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Length scales and statistical characteristics of outer bank roughness for large elongate meander bends: The influence of bank material properties,floodplain vegetation and flow inundation 
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下载免费PDF全文 Kory Konsoer Bruce Rhoads James Best Eddy Langendoen Mick Ursic Jorge Abad Marcelo Garcia 《地球表面变化过程与地形》2017,42(13):2024-2037
This article explores the length scales and statistical characteristics of form roughness along the outer banks of two elongate bends on a large meandering river through investigation of topographic variability of the bank face. The analysis also examines how roughness varies over the vertical height of the banks and when the banks are exposed subaerially and inundated during flood stage. Detailed data on the topography of the outer banks were obtained subaerially using terrestrial LiDAR during low flow conditions and subaqueously using multibeam echo sounding (MBES) during near‐bankfull conditions. The contributions of various length scales of topographic irregularity to roughness for subaerial conditions were evaluated for different elevation contours on the bank faces using Hilbert–Huang Transform (HHT) spectral analysis. Statistical characteristics for discrete areas on the bank faces were determined by calculating the root‐mean‐square of normal distances from a triangulated irregular network (TIN) surface. Results of the HHT analysis show that the characteristics of roughness along bank faces composed primarily of non‐cohesive sediment, and eroding into cropland, vary with bank elevation and exhibit a dominant range of roughness length scales (~15–50 m). However, bank faces composed predominantly of cohesive material and carved into a forested floodplain have relatively uniform topographic roughness characteristics over the vertical extent of the bank face and do not exhibit a dominant roughness length scale or range of length scales. Additionally, comparison between local surface roughness for subaerial versus subaqueous conditions shows that roughness decreases considerably when the banks are submerged, most likely because of the removal of vegetation and eradication of small‐scale erosional features in non‐cohesive bank materials by flow along the bank face. Thus, roughness appears to be linked to the hydraulic conditions affecting the bank, at least relative to conditions that develop when banks are exposed subaerially. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
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David Kirkman † David Tytler † Nao Suzuki † Carl Melis Susan Hollywood Kory James Geoffrey So Dan Lubin † Tridivesh Jena Michael L. Norman Pascal Paschos 《Monthly notices of the Royal Astronomical Society》2005,360(4):1373-1380
We use high-quality echelle spectra of 24 quasi-stellar objects to provide a calibrated measurement of the total amount of Lyα forest absorption (DA) over the redshift range 2.2 < z < 3.2 . Our measurement of DA excludes absorption from metal lines or the Lyα lines of Lyman-limit systems and damped Lyα systems. We use artificial spectra with realistic flux calibration errors to show that we are able to place continuum levels that are accurate to better than 1 per cent. When we combine our results with our previous results between 1.6 < z < 2.2 , we find that the redshift evolution of DA is well described over 1.6 < z < 3.2 as A (1 + z )γ , where A = 0.0062 and γ= 2.75 . We detect no significant deviations from a smooth power-law evolution over the redshift range studied. We find less H i absorption than expected at z = 3 , implying that the ultraviolet background is ∼40 per cent higher than expected. Our data appears to be consistent with an H i ionization rate of Γ∼ 1.4 × 10−12 s−1 . 相似文献
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Neck cutoffs and their resultant oxbow lakes are important and prominent features of riverine landscapes. Detailed field-based research focusing on the morphologic evolution of neck cutoffs is currently insufficient to fully characterize cutoff evolution. High-resolution bathymetric data were collected over 3 years for the purpose of determining channel morphology and morphologic change on three actively evolving neck cutoffs. Results indicate the following general trends in morphologic adjustment: (1) a longitudinal bar in the upstream meander limb that develops near the entrance to the abandoned bend; (2) a deep scour hole in the downstream meander limb immediately downstream of the cutoff channel; (3) erosion of the bank opposite the cutoff in the downstream meander limb; (4) a cutoff bar in the downstream meander limb at the junction corner of the cutoff channel and the downstream meander limb; and (5) perching of the exit of the abandoned bend above the cutoff channel due to channel bed incision. The results presented herein were used to develop a conceptual model that depicts the morphologic evolution of highly curving neck cutoffs. The findings of this research are combined with recent analyses of the three-dimensional flow structure through neck cutoffs to provide a mechanistic explanation for the morphodynamics of neck cutoffs. © 2019 John Wiley & Sons, Ltd. 相似文献
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