Hodge, R.A., Brasington, J., Richards, K.S. (2009). Analysing laser-scanned digital terrain models of gravel bed surfaces: linking morphology to sediment transport processes and hydraulics. Sedimentology, 56(7), 2024-2043. Sponsorship: NERCThe grain-scale topography of a sediment surface is a key component of a fluvial system, affecting aspects including sediment transport, flow resistance and ecology. However, its effect is hard to quantify because of the need for grain-scale elevation data from in situ fluvial gravel surfaces which are difficult to collect. The sediment surface properties are, therefore, commonly estimated as a function of the sediment grain-size distribution; however, because of additional factors, such as grain packing and shape, there is not necessarily a unique relationship between the two. A new methodology has been developed that uses terrestrial laser scanning to collect grain-scale topographic data from in situ fluvial gravel surfaces, from which digital terrain models are created. This paper investigates methods of analysing such digital terrain models, and possible sedimentological interpretations that can be drawn from the analysis. Eleven digital terrain models from exposed gravel surfaces in two contrasting rivers (the River Feshie and Bury Green Brook) were analysed by calculating: the distribution of surface elevations, semivariograms, surface inclinations, surface slopes and aspects and grain orientation. The distribution of surface elevations and surface slope and aspect analysis were found to be most informative. In the River Feshie, grain-size was interpreted as being a dominant control on sediment surface structure and gravel imbrication was identified. In Bury Green Brook, the location of the digital terrain models within the riffle?pool sequence was the dominant control on surface structure and grain orientation. Such digital terrain models therefore provide a new approach to measuring and quantifying the topography of fluvial sediment surfaces.Peer reviewe
The grain-scale morphology of fluvial sediments is an important control on the character and dynamics of river systems; however current understanding of its role is limited by the difficulties of robustly quantifying field surface morphology. Terrestrial Laser Scanning (TLS) offers a new methodology for the rapid acquisition of high-resolution and high-precision surface elevation data from in situ sediments. To date, most environmental and fluvial applications of TLS have focused on large-scale systems, capturing macroscale morphologies. Application of this new technology at scales necessary to characterize the complexity of grain-scale fluvial sediments therefore requires a robust assessment of the quality and sources of errors in close-range TLS data. This paper describes both laboratory and field experiments designed to evaluate close-range TLS for sedimentological applications and to develop protocols for data acquisition. In the former, controlled experiments comprising high-resolution scans of white, grey and black planes and a sphere were used to quantify the magnitude and source of three-dimensional (3D) point errors resulting from a combination of surface geometry, reflectivity effects and inherent instrument precision. Subsequently, a methodology for the collection and processing of grain-scale TLS data is described through an application to a coarse grained gravel system, the River Feshie (D 50 32 to 63 mm). This stepwise strategy incorporates averaging repeat scans and filtering scan artefact and non-surface points using local 3D search algorithms. The sensitivity of the results to the filter parameter values are assessed by careful internal validation of Digital Terrain Models (DTMs) created from the resulting point cloud data. The transferability of this methodology is assessed through application to a second river, Bury Green Brook, dominated by finer gravel (D 50 18 to 33 mm). The factor limiting the resolution of DTMs created from this second dataset was found to be the relative sizes of the laser footprint and smallest grains. Figure 3. Point density scan data from single scanner positions: (a) River Feshie patch from three locations, patches are 1·14 by 0·9 m, (b) Bury Green Brook patch from two locations, patches are 0·85 by 1 m, (c) combined Bury Green Brook patch data.Figure 4. Stages of processing a River Feshie patch: raw data, after RSEV filter, after cone filter, and after local high point filter. Top row (a, c, e, g) shows a surface linearly interpolated at 1 mm from the relevant point cloud, shaded by elevation. Middle row (b, d, f, h) shows point density, in points cm -2 . Bottom (i) shows orthogonal view of final surface. The outlined areas indicate sub-patches used in determining appropriate filter parameter values for the next processing stage.Figure 6. Distributions of E for sub-patches of a River Feshie patch after applications of different filters using the complete range of parameter values tested: (a) RSEV filter, (b) cone filter, (c) local high point filter. Boxes show inter-...
[1] Bedrock rivers exert a critical control over landscape evolution, yet little is known about the sediment transport processes that affect their incision. We present theoretical analyses and field data that demonstrate how grain entrainment, translation and deposition are affected by the degree of sediment cover in a bedrock channel. Theoretical considerations of grain entrainment mechanics and sediment continuity each demonstrate that areas of exposed bedrock and thin sediment depths cause sediment transport to be size-independent, albeit excluding extreme grain sizes. We report gravel and cobble magnetic tracer data from three rivers with contrasting sediment cover: the bedrock River Calder (20% cover), the bedrock South Fork Eel River (80%) and the alluvial Allt Dubhaig (100%). These data sets show that: 1) transport distances in the River Calder are controlled by sediment patch location, whereas in the other rivers transport distances are described by gamma distributions representing local dispersion; 2) River Calder transport distances are size-independent across all recorded shear stresses, whereas the other rivers display size-selectivity; 3) River Calder tracers are entrained at a dimensionless shear stress of 0.038, which is relatively low compared to alluvial rivers; and, 4) virtual grain velocities in the River Calder are higher than in a comparable reach of the Allt Dubhaig. These contrasts result from differences in the thicknesses and spatial distribution of sediment in the three rivers, and support the theoretical analysis. Sediment processes in bedrock rivers systematically vary along a continuum between bedrock and alluvial end-members.Citation: Hodge, R. A., T. B. Hoey, and L. S. Sklar (2011), Bed load transport in bedrock rivers: The role of sediment cover in grain entrainment, translation, and deposition,
[1] The presence of sediment cover in bedrock rivers inhibits saltation-driven incision, and so accurate predictions of the relationship between bedrock exposure (F e ) and relative sediment flux (sediment supply rate over capacity sediment transport rate, Q s /Q t ) are necessary to model incision and hence landscape evolution. Theoretical predictions of a linear or negative exponential form for this relationship are not consistent with laboratory data that instead demonstrate a range of different relationships. Here we use a cellular automaton (CA) model to establish how the relationship between F e and Q s /Q t evolves from the dynamics of, and interactions between, individual sediment grains moving through a bedrock channel. The key model parameter is the probability of grain entrainment, which is altered as a function of the number of neighboring grains in order to reproduce the enhanced mobility of isolated grains on bedrock surfaces. For each model run, an equilibrium sediment cover is attained for a specified sediment input, enabling the relationship between F e and Q s /Q t to be established. As well as both linear and exponential relationships, model runs reproduce other relationships observed in laboratory experiments. These other relationships require isolated grains to be more easily entrained than grains in sediment clusters, which is consistent with field observations of grain mobility. There is therefore a continuum of relationships between F e and Q s /Q t ; the relationship that is most applicable to a particular reach will depend on the role of channel slope, roughness and shear stress in controlling the entrainment of grains from bedrock and alluvial surfaces.Citation: Hodge, R. A., and T. B. Hoey (2012), Upscaling from grain-scale processes to alluviation in bedrock channels using a cellular automaton model,
Riffle–pool sequences are maintained through the preferential entrainment of sediment grains from pools rather than riffles. This preferential entrainment has been attributed to a reversal in the magnitude of velocity and shear stress under high flows; however the Differential Sediment Entrainment Hypothesis (DSEH) postulates that differential entrainment can instead result from spatial sedimentological contrasts. Here we use a novel suite of in situ grain‐scale field measurements from a riffle–pool sequence to parameterize a physically‐based model of grain entrainment. Field measurements include pivoting angles, lift forces and high resolution digital elevation models (DEMs) acquired using terrestrial laser scanning, from which particle exposure, protrusion and surface roughness were derived. The entrainment model results show that grains in pools have a lower critical entrainment shear stress than grains in either pool exits or riffles. This is because pool grains have looser packing, hence greater exposure and lower pivoting angles. Conversely, riffle and pool exit grains have denser packing, lower exposure and higher pivoting angles. A cohesive matrix further stabilizes pool exit grains. The resulting predictions of critical entrainment shear stress for grains in different subunits are compared with spatial patterns of bed shear stress derived from a two‐dimensional computational fluid dynamics (CFD) model of the reach. The CFD model predicts that, under bankfull conditions, pools experience lower shear stresses than riffles and pool exits. However, the difference in sediment entrainment shear stress is sufficiently large that sediment in pools is still more likely to be entrained than sediment in pool exits or riffles, resulting in differential entrainment under bankfull flows. Significantly, this differential entrainment does not require a reversal in flow velocities or shear stress, suggesting that sedimentological contrasts alone may be sufficient for the maintenance of riffle–pool sequences. This finding has implications for the prediction of sediment transport and the morphological evolution of gravel‐bed rivers. Copyright © 2012 John Wiley & Sons, Ltd.
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