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1.
Distribution and dynamics of large woody debris and organic matter in a low-energy meandering stream 总被引：1，自引：1，他引：1
This field-based study, using a combination of bed sediment sampling and large woody debris (LWD) tracking, explores the storage of organic matter and mobility of LWD in Poplar Creek, a low-energy meandering river that contains a diverse arrangement of large woody debris (LWD). This research addresses the following questions: 1) Does LWD influence the spatial pattern of the storage of benthic organic matter in meander bends? 2) How mobile is the LWD in low-gradient meandering systems? 3) Do characteristics of LWD such as length/bankfull channel width ratio, affect movement? Based on detailed sampling of sediment and LWD monitoring, this study provides a detailed representation of the influence of LWD on the storage of benthic organic matter in meander bends, as well as a detailed documentation of the mobility of LWD in the study reach. This study has demonstrated that: 1) LWD strongly influences patterns of storage of benthic organic matter in low-gradient meander bends; 2) removal of LWD from meander bends in low-gradient systems can substantially reduce the storage of benthic organic matter; and 3) the LWD within the study reach at Poplar Creek is highly mobile. The high mobility of LWD, observed in this study, contrasts with the rates observed in high-gradient systems and suggests that LWD-influenced patterns of storage of organic matter are temporally variable and that storage of benthic organic matter occurs in pulses, interspersed with transport, that are keyed to the mobility of LWD. Although LWD may exert an equally significant control over patterns of storage of organic matter in high- and low-gradient systems, the differences in the mobility rates of LWD suggest a fundamental difference in the dynamics of LWD-mediated organic matter between low- and high-gradient systems. 相似文献
2.
A coarse surface layer can help to limit bedload transport rates in channels with cobble and gravel beds. In these systems, periodic boulder-sized clasts often exist with small deposits of fine material in the lee of these large bed elements. A combined field and flume study was conducted to investigate the potential impact of lee deposits with distinctly finer sediment-sizes behind boulders on bedload transport rates. Detailed sediment characterizations were performed on surface, subsurface, and lee sediments in two coarse-bedded Connecticut channels. Bedload measurements also were conducted in a series of flows that approached the bankfull level in these two systems to determine transport rates and the size distribution of bedload material. A 6-m long, 0.5-m wide flume was used to model these systems with fine sediment passing over a fixed bed of sediment particles with uniform-sized, large bed elements. Sediment distributions of the lee deposits in the two Connecticut channels indicate that lee deposits may be produced from winnowing of sediments from the surface layer. Lee deposits also exhibit sediment distributions similar to bedload sediment distributions from low to near-bankfull flow in one of the two channels. Bedload sediments in the second channel were finer than lee deposits, presumably from selective entrainment of fines. Flume experiments demonstrate that bedload transport rates are lower for periods of steady flow relative to periods that include either an increase or decrease in discharge. The results show that lee sediments establish a metastable deposit behind each obstruction for a given discharge. Either increases or decreases in discharge disrupt this temporary stability and increase sediment delivery to the main flow. The study suggests that the influence of the rate of change in discharge may be as important as the absolute magnitude of discharge on sediment transport rates at moderate and low discharges in sediment-limited systems with large bed elements. 相似文献
3.
Channel responses to varying sediment input: A flume experiment modeled after Redwood Creek, California 总被引：1，自引：0，他引：1
At the reach scale, a channel adjusts to sediment supply and flow through mutual interactions among channel form, bed particle size, and flow dynamics that govern river bed mobility. Sediment can impair the beneficial uses of a river, but the timescales for studying recovery following high sediment loading in the field setting make flume experiments appealing. We use a flume experiment, coupled with field measurements in a gravel-bed river, to explore sediment transport, storage, and mobility relations under various sediment supply conditions. Our flume experiment modeled adjustments of channel morphology, slope, and armoring in a gravel-bed channel. Under moderate sediment increases, channel bed elevation increased and sediment output increased, but channel planform remained similar to pre-feed conditions. During the following degradational cycle, most of the excess sediment was evacuated from the flume and the bed became armored. Under high sediment feed, channel bed elevation increased, the bed became smoother, mid-channel bars and bedload sheets formed, and water surface slope increased. Concurrently, output increased and became more poorly sorted. During the last degradational cycle, the channel became armored and channel incision ceased before all excess sediment was removed. Selective transport of finer material was evident throughout the aggradational cycles and became more pronounced during degradational cycles as the bed became armored. Our flume results of changes in bed elevation, sediment storage, channel morphology, and bed texture parallel those from field surveys of Redwood Creek, northern California, which has exhibited channel bed degradation for 30 years following a large aggradation event in the 1970s. The flume experiment suggested that channel recovery in terms of reestablishing a specific morphology may not occur, but the channel may return to a state of balancing sediment supply and transport capacity. 相似文献
4.
黄河中游支流悬移质粒度与含沙量、流量间的复杂关系 总被引：16，自引：4，他引：12
本文以黄河中游若干支流为例,研究了宽变幅水沙两相流河流悬移质泥沙的粒度特征。结果表明,宽变幅水沙两相流河流的悬移质泥沙粒度特征与含沙量、流量之间具有复杂的关系。就同一站点而言,随着含沙量和流量的增大,大于0.05mm的粗颗粒泥沙的百分比迅速减小,并达到最小值;当含沙量和流量进一步增大时,其百分比又迅速增大,表现出明显的双值关系。对于小于0.01mm细泥沙而言,情形正好相反。黄河中游不同的支流之间,悬移质泥沙粒度特征与年均含沙量的关系也是复杂的。这些变化图形可以用非高含沙水流与高含沙水流不同的物理力学行为来解释 相似文献
5.
Fine sediment dynamics in mountain rivers are of concern because of implications for aquatic habitat, channel stability, and downstream sediment yields. Many mountain river systems have episodic fine sediment transport because of infrequent, point-source sediment inputs from landslides; basin instability triggered by land uses such as logging; or infrequent mobilization of the coarse surface layer in channels. Dam removal, which is now more likely along mountain rivers, may also provide a substantial fine sediment input to downstream channel reaches.Fine sediment storage in the interstices of spawning gravels and within pools along mountain rivers is of particular interest because of impacts to aquatic organisms. In this study we focus on sediment dynamics within pools of the North Fork Poudre River in Colorado as an example of the processes controlling fine sediment deposition, storage, and transport within laterally constricted pools. The 1996 release of 7000 m3 of silt-to gravel-sized sediment from a reservoir on the North Fork provided an opportunity to develop a field data set of fine sediment dynamics and to test the predictions of three different one- or two-dimensional sediment transport and hydraulic models against the field observations.The models were calibrated against quantitative measurements of pool scour and fill. One-dimensional HEC-6 results indicate that robust simulations yield the greatest agreement between predicted and measured pool bed elevation change. Model calibration on two pools and validation on one pool indicate that at least 58% of observed bed changes after the sediment release were predicted by HEC-6. Modeling accuracy using quasi-two-dimensional GSTARS 2.0 was considerably more variable, and no pool-wide trends were obtained. The two-dimensional model RMA2 substantially improved the representation of eddy pool hydraulics within a compound pool of the North Fork. Results from the hydraulic modeling, coupled with bed load and total load computations, delineate areas of scour and deposition which are consistent with observations in the field.A conceptual model of sediment delivery and storage for laterally confined pools suggests that persistent deposition of fine sediment within eddies distal from the sediment source may result from sediment releases. The original loss of channel capacity facilitated additional deposition within eddies as sediment within upstream proximal pools became mobilized. At high discharges, the development of a strong shear zone prevents degradation of sediment deposits within the eddy. Central portions of these proximal pools may clear according to existing models, whereas deposition within recirculating zones may be long-term. Water managers could use these models to estimate minimum pool volume for overwinter habitat and residence time of pool sediment. 相似文献
6.
汉江丹江口水库下游河床演变 总被引：1，自引：0，他引：1
丹江口水库建库20多年来,下游河道由堆积性河道变成了冲刷性的河道,河床物质沿程粗化,比降得以调平。水深增加幅度大于河宽,流速并不随着水深增加而加快。深槽、浅滩分布明显化,宽深比不断在减少。整个库下游变化可分为三段:1)近库段,游荡河道向单一限制性曲流转化;2)大支流影响段,河道仍保持着游荡特性;3)下游段,游荡段游荡特性减弱,弯曲段则深蚀作用加强。 相似文献
7.
Large variability in responses of stream sediment and large woody debris (LWD) to severe fire has limited the accurate prediction of the magnitude and duration of fire effects on streams. Conditions in one Absaroka Range stream that was severely burned in 1988 were compared to those in an adjacent, undisturbed stream to improve understanding of fire effects on channel and LWD characteristics beyond the first few years. Ten reaches of each stream were sampled during summer 1999.Average bankfull channel width was greater in burned Jones Creek than in unburned Crow Creek. LWD frequency and overall frequency of LWD accumulations were greater in Crow Creek than Jones Creek. Debris-jam frequency was greater in Jones Creek after accounting for differences in the frequency of pieces with length greater than channel width. Larger piece size and better anchoring contributed to more frequent, small accumulations of LWD in Crow Creek. Differences between streams in LWD frequency are consistent with greater mobility of debris in burned Jones Creek. LWD-associated fine-sediment deposits were thicker but less frequent along Jones Creek than Crow Creek. 相似文献
8.
宽变幅水沙两相流的冲淤双临界现象及其地貌学意义 总被引：3，自引：1，他引：3
通过黄土高原一些河流资料的分析，发现了宽变幅水沙两相流冲淤过程中的2个临界值，位于非高含水流区域中的临界点，可称之为冲淤下临界；位于高含沙水流区域中的临界点，可称之为冲淤上临界，水沙两相流冲淤的双临界现象，对珩多沙河流河道泥沙的输移有重要意义，随着含沙量的增大，排沙比先减小而后增大，2次与代表排沙比等于1的直线相交，这2个交点即分别对应于挟沙水流冲淤的下临界和上临界，运用所揭示的水沙两相流冲淤过程的双临界现象，可以对冲积河流河型的形成进行新的解释，以我国近百条冲积河流的资料为基础，点绘了年均悬移质输沙率与年均流量的关系，并以不同的符号来区分弯曲河型、游荡河型和高含沙曲流河型。结果表明，上述3种河型可以很好于被代表特征含沙量分别为C=60kg/m^3和C=3.3kg/m^3的2条直线所区分，这2个特征含沙量反映了河道水沙两相流冲淤的上、下临界值。 相似文献
9.
黄河内蒙古段洪峰特征及水沙关系变化 总被引：1，自引：0，他引：1
本文通过黄河头道拐站1950-2009 年水沙年系列进行突变点分析, 识别出1969 年、1986 年、1996 年3 个突变点, 将黄河内蒙古段水文过程划分为4 个时期。在此基础上, 利用内蒙古段6 个水文站长系列日流量、日含沙量及泥沙组成数据, 划分了各站的夏汛洪峰, 并统计分析了分期洪峰的水沙分布特征和洪峰水沙关系, 揭示出夏汛洪峰流量和输沙率逐渐减少的规律:1996-2006 年与1954-1968 年相比, 夏汛洪峰水沙量占全年水沙的比例都减少了约一半, 流量级频率与输沙率乘积的峰值对应的流量也降低了约一半。分析认为, 黄河内蒙古段夏汛洪峰特征阶段性变化既有大型水库的影响, 又有气候变化及人类活动的影响, 而且后者的作用越来越大。分析发现, 20 世纪90 年代以前, 黄河上游水沙变化只是改变了内蒙古段来沙中粗颗粒泥沙的水沙关系, 显示出大流量输送粗颗粒泥沙能力相对增强的趋向;进入内蒙段后, 通过泥沙冲淤调整, 分粒径组泥沙水沙关系变化比较复杂。近期在水沙条件巨大变化下, 河流的输沙特性并没有发生根本变化, 但是无论全沙还是分组沙, 水沙关系曲线变陡, 大流量输送泥沙能力相对增强。 相似文献
10.
11.
Small, low order channels located in wet meadows along the Mogollon Rim of northern Arizona that receive the bulk of their flow from spring discharge exhibit a morphology that differs markedly from channels that receive the bulk of their flow from runoff. These small, spring-dominated channels generally have dense vegetation cover, vertical (or near vertical) banks with flat channel beds that are armored with clasts up to 60 mm. Clasts armoring the spring-dominated channels become mobile at 45 to 85% of the bankfull depth. The lack of fine-grained material in the bed of the spring-dominated channels reflects the small drainage size, lack of fine grain input from the spring, and winnowing affect of the consistent discharge. Minor amounts of large woody debris were present in some of the spring-dominated channels, however, unlike previous studies it does not appear to play a role in the spring-dominated channel morphology. Sinuosity values for spring-dominated channels averaged 1.19, while the average sinuosity values for the runoff-dominated channels, 1.08, were significantly lower. Measured width-to-depth ratios averaged 2.4 in the spring-dominated channels, much lower than the average ratio of 11.6 found for the runoff-dominated channels. The standard deviation of width-to-depth ratios was higher for runoff-dominated channels, reflecting a more variable channel profile. A third channel type, here referred to as hybrid channels, receive significant flow from both springs and runoff. These channels have characteristics that fall between spring-dominated and runoff-dominated channels.Elevation, gradient, organic matter content, and sediment grain size distribution of the wet meadows in which the channels are formed do not exhibit significant differences between channel types, suggesting that these factors are not responsible for the observed differences in channel morphologies. The major differences in controls on the channel morphology found between the spring-dominated and runoff-dominated channels are the discharge regime and the sediment input. The hydrology unique to the spring-dominated channels and the lack of fine-grained sediment input combine to create the observed differences. 相似文献
12.
The geomorphic function and characteristics of large woody debris in low gradient rivers, coastal Maine, USA 总被引：1，自引：0，他引：1
F.J. Magilligan K.H. Nislow G.B. Fisher J. Wright G. Mackey M. Laser 《Geomorphology》2008,97(3-4):467-482
The role, function, and importance of large woody debris (LWD) in rivers depend strongly on environmental context and land use history. The coastal watersheds of central and northern Maine, northeastern U.S., are characterized by low gradients, moderate topography, and minimal influence of mass wasting processes, along with a history of intensive commercial timber harvest. In spite of the ecological importance of these rivers, which contain the last wild populations of Atlantic salmon (Salmo salar) in the U.S., we know little about LWD distribution, dynamics, and function in these systems. We conducted a cross-basin analysis in seven coastal Maine watersheds, documenting the size, frequency, volume, position, and orientation of LWD, as well as the association between LWD, pool formation, and sediment storage. In conjunction with these LWD surveys, we conducted extensive riparian vegetation surveys. We observed very low LWD frequencies and volumes across the 60 km of rivers surveyed. Frequency of LWD ≥ 20 cm diameter ranged from 15–50 pieces km− 1 and wood volumes were commonly < 10–20 m3 km− 1. Moreover, most of this wood was located in the immediate low-flow channel zone, was oriented parallel to flow, and failed to span the stream channel. As a result, pool formation associated with LWD is generally lacking and < 20% of the wood was associated with sediment storage. Low LWD volumes are consistent with the relatively young riparian stands we observed, with the large majority of trees < 20 cm DBH. These results strongly reflect the legacy of intensive timber harvest and land clearing and suggest that the frequency and distribution of LWD may be considerably less than presettlement and/or future desired conditions. 相似文献
13.
Long-term morphodynamic changes of a desert reach of the Yellow River following upstream large reservoirs' operation 总被引：6，自引：0，他引：6
China's Yellow River has experienced its dramatically decreasing trend for the flow discharge since the construction and operation of large reservoirs located upstream. This low flow regulation has triggered a severe aggradation of the Ulan Buh Desert channel of the Yellow River because the declining flow exhibits no capability to scour and carry away large amount input of desert sands from the Ulan Buh Desert. Twenty monitoring cross-sections documented the Ulan Buh Desert channel has experienced its increasing aggradational trend in conjunction with its lateral migration decreasing trend from 1966 to 2005, which is opposite to the normal pattern of aggradation with deepening or symmetrical infilling for a channel located downstream of a reservoir. The channel aggradation can also be identified two stages: slow aggradation and rapid aggradation. Slow aggradation is characterized by the channel bed elevation rising 9.5 cm on average between 1968 and 1985, which responded to the operation of the Liujiaxia reservoir. During this period, the flow discharge was similar to pre-dam flow conditions but the sediment transport reduced to half of its pre-dam value. Because of about 0.24 × 108 t of desert sands entering the channel from the Ulan Buh Desert annually, this dilute flow indicated not to scour the channel as expected, but contrarily to cause the channel aggraded. Rapid aggradation followed completion of the Longyangxia reservoir with the channel bed elevation rising by 73 cm on average between 1986 and 2005. In this period, the combined regulation of Liujiaxia and Longyangxia reservoirs has caused the flow discharge decreasing dramatically, which is more beneficial for accumulation of the desert sands (0.19 × 108 t yr− 1 on average) in the desert channel, and led to the channel aggradation rate accelerated rapidly. 相似文献
14.
Flow structure at an asymmetrical stream confluence 总被引：2，自引：0，他引：2
Measurements of downstream and cross-stream velocities at a small, asymmetrical stream confluence show that the structure of low-stage flows is influenced by the tributary/main stem momentum flux ratio, the total discharge of the incoming flows, and the bed morphology. Flow accelerates through the confluence during all three measured events. This acceleration is associated with a downstream reduction in channel capacity caused in part by the presence of a large bar along the inner bank of the downstream channel. As the momentum ratio increases, flow from the lateral tributary increasingly deflects flow from the main stream toward the outer channel bank within the confluence. As a result, the mixing interface between the converging flows also shifts outward. The large bar in the downstream channel deflects flow along the inner bank toward the adjacent scour hole, enhancing flow convergence downstream of the confluence and producing a region of flow separation adjacent to, or in the lee of the bar. The loci of maximum topographic deflection and flow separation vary with momentum ratio and total discharge.Secondary circulation within the downstream channel is characterized by a single large helical cell when the momentum ratio exceeds one, and weak surface-convergent helical cells on opposite sides of the mixing interface when the momentum ratio is less than one. Curvature of the flow, and thus the strength of helical motion, is greatest on the tributary side of the mixing interface. Although the flow events measured in this study did not exceed the threshold for sediment movement, the bed morphology at the confluence can be explained by the flow structure observed during these low-stage events, suggesting that formative flows may have similar downstream and cross-stream velocity fields. 相似文献
15.
黄河下游游荡段河床调整对于水沙组合的复杂响应 总被引：14，自引：3，他引：11
以黄河资料为基础,提示地包括非高含沙水流和高含沙水流在内的河道挟不流的复杂冲淤行为。由于这种复杂冲淤行为的作用和高含沙水流特殊能耗特征的影响,使得河床横断面形态及平面形态对于水沙组合的响应出现非线性特征,这可视为流水地貌系统复杂响应的又一表现形式。这种复杂响应不能用Ｓｃｈｕｍｍ的河床调整来解释,因而需要对这一理论作必要的修正。 相似文献
16.
Molly Pohl 《The Professional geographer》2004,56(3):422-431
Dams are a major source of fragmentation and degradation of rivers. Although substantial research has been conducted on the environmental impacts of large structures in the United States, smaller dams have received less attention. This study evaluated the impact of two dams of moderate size, the Elwha Dams, on the downstream channel system using field data collection at river cross‐sections. The relationship of average boundary shear stress (τo) to critical shear stress (τcr) served as the basis for determining channel bed material mobility under the two‐year and ten‐year flood events. The channel had the greatest channel bed mobility at the natural cross‐section upstream from the dams, low bed mobility between the structures, and an increase in channel bed mobility in the low gradient river segment near the mouth of the river. Low bed mobility tended to be associated with a lack of channel system complexity, including reduction or loss of bars and low alluvial terraces and their associated young riparian communities. Although these run‐of‐the‐river dams do not modify streamflow greatly, the loss of sediment from the channel system has had a substantial impact on bed mobility and geomorphic and biotic complexity of the Elwha River. 相似文献
17.
山区河流准三维水沙输运与河床演变模拟 总被引：1，自引：0，他引：1
在渠道水力学和河流动力学研究的基础上，对不规则河流形态下的纵向流速分布和弯道水流进行了讨论，并分析了任意断面和垂线位置的流场计算公式。在综合各种泥沙动力学研究成果基础之上，探讨了在天然河流中的悬移质输沙能力与沉积条件，分析了河床质起动的控制性因素及推移质输沙方程。提出了受冲淤影响的河床质粒径组成和动态递推公式。结合这些流场与泥沙计算方法及河床形态调整技术，作者提出一个准三维河流动力学模型。该模型只需要河流几何形状、人流条件以及泥沙级配参数，能够模拟出不规则几何形态的河道断面流速分布及平面流场分布，并且对于弯道水流特征与泥沙输送一沉积特征也能得到较好的表现。该模型是对有关河流演变理论和实验成果的综合应用，为研究山区高含沙水流下的河床变形以及洪水演进提供了强有力的工具。 相似文献
18.
Although studies of sediment transport in steep and coarse-grained channels have been more numerous in recent years, the dynamics of sediment transport in step–pool river systems remain poorly understood. This paper investigates displacements of individual clasts through Spruce Creek (Québec, Canada), a classic step–pool channel, and the effects of the channel morphology on the path length of the clasts. Passive integrated transponder tags (PIT) were used to track the displacement of 196 individual particles over a range of discharges including the bankfull stage. Clasts were tracked after five sequences of flood events. The results showed that the distance distributions match a two-parameter Gamma model. Equal mobility transport occurs for the particle size investigated during each sequence of flood events. Mean travel distance of the clasts can be estimated from excess stream power, and the mobility of the clasts is more than an order of magnitude less than the model reported in riffle–pool channels. The dominant morphological length scale of the bed also controls the path length of the clasts. These results confirm some preliminary observations on sediment transport in step–pool channels. 相似文献
19.
Jiongxin Xu 《Geomorphology》2004,57(3-4):321-330
Through an analysis of data collected from the Yellow River and its tributaries on the Loess Plateau of China, the phenomenon of double-thresholds in scour–fill processes of wide-range water-sediment two-phase flows has been shown. Thresholds located in non-hyperconcentrated flows may be called the lower threshold, and that in hyperconcentrated flows the upper threshold. This double-threshold phenomenon leads to complicated sediment transport behavior of heavily sediment-laden rivers. With an increase in suspended sediment concentration, the channel sediment delivery ratio increases initially and becomes higher than 1, followed by a decrease and finally becomes lower than 1 again.Controlled by the double-thresholds in the scour–fill processes, channel adjustment of the lower Yellow River is non-linear and complex. When the suspended concentrations were lower than the lower threshold or higher than the upper threshold, scour or bed downcutting was the dominant channel-forming process. Channel shape tends to be narrower and deeper, and the channel thalweg became more sinuous. When the suspended concentrations lay between the lower and upper thresholds, deposition of sediment was the dominant channel-forming process; channel shape tended to be shallower and wider, and channel thalweg became less sinuous. 相似文献
20.
Amalie Jo Brown 《自然地理学》2013,34(1):82-102
Geometric, hydraulic, and sediment characteristics in arid badlands near Borrego Springs, California, are examined in relation to precipitation events of varying magnitude and frequency. The longitudinal and cross profiles of five ephemeral channels occupying a 2.5 km2 catchment were surveyed under pre-and post-storm conditions during the February 1976-December 1978 period. Such arid region channels offer the opportunity to observe and explain rates and methods of profile change under different flow types in a short period of time. Catchment responses to light winter events include substantial lags between initial precipitation and channel runoff, the limited downstream movement of small slugs of sediment, high losses of discharge into channel alluvium, and prolonged mass movement of debris from adjacent hillslopes into the channels following the storm events thus promoting aggradation along certain channel reaches. Responses to intense summer storms include explosive channel and hillslope runoff and localized scour and fill, both during and following such events, thereby promoting substantial aggradation and erosion along portions of the channels. Although ephemeral flow conditions may produce channel profiles which are distinct from those in perennial streams, the evaluation of the methods of sediment transport and the storage of debris in arid catchments offer useful explanation for other environments. 相似文献