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  1. High resolution topographic data are necessary to understand benthic habitat, quantify processes at the water-sediment interface, and support computational fluid dynamics models for both surface and hyporheic hydraulics. In riverine systems, these data are typically collected using traditional surveying methods (total station, DGPS, etc.), airborne or terrestrial laser scanning, and photogrammetry. Recently, handheld surveying equipment has been rapidly acquiring popularity in part due to its processing capacity, price, size, and versatility. One such device is the iPhone LiDAR, which could have a good balance between precision and ease of use and is a potential replacement for conventional measuring tools. Here, we evaluated the accuracy of the LiDAR sensor and a Structure from Motion (SfM) method based on photos collected using the iPhone Cameras. We compared the LiDAR and SfM elevations to those from a high-precision laser scanner for an experimental rough water-worked gravelbed channel with boulder-like structures. We observed that both the LiDAR and SfM methods captured the overall streambed morphology and detected large (Hs  15 cm) and macro (5cm  Hs < 15cm) scales of topographic variations (Hs, roughness). The SfM technique also captured small scale (Hs <5cm) roughness whereas the LiDAR consistently simplified it with errors of 3.7 mm. 
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    Free, publicly-accessible full text available August 9, 2024
  2. Abstract

    Steep, boulder bed streams often contain sediment patches, which are areas of the bed with relatively well‐defined boundaries that are occupied by distinct grain size distributions (GSD). In sediment mixtures, the underlying GSD affects the critical Shields stress for a given grain size, which is commonly predicted using hiding functions. Hiding functions may vary with reach‐wide bed GSD, but the effect of local GSD on relative sediment mobility between sediment patches is poorly understood. We explore the effects of patch‐scale GSD on sediment mobility using tracer particles combined with local shear stresses to develop hiding functions for different patch classes within a steep stream. Hiding functions for all tested patch classes were similar, which indicates that the same hiding function can be used for different patches. However, the critical Shields stress for a given grain size generally decreased with lower patch median grain size (D50) suggesting that patches control the relative mobility of each size through both the underlying GSD and local shear stresses. The effects of the underlying GSD partly depend on grain protrusion, which we measured for all grain sizes present on each patch class. Protrusion was generally greater for larger grains regardless of patch class, but for a given grain size, protrusion was increased with smaller patchD50. For a given grain size, higher protrusion results in greater applied fluid forces and reduced resisting forces to partly explain our lower critical Shields stresses in finer patches. Patches therefore can importantly modulate relative sediment mobility through bed structure and may need to be included in reach‐scale sediment transport and channel stability estimates.

     
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  3. Abstract

    Wildfires can profoundly alter rates, magnitudes, and ecological influences of aeolian redistribution of sediments and nutrients. This study examines the influence of fire in a semi‐arid ecosystem using 2 years of continuous passive dust trap data in the northern Great Basin, USA. We analyse the mass flux, organic material content, grain size distribution, and geochemistry of the collected samples to trace the fingerprint of the 2015 Soda Fire through space and time. In areas not affected by fire, dust is characterized by silt‐sized median grains, a geochemical signature consistent with a playa source area, and spatially consistent but seasonally variable dust flux rates. Following fire, dust flux increases significantly within and near the burned area. At burned and topographically sheltered sites, dust deposition in the eighth month following fire was 190% higher than dust deposition 2 years post‐revegetation. Topographically exposed sites recorded only modest increases in dust deposition following fire. Analysis of organic matter indicates all dust samples (both burned and unburned) contained an average of 45% organic matter compared to a watershed average of 1.6% organic matter in soils.

    Geochemical and seasonal dust deposition data from 12 dust traps at a range of elevations indicate that with the removal of stabilizing vegetation after wildfire, differences in topographic position and wind direction lead to preferential redistribution of material across a burned landscape over hillslope scales (0–10 km). We posit post‐fire aeolian redistribution of locally derived material to topographically controlled positions is an important control on the spatial variability of soil depth and characteristics in drylands with complex topography. © 2020 John Wiley & Sons, Ltd.

     
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  4. Abstract

    Estimates of the onset of sediment motion are integral for flood protection and river management but are often highly inaccurate. The critical shear stress (τ*c) for grain entrainment is often assumed constant, but measured values can vary by almost an order of magnitude between rivers. Such variations are typically explained by differences in measurement methodology, grain size distributions, or flow hydraulics, whereas grain resistance to motion is largely assumed to be constant. We demonstrate that grain resistance varies strongly with the bed structure, which is encapsulated by the particle height above surrounding sediment (protrusion,p) and intergranular friction (ϕf). We incorporate these parameters into a novel theory that correctly predicts resisting forces estimated in the laboratory, field, and a numerical model. Our theory challenges existing models, which significantly overestimate bed mobility. In our theory, small changes inpandϕfcan induce large changes inτ*cwithout needing to invoke variations in measurement methods or grain size. A data compilation also reveals that scatter in empirical values ofτ*ccan be partly explained by differences inpbetween rivers. Therefore, spatial and temporal variations in bed structure can partly explain the deviation ofτ*cfrom an assumed constant value. Given that bed structure is known to vary with applied shear stresses and upstream sediment supply, we conclude that a constantτ*cis unlikely. Values ofτ*care not interchangeable between streams, or even through time in a given stream, because they are encoded with the channel history.

     
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  5. Abstract

    To explore the causes of history‐dependent sediment transport in rivers, we use a 19‐year record of coarse sediment transport from a steep channel in Switzerland. We observe a strong dependence of the threshold for sediment motion (τc) on the magnitude of previous flows for prior shear stresses ranging from 104 to 340 Pa, resulting in seasonally increasingτcfor 10 of 19 years. This stabilization occurs with and without measureable bedload transport, suggesting that small‐scale riverbed rearrangement increasesτc. Following large transport events (>340 Pa), this history dependence is disrupted. Bedload tracers suggest that significant reorganization of the bed erases memory of previous flows. We suggest that the magnitude of past flows controls the organization of the bed, which then modifiesτc, paralleling the evolution of granular media under shear. Our results support the use of a state function to better predict variability in bedload sediment transport rates.

     
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