ALBERT

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: Our incomplete knowledge of the proportion of mass loss due to frontal ablation (the sum of ice loss through calving and submarine melt) from tidewater glaciers outside of the Greenland and Antarctic ice sheets has been cited as a major hindrance to accurate predictions of global sea level rise. We present a 28 year record (1985–2013) of frontal ablation for 27 Alaska tidewater glaciers (representing 96% of the total tidewater glacier area in the region), calculated from satellite-derived ice velocities and modeled estimates of glacier ice thickness. We account for cross-sectional ice thickness variation, long-term thickness changes, mass lost between an upstream flux gate and the terminus, and mass change due to changes in terminus position. The total mean rate of frontal ablation for these 27 glaciers over the period 1985–2013 is 15.11 ± 3.63 Gt a −1 . Two glaciers, Hubbard and Columbia, account for approximately 50% of these losses. The regional total ablation has decreased at a rate of 0.14 Gt a −1 over this time period, likely due to the slowing and thinning of many of the glaciers in the study area. Frontal ablation constitutes only ~ 4% of the total annual regional ablation, but roughly 20% of net mass loss. Comparing several commonly-used approximations in the calculation of frontal ablation we find that neglecting cross-sectional thickness variations severely underestimates frontal ablation.

Beschreibung: Dynamic equilibrium of short tidal systems with ebb deltas, inlets and basins is poorly understood. Observations suggest the possibility of equilibrium with sediment import balancing export, whilst individual channels and shoals at the local scale remain dynamic. Our objectives are to ascertain 1) whether tidal systems under entirely steady forcing can attain this state and 2) under what conditions cyclic channel-shoal migration occurs. We present experiments of tidal systems developing from an initial breach in the coast. We periodically tilted the entire flume to obtain reversing tidal currents and sediment transports. The surface area of the back-barrier basin with an inlet channel with erodible boundaries continued to enlarge whilst sediment mobility decreased. Experiments with fixed inlet boundaries remained smaller and much more dynamic and had cyclically migrating ebb and flood channels. The same cyclicity with a period of about 80 tides is observed in shifting dominance of the two channels of the inlet and in the shifting channels in the basin. Experiments with stepwise sea level rises resulted in more rapid channel and bar shifting, increased channel dimensions and basin size. We conclude that cyclic migration of channels is coupled between inlet and basin but the ebb delta did not show such cyclicity. Furthermore tidal basins with erodible boundaries slowly enlarge by margin erosion towards a system where sediment mobility is at the threshold for motion, as in braided gravel-bed rivers. Consequently, in nature dimensions of tidal systems are partly determined by naturally-formed cohesive and vegetated margins and geological context.

Beschreibung: No abstract is available for this article.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.