Bioturbation on a south‐east Australian hillslope: estimating contributions to soil flux     

Paul J. Richards  Geoff S. Humphreys 《地球表面变化过程与地形》2011,36(9):1240-1253

The contribution of bioturbation to downslope soil transport is significant in many situations, particularly in the context of soil formation, erosion and creep. This study explored the direct flux of soil caused by Aphaenogaster ant mounding, vertebrate scraping and tree‐throw on a wildfire‐affected hillslope in south‐east Australia. This included the development of methods previously applied to Californian gopher bioturbation, and an evaluation of methods for estimating the volume of soil displaced by tree‐throw events. All three bioturbation types resulted in a net downslope flux, but any influence of hillslope angle on flux rates appeared to be overshadowed by environmental controls over the spatial extent of bioturbation. As a result, the highest flux rates occurred on the footslope and lower slope. The overall contribution of vertebrate scraping (57.0 ± 89.4 g m^?1 yr^?1) exceeded that of ant mounding (36.4 ± 66.0 g m^?1 yr^?1), although mean rates were subject to considerable uncertainty. Tree‐throw events, which individually cause major disturbance, were limited in their importance by their scarcity relative to faunalturbation. However, tree‐throw might be the dominant mechanism of biotic soil flux on the mid‐slope provided that it occurs at a frequency of at least 2–3 events ha^?1 yr^?1. Although direct biotic soil flux appears to be geomorphologically significant on this hillslope, such transport processes are probably subordinate to other impacts of bioturbation at this site such as the enhancement of infiltration following wildfire. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Effects of turbulence on the transport of individual particles as bedload in a gravel‐bed river     

Geneviève Paiement‐Paradis  Geneviève Marquis  André Roy 《地球表面变化过程与地形》2011,36(1):107-116

This study investigates the association between mean and turbulent flow variables and the movement of individual particles in a gravel‐bed river. The experimental design implemented in the Eaton‐North River (Québec, Canada) is based on the simultaneous observations at a high temporal resolution of both particle movements as bedload using an underwater video camera and of the streamwise and vertical flow velocity components using a vertical array of three electromagnetic current meters (ECMs). The frequency and distance of displacement of particles larger than 20 mm that were sliding or rolling on the bed were measured from a 10 minutes long film. Mean and turbulent flow properties obtained for periods without sediment transport are compared to those when particles were sliding and rolling. When particles are sliding, weak differences are present for the mean streamwise velocity and normal vertical stresses. Instantaneous Reynolds shear stresses are significantly lower for sliding events which was not expected but could be explained by the important dominance of Quadrant 3 events (inward interactions). When particles are rolling, only the vertical normal stresses show a weak difference from those observed in the absence of transport but they tend to occur when Quadrant 2 (ejections) dominate the flow field. For both sliding and rolling particles, vertical and/or streamwise fluid accelerations show high magnitude values when compared to periods without transport. For sliding particles, streamwise acceleration is mostly negative and combines most of the time with a positive vertical acceleration. For rolling particles, streamwise and vertical acceleration are predominantly of opposite sign. These results suggest that fluid acceleration or deceleration in the streamwise and vertical directions is affecting the pressure field around particles. In this study, fluid acceleration seems to play a more important role than Reynolds shear stress or normal stresses for bedload movements. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

Accuracy of numerical simulations of contaminant transport in heterogeneous aquifers: A comparative study     

F.A. Radu  N. Suciu  J. HoffmannA. Vogel  O. Kolditz  C.-H. ParkS. Attinger 《Advances in water resources》2011,34(1):47-61

This work deals with a comparison of different numerical schemes for the simulation of contaminant transport in heterogeneous porous media. The numerical methods under consideration are Galerkin finite element (GFE), finite volume (FV), and mixed hybrid finite element (MHFE). Concerning the GFE we use linear and quadratic finite elements with and without upwind stabilization. Besides the classical MHFE a new and an upwind scheme are tested. We consider higher order finite volume schemes as well as two time discretization methods: backward Euler (BE) and the second order backward differentiation formula BDF (2). It is well known that numerical (or artificial) diffusion may cause large errors. Moreover, when the Péclet number is large, a numerical code without some stabilising techniques produces oscillating solutions. Upwind schemes increase the stability but show more numerical diffusion. In this paper we quantify the numerical diffusion for the different discretization schemes and its dependency on the Péclet number. We consider an academic example and a realistic simulation of solute transport in heterogeneous aquifer. In the latter case, the stochastic estimates used as reference were obtained with global random walk (GRW) simulations, free of numerical diffusion. The results presented can be used by researchers to test their numerical schemes and stabilization techniques for simulation of contaminant transport in groundwater.  相似文献   

Adaptive split-operator methods for modeling transport phenomena in porous medium systems     

Sarah E. Gasda 《Advances in water resources》2011,34(10):1268-1282

Split-operator methods are commonly used to approximate environmental models. These methods facilitate the tailoring of different approximation approaches to different portions of the differential operator and provide a means to split large coupled problems into pieces that are more amenable to parallel computation than the original fully-coupled problem. However, split-operator methods introduce an additional source of approximation error into the solution, which is typically either ignored or controlled heuristically. In this work, we develop two methods to estimate and control the error in split-operator methods, which lead to a dynamic adjustment of the temporal splitting step based upon the error estimators. The proposed methods are shown to yield robust solutions that provide the desired control of error. In addition, for a typical nonlinear reaction problem, the new methods are shown to reduce the solution error by more than two orders of magnitude compared to standard methods for an identical level of computational effort. The algorithms introduced and evaluated have widespread applicability in environmental modeling.  相似文献   

Divergence of solutions to perturbation-based advection-dispersion moment equations     

Kenneth D. Jarman  Alexandre M. Tartakovsky 《Advances in water resources》2011,34(5):659-670

Moment equation methods are popular and powerful tools for modeling transport processes in randomly heterogeneous porous media, but the application of these methods to advection-dispersion equations often leads to erroneous oscillations. Perturbative methods, required to close systems of moment equations, become inaccurate for large perturbations; however, little quantitative theory exists for determining when this occurs for advection-dispersion equations. We consider three different methods (asymptotic approximation, Eulerian truncation, and iterative solution) for closing and solving advection-dispersion moment equations describing transport in stratified porous media with random permeability. We obtain approximate analytical expressions for time above which the asymptotic approximation to the mean diverges, in particular quantifying the impact that dispersion has on delaying—but not eliminating—divergence. We demonstrate that Eulerian truncation and iterative solution methods do not eliminate divergent behavior either. Our divergence criteria provide a priori estimates that signal a warning to the practitioner of stochastic advection-dispersion equations to carefully consider whether to apply perturbative approaches.  相似文献   

Assessment of Water Quality in the Elbe River at Low Water Conditions Based on Factor Analysis     

Martina Baborowski  Olaf Büttner  Jürgen W. Einax 《洁净——土壤、空气、水》2011,39(5):437-443

An assessment of water quality measurements during a long‐lasting low water period in the Elbe River is presented. Weekly samples were taken from May to December 2003 at a sampling site in the middle part of the Elbe River. For multivariate data analysis, 34 parameters of 46 samplings were considered. As a result of this analysis, 78% of the variance of the data set is explained by five factors. They can be assigned to the following latent variables: season (37.5%) > tributaries (12.7%) > re‐suspension (10.4%) > discharge (9.4%) > complexation (8.5%). For the investigated sampling site, two key processes were identified as dominating factors on the water quality during low water conditions. First, seasonal phytoplankton development caused changes in redox conditions with consequences for re‐solution of pollutants from sediments. Second, tributaries have a higher impact on the main stream, due to changes in mixing processes. Therefore, in addition to flood investigations, monitoring strategies, and management plans should be developed in order to survey changes in water quality during low water conditions.  相似文献   

基于GIS的公交可达性指数算法   总被引：2，自引：0，他引：2  

陈洋  李燕燕 《测绘与空间地理信息》2011,34(2):177-179

借助于GIS空间分析方法,以福州市金山新区为例,对公交可达性的问题进行了探讨。研究过程中建立了研究区范围内包括交通网络信息的地理空间数据库,并通过综合考虑相关影响因子,提出了一种公交可达性指数的计算方法,并进行实地价。这一算法可为选址分析等各类问题提供相对定量化的公交可达性信息。  相似文献   

南极普里兹湾及其邻近海域表层水镭同位素的分布及应用   总被引：1，自引：0，他引：1  

陈倩娜  任春燕  李琦  李敬轩  贾仁明  郑敏芳  邱雨生  陈敏 《海洋与湖沼》2015,46(1):77-87

中国第27次南极科学考察期间(2010年12月30日至2011年1月16日),对普里兹湾及其邻近海域表层海水进行了226Ra和228Ra的分析,结果表明:226Ra和228Ra比活度的变化范围分别为1.47—2.43Bq/m3和0.17—0.45Bq/m3,平均值分别为2.13Bq/m3和0.29Bq/m3,228Ra/226Ra)A.R.(228Ra与226Ra的活度比)的变化范围为0.08—0.20,平均值为0.14。根据盐度和226Ra的质量平衡方程,计算出研究海域表层水中冰融水、南极夏季表层水和普里兹湾中深层水的份额。研究海域表层水中温度、盐度、226Ra、228Ra、228Ra/226Ra)A.R.和冰融水份额的空间分布显示,在埃默里冰架前沿海域,西侧海域较东侧海域具有低温、高盐、高226Ra、低228Ra、低228Ra/226Ra)A.R.、低冰融水份额的特征,证实埃默里冰架下水体东进西出的运动规律。根据埃默里冰架前沿东、西侧水体228Ra/226Ra)A.R.的差异,估算出埃默里冰架下表层水体东进西出所经历的时间为1.85a。此外,在普里兹湾湾口中部海域(66.5—67.5°S,72°—74°E),观察到次表层水的上升通风作用,该区域较高的228Ra含量和228Ra/226Ra)A.R.证明这些表层水体并非来自湾外绕极深层水的上涌,而可能来自湾内埃默里冰架输出水体。  相似文献   

海岸沙坝剖面和滩肩剖面特征研究     

张洋  邹志利  苟大荀  罗年鹏  尹晶 《海洋学报》2015,37(1):147-157

通过物理模型实验研究了海岸沙坝剖面和滩肩剖面的形成和演化过程,给出了稳定的沙坝剖面和滩肩剖面的几何特征。实验中考虑了两种初始坡度(1∶20和1∶10)和不同波高的规则波和不规则波,讨论了不同初始坡度海岸上破碎波空间分布特征。结果表明,沙坝产生后存在向岸和离岸两种运动形态,但最终将停留在稳定位置。稳定的沙坝剖面对应不同初始坡度和波浪存在不同的大沙坝和小沙坝分布。沙坝剖面由长时间小波高波浪序列作用后可转化为稳定滩肩剖面,该剖面不依赖于波浪和初始坡度。实验也给出了平衡剖面与理论曲线的对比以及剖面上泥沙粒径的分布。  相似文献   

Grain size composition and transport of sedimentary organic carbon in the Changjiang River (Yangtze River) Estuary and Hangzhou Bay and their adjacent waters     

ZHANG Weiyan  JIN Haiyan  YAO Xuying  JI Zhongqiang  ZHANG Xiaoyu  YU Xiaoguo  ZHANG Fuyuan  GAO Aigen 《海洋学报(英文版)》2015,34(10):46-56

Surface sediments from the Changjiang River(Yangtze River) Estuary,Hangzhou Bay,and their adjacent waters were analyzed for their grain size distribution,organic carbon(OC) concentration,and stable carbon isotope composition(δ13C).Based on this analysis,about 36 surface sediment samples were selected from various environments and separated into sand(0.250 mm,0.125–0.250 mm,0.063–0.125 mm) and silt(0.025–0.063 mm)fractions by wet-sieving fractionation methods,and further into silt-(0.004–0.025 mm) and clay-sized(0.004mm) fractions by centrifugal fractionation.Sediments of six grain size categories were analyzed for their OC andδ13C contents to explore the grain size composition and transport paths of sedimentary OC in the study area.From fine to coarse fractions,the OC content was 1.18%,0.51%,0.46%,0.42%,0.99%,and 0.48%,respectively,while theδ13C was –21.64‰,–22.03‰,–22.52‰,–22.46‰,–22.36‰,and –22.28‰,respectively.In each size category,the OC contribution was 42.96%,26.06%,9.82%,5.75%,7.09%,and 8.33%,respectively.The OC content in clay and fine silt fractions(0.025 mm) was about 69.02%.High OC concentrations were mainly found in offshore modern sediments in the northeast of the Changjiang River Estuary,in modern sediments in the lower estuary of the Changjiang River and Hangzhou Bay,and in Cyclonic Eddy modern sediments to the southwest of the Cheju Island.Integrating the distribution of terrestrial OC content of each grain size category with the δ13C of the bulk sediment indicated that the terrestrial organic material in the Changjiang River Estuary was transported seaward and dispersed to the Cyclonic Eddy modern sediments to the southwest of the Cheju Island via two pathways:one was a result of the Changjiang River Diluted Water(CDW) northeastward extending branch driven by the North Jiangsu Coastal Current and the Yellow Sea Coastal Current,while the other one was the result of the CDW southward extending branch driven by the Taiwan Warm Current.  相似文献