Flash flood sediment transport in a steep sand-bed ephemeral stream     

Paolo BILLI 《国际泥沙研究》2011,26(2):193-209

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A simple model accounting for the uptake,transport, and deposition of wind‐eroded mineral particles in the hyperarid coastal Atacama Desert of northern Chile     

Stephane C. Alfaro  Valentina Flores‐Aqueveque  Gilles Foret  Sandrine Caquineau  Gabriel Vargas  Jose A. Rutllant 《地球表面变化过程与地形》2011,36(7):923-932

As previously observed in marine sediments collected downwind of African or South American continental sources, recent studies of sediment cores collected at the bottom of Mejillones Bay in north Chile (23°S) show a laminated structure in which the amount of particles of aeolian origin and their size create significant differences between the layers. This suggests inter‐annual to inter‐decadal variations in the strength of the local southerly winds responsible for (1) the erosion of the adjacent hyperarid surface of the Mejillones Pampa, and (2) the subsequent transport of the eroded particles towards the bay. A simple model accounting for the vertical uptake, transport, and deposition of the particles initially set into motion by wind at the surface of the pampa is proposed. This model, which could be adapted to other locations, assumes that the initial rate of (vertical) uptake is proportional to the (horizontal) saltation flux quantified by means of White's equation, that particles are lifted to a height (H), increasing with the magnitude of turbulence, and that sedimentation progressively removes the coarsest particles from the air column as it moves towards the bay. In this model, the proportionality constant (A) linking the vertical flux of particles with the horizontal flux, and the injection height (H) control the magnitude and size distribution of the deposition flux in the bay. Their values are determined using the wind speed measured over the pampa and the size distribution of particles collected in sediment traps deployed in the bay as constraints. After calibration, the model is used to assess the sensitivity of the deposition flux to the wind intensity variations. The possibility of performing such quantitative studies is necessary for interpreting precisely the variability of the aeolian material in the sediment cores collected at the bottom of Mejillones Bay. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

Evolution of an advancing gravel front: observations from Vedder Canal,British Columbia     

R. I. Ferguson  D. J. Bloomer  M. Church 《地球表面变化过程与地形》2011,36(9):1172-1182

Channelization of the lowermost part of Vedder River in 1922 initiated a natural experiment relevant to the unresolved question of how abrupt gravel–sand transitions develop along rivers. The new channel (Vedder Canal) had a fine bed and a much lower slope than the gravel‐bed river immediately upstream. Changes in morphology and sedimentology as gravel advanced into and along the Canal are documented using air photos, historical surveys, and fieldwork. The channel aggraded and steepened until stabilized by occasional gravel extraction in recent decades. The deposited material fines progressively along the Canal but the gravel front has retained an abrupt appearance because it has advanced by the sequential development of discrete gravel tops on initially sandy alternate bars. Near the gravel front the bed is highly bimodal and there is a sharper drop in the extent of gravel‐framework surface facies than in bulk gravel content. Ahead of the front, gravel is restricted to thin ribbons which often become buried by migrating sand. Calculations show that even though the gravel bed at the head of the Canal is almost unimodal, size‐selective transport during floods can account for the strong bimodality farther downstream. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

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   

青藏高原一次强对流过程对水汽垂直输送的数值模拟   总被引：1，自引：0，他引：1  

朱士超  银燕  金莲姬 《大气科学》2011,35(6):1057-1068

本文采用中尺度天气研究预测模式(WRF)模拟了青藏高原那曲地区的一次强对流过程,分析了强对流对水汽的垂直输送量及对模式不同云微物理参数化方案的敏感性.通过与实测资料的比较,发现此次模拟在对流发生时间、地点、降水时间等方面均与实际接近.敏感性试验表明:当对流发生时,对流区域向上的水汽通量随海拔高度呈先增大后减小的趋势,该...  相似文献   

一次东风波暴雨过程诊断分析     

曹楚  王忠东 《气象科技》2011,39(3):308-314

利用常规气象资料和NCEP 1°×1°再分析资料,结合自动站加密资料对2009年9月29日20:00至10月1日20:00发生在浙江东部沿海的一次东风波暴雨过程进行了分析.结果发现:0916号台风"凯萨娜"外围环流为此次东风波的形成提供了扰动作用,副热带高压的加强使得低层形成了明显的东南急流,为暴雨或强降水的发生提供了...  相似文献   

Simulation of the East Asian Subtropical Westerly Jet Stream with GFDL AGCM (AM2.1)          下载免费PDF全文

HUANG Gang  LIU Yong 《大气和海洋科学快报》2011,4(1):24-29

The present study validated the capability of the AM2.1, a model developed at NOAA’s Geophysical Fluid Dynamics Laboratory (GFDL), in reproducing the fundamental features of the East Asian Subtropical Westerly Jet Stream (EASWJ). The main behaviors of the EASWJ are also investigated through the reanalysis of observational NCEP/NCAR data. The mean state of the EASWJ, including its intensity, location, structure, and seasonal evolution is generally well-portrayed in the model. Compared with the observation, the model tends to reproduce a weaker jet center. And, during summer, the simulated jet center is northward-situated. Results also demonstrate the model captures the variability of EASWJ during summer well. The results of the empirical orthogonal function (EOF) applied on the zonal wind at 200 hPa (U200) over East Asia for both the observation and simulation indicate an inter-decadal shift around the late 1970s. The correlation coefficient between the corresponding principle components is as great as 0.42 with significance at the 99% confidence level.  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   

中国南方地区水系沉积物中元素丰度   总被引：6，自引：0，他引：6       下载免费PDF全文

程志中  谢学锦  潘含江  杨榕  商云涛 《地学前缘》2011,18(5):289-295

从中国南方12个省自治区230万km2面积上所采集的水系沉积物中按每1∶5万图幅一个组合样的密度组合成5 244件组合样,采用以ICP-MS、ICP-AES和XRF为主的先进的分析测试方法和严格的质量控制方法,准确测试了其中的76种元素。统计计算了中国南方地区76种元素的平均值,给出了中国南方地区水系沉积物中76种元素...  相似文献   

Modelling controls on aeolian dune‐field pattern evolution     

ERIN EASTWOOD  JOANNA NIELD  ANDREAS BAAS  GARY KOCUREK 《Sedimentology》2011,58(6):1391-1406

A second‐generation, source‐to‐sink cellular automaton‐based model presented here captures and quantifies many of the factors controlling the evolution of aeolian dune‐field patterns by varying only a small number of parameters. The role of sediment supply, sediment availability and transport capacity (together defined as sediment state) in the development and evolution of an aeolian dune‐field pattern over long time scales is quantified from model simulations. Seven dune‐field patterns can be classified from simulation results varying the sediment supply and transport capacity that control the type and frequency of dune interactions, the sediment availability of the system and, ultimately, the development of dune‐field patterns. This model allows predictions to be made about the range of sediment supply and wind strengths required to produce the dune‐field patterns seen in the real world. A new clustered dune‐field pattern is identified from model results and used to propose an alternative mechanism for the formation of superimposed dunes. Bedforms are hypothesized to cluster together, simultaneously forming two spatial scales of bedforms without first developing a large basal dune with small superimposed dunes. Manipulation of boundary conditions produces evolving dune fields with different spatial configurations of sediment supply. Trends of spacing and crest length increase with decreasing variability as the dune field matures. This simple model is a valuable tool which can be used to elucidate the dominant control of aeolian sediment state on the construction and evolution of aeolian dune‐field patterns.  相似文献