Apparent redox potential discontinuity (aRPD) depth as a relative measure of sediment oxygen content and habitat quality

As hypoxic conditions spread in our oceans, indices that quickly and efficiently assess oxygen content in sediment pore water, and habitat quality are increasingly becoming desirable. Depth to the appa...     

Well-balanced numerical modelling of non-uniform sediment transport in alluvial rivers

The last two decades have witnessed the development and application of well-balanced numerical models for shallow flows in natural rivers.However,until now there have been no such models for flows with non-uniform sediment transport.This paper presents a 1D well-balanced model to simulate flows and non-capacity transport of non-uniform sediment in alluvial rivers.The active layer formulation is adopted to resolve the change of bed sediment composition.In the framework of the finite volume Slope Llmiter Centred(SLIC) scheme,a surface gradient method is incorporated to attain well-balanced solutions to the governing equations.The proposed model is tested against typical cases with irregular topography,including the refilling of dredged trenches,aggradation due to sediment overloading and flood flow due to landslide dam failure.The agreement between the computed results and measured data is encouraging.Compared to a non-well-balanced model,the well-balanced model features improved performance in reproducing stage,velocity and bed deformation.It should find general applications for non-uniform sediment transport modelling in alluvial rivers,especially in mountain areas where the bed topography is mostly irregular.     

预应力锚杆对岩体板裂化的控制机制研究

为探究预应力锚杆对岩体板裂化破坏的控制机制,首先进行了岩体板裂化破坏的室内物理模型试验,并运用FLAC数值模拟技术,模拟了平面应变状态下板裂化破坏的形成过程;在此基础上,通过3种不同的预应力锚杆施加方案,进行预应力锚杆对岩体板裂化破坏控制机制的数值试验。研究结果表明：预应力锚杆的作用削弱了裂隙尖端应力集中现象,有效地抑制了岩体内裂隙的扩展与贯通、板裂化破坏的形成;作用在板裂区边界的预应力锚杆,不仅能够抑制板裂化破坏的形成,而且能在一定程度上控制板裂化破坏的范围;作用在板裂破坏区内的预应力锚杆,可有效限制板裂岩体向临空面的位移,体现出提高板裂岩体整体变形刚度的作用。研究结果对认识深埋隧洞围岩板裂化破坏的形成机制、板裂化破坏的合理支护控制以及岩爆防治具有重要指导价值。     

Perfluoroalkyl and polyfluoroalkyl substances in sediments from South Bohai coastal watersheds,China

This study investigated the concentrations and distribution of Perfluoroalkyl and polyfluoroalkyl substances (PFAS) in sediments of 12 rivers from South Bohai coastal watersheds. The highest concentrations of ΣPFAS (31.920 ng g⁻¹ dw) and PFOA (29.021 ng g⁻¹ dw) were found in sediments from the Xiaoqing River, which was indicative of local point sources in this region. As for other rivers, concentrations of ΣPFAS ranged from 0.218 to 1.583 ng g⁻¹ dw were found in the coastal sediments and from 0.167 to 1.953 ng g⁻¹ dw in the riverine sediments. Predominant PFAS from coastal and riverine areas were PFOA and PFBS, with percentages of 30% and 35%, respectively. Partitioning analysis showed the concentrations of PFNA, PFDA and PFHxS were significantly correlated with organic carbon. The results of a preliminary environmental hazard assessment showed that PFOS posed the highest hazard in the Mi River, while PFOA posed a relative higher hazard in the Xiaoqing River.     

History of human activity in coastal southern Brazil from sediment

This study reports results of analysis of sediment cores collected from the Patos Lagoon estuary. This estuary receives materials from land runoff into Patos and Mirim lagoons and from exchange with the adjacent South Atlantic Ocean through a narrow inlet. Sediment from these sources is mostly natural, but additional contributions associated with source/activities related to the port of Rio Grande. The aim of this study was to estimate rates of accumulation of the sediments and to assess the significance of metal inputs from these activities relative to natural inputs. Our results indicate an average sedimentation rate of 0.3 cm/yr and that the transport of suspended solids from offshore sources into the estuary was enhanced after the deepening of the channel and construction of the breakwaters (in the early 1900). Results for metal accumulation in these sediments suggest that there have been only minor enrichments which can be attributed to anthropogenic sources.     

Anthropogenic lead concentrations and sources in Baltic Sea sediments based on lead isotopic composition

The Gulf of Gdańsk is influenced by heavy metals of anthropogenic origin. In this study, temporal concentration changes of Pb, Zn, Cd, and Cu were studied in six, 50 cm long sediment cores. The main aim of the study was to concentrate on the history of Pb fluxes and Pb isotopic composition (²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁶Pb) to trace Pb sources. The lowest Pb concentrations (19 μg g⁻¹) were measured in sediments deposited circa 1860, while the highest Pb concentrations (63–147 μg g⁻¹) were measured in sediments deposited between 1960s and 70s. Pre-industrial Pb fluxes were 7 Pb m² year⁻¹, while after WWII they reached 199 Pb m² year⁻¹. Highest ²⁰⁶Pb/²⁰⁷Pb ratios (∼1.22) were measured in the oldest sediment layers, and the lowest ²⁰⁶Pb/²⁰⁷Pb ratios (∼1.165) were measured in the sediments deposited in 1970s–90s. During the period of highest Pb contamination, the anthropogenic Pb fraction reached up to 93%. A general discussion of the Pb sources, emissions, and loads for Poland is included.     

Regional grain size variations in aeolian sediments along the transition between Tibetan highlands and north‐western Chinese deserts – the influence of geomorphological settings on aeolian transport pathways

Sediment distribution is investigated applying grain size analysis to 279 surface samples from the transitional zone between high mountains (Qilian Shan) and their arid forelands (Hexi Corridor) in north‐western China. Six main sediment types were classified. Medium scale (10³ m) geomorphological setting is carefully considered as it may play an important role concerning sediment supply and availability. A tripartite distribution of sedimentological landscape units along the mountain to foreland transition is evident. Aeolian sediments (e.g. loess and dune sands) are widespread. They are used to identify aeolian transport pathways. The mU/fS‐ratio (5–11 µm/48–70 µm) among primary loess opposes the two grain size fractions being most sensitive to varying accumulation conditions. The first fraction is attributed to long‐distance transport in high suspension clouds whereas the latter represents local transport in saltation mode. The ratio shows strong correlation with elevation (R² = 0.77). Thus, it indicates a relatively higher far‐traveled dust supply in mountainous areas (>3000 m above sea level [a.s.l.]) compared to the foreland. The contribution of westerlies to high mountain loess deposits is considered likely. Hereby, the influence of the geomorphological setting on grain size composition of aeolian sediments becomes apparent: the contribution from distant dust sources is ubiquitous in the study area. However, the far‐distance contribution may be reduced by the availability of fine sand provided in low topography settings. Plain foreland areas support fine sand deflation from supplying river beds, allowing the formation of sandy loess in foreland areas and intramontane basins. In contrast, high mountain topography inhibits strong sand deflation into loess deposits. Eastern parts of the Hexi Corridor show higher aeolian sand occurrence. In contrast, the western parts are dominated by gravel gobi surfaces. This is attributed to higher sand supply in eastern parts provided by the Badain Jaran Desert and fluvial storages as sand sources. Copyright © 2014 John Wiley & Sons, Ltd.     

First-principles derivation of reactive transport modeling parameters for particle tracking and PDE approaches

Both Eulerian and Lagrangian reactive transport simulations in natural media require selection of a parameter that controls the “promiscuity” of the reacting particles. In Eulerian models, measurement of this parameter may be difficult because its value will generally differ between natural (diffusion-limited) systems and batch experiments, even though both are modeled by reaction terms of the same form. And in Lagrangian models, there previously has been no a priori way to compute this parameter. In both cases, then, selection is typically done by calibration, or ad hoc. This paper addresses the parameter selection problem for Fickian transport by deriving, from first principles and D (the diffusion constant) the reaction-rate-controlling parameters for particle tracking (PT) codes and for the diffusion–reaction equation (DRE). Using continuous time random walk analysis, exact reaction probabilities are derived for pairs of potentially reactive particles based on D and their probability of reaction provided that they collocate. Simultaneously, a second PT scheme directly employing collocation probabilities is derived. One-to-one correspondence between each of D, the reaction radius specified for a PT scheme, and the DRE decay constant are then developed. These results serve to ground reactive transport simulations in their underlying thermodynamics, and are confirmed by simulations.     

Peak discharge increase in hyperconcentrated floods

The peak of river floods usually decreases in the downstream direction unless it is compensated by freshwater inflow from tributaries. In the Yellow River (China) the opposite is regularly observed, where the peak discharge of river floods increases in the downstream direction (at a rate far exceeding the contribution from tributaries). This flood peak discharge increase is probably related to rapid morphological changes, to a modified bed friction, or to a combination of both. Yet the relative role of these processes is still poorly understood. This paper aims to analyze the relative contribution of bed erosion and friction change to the peak discharge increase, based on available data and a recently developed numerical model. Using this high-resolution, fully coupled morphodynamic model of non-capacity sediment transport, two hyperconcentrated floods characterized by downstream peak discharge increase are numerically reproduced and analyzed in detail. The results reveal that although erosion effects may contribute to the downstream discharge increase (especially in case of extreme erosion), for most cases the increase must be mainly due to a reduction in bed friction during peak discharge conditions. Additionally, based on the concept of channel storage reduction, the effects of decreasing bed friction and (very strong) bed erosion can be integrated in explaining the peak discharge increase.     

Numerical investigation of local scour at two adjacent cylinders

Local scour around cylinders in a side-by-side or tandem arrangement under clear-water conditions is investigated numerically. Large eddy simulations with a Smagorinsky subgrid model are combined with a ghost-cell immersed boundary method, and details of the bed scouring are realized with sophisticated sediment and morphodynamic models. The scour patterns and depths in the two-cylinder cases are shown to be significantly influenced by the cylinder spacing. The features of the scour evolution, depth, and flow fields for a range of cylinder spacings are discussed. The maximum scour depth in the side-by-side cylinder cases increases as the distance between the cylinders decreases, whereas in the tandem cases, it tends to initially increase with increasing distance between the cylinders, after which it gradually decreases beyond the peak point. The maximum scour depths and trends computed using the present model show good agreement with the measured data in the literature.