Abstract Sediment transport by wind or water near the threshold of grain motion is dominated by rare transport events. This intermittency makes it difficult to calibrate sediment transport laws, or to define an unambiguous threshold for grain entrainment, both of which are crucial for predicting sediment transport rates. We present a model that captures this intermittency and shows that the noisy statistics of sediment transport contain useful information about the sediment entrainment threshold and the variations in driving fluid stress. Using a combination of laboratory experiments and analytical results, we measure the threshold for grain entrainment in a novel way and introduce a new property, the “shear stress variability”, which predicts conditions under which transport will be intermittent. Our work suggests strategies for improving measurements and predictions of sediment flux and hints that the sediment transport law may change close to the threshold of motion.
<p>Glaciers are an effective agent of erosion and landscape evolution, capable of driving high rates of erosion and sediment production. Glacial erosion is therefore an important process mediating the effect of climate on erosion rates and tectonics. Further, as a source of sediment, glacial erosion also has implications for the carbon and silicate cycles, with the potential for longterm feedbacks. &#160;Understanding the interaction of climate, tectonics, glacial erosion and topography will lead to more insight into how glaciers can impact these processes. Simple, analytical long-profile models of fluvial incision are fundamental in tectonic geomorphology and critical for addressing fluvial analogues of problems such as those posed above. The advantage of these simple long-profile models is that they can be applied when information about forcing and boundary conditions is minimal (e.g. in deep time), and they can aid in the development of intuition about how such systems respond in general to different forcing. While models of glacial erosion have existed for quite some time, they tend to be complicated and computationally expensive. Currently, analytical long-profile models do not exist for glacial systems. At the same time, the patterns of glacial erosion and sediment transport, and how these processes respond to climate is fundamentally different than fluvial systems, and cannot be addressed properly with purely fluvial models.</p><p>Building on previous work, we introduce several simplifications to make the equations for coupled glacier-fluvial long-profile models easier to use and show that these simplifications have minimal effect on the steady state solution. We then use these new equations to develop an analytical solution for glacier-fluvial long-profiles at erosional steady state. The solution provides glacier geometry, including length and slope, ice thickness, and overall orogen relief for a given uplift rate, rock erodibility, profile length and climatic conditions. To explore the effect of glaciation on the balance between climate, erosion and orogen geometry, we integrate this solution into a critical wedge orogen theory. We find that the total orogen relief should be closely tied to the equilibrium line altitude (ELA), in line with the glacial buzzsaw theory. In addition, our theory predicts that the geometry and average uplift rate of glaciated critical wedge orogens respond more sensitively to changes in climate than those dominated by fluvial erosion. We suggest that the lowered ELA during glacial maxima over the last few million years could have triggered narrowing of critical orogens, with an associated increase in uplift rates within the active orogen core.&#160;</p>
Data from DEM-LBM simulations of round sediment particles and continuum models:-single_sphere: single sphere tests of DEM-LBM for validation-flume: DEM-LBM flume tests, compared with the corresponding experiments-wide_wall_free: DEM-LBM simulations, wide wall free cases-continuum: continuum modeling results-fluid_bc: pure fluid LBM tests to validate the boundary condition for the flume tests
data/: Data from DEM-LBM simulations of round sediment particles and continuum models -single_sphere: single sphere tests of DEM-LBM for validation -flume: DEM-LBM flume tests, compared with the corresponding experiments -wide_wall_free: DEM-LBM simulations, wide wall free cases -continuum: continuum modeling results -fluid_bc: pure fluid LBM tests to validate the boundary condition for the flume tests make_figures/: The figures can be generated using the Matlab files Code/: The programs used to get the DEM-LBM results and continuum modeling results are available
data/: Data from DEM-LBM simulations of round sediment particles and continuum models -single_sphere: single sphere tests of DEM-LBM for validation -flume: DEM-LBM flume tests, compared with the corresponding experiments -wide_wall_free: DEM-LBM simulations, wide wall free cases -continuum: continuum modeling results -fluid_bc: pure fluid LBM tests to validate the boundary condition for the flume tests make_figures/: The figures can be generated using the Matlab files Code/: The programs used to get the DEM-LBM results and continuum modeling results are available
Data from DEM-LBM simulations of round sediment particles and continuum models:-single_sphere: single sphere tests of DEM-LBM for validation-flume: DEM-LBM flume tests, compared with the corresponding experiments-wide_wall_free: DEM-LBM simulations, wide wall free cases-continuum: continuum modeling results-fluid_bc: pure fluid LBM tests to validate the boundary condition for the flume tests
Abstract. Following the tradition of modeling fluvial landscape evolution, a novel approach describing glacial erosion based on an empirical stream power law was proposed. This approach differs substantially from well established process-based models applied to describe glacial erosion in mountain landscapes. Outstanding computational performance but a number of potential limitations compared to process-based models requires extensive testing to evaluate the applicability of this novel approach. In this study, we test the validity of the glacial stream power law and its implementation into a 2-D landform evolution model (OpenLEM) by benchmarking it against a state of the art surface process model based on the integrated second order shallow-ice approximation (iSOSIA). Despite completely different approaches, OpenLEM and iSOSIA predict similar ice flow patterns and erosion rates for a wide range of climatic conditions without re-adjusting a set of calibrated scaling parameters. This parameter set is valid for full glacial conditions where the entire precipitation is converted to ice but also for an altitude-dependent glacier mass balance as characteristic for most glaciated mountain ranges on Earth. In both models characteristic glacial features, such as overdeepenings, hanging valleys, and steps at confluences emerge roughly at the same locations resulting in a consistent altitude-dependent adjustment of channel slope and relief. Compared to iSOSIA, however, distinctly higher erosion rates occur in OpenLEM at valley flanks during the initial phase of the fluvial to glacial transition. This is mainly due to the simplified description of glacier width and ice surface in OpenLEM. In this respect, we found that the glacial stream power approach cannot replace process-based models such as iSOSIA, but is complementary to them by addressing research questions that could not previously be answered due to a lack of computational efficiency. The implementation of the glacial stream power law is primarily suitable for large-scale simulations investigating the evolution of mountain topography in the interplay of tectonics and climate. As coupling glacial and fluvial erosion with sediment transport shows nearly the same computationally efficiency as its purely fluvial counterpart, mountain range scale simulations at high spatial resolution are not exclusively restricted to the fluvial domain anymore and a series of exciting research questions can be attacked by this novel approach.
Near the threshold of grain motion, sediment transport is “on-off” intermittent, characterized by large but rare bursts separated by long periods of low transport. Without models that can predict the presence of intermittency, measurements of average sediment flux can be in error by up to an order of magnitude. Despite its known presence and impact, it is not clear whether on-off intermittency arises from the grain activity (the number of moving grains) or grain velocities, which together determine the sediment flux. We use laboratory flume experiments to show that the on-off intermittency has its origins in the velocity distributions of grains that move by rolling along the bed, whereas grain activity is not on-off intermittent. Improved predictions of sediment flux require that the types of intermittency we identify be incorporated into stochastic models of sediment flux. Their recognition opens the door to physically based uncertainty estimates of time-averaged sediment flux.
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