首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 140 毫秒
1.
Six plains cottonwoods along the axis of a meander were excavated to determine if dendrochronology could identify the year and location of germination and date past overbank sedimentation events. Samples from all excavated trees showed clear anatomical changes associated with burial, including increased vessel size, decreased definition of annual ring boundaries, and decreased ring widths. Some of these burial signatures were created by deposition of only a few centimeters of sediment, and most burial events were detected by multiple samples from the same tree. Four of the trees germinated at or near the upper surfaces of bar deposits, while two germinated within thin overbank deposits draped over bar deposits, indicating that germination is closely associated with bars. Dates and inferred thicknesses of overbank sedimentation events are consistent with repeated topographic surveys and data obtained from cesium-137 (137Cs) analyses. However, the record of overbank sedimentation extracted from the trees does not entirely reflect the history of past peak discharges documented by stream gaging, largely because individual trees are progressively less likely to be flooded through time as the river migrates farther away. Germination dates and locations closely track past positions of the river channel. Germination elevations and the elevations of the tops of point bars appear to be decreasing with time as the bend migrates, implying vertical incision by Powder River at a rate of 7.1 ± 4.3 mm/yr. The rate of floodplain growth determined by elevation changes decreases progressively through time, ultimately reaching an apparent plateau after 0.8–1.3 m of vertical accretion. While similar patterns of vertical accretion have previously been interpreted as resulting from decreasing flood probability with increasing floodplain elevation, distance from the channel is also a first-order control on vertical floodplain growth. © 2019 John Wiley & Sons, Ltd.  相似文献   

2.
This article addresses spatial variability of comtemporary floodplain sedimentation at the event scale. Measurements of overbank deposition were carried out using sediment traps on 11 floodplain sections along the rivers Waal and Meuse in The Netherlands during the high-magnitude flood of December 1993. During the flood, sand sheets were locally deposited behind a natural levee. At distances greater than 50 to 100 m from the river channel the deposits consisted mainly of silt- and clay-sized material. Observed patterns of deposition were related to floodplain topography and sediment transporting mechanisms. Though at several sites patterns were observed that suggest transport by turbulent diffusion, convection seems the dominant transporting mechanism, in particular in sections that are bordered by minor embankments. The average deposition of overbank fines ranged between 1·2 and 4·0 kg m−2 along the river Waal, and between 1·0 and 2·0 kg m−2 along the river Meuse. The estimated total accumulation of overbank fines (not including sand sheets) on the entire river Waal floodplain was 0·24 Mton, which is 19 per cent of the total suspended sediment load transported through the river Waal during the flood. © 1998 John Wiley & Sons, Ltd.  相似文献   

3.
We analyzed variation of channel–floodplain suspended sediment exchange along a 140 km reach of the lower Amazon River for two decades (1995–2014). Daily sediment fluxes were determined by combining measured and estimated surface sediment concentrations with river–floodplain water exchanges computed with a two‐dimensional hydraulic model. The average annual inflow to the floodplain was 4088 ± 2017 Gg yr?1 and the outflow was 2251 ± 471 Gg yr?1, respectively. Prediction of average sediment accretion rate was twice the estimate from a previous study of this same reach and more than an order of magnitude lower than an estimate from an earlier regional scale study. The amount of water routed through the floodplain, which is sensitive to levee topography and increases exponentially with river discharge, was the main factor controlling the variation in total annual sediment inflow. Besides floodplain routing, the total annual sediment export depended on the increase in sediment concentration in lakes during floodplain drainage. The recent increasing amplitude of the Amazon River annual flood over two decades has caused a substantial shift in water and sediment river–floodplain exchanges. In the second decade (2005–2014), as the frequency of extreme floods increased, annual sediment inflow increased by 81% and net storage increased by 317% in relation to the previous decade (1995–2004). Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

4.
Along the lower reaches of the Waipaoa River, New Zealand, cross‐section survey data indicate there was a 23 per cent decrease in bankfull width and a 22 per cent reduction in channel cross‐section area between 1948 and 2000, as the channel responded to increased inputs of fine (suspended) sediment following deforestation of the headwaters in late C19 and early C20. We determined the bankfull discharge within a ～39 km long reach by routing known discharges through the one‐dimensional MIKE 11 flow model. The model runs suggest that the bankfull discharge varies between ～800 and ～2300 m3 s?1 and that the average recurrence interval is 4 ± 2 years on the annual maximum series; by contrast, the effective flow (360 m3 s?1) is equaled or exceeded three times a year. The variability in bankfull discharge arises because the banks tend to be lower in places where flood flows are constricted than in reaches where overbank flow is dispersed over a wide area, and because scour has counteracted aggradation in some locations. There is no downstream variation in Shields stress, or in relative shear stress, within the study reach. Bankfull shear stress is, on average, five times greater than the shear stress required to initiate motion. At the effective discharge it is more than twice the threshold value. The effective discharge probably has more relevance than the bankfull discharge to the overall picture of sediment movement in the lower reaches of the Waipaoa River but, because width is constrained by the stability and resistance of the bank material to erosion during high flows that also scour the bed, the overall channel geometry is likely determined by discharges at or near bankfull. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

5.
A large amount of the total sediment load in the Chinese Yellow River is transported during hyperconcentrated floods. These floods are characterized by very high suspended sediment concentrations and rapid morphological changes with alternating sedimentation and erosion in the main channel, and persistent sedimentation on the floodplain. However, the physical mechanisms driving these hyperconcentrated floods are still poorly understood. Numerical modelling experiments of these floods reveal that sedimentation is largely caused by large vertical concentration gradients, both in the channel during the rising stage of the flood, as well as on the floodplains, during a later stage of the flood. These vertical concentration gradients are large because the turbulent mixing rates are reduced by the increased sediment‐induced density gradients, resulting in a positive feedback mechanism that produces high deposition rates. Erosion prevails when the sediment is largely held in suspension due to hindered settling, and is strengthened by the reduced wetted cross‐section caused by massive sedimentation on the floodplain. Observed patterns of erosion and sedimentation during these floods can be qualitatively reproduced with a numerical model in which sediment‐induced density effects and hindered settling are included. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

6.
The potential for using fallout (unsupported) Pb-210 (210Pb) measurements to estimate rates of overbank sediment deposition on the floodplains of lowland rivers is explored. A model which distinguishes the contribution from direct atmospheric fallout and the catchment-derived input associated with the deposition of suspended sediment has been developed to interpret the fallout Pb-210 inventories at floodplain sampling sites and to estimate average sediment accumulation rates over the past 100 years. The approach has been successfully used to estimate rates of overbank sedimentation on the floodplains of the Rivers Culm and Exe in Devon, U.K. A detailed investigation of the pattern of longer-term sedimentation rates within a small reach of the floodplain of the River Culm indicated a range of deposition rates between 0.07 and 0.59 g cm−2 a−1, which was in close agreement with estimates of current sedimentation rates obtained using sedimentation traps.  相似文献   

7.
《Continental Shelf Research》2006,26(17-18):2205-2224
On the eastern Raukumara Ranges of the New Zealand East Coast, active tectonics, vigorous weather systems, and human colonisation have combined to cause widespread erosion of the mudstone- and sandstone-dominated hinterland. The Waipaoa River sedimentary dispersal system is an example that has responded to environmental change, and is now New Zealand's second largest river in terms of suspended sediment discharge. This paper presents new sediment accumulation rates for the continental shelf and slope that span century to post-glacial time scales. These data are derived from radiochemical tracer, palynological, tephrostratigraphic, and seismic methods. We hypothesise on the temporal and spatial complexity of post-glacial sedimentation across the margin and identify the broad extent of sediment dispersal from the Waipaoa system. The ∼15 km3 Poverty Bay mid-shelf basin lies adjacent to the mouth of the Waipaoa River, reaching a maximum thickness of ∼45 m. A post-glacial mud lobe of an additional ∼3 km3 extends through the Poverty Gap and out onto the uppermost slope, attaining 40 m thickness in a structurally controlled sub-basin. Here, an offset in the last-glacial erosion surface indicates that deposition was sympathetic with fault activity and the creation of accommodation space, implying that sedimentation was not supply limited. Contrary to classical shelf sedimentation models, the highest modern accumulation rate of 1 cm y−1 occurs on the outer-shelf sediment lobe, approximately ∼2 times the rate recorded at the mid-shelf basin depocentre, and ∼10 times faster than the excess 210Pb rates estimated from the slope. Pollen records from slope cores fingerprint Polynesian then European settlement, and broaden the spatial extent of post-settlement sedimentation initially documented from the Poverty Bay mid-shelf. Changes in sub-millennial sedimentation infer a 2–3-times increase in post-settlement accumulation on the shelf but a smaller 1–2 times increase on the slope. Over longer time scales, seismic evidence infers slower but steady sedimentation since the last transgression, and that significant cross-shelf sediment pathways pre-date the increase in sedimentation resulting from colonisation and deforestation. From a summation of coastal bedload, shelf and slope sediment mass accumulation, the total sediment budget for the Holocene is ∼1 Mt y−1. Under modern conditions a larger proportion of the Waipaoa sediment dispersal system likely extends onto the slope and beyond.  相似文献   

8.
In this study, we captured how a river channel responds to a sediment pulse originating from a dam removal using multiple lines of evidence derived from streamflow gages along the Patapsco River, Maryland, USA. Gages captured characteristics of the sediment pulse, including travel times of its leading edge (~7.8 km yr−1) and peak (~2.6 km yr−1) and suggest both translation and increasing dispersion. The pulse also changed local hydraulics and energy conditions, increasing flow velocities and Froude number, due to bed fining, homogenization and/or slope adjustment. Immediately downstream of the dam, recovery to pre-pulse conditions occurred within the year, but farther downstream recovery was slower, with the tail of the sediment pulse working through the lower river by the end of the study 7 years later. The patterns and timing of channel change associated with the sediment pulse were not driven by large flow or suspended sediment-transporting events, with change mostly occurring during lower flows. This suggests pulse mobility was controlled by process-factors largely independent of high flow. In contrast, persistent changes occurred to out-of-channel flooding dynamics. Stage associated with flooding increased during the arrival of the sediment pulse, 1 to 2 years after dam removal, suggesting persistent sediment deposition at the channel margins and nearby floodplain. This resulted in National Weather Service-indicated flood stages being attained by 3–43% smaller discharges compared to earlier in the study period. This study captured a two-signal response from the sediment pulse: (1) short- to medium-term (weeks to months) translation and dispersion within the channel, resulting in aggradation and recovery of bed elevations and changing local hydraulics; and (2) dispersion and persistent longer-term (years) effects of sediment deposition on overbank surfaces. This study further demonstrated the utility of US Geological Survey gage data to quantify geomorphic change, increase temporal resolution, and provide insights into trajectories of change over varying spatial and temporal scales.  相似文献   

9.
Reconstruction of overbank sedimentation rates over the past decades gives insight into floodplain dynamics, and thereby provides a basis for efficient and sustainable floodplain management. We compared the results of four independent reconstruction methods – optically stimulated luminescence (OSL) dating, caesium‐137 (137Cs) dating, heavy metal analysis, and flood bed interpretation – applied at three embanked floodplain sites along lower Rhine River distributaries in the Netherlands. All methods indicate significant sedimentation rates on the floodplains, varying between 2–7 mm/a in the distal zones and 3–9 mm/a in the proximal zones. On a rapidly developing sand bar along a natural levee sedimentation rates of 9 to 25 mm/a were found. Except for some minor inconsistencies in 137Cs dating results, all methods show decreasing sedimentation rates with increasing distance from the river channel. Intercomparison of the results of the different dating methods revealed the potential errors associated with each method, particularly where disagreement among the results were found. Uncertainties may arise due to (1) grain‐size dependent downward migration of 137Cs, (2) smoothing of the vertical heavy metal and 137Cs profiles, (3) delayed sediment‐associated input of 137Cs in addition to direct atmospheric fall‐out, (4) overestimation of the burial age in OSL dating due to incomplete resetting of the OSL signal, or (5) non‐linear relationships between sediment deposition and flood magnitude in the count‐from‐the‐top correlation between sediment lamination and past observed flood records. Still, taking the uncertainties associated with each method into account, the results are generally in good agreement. Using the results we indicate the optimal spatial range of application of each method, depending on sediment texture and sedimentation rate. The optimal spatial and temporal ranges differ for each method, but show significant overlap. A combination of the methods will thus provide maximum information for accurate estimation of sedimentation rates on a decadal time scale. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

10.
It is often believed that extreme but infrequent events are most important in the development of landforms. When evaluating the overall effect of large floods on floodplain sedimentation, quantitative measurements of both high- and low-magnitude events should be considered. To analyse the role of flood magnitude on floodplain sedimentation, we measured overbank sedimentation during floods of different magnitude and duration. The measurements were carried out on two embanked floodplain sections along the rivers Rhine and Meuse in The Netherlands, using sediment traps made of artificial grass. The results showed an increase in total sediment accumulation with flood magnitude, mainly caused by enhanced accumulation of sand. At low floodplain sections the increase in sediment deposition was smaller than expected from the strong increase in suspended sediment transport in the river. Spatial variability in sediment accumulation was found to depend both on flood magnitude and duration. Deposition of sand on natural levees mainly takes place during high-magnitude floods, whilst low floods and slowly receding floods are important for the deposition of silt and clay in low-lying areas, at greater distance from the main channel. © 1998 John Wiley & Sons, Ltd.  相似文献   

11.
Suspended sediment is the primary source for a sustainable agro‐ecosystem in the Mekong Delta by providing nutrient input for the subsequent cropping season. In addition, the suspended sediment concentration (SSC) plays an important role in the erosion and deposition processes in the Delta; that is, it influences the morphologic development and may counteract the deltaic subsidence and sea level rise. Despite this importance, little is known about the dynamics of suspended sediment in the floodplains of the Mekong Delta. In particular, quantitative analyses are lacking mainly because of data scarcity with respect to the inundation processes in the floodplains. In 2008, therefore, a comprehensive in situ system to monitor the dynamics of suspended sediment in a study area located in the Plain of Reeds was established, aiming at the characterization and quantification of suspended sediment dynamics in the deeply inundated parts of the Vietnamese part of the Mekong Delta. The monitoring system was equipped with seven water quality–monitoring stations. They have a robust design and autonomous power supply suitable for operation on inundated floodplains, enabling the collection of reliable data over a long period of time with a high temporal resolution. The data analysis shows that the general seasonal dynamics of suspended sediment transport in the Delta is controlled by two main mechanisms: the flood wave of the Mekong River and the tidal backwater influences from the coast. In the channel network, SSC decreases exponentially with distance from the Mekong River. The anthropogenic influence on SSC could also be identified for two periods: at the start of the floodplain inundation and at the end of the flood period, when subsequent paddy rice crops are prepared. Based on the results, we recommend an operation scheme for the sluice gates, which intends to distribute the sediment and thus the nutrients equally over the floodplain. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

12.
Channelization of the severely polluted Odra and Vistula Rivers in Poland induced intensive accumulation of fine‐grained deposits rich in organic matter and heavy metals. These sediments have been identified in vertical profiles in a narrow zone along river banks both in groyne‐created basins and on the floodplain. Grain size, organic matter, zinc (Zn), lead (Pb), copper (Cu) content and cesium‐137 (137Cs) was used for sediment dating and, stratigraphy and chemistry have been diagnostic features for these deposits, named industrial alluvium. In the most polluted river reaches stabilized by bank reinforcements and groynes, 2‐m‐thick slack water groyne deposits are composed of uniform strata of polluted silts with organic matter content over 10%, Zn content over 1000 mg/kg and average Cu and Pb over 100 mg/kg. The average rate of sediment accretion in groynes is higher than on the floodplain and reaches 5 cm/yr. Stratification which appears at higher levels in the groyne fields and on the levees reflects a change from in‐channel to overbank deposition and is typified by dark layers separated by bright, sandy, and less polluted strata. Stratified, 4‐m‐thick, sediment sequences have been found in groyne fields of incised river reaches. The average rate of sediment accretion in these reaches is of the order of 5 cm/yr. In stable and relatively less polluted river reaches, vertical‐accretion organic deposits are finely laminated and the average rate of deposition amounts to a few millimeters per year. Investigations indicate that groyne construction favors conditions for long‐term storage of sediments at channel banks. For this reason, groynes should be considered as structures that efficiently limit sudden release of sediment‐associated heavy metals stored in channels and in floodplains of the historically polluted rivers. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

13.
The Amazon Macrotidal Mangrove Coast (AMMC) is a large (~7500 km2) contiguous mangrove fringe eastwards from the Amazon River mouth. It encompasses dozens of interconnected bays intercalated with mangrove peninsulas. Mud accumulates on the mangrove flats, whereas the bed of the bays and channels is generally sandy. In this study we investigated the circulation, sediment transport and deposition in a central site at one of these mangrove peninsulas. The study was undertaken during the dry period, when there is no influence of the Amazon River plume and minimum local freshwater inflow. Current and suspended-sediment concentration were monitored in a feeder channel on the mangrove flat along a ~1000 m section oriented along the peninsula axis. Sediment deposition was also measured on the flat. Our results show there was a strong exchange between the neighboring bays. Channel currents were flood dominant, reaching up to >1 m s−1, with residual water and sediment transport westwards. Suspended sediment concentration (SSC) in the channel was directly related to velocity magnitude, ranging between 50 and 350 mg L−1. The flat was flooded in a way that indicated the tidal wave evolves westwards, nearly parallel to the AMMC shoreline. Currents on the flats were much slower than those in the channel and showed slight ebb dominance. However, SSC was higher during the flood than ebb, clearly indicating settling during the current deceleration and limited erosion during the following ebb–flow acceleration. The net sediment transport across the section was 60 tons westwards for the period of the experiment (~4 days). The mean deposition rate was 0.006 kg m−2 s−1 (or 1.4 kg m−2 per tide), which was higher than rates from other reported assessments in mangroves. The set of results indicate very large internal sediment reworking in the AMMC. © 2019 John Wiley & Sons, Ltd.  相似文献   

14.
A wide range of approaches has been adopted for collecting and reporting vertical accretion rates, hence evaluation of results from different catchments, or even from different reaches within the same basin, is difficult as they may not be directly comparable. The present study compares depth‐ and volume‐based estimates of sediment accumulation for well‐dated vertical accretion sequences at Broomhaugh Island in the Tyne basin, northern England. High resolution dating control (based on heavy metal stratigraphy and cartography) coupled with detailed reconstruction of channel geometry allows accumulation rates to be calculated for the last 300–400 years. The results show a marked disparity in the magnitude and pattern of vertical accretion rates between the two approaches. The average annual thickness of sediment increases progressively, with a six‐fold difference between the seventeenth and twentieth centuries. The volume figures are more consistent, with a two‐fold difference between the highest and lowest rates, both of which occurred in the twentieth century. A major control on vertical accretion is found to be changing channel shape and capacity. Enlargement of the channel by 76 per cent since the seventeenth century, associated with channel bed incision, has resulted in fewer overbank flows. Hence floods, with their associated suspended sediment loads, which once inundated the floodplain are now confined within the channel. The reduced areal extent of surfaces available for deposition has resulted in greater annual thickness of sediment accumulated, despite an overall reduction in volume. This study emphasizes that caution is needed when attempting to identify causal linkages between changes in sedimentation rates and catchment land use and/or climatic factors and the geomorphological setting of the site must be taken into account. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

15.
Red Creek, in the Red Desert area of the Great Divide Basin, Wyoming, is an arid-region anastomosing stream. The narrow, deep, and sinuous main channel is flanked by anastomosing flood channels, or anabranches. Most anabranches are initiated at meander bends. The primary mechanism of anabranch initiation is avulsion during overbank floods. Anabranch enlargement occurs by headward erosion. Anabranches act as distributary channels during floods, when water and sediment from overbank flows are transported to and deposited on the floodplain via the anabranches. During periods of low discharges, the anabranches act as tributaries to the main channel, transporting runoff from the floodplain and surrounding hillslopes to the main channel of Red Creek. Aggradation is occurring in the main channel and on the floodplain throughout the study reach. Infilling of the main channel occurs primarily by lateral accretion, while the floodplain accretes vertically through deposition of overbank sediment from the main channel and anabranches. Infilling of the main channel may cause avulsion of the main channel into an anabranch. The abandoned main channel segment may then fill completely or act as an anabranch. Because lateral migration of channels is inhibited by the high cohesion of the silt and clay channel sediment, periodic avulsion is the primary form of lateral mobility in the system.  相似文献   

16.
This study investigates the relation between channel changes, as mapped from aerial photography, and bed‐material transport along Chilliwack River, British Columbia. Detailed mapping of channel features was completed for five dates between 1952 and 1991 using an analytical stereoplotter. Data were transferred to a geographic information system (GIS) to analyse changes during four consecutive periods. Erosion and deposition volumes along channel reaches were estimated by multiplying measured areal changes by the bed‐material depth along each reach. Bed‐material transport rates are related to morphologic changes using a sediment budget approach. The highest rate of transport for the four study periods is estimated as 55 000 ± 10 000 m3 a−1 between 1983 and 1991. These rates are compared with estimates from short‐term (1–2 year) changes along the lower reach to investigate variations in sediment flux that may otherwise remain undetected. Significant morphologic change occurs roughly once every 5 years when flows are large enough to erode and entrain large volumes of bed material stored within the contemporary floodplain. In the absence of large floods, transport rates decline and vegetation begins to establish new floodplain. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

17.
The methodology and errors involved in determining the amount of sediment produced during two (19·5 and 33·2 year) periods by 11 (c. 0·01 − >0·20 km2) gullies within a 4 km2 area in the headwaters of the Waipaoa River basin, New Zealand, using sequential digital elevation models are described. Sediment production from all gullies within the study area was 0·99 ± 0·03 × 106 t a−1 (2480 ± 80 t ha−1 a−1) during the period from 1939 to 1958. It declined to 0·62 ± 0·02 × 106 t a−1 (1550 ± 50 t ha−1 a−1) during the period from 1958 to 1992, when many of the smaller gullies were stabilized by a programme of afforestation, which commenced in 1960. Both figures are very high by global standards. The two largest (the Tarndale and Mangatu) gully complexes together generated 73 and 95 per cent of the sediment in the specified time periods, but the latter amount is equivalent to only c. 5 per cent of the total annual sediment load of the Waipaoa River. © 1998 John Wiley & Sons, Ltd.  相似文献   

18.
This paper outlines a numerical model for the prediction of floodplain inundation sequences, overbank deposition rates and deposit grain size distributions. The model has two main components: first, a simplified hydraulic scheme which predicts floodwater flow depths and velocities, and second, a sediment transport element which employs a mass balance relation describing suspended sediment dispersion by convective and diffusive processes and sediment deposition as a function of particle settling rates. These relationships are solved numerically on a finite difference grid that accurately replicates the complex topographic features typical of natural river floodplains. The model is applied to a 600 m reach of the River Culm, Devon, U.K. using data derived from a range of field and laboratory techniques. Continuous records of river stage and suspended sediment concentration provide the model's upstream boundary input requirements. These are supplemented by measurements of the in situ settling characteristics of the suspended sediment load. The model's sediment transport component is calibrated with the aid of a dataset of measured overbank deposition amounts derived from flood events over a 16 month period. The model is shown to predict complicated floodwater inundation sequences and patterns of suspended sediment dispersion and deposition, which are largely a product of the complex topography of the floodplain. These results compare favourably with observations of overbank processes and are an improvement over those of previous models which have employed relatively simple representations of floodplain geometry. © 1997 by John Wiley & Sons, Ltd.  相似文献   

19.
《国际泥沙研究》2022,37(5):619-638
The large confluence between the Yangtze River and the outflow channel of Poyang Lake is receiving attention due to its importance in flood control and ecological protection in the Yangtze River basin. There is a large floodplain along the outflow channel of Poyang Lake, which is submerged during high flow and dry during low flow. The effects of the submergence of this floodplain on sediment and morphological characteristics at this large confluence have not been known. Hence, a field investigation was done in March 2019 (relatively high flow, Survey 3) to complement the previous field studies done in August (high flow, Survey 1) and December 2018 (low flow, Survey 2) to identify the temporal variations of sediment and morphological characteristics considering the submergence of this large floodplain. The predominant sediment transport modes were wash load for Poyang Lake and confluence particles and mixed bedload/suspended load for the Yangtze River particles. The sediment transport processes were largely affected by both the secondary flows and the water density contrast between the tributaries with a lock-exchange sediment rich, denser flow moving across the inclined mixing interface in Surveys 1 and 2. The sediment flux across the mixing interface was weakened in Survey 3 when the density contrast was very small. The stagnation zone near the confluence apex had a low sediment concentration and played a role in preventing the sediment flux exchange between the two flows, and its size, and, thus, its importance as a barrier to sediment mixing were related to the submergence of the floodplain. The bed morphology with the local scour holes at the confluence was largely affected by the large-size helical cells, and this kind of effect was weakened as the secondary flows got restricted in Survey 3. The current results expand the database and knowledge on the sediment transport and morphological features of large river confluences.  相似文献   

20.
Large (>0.1 km2) gully–mass movement complexes (badass gullies) are significant contributors to the sediment cascade in New Zealand's steepland East Coast Region catchments. The scale of change taking place in these gully systems allows significant evolution in morphology and sediment dynamics to be tracked at annual to decadal timescales. Here we document changes in two adjacent badass gullies in Waipaoa catchment (Tarndale and Mangatu) to infer sediment generation processes and connectivity using a morphological budgeting approach. A baseline dataset for this study is provided by a LiDAR-derived digital elevation model (DEM) in 2005. We produced new DEMs and orthophoto mosaics using photogrammetry in 2017, 2018, and 2019 to quantify gully morphodynamics and associated volumes of sediment erosion and deposition in both systems as they co-evolved. Results indicate ongoing rapid development of both gully complexes. Severe erosion took place at the gully heads with lowering and migration (up to 25 m vertically and laterally) of the topographic divide separating the two gullies between 2005 and 2019. Over the same period, net lowering of each gully system was ~250 mm year−1. Key sediment-generating processes included surface erosion, deep-seated landslides, and debris flows. Longer term, the overall contribution of sediment from both badass gullies to the Waipaoa catchment has been declining. In the mid-20th century, both gullies yielded in excess of 300 kt year−1. From 2005 to 2019, 80 kt year−1 was yielded from Tarndale and 110 kt year−1 from Mangatu. Our most recent surveys demonstrated considerable variability in sediment yield, ranging from 76 kt year−1 (2017–2018) to 291 kt year−1 (2018–2019). The annual variability observed reflects the complex morphodynamics of discrete hillslopes and tributary fans in these badass gully systems and underlines the importance of integrating decadal and annual surveys when assessing system trajectory. © 2020 John Wiley & Sons, Ltd.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号