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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.
The stability of river channels and their suitability as habitat for aqueous organisms is strongly controlled by the rate of bedload transport. Quantification of bedload transport rates in rivers is difficult, not only because of the temporal variation in transport, but also because of the cross‐channel variation in transport. The objectives of this study were: (i) to determine the effect of cross‐channel variation in bedload transport on the uncertainty of width‐integrated transport rates; and to use this knowledge (ii) to improve guidelines for bedload sampling. This was done through a thorough statistical evaluation of stochastic and systematic uncertainties involved in bed‐load transport measurements. Based on this evaluation, new guidelines are presented for determination of the number of samples and sampling positions across the channel that are required for bedload measurements in several types of sand‐bed rivers and gravel‐bed rivers. The guidelines relate to bedload measurements made with pressure‐difference (Helley‐Smith type) samplers that require numerous bedload samples of short duration at several positions across the channel. The results show that generally more sampling positions across the channel are required in gravel‐bed rivers than in sand‐bed rivers. For gravel‐bed rivers with unknown cross‐channel distribution of transport, at least 10 sampling positions are recommended, whereas for most sand‐bed rivers five positions suffice. In addition, at least 12 short‐duration samples are required at each position to obtain bedload estimates with uncertainties below 20%. If the same level of uncertainty is desired in the case of high spatial and temporal variation in transport rates, the number of short‐duration samples needed per sampling position increases to 40. 相似文献
3.
Flow and sediment dynamics in a low sinuosity, braided river: Calamus River, Nebraska Sandhills 总被引：1，自引：0，他引：1
The interaction between channel geometry, flow, sediment transport and deposition associated with a midstream island was studied in a braided to meandering reach of the Calamus River, Nebraska Sandhills. Hydraulic and sediment transport measurements were made over a large discharge range using equipment operated from catwalk bridges. The relatively low sinuosity channel on the right-hand side of the island carries over 70% of the water discharge at high flow stages and 50–60% at low flow stages. As a result, mean velocity, depth, bed shear stress and sediment transport rate tend to be greater here than in the more strongly curved left-hand channel. The loci of maximum flow velocity, depth and bed shear stress are near the centre of the channel upstream of the island, but then split and move towards the outer banks of both channels downstream. Variations in these loci depend on the flow stage. Topographically induced across-stream flows are generally stronger than the weak, curvature-induced secondary circulations. Water surface topography is controlled mainly by centrifugal accelerations and local changes in downstream flow velocity. The averaged water surface slope of the study reach varies very little with discharge, having values between 0·00075 and 0·00090. As bed shear stress generally varies in a similar way to mean velocity, friction coefficients vary little, normally being in the range 0·07–0·13. These values are similar to those in straight channels with sandy dune-covered beds. Bedload is moved mainly as dunes at all flow stages. Grain size is mainly medium sand with coarse sand moved in thalwegs adjacent to the cut banks, and with fine sand at the downstream end of the island. These patterns of flow velocity, depth, water surface topography, bed shear stress, bedload transport rate and mean grain size can be accurately predicted using theoretical models of flow, bed topography and sediment transport rate in single river bends, applied separately to the left and right channels. During high flow stages deposition occurs persistently near the downstream end of the island, and cut banks are eroded. Otherwise, erosion and deposition occurs only locally within the channel as discharge varies. Abandonment and filling of a strongly curved channel segment may occur by migration of an upstream bar into the channel entrance at a high flow stage. 相似文献
4.
The UlanBuh Desert reach belongs to the upper reaches of the Yellow River located in Inner Mongolia Autonomous Region (China), which has developed a sandy bed. This reach that flows across aeolian dunes field of the UlanBuh Desert has caused large lateral infusion of aeolian sands into riverbed, an abrupt addition of the bedload and a sharp channel widening, forming a typical braided channel. In addition to traditional driving factors such as discharge, suspended sediments and slope, here we also focus on the lateral infusion of aeolian sands into the Yellow River, which may have a significant impact on channel form. To further recognize the desert braided channel change, here based on a long-time series of field dataset from 1966 to 2013, results show that lateral infusion of aeolian sand is the same as discharge as a significant factor to channel form, which is mainly controlled by aeolian activity and discharge conditions. Before the early-1990s, higher discharge and greater intensity of aeolian activity caused a frequent lateral movement and large lateral inputs of aeolian sands into river; this reach showed an unsteady and wider braided channel with a higher braiding intensity. From the early-1990s to 2013, a reduced discharge that caused a decreasing lateral movement, coupled with the decreasing aeolian activity, have resulted in the main flow of the Yellow River far away from sand dunes, which led to a reduction in lateral inputs of aeolian sands. Thus, the braided channel of the UlanBuh Desert reach presented a shrinkage state, gradually changing from a wider and unsteady braided channel to a narrower and relative steady channel form. 相似文献
5.
Supply limited sediment transport in a high-discharge event of the tropical Burdekin River, North Queensland, Australia 总被引：1，自引：0，他引：1
Kathryn J. Amos Jan Alexander Anthony Horn† Geoff D. Pocock‡ Chris R. Fielding§ 《Sedimentology》2004,51(1):145-162
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
6.
Joanna C. Curran 《Sedimentary Geology》2007,202(3):572-580
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
7.
The response of subaqueous dunes to floods in sand and gravel bed reaches of the Dutch Rhine 总被引：4，自引：0，他引：4
Abstract The branches of the River Rhine in the Netherlands, characterized by a sand–gravel bed in the upstream part and a sand bed in the downstream part of the river system, show migrating dunes, especially during floods. In the last 20 years, these dunes have been studied extensively. High-resolution echo-sounding measurements of these dunes, made with single and multibeam equipment, were analysed for three different sections of the Rhine river system during several floods. This analysis was done to quantify the growth, decay and migration rates of the dunes during floods. In addition, the migrating dunes were used to calculate bedload transport rates with dune tracking. The results of dune growth and decay and migration rate are shown to be very different for the various sections during the various floods, and these differences are related to differences in grain size of the bed and to differences in the distribution of discharge over the main channel and the floodplain. The relations are used to show that the growth and migration rate of dunes, and the calculated bedload transport rates during the rising stage of a flood wave can be predicted from the mobility of the bed material with simple power relations. 相似文献
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9.
《Sedimentology》2018,65(5):1590-1610
Downstream variation in grain size associated with changes in river pattern is a topic that interests multiple disciplines. How grain size varies between adjacent reaches with strongly contrasting river pattern is an outstanding question. This study presents a combined field and numerical modelling investigation of a river with a downstream planform change from single channel to anabranching, where the planform is controlled by a change in underlying lithology. This approach enabled exploration of the controls on sedimentology in a river for which there is very limited opportunity to collect flow and sediment transport data. This study shows that the surficial grain size decreases as a result of the downstream change in planform. This is because of a decrease in flow velocity and shear stress associated with a decrease in channel depth related to the planform change. Channel geometries in both the field and modelling data fit into distinct groups based on channel depth, the deepest being the single channel reach and the shallowest being the anabranching. This downstream reduction in channel dimension (depth) is caused because the total discharge is split from one channel into multiple channels. The coarsest grain sizes (cobble) are deposited at the terminus of the single channel and in the distributary channels; anabranching channels contain sand‐size sediments. This study shows that, in a transition from single channel to anabranching, the channel dimensions decrease as the number of channels increases, resulting in a decrease in bed shear stress and the fining of bed material downstream. 相似文献
10.
Sand transport in the Colorado River in Marble and Grand canyons was naturally limited by the upstream supply of sand. Prior to the 1963 closure of Glen Canyon Dam, the river exhibited the following four effects of sand supply limitation: (1) hysteresis in sediment concentration, (2) hysteresis in sediment grain size coupled to the hysteresis in sediment concentration, (3) production of inversely graded flood deposits, and (4) development or modification of a lag between the time of a flood peak and the time of either maximum or minimum (depending on reach geometry) bed elevation. Construction and operation of the dam has enhanced the degree to which the first two of these four effects are evident, and has not affected the degree to which the last two effects of sand supply limitation are evident in the Colorado River in Marble and Grand canyons. The first three of the effects involve coupled changes in suspended-sand concentration and grain size that are controlled by changes in the upstream supply of sand. During tributary floods, sand on the bed of the Colorado River fines; this causes the suspended sand to fine and the suspended-sand concentration to increase, even when the discharge of water remains constant. Subsequently, the bed is winnowed of finer sand, the suspended sand coarsens, and the suspended-sand concentration decreases independently of discharge. Also associated with these changes in sand supply are changes in the fraction of the bed that is covered by sand. Thus, suspended-sand concentration in the Colorado River is likely regulated by both changes in the bed-sand grain size and changes in the bed-sand area. A physically based flow and suspended-sediment transport model is developed, tested, and applied to data from the Colorado River to evaluate the relative importance of changes in the bed-sand grain size and changes in the bed-sand area in regulating suspended-sand concentration. Although the model was developed using approximations for steady, uniform flow, and other simplifications that are not met in the Colorado River, the results nevertheless support the idea that changes in bed-sand grain size are much more important than changes in bed-sand area in regulating the concentration of suspended sand. 相似文献
11.
The Burdekin River is an example of a class of tropical streams which experience two to four orders of magnitude variation in discharge, in response to seasonal but erratic monsoonal rainfall. Floods of the Burdekin rise abruptly, reaching peak discharges of up to 40,000 m3 s-1 in less than 24 h; maintain peak flow for up to a few days, and recede exponentially. The geomorphology and deposits of these rivers reflect the extreme discharge fluctuations, and have not previously been described. A stretch of the upper Burdekin River comprising four bends and one straight reach was examined near the town of Charters Towers. The river bed is largely exposed for most of any year, with a small, misfit perennial channel carrying low stage flow. Major geomorphic elements of bends include point bars with ridge-and-swale topography, three distinct types of chute channels, avalanche slipfaces up to 5 m or more high around the downstream edges of bars, and on the outer part of one point bar an elevated, vegetated ridge. Straight reaches are flat or gently inclined, sand- and gravel-covered surfaces. Much of the river bed is covered by well sorted, in places gravelly, coarse to very coarse-grained sand with local accumulations of pebble to boulder gravel. Lower parts of the river bed are periodically draped by mud which is desiccated on exposure. Dunes and plane beds are the most commonly occurring bedforms, with local development of gravelly antidunes. Most bank tops and upper, vegetated bars are covered by silt and fine-grained sand. The river bed also hosts a low-diversity but locally high-abundance, flood-tolerant flora dominated by the paperbark tree Melaleuca argentea, which plays an important role in controlling the distribution of sediment. The gross geomorphology of the river bed and most of the sedimentary features are interpreted as having formed during major (bankfull or near bankfull) flows, which have a recurrence of about 18 years (based on 65 years hydrographic data). The initial rapid drop in discharge following flood peaks appears to preserve flood peak features on upper bars more or less intact, whereas lower areas are subjected to variable degrees of modification during falling stage and by more frequent, non-bankfull discharge events. 相似文献
12.
海洋沉积物动力学的示踪物方法 总被引：4，自引：1，他引：3
本文的论题是示踪物方法在海洋物源追踪和沉积物输运率计算中的应用 ,以及示踪物方法的普适性理论框架。定量的物源追踪需要适当的示踪标记和物质混合模型 ,有必要根据沉积动力过程的研究来确定示踪标记的变换函数 ,从而将改进的示踪标记用于混合模型分析。人工示踪物实验的现有方法以空间积分法最为常用 ,其关键是示踪物质心运动和沉积物活动层的界定。天然示踪物的质心位置是无法定义的 ,因此不能借用传统人工示踪物方法来估算物质输运率 ;在某些特殊情形下 ,可以利用天然示踪物的质量守衡原理来获得物质输运信息。今后 ,以示踪物质的连续方程为基础 ,有可能建立一种同时适用于人工和天然示踪物的普适理论框架。其中需解决的问题包括示踪物与现场物质的差异、沉积速率和活动层厚度的时间尺度、沉积物扩散过程等。 相似文献
13.
指出原状土与散粒体的起动流速、输沙率存在明显的差异,这使得新开挖河道动床冲刷模型设计方法与天然河道动床模型设计方法有所区别,也复杂得多。提出了新开挖河道动床冲刷模型的设计思想和设计方法,泰州引江河道动床冲刷模型论证了该方法的正确性,并认为该方法具有普遍意义。 相似文献
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15.
Geometry and kinematics of dunes during steady and unsteady flows in the Calamus River, Nebraska, USA 总被引：3，自引：0，他引：3
SHARON L. GABEL 《Sedimentology》1993,40(2):237-269
The geometry and kinematics of river dunes were studied in a reach of the Calamus River, Nebraska. During day-long surveys, dune height, length, steepness, migration rate, creation and destruction were measured concurrently with bedload transport rate, flow depth, flow velocity and bed shear stress. Within a survey, individual dune heights, lengths and migration rates were highly variable, associated with their three-dimensional geometry and changes in their shape through time. Notwithstanding this variability, there were discernible changes in mean dune height, length and migration rate in response to changing discharge over several days. Changes in mean dune height and length lagged only slightly behind changes in discharge. Therefore, during periods of both steady and unsteady flow, mean dune lengths were quite close to equilibrium values predicted by theoretical models. Mean dune steepnesses were also similar to predicted equilibrium values, except during high, falling flows when the steepness was above that predicted. Variations in mean dune height and length with discharge are similar to those predicted by theory under conditions of low mean dune excursion and discharge variation with a short high water period and long low water period. However, the calculated rates of change of height of individual dunes vary considerably from those measured. Rates of dune creation and destruction were unrelated to discharge variations, contrary to previous results. Instead, creations and destructions were apparently the result of local variations in bed shear stress and sediment transport rate. Observed changes in dune height during unsteady flows agree with theory fairly well at low bed shear stresses, but not at higher bed shear stresses when suspended sediment transport is significant. 相似文献
16.
A 2D depth-averaged model for hydrodynamic sediment transport and river morphological adjustment was established. The sediment transport submodel takes into account the influence of non-uniform sediment with bed surface armoring and considers the impact of secondary flow in the direction of bed-load transport and transverse slope of the river bed. The bank erosion submodel incorporates a simple simulation method for updating bank geometry during either degradational or aggradational bed evolution. Comparison of the results obtained by the extended model with experimental and field data, and numerical predictions validate that the proposed model can simulate grain sorting in river bends and duplicate the characteristics of meandering river and its development. The results illustrate that by using its control factors, the improved numerical model can be applied to simulate channel evolution under different scenarios and improve understanding of patterning processes. 相似文献
17.
N. Kosheleva E. Karabanov A. Kositskiy D. Williams S. Armstrong 《Journal of Geochemical Exploration》2006,88(1-3):118
A channel account approach is proposed to estimate longitudinal changes in runoff along large river systems. This new method provides a quantitative basis for describing the fluvial transport of suspended particulate material and dissolved substances. This method includes an evaluation of basic elements of water balance in separate sections of the river network and subsequent correction of channel accounting equations for the entire system using a maximum likelihood principle. To calculate water discharges of tributaries that have no hydrological information, structural analysis of river network is performed. This approach provides less error in comparison with traditional methods of estimating lateral inflow. The method is used to trace water discharge with increasing distance along the Lena river basin and to evaluate the contribution of geologically and lithologically uneven sub-basins in water discharge formation during a summer low water period. 相似文献
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19.
Complex variations in sediment transport at three large river bifurcations during discharge waves in the river Rhine 总被引：1，自引：0，他引：1
River bifurcations strongly control the distribution of water and sediment over a river system. A good understanding of this distribution process is crucial for river management. In this paper, an extensive data set from three large bifurcations in the Dutch Rhine is presented, containing data on bed‐load transport, suspended bed sediment transport, dune development and hydrodynamics. The data show complex variations in sediment transport during discharge waves. The objective of this paper is to examine and explain these measured variations in sediment transport. It is found that bend sorting upstream of the bifurcations leads to supply limitation, particularly in the downstream branch that originates in the outer bend of the main channel. Tidal water level variations lead to cyclical variations in the sediment distribution over the downstream branches. Lags in dune development cause complex hysteresis patterns in flow parameters and sediment transport. All bifurcations show evidence of sediment waves, which probably are intrinsic bifurcation phenomena. The complex transport processes at the three bifurcations cause distinct discontinuities in the downstream fining trend of the river. Differences among the studied river bifurcations are mainly due to differences in sediment mobility (Shields value). Because the variations in sediment transport are complex and poorly correlated with the flow discharge, prediction of the sediment distribution with existing relationships for one‐dimensional models is problematic. 相似文献
20.
辫状河储层的夹层预测是油藏描述的重点内容。目前夹层的预测主要集中于夹层发育模式研究和心滩坝体的构型单元解剖,且多运用单一的预测方法。南苏丹P油田辫状河储层夹层类型多、规模差异大、分布复杂,定量表征难度较大,在文献调研的基础上,从夹层的沉积成因入手,依据不同沉积方式形成的沉积砂体及其内部泥质夹层形态与结构不同的特点,综合岩心、测井与地震等多种资料,提出多信息关联的辫状河储层夹层预测方法。在密井网区建立骨架剖面与三角网小剖面,运用测井资料的垂向高分辨率与地震资料的横向强连续性特征确定不同类型夹层的井间发育规模;在建立岩相模型的基础上,以隔层厚度分布图为约束条件,采用确定性建模方法建立稳定泥岩隔层分布模型;以沉积微相研究结果和夹层规模预测结果为约束条件,采用随机建模方法分别在砂岩相和泥岩非隔层相中模拟心滩坝、河道和各类型夹层的分布;最终确定了研究区主要存在4种成因类型的夹层,并在多信息关联的基础上建立反映多类型夹层空间分布的辫状河储层精细地质模型。研究发现,对于厚度大于2 m的夹层可以通过井震结合的方法验证其井间规模,定量确定不同层位、不同类型夹层顺物源与切物源的发育规模,为夹层模型的建立奠定基础;基于克里金插值方法建立的岩相概率模型增加岩相模型准确率至94%;以隔层厚度平面分布图为约束条件的确定性建模方法可准确建立砂组及小层间隔层分布模型;在各成因类型夹层井间规模预测的基础上,基于目标的随机模拟方法可以针对不同成因类型夹层的发育形态、数量、规模和趋势分别设定模拟参数,确定性与随机性相结合,实现了辫状河储层精细地质模型的建立。同时,对相关储层的夹层预测具有一定的指导作用。 相似文献