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1.
In settings where the transport of sand is partially or fully supply limited, changes in the upstream supply of sand are coupled to changes in the grain size of sand on the bed. In this manner, the transport of sand under the supply-limited case is ‘grain-size regulated’. Since the closure of Glen Canyon Dam in 1963, the downstream reach of the Colorado River in Marble and Grand Canyons has exhibited evidence of sand-supply limitation. Sand transport in the river is now approximately equally regulated by changes in the discharge of water and changes in the grain sizes of sand on the channel bed and eddy sandbars. Previous work has shown that changes in the grain size of sand on the bed of the channel (driven by changes in the upstream supply of sand owing to both tributary floods and high dam releases) are important in regulating sand transport over timescales of days to months. In this study, suspended-sand data are analysed in conjunction with bed grain-size data to determine whether changes in the grain size of sand on the bed of the channel or changes in the grain size of sand on the surface of eddy sandbars have been more important in regulating sand transport in the post-dam Colorado River over longer, multi-year timescales. The results of this study show that this combined theory- and field-based approach can be used to deduce which environments in a complicated setting are the most important environments for regulating sediment transport. In the case of the regulated Colorado River in Marble and Upper Grand Canyons, suspended-sand transport has been regulated mostly by changes in the surface grain size of eddy sandbars.  相似文献   

2.
依据三峡水库修建以前的资料,运用数理统计方法对含沙量和悬沙粒径变化对长江宜昌-汉口河段年冲淤量的影响进行研究,以期为三峡水库修建以后库下游河道冲淤特性的预测提供参考。建立了1980-1997年间宜昌-汉口#河段年冲淤量与宜昌站年均含沙量C宜昌之间的回归方程,据此估算出使宜昌-汉口#河段处于不冲不淤状态的宜昌站临界年均含沙量为0.734 kg/m3。以宜昌-汉口冲淤量作为因变量,以宜昌站的含沙量、悬沙中径D50、最大流量和三口分流比作为影响变量,建立了多元回归方程。基于1980-1997年资料的方程表明,宜昌站含沙量越高,悬沙中径越粗,宜昌站洪水流量越大,宜昌-汉口河段年淤积量越大;三口分流比越小,宜昌-汉口河段年淤积量越大。  相似文献   

3.
钟亮  许光祥  曾锋 《水科学进展》2013,24(1):111-117
沙粒当量粗糙度ks的准确确定是计算沙粒阻力的关键。针对沙粒轮廓剖面具有自相似精细结构的特点,提出采用分形维数对其复杂性及不规则性进行定量描述;通过粘沙平整床面、初始静平整床面和清水冲刷粗化床面的沙粒阻力试验研究ks的分形表达方法,建立了静平整床面时ks的分形表达式,并对公式进行了验证、比较和讨论;结果表明,公式引入的中值粒径d50、分形维数D和弗劳德数Fr 3个参数,可定量地综合反映床沙颗粒大小、级配组成、随机排列以及水流条件对ks的影响。  相似文献   

4.
For more than a century, studies of sedimentology and sediment transport have measured bed-sediment grain size by collecting samples and transporting them back to the laboratory for grain-size analysis. This process is slow and expensive. Moreover, most sampling systems are not selective enough to sample only the surficial grains that interact with the flow; samples typically include sediment from at least a few centimeters beneath the bed surface. New hardware and software are available for in situ measurement of grain size. The new technology permits rapid measurement of surficial bed sediment. Here we describe several systems we have deployed by boat, by hand, and by tripod in rivers, oceans, and on beaches.  相似文献   

5.
Aeolian sand entrainment, saltation and deposition are important and closely related near surface processes. Determining how grains are sorted by wind requires a detailed understanding of how aerodynamic sand transport processes vary within the saltating layer with height above the bed. Grain‐size distribution of sand throughout the saltation layer and, in particular, how the associated flux of different grain size changes with variation in wind velocity, remain unclear. In the present study, a blowdown wind tunnel with a 50 cm thick boundary layer was used to investigate saltating sand grains by analyzing the weight percentage and transport flux of different grain‐size fractions and the mean grain size at different wind velocities. It was found that mean grain size decreases with height above the sand bed before undergoing a reversal. The height of the reversal point ranges from 4 to 40 cm, and increases with wind velocity following a non‐linear relationship. The content of the finer fractions (very fine and fine sand) initially increases above the sand bed and then decreases slightly with height, whereas that of the coarser fractions (medium and coarse sand) exhibits the opposite trend. The content of coarser grains and the mean grain size of sand in the saltation layer increase with wind velocity, indicating erosional selectivity with respect to grains in multi‐sized sand beds; but this size selectivity decreases with increasing wind velocity. The vertical mass flux structure of fine sand and very fine sand does not obey a general exponential decay pattern under strong wind conditions; and the coarser the sand grain, the greater the decrease rate of their transport mass with height. The results of these experiments suggest that the grain‐size distribution of a saltating sand cloud is governed by both wind velocity and height within the near‐surface boundary layer.  相似文献   

6.
Interactions between catchment variables and sediment transport processes in rivers are complex, and sediment transport behaviour during high‐flow events is not well documented. This paper presents an investigation into sediment transport processes in a short‐duration, high‐discharge event in the Burdekin River, a large sand‐ and gravel‐bed river in the monsoon‐ and cyclone‐influenced, semi‐arid tropics of north Queensland. The Burdekin's discharge is highly variable and strongly seasonal, with a recorded maximum of 40 400 m3 s?1. Sediment was sampled systematically across an 800 m wide, 12 m deep and straight reach using Helley‐Smith bedload and US P‐61 suspended sediment samplers over 16 days of a 29‐day discharge event in February and March 2000 (peak 11 155 m3 s?1). About 3·7 × 106 tonnes of suspended sediment and 3 × 105 tonnes of bedload are estimated to have been transported past the sample site during the flow event. The sediment load was predominantly supply limited. Wash load included clay, silt and very fine sand. The concentration of suspended bed material (including very coarse sand) varied with bedload transport rate, discharge and height above the bed. Bedload transport rate and changes in channel shape were greatest several days after peak discharge. Comparison between these data and sparse published data from other events on this river shows that the control on sediment load varies between supply limited and hydraulically limited transport, and that antecedent weather is an important control on suspended sediment concentration. Neither the empirical relationships widely used to estimate suspended sediment concentrations and bedload (e.g. Ackers & White, 1973) nor observations of sediment transport characteristics in ephemeral streams (e.g. Reid & Frostick, 1987) are directly applicable to this river.  相似文献   

7.
The paper presents the results of granulometric and geochemical analyses of the superficial bed sediments of the St. George branch, one of three Danube River distributaries. Three cutoff meander reaches (the Mahmudia, Dunavăţ de Sus (Upper Dunavăţ) and Dunavăţ de Jos (Lower Dunavăţ) meanders, noted here as M, DS and DJ meanders) was selected as an example to analyse the impact of the meanders rectification on the grain size and sediment quality. The cut-offs of the meanders by navigational canals since 1984–1988 caused dramatic changes in the distribution of velocities, discharge and sediment fluxes (Popa, 1997; Panin, 2003, Tiron and Provansal, 2010, Tiron Dutu et al., 2014). The correction of the studied meanders induced different hydro-sedimentary impacts: strong modifications by acceleration of the fluxes through the artificial canals combined with dramatically enhanced deposition in the former meander in two cases (Mahmudia and Dunavăţ de Jos meanders) and slight modifications in Dunavăţ de Sus meander, where the natural channel still keeps almost its initial conveyance capacity. Sediment samples were acquired throughout several cross-sections of both natural and artificial canals of the three rectified meanders, in order to investigate the bed sediment characteristics. Grain size parameters (such as Median, Standard deviation, Skewness) show the predominance of fine fraction (silt and medium/ fine sand), relatively good sorted depending of the sample location (on the natural course of the branch, on the rectification canals or the former meanders. The statistical analysis of chemical data evidenced a high compositional variability, with concentration variation coefficients ranging between 27.4% – Sr and 178.3% – TOC (Total Organic Carbon). Generally, the highest concentrations of terrigenic compounds appear in the samples collected from the rectified natural canals. These results are validated by comparison with aDcp measurements made on each investigated profile.  相似文献   

8.
We report new mapping, soils, survey, and geochronologic (luminescence, U-series, and cosmogenic-nuclide) data from Pleistocene deposits in the arid setting of eastern Grand Canyon. The result is a stratigraphic framework of inset fill gravels and associated terraces that provide a record of the responses of hillslopes, tributary streams, and the Colorado River to the last 400 kyr of glacial–interglacial climate change. The best-preserved last 80 kyr of this record indicates a stratigraphic–chronologic disconnect between both deposition and incision along the Colorado River versus along the trunks of local tributaries. For example, the Colorado River finished aggrading and had already begun incising before the main pulse of aggradation in the trunks of local catchments during Marine Isotope Stage 3, and then tributary incision followed during the millennial-scale fluctuations of the last glacial epoch, potentially concurrent with mainstem aggradation. The mainstem record appears to broadly correlate with regional paleoclimate and upstream geomorphic records and thus may be responding to climatic–hydrologic changes in its mountain headwaters, with aggradation beginning during full-glacial times and continuing into subsequent interglacials. The contrasting lag time in responses of the dryland catchments within Grand Canyon may be largely a function of the weathering-limited nature of hillslope sediment supply.  相似文献   

9.
It is generally accepted that a gravel-bed river will aggrade if the supply of sediment to the river is increased. In a series of flume experiments using constant discharge and gravel feed rate, sand feed rates were increased to 6.1 times that of gravel. The slope of the bed decreased with increasing sand supply, indicating that the increased sediment load could be transported at the same rate due to a decrease in shear stresses. These results extend previous experiments to a wider range of boundary conditions. A recent surface transport model is used to predict the changes in bed composition and transport using the same sediment supply composition and feed rates as in the laboratory experiments. This model reasonably predicts a decrease in the reference shear stresses of the sand and gravel fractions as the sand supply is increased. An increase in sand supply can increase the mobility of gravel fractions in the stream bed, which can lead to bed degradation and preferential evacuation of these sediments from the river.  相似文献   

10.
A distinct suite of sand bedforms has been observed to occur in laboratory flows with limited sand supply. As sand supply to the bed progressively increases one observes sand ribbons, discrete barchans and, eventually, channel spanning dunes; but there are relatively few observations of this sequence from natural river channels. Furthermore, there are few observations of transitions from limited sand supply to abundant supply in the field. Bedforms developed under limited, but increasing, sand supply downstream of the abrupt gravel–sand transition in the Fraser River, British Columbia, are examined using multi‐beam swath‐bathymetry obtained at high flow. This is an ideal location to study supply‐limited bedforms because, due to a break in river slope, sand transitions from washload upstream of the gravel–sand transition to bed material load downstream. Immediately downstream, barchanoid and isolated dunes are observed. Most of the bedform field has gaps in the troughs, consistent with sand moving over a flat immobile or weakly mobile gravel bed. Linear, alongstream bedform fields (trains of transverse dunes formed on locally thick, linear deposits of sand) exhibit characteristics of sand ribbons with superimposed bedforms. Further downstream, channel spanning dunes develop where the bed is composed entirely of sand. Depth scaling of the dunes does not emerge in this data set. Only where the channel has accumulated abundant sand on the bed do the dunes exhibit scaling congruent with previous data compilations. The observations suggest that sediment supply plays an important, but often overlooked, role in bedform scaling in rivers.  相似文献   

11.
Engelund's (1974a, b) theoretical bed topography model for curved channels is modified and shown to fit natural channels when using values for the dynamic bed load friction coefficient of about 0·4–0·5. The dependence of this coefficient on grain size for flow in the lower flow regime is discussed. Using the bed topography model simultaneously, Allen's (1970a, b) theory for grain size variation over point bar profiles is shown to be broadly applicable. The interaction between sediment, flow and bed topography in bends is therefore adequately described.  相似文献   

12.
Grain‐size breaks are surfaces where abrupt changes in grain size occur vertically within deposits. Grain‐size breaks are common features in turbidites around the world, including ancient and modern systems. Despite their widespread occurrence, grain‐size breaks have been regarded as exceptional, and not included within idealized models of turbidity current deposition. This study uses ca 100 shallow sediment cores, from the Moroccan Turbidite System, to map out five turbidite beds for distances in excess of 2000 km. The vertical and spatial distributions of grain‐size breaks within these beds are examined. Five different types of grain‐size break are found: Type I – in proximal areas between coarse sand and finer grained structureless sand; Type II – in proximal areas between inversely graded sand overlain by finer sand; Type III – in proximal areas between sand overlain by ripple cross‐laminated finer sand; Type IV – throughout the system between clean sand and mud; and Type V – in distal areas between mud‐rich (debrite) sand and mud. This article interprets Types I and V as being generated by sharp vertical concentration boundaries, controlled by sediment and clay concentrations within the flows, whilst Types II and III are interpreted as products of spatial/temporal fluctuations in flow capacity. Type IV are interpreted as the product of fluid mud layers, which hinder the settling of non‐cohesive grains and bypasses them down slope. Decelerating suspensions with sufficient clay will always form cohesive layers near to bed, promoting the generation of Type IV grain‐size breaks. This may explain why Type IV grain‐size breaks are widespread in all five turbidites examined and are commonplace within turbidite sequences studied elsewhere. Therefore, Type IV grain‐size breaks should be understood as the norm, not the exception, and regarded as a typical feature within turbidite beds.  相似文献   

13.
为了研究长江口-杭州湾外近海沉积物粒径的现状和近期变化,于2008年4月在该区域用箱式取样器取得了33个表层样,用震动活塞采样器取得7个柱状样;在室内用激光粒度仪对沉积物样品进行了粒度分析并与历史时期的研究成果进行对比。结果表明,1）表层沉积物总体上呈东粗（砂）西细（泥）的特点,粒径总体上的空间格局与历史时期相似;但砂-泥区界线在研究区北部（长江口和杭州湾外）有向西迁移（蚀退）迹象（西移10~30 km）,而在南部（舟山群岛以南）则出现明显向东迁移（淤进）现象（最大超过50 km）。2）当前的表层沉积物形成一条南北连续的泥质带（粒径向南逐渐变细）,反映过去存在于舟山群岛以东的最细组分（“黏土”）带不连续现象在表层现已趋于消失。3）表层沉积物粒径趋势分析揭示长江入海泥沙的运移方向主要是向南-东南。4）泥质区柱状沉积物具有粉砂和黏土为主但在垂向上（反映在时间上）有粗细多变的特点（侵蚀区表层沉积物有粗化迹象）。研究认为,长江口-杭州湾邻近海域现代沉积物对流域和沿海重大人类活动有较敏感的响应。  相似文献   

14.
The grain/bed collision in sand transport by wind   总被引：6，自引：0，他引：6  
In the last decade much progress has been made toward the development of a comprehensive model of aeolian sediment transport in which the grain/bed collision has been identified as having a significant role. The grain/bed collision has been studied by both physical experiments and numerical simulation. A principal objective of these studies has been to gather sufficient data to characterize the collision in order that it may be represented as an empirical function in numerical models of the sand transport system. Thus the study of the exact physical mechanism of the collision has to some extent been neglected. The transport of larger particles in saltation over a loose surface is known to promote the release of finer particles from that surface into suspension. Thus the precise physics of the grain/bed collision become highly significant with regard to the physics of dust release from a surface. This paper re-examines previous collision data and compares them with a simple collision model. This model proposed that the impinging grain strikes a single surface grain and rebounds: an alternative mechanism might be that the impinging grain ploughs through the surface striking a number of bed grains before rebounding. The collision data are shown to support the model: first, because the duration of the collisions observed on the high speed film supports a short contact time with the surface and, second, because the data fit well with the model.  相似文献   

15.
P. A. WOOD 《Sedimentology》1977,24(3):437-445
Suspended sediment concentrations were determined for samples collected from the River Rother, West Sussex, and rating loops constructed for several hydrographs. The rating loops often exhibit hysteresis with a greater suspended sediment concentration for a given discharge occurring on the rising limb than on the falling limb. A comparison of these loops indicates that the hysteresis, and the suspended sediment concentration (for a given discharge) become progressively reduced when storm events occur in rapid succession. Various types of rating curves can be identified, and a model of suspended sediment concentration and discharge constructed. Controlling factors of suspended sediment concentration are river discharge, duration and frequency of the storm event, the length of time between successive events, and the time of year.  相似文献   

16.
洞庭湖砂纹淤泥质土固结过程微观结构变化   总被引：1，自引：1，他引：0       下载免费PDF全文
砂纹淤泥质土是一种特殊的夹有微薄层粉细砂的洞庭湖湖相沉积软土,具有独特的薄砂层构造。该软土固结过程中的微观结构变化,对解释宏观的工程现象、揭示土体变形的本质和采取合理的地基处理措施具有重要意义。本文利用X射线微区衍射试验和扫描电子显微镜试验,研究洞庭湖砂纹淤泥质土的矿物成分和微观结构,同时结合固结试验探讨砂纹淤泥质土微观孔隙与结构单元体参数变化特征。研究结果表明:洞庭湖砂纹淤泥质土主要矿物成分为绿泥石、白云母和石英,微观结构为片状或板状聚体结构;随着固结压力增加,等效粒径增长呈先快速后放缓的规律,结构单元体形态参数则先增大后减小;砂纹淤泥质土压缩系数与竖直切面结构单元体定向概率熵呈明显负相关关系。  相似文献   

17.
为了研究河流沉积物粒度对区域气候变化的指示作用,对濠河中游、下游、濠河入淮河口3个典型河床沉积物剖面及石英尾砂的粒度进行了测量。结果显示:3个沉积物剖面按粒度的变化特征均可分成四层,第一层沉积物以细粉砂为主,第二、三、四层以粗粉砂为主,其中第二、三、四层沉积物的粒度频率分布曲线与石英尾砂非常相似,说明大量石英尾砂排入濠河后,沉积物粒度特征由石英尾砂控制。在近30年中,濠河沉积物的粒度随河水动力的变化而出现波动,这种波动与降雨量的年际变化、洪水的发生等自然因素有关,沉积物平均粒径的峰值与降雨量大或洪水发生的年份存在对应关系。濠河沉积物的粒度特征具有濠河流域气候干湿变化的指示意义:粒径粗指示降雨量较多的湿润气候,粒径细指示降雨量较少的干旱气候。  相似文献   

18.
The links between large‐scale turbulence and the suspension of sediment over alluvial bedforms have generated considerable interest in the last few decades, with past studies illustrating the origin of such turbulence and its influence on flow resistance, sediment transport and bedform morphology. In this study of turbulence and sediment suspension over large sand dunes in the Río Paraná, Argentina, time series of three‐dimensional velocity, and at‐a‐point suspended sediment concentration and particle‐size, were measured with an acoustic Doppler current profiler and laser in situ scattering transmissometer, respectively. These time series were decomposed using wavelet analysis to investigate the scales of covariation of flow velocity and suspended sediment. The analysis reveals an inverse relationship between streamwise and vertical velocities over the dune crest, where streamwise flow deceleration is linked to the vertical flux of fluid towards the water surface in the form of large turbulent fluid ejections. Regions of high suspended sediment concentration are found to correlate well with such events. The frequencies of these turbulent events have been assessed from wavelet analysis and found to concentrate in two zones that closely match predictions from empirical equations. Such a finding suggests that a combination and interaction of vortex shedding and wake flapping/changing length of the lee‐side separation zone are the principal contributors to the turbulent flow field associated with such large alluvial sand dunes. Wavelet analysis provides insight upon the temporal and spatial evolution of these coherent flow structures, including information on the topology of dune‐related turbulent flow structures. At the flow stage investigated, the turbulent flow events, and their associated high suspended sediment concentrations, are seen to grow with height above the bed until a threshold height (ca 0·45 flow depth) is reached, above which they begin to decay and dissipate.  相似文献   

19.
In measured sequences of limestone- and greywacke-turbidites the bed-thickness is found to vary proportionally with the fall velocity of the maximum grain size, found at the base of the bed. A simple theoretical model, based on the decay of isotropic turbulence, suggests that bed-thickness should be a function not only of this fall velocity, but also of bottom slope, flow depth and the concentration and grain-size distribution of the sediment in the turbidity current. The field data do show some influence of these additional factors. Nevertheless, for many natural sequences of turbidites the flows must have carried very poorly sorted sediments and the inferred flow volumes and densities must cluster tightly about modal values. Thus, grain size remains the primary variable and the modal regression curve of bed-thickness on maximum grain size is well defined and resembles a fall-velocity curve. Relatively steep basin floors near to source can, theoretically, cause these modal regressions for distal and very proximal parts of a turbidite to diverge, introducing a crudely parabolic appearance in the form of the total regression curve. The form of this parabolic curve predicts the deposition of thin but relatively coarse proximal beds. Such beds do occur. They are different from the thin, but relatively fine, proximal beds that have been interpreted as the result of a fractionation of a turbidity current during levee-forming processes.  相似文献   

20.
Sediment dynamics in the lowermost Mississippi River   总被引：1，自引：0，他引：1  
There is much to be gained from investigating sediment dynamics in the lower Mississippi system, the largest river in terms of discharge and sediment load in North America. Such work can improve conceptual knowledge concerning downstream changes at the lower end of large river systems and can be applied to manage sediment diversions for wetland restoration in south Louisiana. Suspended sediment dynamics in the lowermost Mississippi River system in Louisiana are characterized using three approaches: (1) temporal changes in discharge-suspended sediment relationships showing interannual variations and the effects of floods over short timescales; (2) empirical relationships between discharge and suspended sediment variables at various locations; and (3) downstream changes in discharge-suspended sediment relationships. Interpretation of this data set is enhanced with other secondary data regarding processes, morphology, and bed materials.

Upstream, near Old River, LA, empirical relationships show nonlinearity, particularly in fine sediments, with decreased concentrations at highest discharges. During high discharge years, suspended sediment concentration peaks precede discharge crests by 40–85 days. The lead generally decreases with decreasing discharge maxima so that in low discharge years sediment peaks and discharge crests closely coincide in time. Downstream, near Belle Chasse, LA, fine bottom materials are resuspended and the timing of sediment peaks and discharge crests is coincident, regardless of flow magnitude. Conceptually, results suggest caution when generalizing about the relative timing of the sediment wave and flood wave and their downstream progression. These phenomena are influenced by local bed material and hydraulic conditions, and depend on the causative factors of sediment peaks. From an applied perspective, diversions should be managed differently depending upon where they are constructed along the river and upon the magnitude of the annual maximum flow. During high discharge years, when concerns for navigation and water supply are minimal, flow should be diverted on the rising limb upstream, near Old River, and during the discharge crest downstream near New Orleans.  相似文献   


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