock valley type; that bedrock-bound channels can evolve under steady forcing from alluvial states; and that weak bedrock and low vertical incision rates favor wide, shallow valleys, while resistant bedrock and high vertical incision rates favor narrow, deep valleys. During vertical incision, sustained planation of the valley floor is favored when bedrock boundaries restrict channel migration to a zone of thin sediment fill. The inherent unsteadiness of river meandering in space and time is enhanced by evolving spatial contrasts in bank strength between sediment and bedrock and can account for several valley features—including strath terraces and underfit valleys—commonly ascribed to external drivers.

Description: We describe a geophysical technique to measure englacial vertical velocities through to the beds of ice sheets without the need for borehole drilling. Using a ground-based phase-sensitive radio-echo sounder (pRES) during seven Antarctic field seasons, we measure the temporal changes in the position of englacial reflectors within ice divides up to 900 m thick on Berkner Island, Roosevelt Island, Fletcher Promontory and Adelaide Island. Recorded changes in reflector positions yield 'full-depth' profiles of vertical ice velocity that we use to examine spatial variations in ice flow near the divides. We interpret these variations by comparing them to the results of a full-Stokes simulation of ice-divide flow, qualitatively validating the model and demonstrating that we are directly detecting an ice-dynamical phenomenon called the Raymond Effect. Using pRES, englacial vertical ice velocities can be measured in higher spatial resolution than is possible using instruments installed within the ice. We discuss how these measurements could be used with inverse methods to measure ice rheology, and to improve ice-core dating by incorporating pRES-measured vertical velocities into age modelling.

Description: Many tidewater outlet glacier fjords surround the coast of Greenland, and their dynamics and circulation are of great importance for understanding the heat transport towards glaciers from the Ice Sheet. Thus, fjord circulation is a critical aspect for assessing the threat of global sea level rise due to melting of the ice sheet. However, very few observational studies describe the seasonal dynamics of fjord circulation. Here, we present the first continuous current measurements (April-November) from a deep mooring deployed in a West Greenland tidewater outlet glacier fjord. Four distinct circulation phases are identified during the period, and they are related to exchange processes with coastal waters, tidal mixing and melt processes on the Greenland Ice Sheet. During early summer, warm intermediate water is transported towards the glacier at an average velocity of about 7 cm s -1 . In late summer, the average velocity decreases to 3 cm s -1 during a period with significant subglacial freshwater discharges. During this period, a large variability in current velocities is also observed. The associated average heat transport in an intermediate depth range corresponds to 568 GW in early summer and is reduced to 287 GW in late summer. These heat fluxes are at the higher end of previously reported fluxes. Our measurements show that the intermediate heat transport varies over time and during summer provides a major contribution to the heat budget and, thereby, potentially to glacial melt. We suggest that intermediate heat transport may play a similar important role in other fjords around Greenland.

Description: Although the Greenland Ice Sheet (GrIS) is losing mass at an accelerating rate, much uncertainty remains about how surface runoff interacts with the subglacial drainage system and affects water pressures and ice velocities, both currently, and into the future. Here, we apply a physically-based, subglacial hydrological model to the Paakitsoq region, west Greenland, and run it into the future to calculate patterns of daily subglacial water pressure fluctuations in response to climatic warming. The model is driven with moulin input hydrographs calculated by a surface routing model, forced with distributed runoff. Surface runoff and routing are simulated for a baseline year (2000), before the model is forced with future climate scenarios for the years 2025, 2050 and 2095, based on the IPCC's Representative Concentration Pathways (RCPs). Our results show that as runoff increases throughout the 21 st century, and/or as RCP scenarios become more extreme, the subglacial drainage system makes an earlier transition from a less efficient network operating at high water pressures, to a more efficient network with lower pressures. This will likely cause an overall decrease in ice velocities for marginal areas of the GrIS. However, short-term variations in runoff, and therefore subglacial pressure, can still cause localized speedups, even after the system has become more efficient. If these short-term pressure fluctuations become more pronounced as future runoff increases, the associated late-season speedups may help to compensate for the drop in overall summer velocities, associated with earlier transitioning from a high to a low pressure system.

Description: No abstract is available for this article.

Description: We investigate the morphodynamic equilibrium of tidally dominated alluvial estuaries, extending previous works concerning the purely tidal case and the combined tidal-fluvial case with a small tidal forcing. We relax the latter assumption and seek the equilibrium bed profile of the estuary, for a given planform configuration with various degrees of funneling, solving numerically the 1D governing equation. The results show that with steady fluvial and tidal forcings an equilibrium bed profile of estuaries exists. In the case of constant width estuaries, a concave down equilibrium profile develops through most of the estuary. Increasing the amplitude of the tidal oscillation, progressively higher bed slopes are experienced at the mouth whilst the river-dominated portion of the estuary experiences an increasing bed degradation. The fluvial-marine transition is identified by a ‘tidal length’ that increases monotonically as the river discharge and the corresponding sediment supply are increased while the river attains a new morphological equilibrium configuration. Tidal length also increases if, for a fixed river discharge and tidal amplitude, the sediment flux is progressively reduced with respect to the transport capacity. In the case of funnel-shaped estuaries the tidal length strongly decreases, aggradation is triggered by channel widening and tidal effects are such to enhance the slope at the inlet and the net degradation of the river bed. Finally, results suggest that alluvial estuaries in morphological equilibrium cannot experience any amplification of the tidal wave propagating landward. Hence, hyper-synchronous alluvial estuaries cannot be in equilibrium.