Empirical and conceptual modelling of the suspended sediment dynamics in a large tropical African river: the Upper Niger river basin     

C. Picouet  B. Hingray  J. C. Olivry 《Journal of Hydrology》2001,250(1-4):19-39

A 7-year sediment transport monitoring on the Upper Niger rivers was used to study the relationship between suspended sediment concentration and river discharge. During annual floods, these relationships show positive hysteresis. This paper presents the results of two models that estimate the time evolution of suspended sediment concentration using water discharge data only. The first model is based on a statistical approach using two relationships, one for the rising stage period of the flood and one for the recession period of the annual flood; the second model is a lumped conceptual one; it supposes that the sediment flux observed in the river comes from two different sources of sediment and that these two sources may be regarded as two different reservoirs. The erosion of the first reservoir represents hillslope erosion observed during the runoff season. Sediment supply from this ‘reservoir’ is limited in time because depletion occurs during the runoff season. The second reservoir is unlimited in time and quantity and its erosion represents contributions coming from bank erosion and mobilisation of deposits in the channel network.

Both of the models are compared with a simple rating curve based model. The model results show that the conceptual model has the highest efficiency to reproduce from weekly discharge only the time evolution of weekly suspended sediment concentrations, the time evolution of weekly sediment fluxes, and the global annual sediment yields.  相似文献   

ON THE DEPOSITIONAL DYNAMICS OF GUANHE ESTUARY     

ZHANG Dongsheng  ZHANG Changkuan  DING Xianrong 《国际泥沙研究》1995,(3)

ＯＮＴＨＥＤＥＰＯＳＩＴＩＯＮＡＬＤＹＮＡＭＩＣＳＯＦＧＵＡＮＲＥＥＳＴＵＡＲＹＺＨＡＮＧＤｏｎｇｓｈｅｎｇ；ＺＨＡＮＧＣｈａｎｇｋｕａｎａｎｄＤＩＮＧＸｉａｎｒｏｎｇ（Ｐｒｏｆ．ＨｏｈａｉＵｎｉｖｅｒｓｉｔｙ，Ｎａｎｊｉｎｇ，Ｃｈｉｎａ）（Ａｓｓ，...  相似文献   

Bed-material transport estimated from channel surveys: Vedder River,British Columbia     

Yvonne Martin  Michael Church 《地球表面变化过程与地形》1995,20(4):347-361

This study investigates the possibility to estimate bed-material transfer in gravel-bed rivers by analysis of morphological changes along Vedder River, British Columbia. Data from repeated cross-section surveys are used to estimate volume changes along the length of an 8 km reach. Gravel budgets are based on a continuity approach. An error analysis is performed to evaluate the uncertainty in the best estimate of transport rates. The mean annual gravel transport into the reach over a 9 year period was estimated to be 36600 ± 5600 m³ yr^?1. The sediment transport regime along the length of the river is evaluated and examined in relation to peak flood flows. Significant spatial and temporal variability in transport rates is demonstrated, making dubious the generalization of transport estimates from hydraulic calculations, or from sample measurements at a single cross-section. The assumptions, procedures and limitations of the ‘morphological approach’ to sediment transport analysis are discussed. It is concluded that this approach provides information of quality comparable or superior to that of direct measurements of transport, yet requires less field effort. It also provides additional information about river morphological changes, making it a preferred method for geomorphlogical investigations and for many river management concerns.  相似文献   

Generation of three dimensional flow fields for statistically anisotropic heterogeneous porous media     

M. A. Cushy  A. Bellin  Y. Rubin 《Stochastic Environmental Research and Risk Assessment (SERRA)》1995,9(1):89-104

A methodology for generating three dimensional (3D) flow fields for statistically anisotropic heterogeneous porous media is presented and demonstrated. The simulated flow fields are shown to exhibit the input spatial correlation structure and observe mass continuity. Sample flow fields are presented in the form of cross sectional slices of the 3D formation. These cross sections demonstrate visually the characteristics of subsurface flow. The method was found to be faster than traditional techniques in terms of its computational requirements. Given this method, it is possible to generate the large number of realizations of a velocity field necessary to compute high order statistics in transport problems.  相似文献   

A resumé of nonlocal transport theories     

J. H. Cushman  B. X. Hu 《Stochastic Environmental Research and Risk Assessment (SERRA)》1995,9(2):105-116

Analysis from a number of different perspectives has shown diffusion and dispersion in natural porous formations to generally be nonlocal in character, i.e., the mass balance involves integro-partial differential equations. Only in certain asymptotic limits do these laws localize to classical partial differential equations. Compiled within is a resume of nonlocal laws that our group has developed over the last few years for systems with physical, chemical and biological heterogeneity. Analytical tools used to obtain these laws are nonequilibrium and equilibrium statistical mechanics, and first-order spectral-perturbation methods. This paper is an expansion of the material presented at the Waterloo conference held in the memory of Dr. Unny.  相似文献   

Distribution of clay minerals in surface sediments from the eastern Barents and south-western Kara seas   总被引：2，自引：0，他引：2  

D. Nürnberg  M. A. Levitan  J. A. Pavlidis  E. S. Shelekhova 《International Journal of Earth Sciences》1995,84(3):665-682

Surface samples from the eastern Barents and south-western Kara seas have been analysed for clay mineralogy. Transport paths, the role of regional sources and local bedrock outcrops and the influence of hydrodynamic and glacigenous processes for clay distribution on the shelves are discussed in relation to central Arctic Ocean deep sea and sea ice sediments. Franz Josef Land and Novaya Zemlya show significantly different clay mineral associations. Although smectite concentrations are fairly high, Franz Josef Land can be excluded as a source for central Arctic sea ice sediments, which are relatively rich in smectite. In the Kara Sea, smectite concentrations in coastal sediments surpass even the Franz Josef Land concentrations. The large cyclonic gyre in the eastern Barents Sea between Novaya Zemlya and Franz Josef Land, which serves as a mixing zone between Arctic and North Atlantic water, is apparently reflected within the smectite distribution pattern. With the exception of Franz Josef Land, the area of investigation is typically low in kaolinite. In particular, coastal areas and areas north of Novaya Zemlya, influenced by the inflow of Arctic waters, show the lowest kaolinite concentrations. A high kaolinite occurrence within the Nansen Basin is most probably related to Franz Josef Land and emphasizes the importance of long-range downslope transport of sediments across the continental slope. The surface water circulation pattern in close interaction with local outcrops onshore Novaya Zemlya and locally restricted occurrences within the eastern Barents Sea significantly alter the illite dispersal pattern. Illite concentrations are lowest around Franz Josef Land. Chlorite is generally low in the area of investigation. Submarine outcrops and important chlorite occurrences onshore Novaya Zemlya bias its distribution pattern.  相似文献   

Dispersive Particle Transport: Identification of Macroscale Behavior in Heterogeneous Stratified Subsurface Flows     

Ali Fadili  Rachid Ababou  Roland Lenormand 《Mathematical Geology》1999,31(7):793-840

Dispersive mass transport processes in naturally heterogeneous geological formations (porous media) are investigated based on a particle approach to mass transport and on its numerical implementation using LPT3D, a Lagrangian Particle Tracking 3D code. We are currently using this approach for studying microscale and macroscale space–time behavior (advection, diffusion, dispersion) of tracer plumes, solutes, or miscible fluids, in 1,2,3-dimensional heterogeneous and anisotropic subsurface formations (aquifers, petroleum reservoirs). Our analyses are based on a general advection-diffusion model and numerical scheme where concentrations and fluxes are discretized in terms of particles. The advection-diffusion theory is presented in a probabilistic framework, and in particular, a numerical analysis is developed for the case of advective transport and rotational flows (numerical stability of the explicit Euler scheme). The remainder of the paper is devoted to the behavior of concentration, mass flux density, and statistical moments of the transported tracer plume in the case of heterogeneous steady flow fields, where macroscale dispersion occurs due to geologic heterogeneity and stratification. We focus on the case of perfectly stratified or multilayered media, obtained by generating many horizontal layers with a purely random transverse distribution of permeability and horizontal velocity. In this case, we calculate explicitly the exact mass concentration field C(x, t), mass flux density field f(x, t), and moments. This includes spatial moments and dispersion variance ² _x (t) on a finite domain L, and temporal moments on a finite time scale T, e.g., the mass variance of arrival times ² _T (x). The moments are related to flux concentrations in a way that takes explicitly into account finite space–time scales of analysis (time-dependent tracer mass; spatially variable flow through mass). The multilayered model problem is then used in numerical experiments for testing different ways of recovering information on tracer plume migration, dispersion, concentration and flux fields. Our analyses rely on a probabilistic interpretation that emerges naturally from the particle approach; it is based on spatial moments (particle positions), temporal moments (mass weighted arrival times), and probability densities (both concentrations and fluxes). Finally, as an alternative to direct estimations of the flux and concentration fields, we formulate and study the Moment Inverse Problem. Solving the MIP yields an indirect method for estimating the space–time distribution of flux concentrations based on observed or estimated moments of the plume. The moments may be estimated from field measurements, or numerically computed by particle tracking as we do here.  相似文献   

2017年湖南一次极端降雨过程特征及成因分析     

许霖  姚蓉  陈红专  苏涛  周莉 《沙漠与绿洲气象(新疆气象)》2020,14(1):31-38

2017年6月22日-7月2日湖南出现了一次罕见的大范围持续暴雨、大暴雨过程，持续强降雨造成了重大人员伤亡和财产损失。本文利用地面常规探测资料、自动站资料和NCEP/NCAR再分析资料，对此次极端降雨过程的成因进行了分析。结果表明：（1）过程发生在单阻型稳定背景条件下，西太平洋副高较常年位置偏南，带状副高与北方冷涡对峙，导致了梅雨锋雨带在湖南摆动；（2）高低空急流加强了低层辐合高空辐散的动力作用，同时高能高湿激发了对流不稳定的产生，为大暴雨产生提供了动、热力条件；（3）水汽异常偏高，整层可降水量超过常年同期1-2个标准差，其大值区的演变与实际强降水区域对应较好；（4）湖南地形对降水有一定的增幅作用，地形坡度绝对值与降水量变化十分吻合，地形变化越剧烈降水强度越大，且在地形复杂的2个山峰之间区域通常对应着降水的峰值。  相似文献   

北京两次重污染沙尘天气成因及动力传输特征的对比研究     

王华  轩春怡  吴方  金晨曦  李梓铭  李靖  郭金兰 《沙漠与绿洲气象(新疆气象)》2020,14(4):18-26

应用气象、空气质量和ERA-Interim再分析资料,结合HYSPLIT后向轨迹模型和地理信息,对比分析了2018年北京两次重污染沙尘天气的成因、传输特征及影响要素等。结果表明:两次过程的沙尘源地均为蒙古国,但3月28日沙尘天气由蒙古气旋引发,传输出现折向,以东北和偏东两条路径输送入京,5月28日沙尘受低涡和发展的低压冷锋影响,为典型的西北传输路径;高空的引导气流与低层冷空气的强度以及地形是导致沙尘传输出现显著差异的主要影响因素;高空为偏西气流,低层冷平流较弱,受到山脉的阻挡,沙尘气团东移至地势较低的区域后,在气压梯度力的驱动下传输方向发生转折,以偏东路径绕流进入北京;高空的偏北气流和低层冷平流较强,沙尘气团可直接翻越山脉从西北路径入京,沙尘粒子的运移高度相对较高。  相似文献   

The Vertical Transport of Air Pollutants by Convective Clouds. Part I: A Non-Reactive Cloud Transport Model   总被引：1，自引：0，他引：1  

Kong Fanyou  Qin Yu 《大气科学进展》1993,10(4):415-427

A convective cloud transport model, without chemical processes, is developed by joining a set of concentration conservative equations into a two-dimensional, slab-symmetric and fully elastic numerical cloud model, and a numerical experiment is completed to simulate the vertical transport of ground-borne, inert gaseous pollutant by deepthunderstorm. The simulation shows that deep convective storm can very effectively transport high concentrated pollutant gas from PBL upward to the upper troposphere in 30 to 40 minutes, where the pollutant spreads laterally outward with strong anvil outflow, forming an extensive high concentration area. Meanwhile, relatively low concentration areas are formed in PBL both below and beside the cloud, mainly caused by dynamic pumping effect and sub-cloud downdraft flow. About 80% of the pollutant gas transported to the upper troposphere is from the layer below 1.5 km AGL (above ground level).  相似文献