Description: A reduced-complexity model for river delta formation – Part 2: Validation of the flow routing scheme Earth Surface Dynamics Discussions, 2, 871-910, 2014 Author(s): M. Liang, N. Geleynse, D. A. Edmonds, and P. Passalacqua In a companion paper (Liang et al., 2014) we introduced a Reduced-Complexity Model (RCM) for river delta formation, developed using a parcel-based "weighted random walk" method for routing water and sediment flux. This model (referred to as "DeltaRCM") consists of a flow routing scheme as the hydrodynamic component (referred to as "FlowRCM") and a set of sediment transport rules as the morphodynamic component. In this work, we validate the flow routing scheme (FlowRCM) via a series of hydrodynamic tests by comparing the model outputs to Delft3D and theoretical predictions. These tests are designed to reveal the capability of FlowRCM to resolve flow field features that are critical to delta dynamics at the level of channel processes. In particular, we focus on (1) backwater profile, (2) flow around a mouth bar, (3) flow through a single bifurcation, and (4) flow through a distributary channel network. We show that while the simple rules are not able to reproduce all fine-scale flow structures, FlowRCM does capture flow field features that are essential to deltaic processes such as bifurcations and avulsions. Finally, we discuss advantages and limitations of FlowRCM and identify environments most suitable for it.

Description: A reduced-complexity model for river delta formation – Part 1: Modeling deltas with channel dynamics Earth Surface Dynamics Discussions, 2, 823-869, 2014 Author(s): M. Liang, V. R. Voller, and C. Paola We develop a reduced-complexity model (RCM) delta formation model, in contrast to reductionist models based on high-resolution computational fluid dynamics. The basic framework of this model (referred in this paper as "DeltaRCM") consists of stochastic parcel-based cellular routing schemes for water and sediment and a set of phenomenological rules for sediment deposition and erosion. The outputs of the model include flow field, water surface topography and bed topography that evolves in time. Results show that DeltaRCM is able to: (1) resolve a wide range of channel dynamics, including elongation, bifurcation, avulsion and migration; (2) in response to the changes in input parameters, produce different types of deltas such as alluvial fan deltas at experimental scale. We also identify three key areas of particular model sensitivity, even at the RCM level: (1) avulsion dynamics is sensitive to dynamic free-surface topography; (2) channel network structure is sensitive to instability at channel mouths which creates bars; and (3) out-of-channel sedimentation is sensitive to water surface slope along channel margins. We also demonstrate a simple stratigraphy tracking component which can display the structure of the deposit in terms of distribution of coarse and fine materials along with the age of the deposit. DeltaRCM is a useful tool for understanding the dynamics of river deltas within a relatively simple cellular representation of water and sediment transport.

Description: Recalculation of bedload transport observations in Swiss mountain rivers using the model sedFlow Earth Surface Dynamics Discussions, 2, 773-822, 2014 Author(s): F. U. M. Heimann, D. Rickenmann, M. Böckli, A. Badoux, J. M. Turowski, and J. W. Kirchner Only few validated numeric models are available for the simulation of bedload transport dynamics in mountain streams. In this study, the recently developed modelling tool sedFlow has been applied to simulate bedload transport in two Swiss mountain streams. It is shown that sedFlow can be used to successfully reproduce observations from historic bedload transport events with reasonable parameter set-ups. The simulation results shed light on the difficulties that arise with traditional flow resistance estimation methods when macro-roughness is present. In addition, our results demonstrate that greatly simplified hydraulic routing schemes, such as kinematic wave or uniform discharge approaches, are probably sufficient for a good representation of bedload transport processes in steep mountain streams. The influence of different parameters is qualitatively evaluated in a simple sensitivity study. This proof-of-concept study demonstrates the usefulness of sedFlow for a range of practical applications in alpine mountain streams.

Description: sedFlow – an efficient tool for simulating bedload transport, bed roughness, and longitudinal profile evolution in mountain streams Earth Surface Dynamics Discussions, 2, 733-772, 2014 Author(s): F. U. M. Heimann, D. Rickenmann, J. M. Turowski, and J. W. Kirchner Especially in mountainuous environments, the prediction of sediment dynamics is important for managing natural hazards, assessing in-stream habitats, and understanding geomorphic evolution. We present the new modelling tool sedFlow for simulating fractional bedload transport dynamics in mountain streams. The model can deal with the effects of adverse slopes and uses state of the art approaches for quantifying macro-roughness effects in steep channels. Local grain size distributions are dynamically adjusted according to the transport dynamics of each grain size fraction. The tool sedFlow features fast calculations and straightforward pre- and postprocessing of simulation data. The model is provided together with its complete source code free of charge under the terms of the GNU General Public License ( www.wsl.ch/sedFlow ). Examples of the application of sedFlow are given in a companion article by Heimann et al. (2014).

Description: The role of velocity, pressure, and bed stress fluctuations in bed load transport over bed forms: numerical simulation downstream of a backward-facing step Earth Surface Dynamics Discussions, 2, 715-732, 2014 Author(s): M. W. Schmeeckle Bed load transport over ripples and dunes in rivers exhibits strong spatial and temporal variability due to the complex turbulence field caused by flow separation at bedform crests. A turbulence-resolving flow model downstream of a backward-facing step, coupled with a model integrating the equations of motion of individual sand grains, is used to investigate the physical interaction between bed load motion and turbulence downstream of separated flow. Large bed load transport events are found to correspond to low-frequency, positive pressure fluctuations. Episodic penetration of fluid into the bed increases the bed stress and moves grains. Fluid penetration events are larger in magnitude near the point of reattachment than further downstream. Models of bed load transport over ripples and dunes must incorporate the effects of these penetration events of high stress and sediment flux.

Description: Ancient pre-glacial erosion surfaces preserved beneath the West Antarctic Ice Sheet Earth Surface Dynamics Discussions, 2, 681-713, 2014 Author(s): K. C. Rose, N. Ross, R. G. Bingham, H. F. J. Corr, F. Ferraccioli, T. A. Jordan, A. M. Le Brocq, D. M. Rippin, and M. J. Siegert We present ice-penetrating radar evidence for ~150 km wide planation surfaces beneath the upstream Institute and Möller Ice Streams, West Antarctica. Accounting for isostatic rebound under ice-free conditions, the surfaces would be around sea level. We, thus, interpreted the surfaces as ancient, marine erosion (wave-cut) platforms. The scale and geometry of the platforms are comparable to erosion surfaces identified in the Ross Sea embayment, on the opposite side of West Antarctica. Their formation is likely to have begun after the development of the deep ocean basin of the Weddell Sea (~160 Myr ago). In order to form wave-cut platforms, the sea must be relatively free of sea ice for a sustained period to allow wave erosion at wave base. As a consequence, the most recent period of sustained marine erosion is likely to be the Mid-Miocene Climatic Optimum (17–15 Ma), when warm atmospheric and oceanic temperatures would have prevented ice from blanketing the coast during periods of ice-sheet retreat. The erosion surfaces are preserved in this location due to the collective action of the Pirrit and Martin–Nash Hills on ice-sheet flow, which results in a region of slow flowing, cold-based ice downstream of this major topographic barrier. This investigation shows that smooth, flat subglacial topography does not always correspond with regions of either present or former fast ice flow, as has previously been assumed.

Description: Neotectonics, flooding patterns and landscape evolution in southern Amazonia Earth Surface Dynamics Discussions, 2, 635-679, 2014 Author(s): U. Lombardo The paper examines the role of neotectonic activity in the evolution of the landscape in southern Amazonia during the Holocene. It uses both new and published data based on the analysis of remote sensing imagery and extensive field work in the Llanos de Moxos, Bolivian Amazon. The study of the region's modern and paleo rivers, ria lakes, paleosols and topography provides a strong case in favour of the thesis that the northern part of the Llanos de Moxos constitutes the southern margin of the Fitzcarrald arch and that it has experienced uplift during the Holocene. The paper assesses the extent and timing of the neotectonic activity in light of the new data and reconstructs the evolution of the landscape since the late Pleistocene. The evidence suggests that at least two uplift events took place: a first uplift in the late Pleistocene, which caused the formation of Lake Oceano, and a second uplift during the mid-Holocene, which formed Lake Rogaguado. These two uplifts appear to be linked to the knickpoints observed close to the towns of Guayaramerín and Puerto Siles respectively. The backwater effect due to these uplifts transformed the region's major rivers in seasonal ria lakes, causing the deposition of thick organic clay layers along the Beni, Mamoré and Madre de Dios river banks. I argue that neotectonic episodes could have dramatically changed the drainage of the Llanos, determining its flooding regime, soil properties and forest-savannah ecotone. These results stress the need for geomorphologists, paleoecologists and archaeologists to take into account neotectonics when reconstructing the region's past.

Description: An overview of underwater sound generated by inter-particle collisions and its application to the measurements of coarse sediment bedload transport Earth Surface Dynamics Discussions, 2, 605-633, 2014 Author(s): P. D. Thorne Over the past two to three decades the concept of using sound generated by the interparticle collisions of mobile bed material, has been investigated to assess if underwater sound can be utilised as a proxy for the estimation of bedload transport. In principle the acoustic approach is deemed to have the potential to provide non-instrusive, continuous, high temporal resolution measurements of bedload transport. It has been considered that the intensity of the sound radiated should be related to the ammount of mobile material and the frequency spectrum to the size of the material. To be able to fully realise this use of acoustics requires an understanding of the parameters which control the generation of sound as particles impact. In the present work the aim is to provide marine scientists developing acoustics to measure bedload transport with a description of how sound is generated when particles undergo collision underwater. To investigate the properties of the sound generated, examples are provided under different conditions of impact. It is considered that an understanding of the origins of the sound generation, will provide a basis for the interpretation of acoustic data collected in the marine environment, for the study of bedload sediment transport processes.

Description: Does deposition depth control the OSL bleaching of fluvial sediment? Earth Surface Dynamics Discussions, 2, 575-603, 2014 Author(s): A. C. Cunningham, J. Wallinga, N. Hobo, A. J. Versendaal, B. Makaske, and H. Middelkoop The Optically Stimulated Luminescence (OSL) signal from fluvial sediment often contains a remnant from the previous deposition cycle, leading to a partially bleached equivalent-dose distribution. Although identification of the burial dose is of primary concern, the degree of bleaching could potentially provide insights into geomorphic processes. However, comparison of bleaching between samples is complicated by sample-to-sample variation in aliquot size and luminescence sensitivity. Here we develop an age model to account for these effects. With measurement data from multi-grain aliquots, we use Bayesian computational statistics to estimate the burial dose and bleaching parameters of the single-grain dose distribution. We apply the model to 46 samples taken from fluvial sediment of Rhine branches in the Netherlands, and compare the results with environmental predictor variables (depositional energy and environment, sample depth, depth relative to mean water level, dose rate). We find no significant correlations between any predictor variable and the bleaching parameters, although large uncertainties may be obscuring relationships. However, the best bleached samples are found close to the mean water level. Based on these results, we hypothesize that bleaching occurs mainly during fluvial transport rather than upon deposition, with extra bleaching possible for sediments near the transition of channel to overbank deposits due to local reworking after deposition either by wind or water.

Description: Controls on slope-wash erosion rates in the Mojave Desert Earth Surface Dynamics Discussions, 2, 535-574, 2014 Author(s): O. Crouvi, V. O. Polyakov, J. D. Pelletier, and C. Rasmussen This study estimates the rates of soil erosion by slope wash in an arid region and the various factors that control these rates. Decadal-scale erosion rates were estimated on hillslope scales using inventories of 137 Cs that were sampled from 46 soil profiles in four different study sites in the Mojave Desert. Calculated mean soil erosion rates per site range from −3.6 to −24.3 t ha −1 yr −1 . Higher mean rates were associated with gently sloping sites that exhibit low percentage of rock and vegetation coverage, whereas lower mean rates corresponded to steep and rocky sites. Individual erosion rates were not correlated to slope gradient or curvature but were negatively correlated with the volume fraction of rocks in the upper soil profile (i.e., upslope rock coverage). Since the slopes get rockier as they get steeper, any increase in erosion rates with increasing slope is outweighed by the inhibiting effect of greater rock cover. This, together with sandy-loam soil texture on the steep slopes hinders runoff and erosion. Our findings are supported by soil data that show greater heterogeneity in the degree of calcic soil development and higher soluble salt contents in more gently sloping sites that are characterized by high erosion rates. The erosion rates reported here for the gently sloping sites are higher than rates calculated for semi-arid regions, probably due to the lower rock and vegetation coverage in these sites compared to wetter areas. These rates are also higher than millennial-scale rates estimated for the Mojave Desert on watershed scales, and suggest that at least part of the eroded sediments are stored in the adjacent streams and do not reach the piedmonts.