ALBERT

Description: Detrital provenance analyses in orogenic settings, in which sediments are collected at the outlet of a catchment, have become an important tool to es-timate how erosion varies in space and time. Here we present how Raman Spectroscopy on Carbonaceous Material (RSCM) can be used for provenance analysis. RSCM provides an estimate of the peak temperature (RSCM-T) experienced during metamorphism. We show that we can infer modern ero-sion patterns in a catchment by combining new measurements on detrital sands with previously acquired bedrock data. We focus on the Whataroa catch-ment in the Southern Alps of New Zealand and exploit the metamorphic gra-dient that runs parallel to the main drainage direction. To account for po-tential sampling biases, we also quantify abrasion properties using flume ex-periments and measure the total organic carbon content in the bedrock that produced the collected sands. Finally, we integrate these parameters into a mass-conservative model. Our results fist demonstrate that RSCM-T can be used for detrital studies. Second, we finnd that spatial variations in tracer concentration and erosion have a rst-order control on the RSCM-T distri-butions, even though our flume experiments reveal that weak lithologies pro-duce substantially more ne particles than do more durable lithologies. This result implies that sand specimens are good proxies for mapping spatial vari-ations in erosion when the bedrock concentration of the target mineral is quan-tied. The modeling suggests highest present-day erosion rates (in Whataroa catchment) are not situated at the range front, but around 10 km into the mountain belt.

Description: In gravel-bed rivers, bed topography and the bed surface grain size distribution evolve simultaneously, but it is not clear how feedbacks between topography and grain sorting affect channel morphology. In this, the second of a pair of papers examining interactions between bed topography and bed surface sorting in gravel bed rivers, we use a two-dimensional morphodynamic model to perform numerical experiments designed to explore the coevolution of both free and forced bars and bed surface patches. Model runs were carried out on a computational grid simulating a 200 m long, 2.75 m wide, straight, rectangular channel, with an initially flat bed at a slope of 0.0137. Over five numerical experiments, we varied a) whether an obstruction was present, b) whether the sediment was a gravel mixture or a single size, and c) whether the bed surface grain size feeds back on the hydraulic roughness field. Experiments with channel obstructions developed a train of alternate bars that became stationary and were connected to the obstruction. Freely migrating alternate bars formed in the experiments without channel obstructions. Simulations incorporating roughness feedbacks between the bed surface and flow field produced flatter, broader, and longer bars than simulations using constant roughness or uniform sediment. Our findings suggest that patches are not simply a byproduct of bed topography, but they interact with the evolving bed and influence morphologic evolution.

Description: Accurate numerical simulation can provide crucial information useful for a greater understanding of destructive granular mass movements such as rock avalanches, landslides and pyroclastic flows. It enables more informed and relatively low cost investigation of significant risk factors, mitigation strategy effectiveness and sensitivity to initial conditions, material or soil properties. In this paper, a granular avalanche experiment from the literature is re-analysed and used as a basis to assess the accuracy of Discrete Element Method (DEM) predictions of avalanche flow. Discrete granular approaches such as DEM simulate the motion and collisions of individual particles and are useful for identifying and investigating the controlling processes within an avalanche. Using a super-quadric shape representation, DEM simulations were found to accurately reproduce transient and static features of the avalanche. The effect of material properties on the shape of the avalanche deposit was investigated. The simulated avalanche deposits were found to be sensitive to particle shape and friction, with the particle shape causing the sensitivity to friction to vary. The importance of particle shape, coupled with effect on the sensitivity to friction highlights the importance of quantifying and including particle shape effects in numerical modelling of granular avalanches.

Description: No abstract is available for this article.

Description: We investigate numerically the failure, collapse and flow of a two-dimensional brittle granular column over a horizontal surface. In our discrete element simulations, we consider a vertical monolayer of spherical particles that are initially held together by tensile bonds, which can be irreversibly broken during the collapse. This leads to dynamic fragmentation within the material during the flow. Compared to what happens in the case of a non-cohesive granular column, the deposit is much rougher, and the internal stratigraphic structure of the column is not preserved during the collapse. As has been observed in natural rockslides, we find that the deposit consists of large blocks laying on a lower layer of fine fragments. The influence of the aspect ratio of the column on the run-out distance is the same as in the non-cohesive case. Finally, we show that for a given aspect ratio of the column, the run-out distance is higher when the deposit is highly fragmented, which confirms previous hypotheses proposed by Davies et al . [1999].

Description: We describe a theoretical and numerical framework that has been developed to investigate the compatibility of the ICE-6G_C reconstruction of the glaciation histories of the Greenland and Antarctic ice sheets with the latest understanding of ice physics. The ICE-6G_C reconstruction has been produced solely on the basis of the theory of the glacial isostatic adjustment (GIA) process and it has remained an issue as to whether such reconstructions of the time dependent thickness variations of grounded continental ice sheets were compatible with physics-based ice mechanical considerations. Our analyses focus on the evolution over the last glacial cycle of these extent ice sheet complexes and demonstrate that the GIA inferred models are entirely consistent with such considerations when uncertainties in (net) mass balance history are taken fully into account.

Description: Dynamic equilibrium theory is a fruitful concept, which we use to systematically explain the tidal flat morphodynamic response to tidal currents, wind waves, sediment supply and other sedimentological drivers. This theory stems from a simple analytical model that derives the tide- or wave-dominated tidal flat morphology by assuming morphological equilibrium is associated with uniform bed shear stress distribution. Many studies based on observation and process-based modeling tend to agree with this analytical model. However, a uniform bed shear stress rarely exists on actual or modeled tidal flats, and the analytical model cannot handle the spatially and temporally varying bed shear stress. In the present study, we develop a model based on the dynamic equilibrium theory and its core assumption. Different from the static analytical model, our model explicitly accounts for the spatiotemporal bed shear stress variations for tidal flat dynamic prediction. To test our model and the embedded theory, we apply the model for both long-term and short-term morphological predictions. The long-term modeling is evaluated qualitatively against previous process-based modeling. The short-term modeling is evaluated quantitatively against high-resolution bed-level monitoring data obtained from a tidal flat in the Netherlands. The model results show good performances in both qualitative and quantitative tests, indicating the validity of the dynamic equilibrium theory. Thus, this model provides a valuable tool to enhance our understanding of the tidal flat morphodynamics and to apply the dynamic equilibrium theory for realistic morphological predictions.

Description: Riverbeds frequently display a spatial structure where the sediment mixture composing the channel bed has been sorted into discrete patches of similar grain size. Even though patches are a fundamental feature in gravel-bed rivers, we have little understanding of how patches form, evolve, and interact. Here, we present a two-dimensional morphodynamic model that is used to examine in greater detail the mechanisms responsible for the development of forced bed surface patches and the coevolution of bed morphology and bed surface patchiness. The model computes the depth-averaged channel hydrodynamics, mixed-grain-size sediment transport, and bed evolution by coupling the river morphodynamic model FaSTMECH with a transport relation for gravel mixtures and the mixed-grain-size Exner equation using the active layer assumption. To test the model, we use it to simulate a flume experiment in which the bed developed a sequence of alternate bars and temporally and spatially persistent forced patches with a general pattern of coarse bar tops and fine pools. Cross-stream sediment flux causes sediment to be exported off of bars and imported into pools at a rate that balances downstream gradients in the streamwise sediment transport rate, allowing quasi-steady bar-pool topography to persist. The relative importance of lateral gravitational forces on the cross-stream component of sediment transport is a primary control on the amplitude of the bars. Because boundary shear stress declines as flow shoals over the bars, the lateral sediment transport is increasingly size-selective and leads to the development of coarse bar tops and fine pools.

Description: Roughness elements of varied scale and geometry commonly appear on the surfaces of sedimentary deposits in a wide range of planetary environments. They perturb the local fluid flow so that the entrainment, transport and deposition of particles surrounding each element is fundamentally altered. Fluid dynamists have expended much effort in examining the flow structures surrounding idealized elements mounted on fixed, planar walls. However self-regulation occurs in sedimentary systems as a result of the bed surface undergoing rapid topographic modification with sediment transport, until it reaches a stable form that enhances the net physical roughness. The present wind tunnel study examines how the flow pattern surrounding an isolated cylinder, a problem extensively studied in classical fluid mechanics, is altered through morphodynamic development of a deep well that envelopes the windward face and side walls of the roughness element. Spatial patterns in the fluid velocity, turbulence intensity and Reynolds stress obtained from laser Doppler anemometer (LDA) measurements suggest that the flow structures surrounding such a cylinder are fundamentally altered through self-regulation of the bed topography as it reaches steady state. For example, flow stagnation and the turbulent dissipation of momentum are substantially increased at selected points surrounding the upwind face and side walls of the cylinder, respectively. Along the center-line of the wake flow to the rear of the cylinder, several structures arising from flow separation are annihilated by strong upwelling of the airflow exhausted from the terminus of the well. Feedback plays a complex, time-dependent role in this system.

Description: We use a new discretization technique to solve the higher order thermomechanically coupled equations of glacier evolution. We find that under radially symmetric continuum equations, small perturbations in symmetry due to the discretization are sufficient to produce the initiation of non-symmetric thermomechanical instabilities which we interpret as ice streams, in good agreement with previous studies which have indicated a similar instability. We find that the inclusion of membrane stresses regularizes the size of predicted streams, eliminating the ill-posedness evident in previous investigations of ice stream generation through thermomechanical instability. Ice streams exhibit strongly irregular periodicity which is influenced by neighboring ice streams and the synoptic state of the ice stream. Ice streams are not always the same size, but instead appear to follow a temperature dependent distribution of widths that is robust to grid refinement. The morphology of the predicted ice streams corresponds reasonably well to extant ice streams in physically similar environments.