ALBERT

Beschreibung: No abstract is available for this article.

Beschreibung: The comparative role of sub-tidal water level and wave height variations on seasonal shoreline changes was investigated at a reef-fringed beach in southwestern Australia. The dataset consisted of continuous sea level and wave records, monthly topographic beach surveys over a 2 year period, and bi-monthly high-resolution aerial images over a 7 year period. Shorelines were extracted from images and topographic surveys and then an empirical orthogonal function (EOF) analysis was applied to both datasets. The temporal amplitudes of the first EOF mode of the image-derived shoreline data set (∼60% of the variance) were most correlated with 30-day averages of the sub-tidal water level (variations up to +/-0.2 m) driven by geostrophic adjustment of the Leeuwin Current. The geostrophic response of the Leeuwin Current was found to be further correlated with the phasing of the El Niño Southern Oscillation (ENSO), leading to higher water levels during La Niña and lower levels during El Niño, and as a consequence, resulting in inter-annual variations in the shoreline behavior reflected in the first EOF mode. The temporal amplitudes of the second EOF mode (∼20% of the variance) were most correlated with 50-day averages in offshore wave height. Our results indicate the seasonal beach response was primarily influenced by seasonal variations in offshore water level rather than by wave heights as has been generally observed in exposed beaches. A simple EOF based model is presented which reproduces the alongshore-variable shoreline response and its seasonal and inter-annual modulation due to ENSO events over the study period.

Beschreibung: The Earth's large continental ice sheets contain a variety of naturally-occurring impurities, both soluble and insoluble. Understanding how these impurities affect the rheology, intrinsic thermodynamic properties, and fate of these ice sheets is not well understood. To investigate the effects that trace amounts of H 2 SO 4 have on the flow and ductility of polycrystalline ice, a series of mechanical tests were conducted at -6°C, -10°C, -12.5°C, and -20°C using laboratory-prepared specimens of polycrystalline ice doped with 1-15 ppm of H 2 SO 4 . Parallel tests were performed on identical, but undoped specimens of polycrystalline ice. Mechanical testing included constant-load tensile creep tests at an initial stress of 0.75 MPa and compression tests at constant displacement rates with initial strain rates ranging from 1x10 -6 s -1 to 1x10 -4 s -1 . It was found that H 2 SO 4 -doped specimens of ice exhibited faster creep rates in tension and significantly lower peak stresses in compression, when compared to the undoped ice. Post-mortem microstructural analyses were performed using cross-polarized light thin section imaging, X-ray computed microtomography, Raman spectroscopy, and electron backscatter diffraction. These analyses showed that H 2 SO 4 -doped specimens had a larger grain size at strains ≤15%, and an earlier onset of micro-cracking at lower strain rates than the undoped ice. Strain-induced grain boundary migration was found to be the predominant mechanism of recrystallization in both doped and undoped specimens. Further, an aqueous phase of H 2 SO 4 was found to exist at the grain boundaries and triple junctions of the doped ice, which is thought to have significantly contributed towards its increase in ductility.

Beschreibung: No abstract is available for this article.

Beschreibung: Due to the challenges in upscaling daily climatic forcing to geological time, physically realistic models describing how rainfall drives fluvial erosion are lacking. To bridge this gap between short-term hydrology and long-term geomorphology we derive a theoretical framework for long-term fluvial erosion rates driven by realistic climate by integrating an established stochastic-mechanistic model of hydrology into a threshold-stochastic formulation of stream power. The hydrological theory provides equations for the daily streamflow probability distribution as a function of climatic boundary conditions. The new parameters introduced are rooted firmly in established climatic and hydrological theory. This allows us to account for how fluvial erosion rates respond to changes in rainfall intensity, frequency, evapotranspiration rates, and soil moisture dynamics in a way that is consistent with existing theories. We use this framework to demonstrate how hydroclimatic conditions and erosion threshold magnitude control the degree of nonlinearity between steepness index and erosion rate. We find that hydrological processes can have a significant influence on how erosive a particular climatic forcing will be. By accounting for the influence of hydrology on fluvial erosion, we conclude that climate is an important control on erosion rates and longterm landscape evolution.

Beschreibung: Sediment particles transported as bed load undergo alternating periods of motion and rest, particularly at weak flow intensity. Bed-load transport can be investigated by either following the motion of individual particles (Lagrangian approach) or observing the phenomenon at prescribed locations (Eulerian approach). In this paper, the Lagrangian and Eulerian descriptions are merged into a unifying framework that includes definitions for quantities used to describe the kinematics of particle motion, as well as the relationships among these quantities. The alternation of motion and rest is represented by two complementary descriptions: (i) proportion of motion, indicating either the relative time spent in motion by an individual particle or the relative number of moving particles; (ii) persistence of motion , indicating the extent to which the process consists of relatively few long periods of motion or of many short ones. The framework only involves first moments of the key quantities. The conceptual developments are tested against results from an experiment with weak bed-load transport, demonstrating the soundness of the approach. From an operational point of view, a Lagrangian observation is difficult to perform, since the particle motion is usually investigated for finite spatial domains (e.g., a measurement window within a laboratory or natural reach). Strategies to overcome such limitations are described, suggesting the possibility of obtaining unbiased mean values for Lagrangian descriptors. The proposed framework can be used in any study aimed at parameterizing the kinematic properties of bed-load particles as functions of the hydrodynamic conditions.

Beschreibung: Island formation and distributary channel branching are important processes in prograding river deltas. We develop and test a new theory predicting the distance to islands and channel bifurcations based on fluid mass conservation and radially symmetric transport conditions. We analyze channelization and island formation using 9 new and 5 existing delta experiments as well as 4 field deltas. The new experiments were designed to produce islands from initial deposition of a mouth bar. Before island formation, each bar evolved into a radially symmetric deposit with unchannelized flow over its top previously described as a topographic flow expansion. This morphology was stable to topographic perturbations, and its distal limit prograded basinward while maintaining a characteristic flow depth. Island formation and channel branching occurred on top of this deposit. We hypothesize that this distance ( Ψ ) is set by the location where boundary shear stress applied by expanding, radially-averaged flow falls below the threshold of sediment motion. The model predicts that the distance to the first island scales with water discharge, scales inversely with flow depth, and scales with the inverse square-root of median grain diameter. From experiment to field scales, distances to island locations are predicted within a factor of two.

Beschreibung: Coupled bed-flow direct numerical simulations investigating the early stages of pattern formation and bedform (ripple) interactions were examined in a previous paper (Part I), making use of the resolved flow field. In this paper (Part II), we compare our results to published experimental data and provide an extensive quantitative analysis of the bed using spectral analysis and two-point correlations. The effect of the mobile rippled bed on the flow structure as well as turbulence is investigated locally (at specific streamwise locations) as well as over the entire computational domain. We show that developing ripples attain a self-similar profile in both the shape and the corresponding bed shear stress. We demonstrate the importance of neighbouring structures, especially upstream neighbours, on bedform dynamics in terms of the growth, decay, and speed of ripples. Finally, we examine the defect-free interactions in the later stages of bed evolution, which primarily lead to wave coarsening.

Beschreibung: We present results of coupled direct numerical simulations between flow and a deformable bed in a horizontally periodic, turbulent open channel at a shear Reynolds number of Re τ  = 180 . The feedback between the temporally and spatially evolving bed and the flow is enforced via the immersed boundary method. Using the near-bed flow field, we provide evidence on the role of locally intense near-bed vortical structures during the early stages of bed formation, from the emergence of quasi-streamwise streaks, to the formation of incipient bedform crestlines. Additionally, we take a new look at a number of defect-related bedform interactions, including lateral linking, defect and bedform repulsion, merging, as well as defect creation and show that the underlying mechanisms, in these flow-aligned interactions, are very similar to each other. Consequently, the interactions are labelled differently depending on the geometry of interacting structures and the outcome of the interaction. In the companion paper, we compare our results to published experimental data and provide an extensive quantitative analysis of the bed, where we demonstrate the importance of neighbouring structures, especially upstream neighbours, on bedform dynamics (growth/decay and speed) and wave coarsening. Video files of bed evolution are available in supplemental materials.

Beschreibung: This study expands the widely-used one-dimensional, four-equation model for turbidity currents to account for mass, momentum, and energy sinks associated with flow stripping and overspill processes acting upon the turbidity current suspension cloud. The suspension cloud is defined as any portion of the flow extending above the channel levees. The expanded model allows steady turbidity currents to evolve to a uniform, or equilibrium, state where mass, momentum, and energy gained through sediment and clear-water entrainment processes are balanced by the mass, momentum, and energy lost to flow stripping and overspill. We perform a sensitivity analysis of the expanded model to assess how changes in channel (e.g., slope, channel height, width, bed friction, and radius of curvature) and flow (e.g., sediment grain-size, suspension cloud concentration, turbulence) properties affect the equilibrium flow conditions. By varying the model inputs from half to double their base-case values, we find that the equilibrium values can change by up to a factor of two. We find that the equilibrium conditions, including the flow Richardson number, were generally most sensitive to changes in slope, channel height, grain-size, and suspension cloud concentration. Additionally, we identify model parameter space where Richardson subcritical equilibrium flow is possible. The fact that turbidity currents can attain equilibrium, and particularly subcritical equilibrium, might help elucidate the long turbidity current runout distances on low slopes inferred from extensive submarine channel-levee systems.