Sediment Transport Dynamics Research Articles

Going with the flow: Experimental simulation of sediment transport from a foraminifera perspective

AbstractTransport of continental shelf sediments to the deep ocean can be studied from displaced symbiont‐bearing larger benthic foraminifera found in turbidity current deposits. The larger benthic foraminifera habitat depth, physical characteristics and preservation serve as indicators for understanding sediment transport dynamics near the seabed and in the water column. Here, an experiment was designed to explore sediment transport in a closed flume system using simulated high current velocities. Shelf sediments from the Gulf of Eilat/Aqaba, dominated by Amphistegina papillosa and Operculina ammonoides, were subjected to 60 cm s−1 and 80 cm s−1 current velocities while collected in a 10 cm vertical sediment trap. Larger benthic foraminifera abundance, shell physical properties and preservation were analyzed and compared with the original bulk sediments. The experiment results showed that at 80 cm s−1 velocity, larger benthic foraminifera shells of all sizes and preservations are efficiently resuspended and transported in large quantities throughout the water column, as opposed to their transport as bedload by the lower velocity current. Larger benthic foraminifera shape also has a role in the transport distances and accumulation depths. Operculina ammonoides shells were found to be more portable, compared to Amphistegina papillosa, due to their flatter discoid shape. The results suggest that a threshold velocity of ca 80 cm s−1 was needed to generate the thick coarse deposits found in the Gulf of Eilat/Aqaba slope sedimentary record, which were previously suggested to be triggered by large magnitude seismic events. Lower velocities probably winnowed minor amounts of larger benthic foraminifera shells (with little or no coarser sediments) that were deposited as a thin sand layer may point to lower magnitude seismic triggers. In conclusion, larger benthic foraminifera shells are transported and deposited in accordance with their hydrodynamic properties, resulting in assemblage differentiation along the transport pathway. This study shows that the fossil biogenic composition in slope sediments includes valuable information on current velocities, transport dynamics and possible triggers in the geological record.

Glacial Catchment Erosion From Detrital Zircon (U‐Th)/He Thermochronology: Patagonian Andes

AbstractAlpine glacial erosion exerts a first‐order control on mountain topography and sediment production, but its mechanisms are poorly understood. Observational data capable of testing glacial erosion and transport laws in glacial models are mostly lacking. New insights, however, can be gained from detrital tracer thermochronology. Detrital tracer thermochronology works on the premise that thermochronometer bedrock ages vary systematically with elevation, and that detrital downstream samples can be used to infer the source elevation sectors of sediments. We analyze six new detrital samples of different grain sizes (sand and pebbles) from glacial deposits and the modern river channel integrated with data from 18 previously analyzed bedrock samples from an elevation transect in the Leones Valley, Northern Patagonian Icefield, Chile (46.7°S). We present 622 new detrital zircon (U‐Th)/He (ZHe) single‐grain analyses and 22 new bedrock ZHe analyses for two of the bedrock samples to determine age reproducibility. Results suggest that glacial erosion was focused at and below the Last Glacial Maximum and neoglacial equilibrium line altitudes, supporting previous modeling studies. Furthermore, grain age distributions from different grain sizes (sand, pebbles) might indicate differences in erosion mechanisms, including mass movements at steep glacial valley walls. Finally, our results highlight complications and opportunities in assessing glacigenic environments, such as dynamics of sediment production, transport, transient storage, and final deposition, that arise from settings with large glacio‐fluvial catchments.

Soil erosion and sediment transport in Tanzania: Part I – sediment source tracing in three neighbouring river catchments

AbstractWater bodies in Tanzania are experiencing increased siltation, which is threatening water quality, ecosystem health, and livelihood security in the region. This phenomenon is caused by increasing rates of upstream soil erosion and downstream sediment transport. However, a lack of knowledge on the contributions from different catchment zones, land‐use types, and dominant erosion processes, to the transported sediment is undermining the mitigation of soil degradation at the source of the problem. In this context, complementary sediment source tracing techniques were applied in three Tanzanian river systems to further the understanding of the complex dynamics of soil erosion and sediment transport in the region. Analysis of the geochemical and biochemical fingerprints revealed a highly complex and variable soil system that could be grouped in distinct classes. These soil classes were unmixed against riverine sediment fingerprints using the Bayesian MixSIAR model, yielding proportionate source contributions for each catchment. This sediment source tracing indicated that hillslope erosion on the open rangelands and maize croplands in the mid‐zone contributed over 75% of the transported sediment load in all three river systems during the sampling time‐period. By integrating geochemical and biochemical fingerprints in sediment source tracing techniques, this study demonstrated links between land use, soil erosion and downstream sediment transport in Tanzania. This evidence can guide land managers in designing targeted interventions that safeguard both soil health and water quality.

Soil erosion and sediment transport in Tanzania: Part II – sedimentological evidence of phased land degradation

AbstractSoil resources in parts of Tanzania are rapidly being depleted by increased rates of soil erosion and downstream sediment transport, threatening ecosystem health, water and livelihood security in the region. However, incomplete understanding to what effect the dynamics of soil erosion and sediment transport are responding to land‐use changes and climatic variability are hindering the actions needed to future‐proof Tanzanian land‐use practices. Complementary environmental diagnostic tools were applied to reconstruct the rates and sources of sedimentation over time in three Tanzanian river systems that have experienced changing land use and climatic conditions. Detailed historical analysis of sediment deposits revealed drastic changes in sediment yield and source contributions. Quantitative sedimentation reconstruction using radionuclide dating showed a 20‐fold increase in sediment yield over the past 120 years. The observed dramatic increase in sediment yield is most likely driven by increasing land‐use pressures. Deforestation, cropland expansion and increasing grazing pressures resulted into accelerating rates of sheet erosion. A regime shift after years of progressive soil degradation and convergence of surface flows resulted into a highly incised landscape, where high amounts of eroded soil from throughout the catchment are rapidly transported downstream by strongly connected ephemeral drainage networks. By integrating complementary spatial and temporal evidence bases, this study demonstrated links between land‐use change, increased soil erosion and downstream sedimentation. Such evidence can guide stakeholders and policy makers in the design of targeted management interventions to safeguard future soil health and water quality.

Assessing and Modelling the Interactions of Instrumented Particles with Bed Surface at Low Transport Conditions

Sediment transport at near threshold to low transport stages (below the continuous transport) can still be affected by flow turbulence and its dynamics can benefit from further comprehensive studies. This study uses an instrumented particle embedded with micro electromechanical sensors (MEMS) to allow tracking the motions and forces acting on it, leading to and during its transport. Instrumented particle transport experiments were carried out at laboratory flume under a range of flow conditions. The probability distributions functions (PDFs) of bed load particle instantaneous velocities, hop distances and associated travel times (measured from start to stop of transport) were obtained for all the performed experiments with varying flow rates and particle density. The modelled distributions are useful and enable a deeper understanding of bed load sediment transport dynamics from a Lagrangian perspective. Furthermore, the results analyzed from the instrumented particle (including the particle’s transport mode) were validated using visual particle tracking methods (top and side cameras). The findings of this study demonstrate that for the range of turbulent flows trialed herein, the instrumented particle can be a useful, accessible, and low-cost tool for obtaining particle transport dynamics, having demonstrated satisfactory potential for field deployment in the near future.

Contrasting Impacts of a Hotter and Drier Future on Streamflow and Catchment Scale Sediment Flux in the High Andes

AbstractFluvial sediment transport in high mountain regions is uniquely sensitive to changes in hydroclimate that alter the timing and magnitude of streamflow contributions from snowmelt, glaciers, and other cryoforms. In the high Andes of Argentina and Chile, mountain river systems serve millions of people in foothill communities and large metropolitan areas alike, yet these same rivers can also carry high sediment loads that compromise water quality and infrastructure. Regional climate predictions are also increasingly forecasting a warmer and drier future that is likely to alter hydrologic regimes and sediment transport dynamics. In this study, we use hydrologic modeling techniques and sediment rating curves compiled from historical data to propagate a suite of endmember climate projections to estimate changes in streamflow and consequent impacts on catchment‐scale sediment transport on both the Argentine and Chilean sides of the high Andes. Our results show reductions in streamflow of up to 57% under scenarios with reduced precipitation and even larger decreases in sediment flux, while outcomes under a warming scenario with no changes to precipitation were highly variable. Our results further show that high winter streamflow events may increase in magnitude in Chile while mountain communities in Argentina may increasingly face water shortages. These results indicate that different adaptation and mitigation strategies may be needed on either side of the Andes in response to a changing hydroclimate.

Drivers, Impacts and Mitigation of Increased Sedimentation in the Hydropower Reservoirs of East Africa

Hydropower reservoirs are essential for the climate-neutral development of East Africa. Hydropower production, however, is threatened by human activities that lead to a decrease in water storage capacity of reservoirs. Land use/land cover and climatic changes are driving accelerated soil erosion in semi-arid East Africa, which ultimately increases reservoir sedimentation and decreases energy production. Sediment delivery dynamics at the catchment scale are complex, involving the interaction of multiple factors and processes on different spatial and temporal scales. A lack of understanding of these processes and their interactions may impede the efficiency of sediment mitigation and control strategies. A deep understanding of the processes of erosion and connectivity of the land to river channel, as well as storage of eroded material within hillslopes and floodplains, and sediment accumulation in the reservoirs supports selection of future dam locations and sustainable management of reservoirs. The sediment budget approach can provide such a holistic perspective by accounting for the various sediment sources, transport, sinks, and redistribution when the sediment is routed through that catchment. Constructing sediment budgets is challenging, but the potential for integrating a number of different techniques offers new opportunities to collect the required information. In East Africa, the spatial planning of dams is mainly dominated by political and financial motives, and impacts of land use and climate on the sediment transport dynamics are not adequately considered. Production of sediment budgets under different scenarios of land use and climate change should be an essential step when deciding the location and management strategies for dams. Selection of new hydroelectric reservoir sites must consider long-term scientific data on climate change, and the sediment budget components for sustainable land management planning, hydropower sustainability.

Sediment transport dynamics in small agricultural catchments in a cold climate: A case study from Norway

Increased nutrient and soil losses from agricultural areas into water bodies constitute a global problem. Phosphorus is one of the main nutrients causing eutrophication in surface waters. In arable land, phosphorus losses are closely linked to sediment losses. Therefore, a better understanding of the sediment-runoff processes in agricultural areas is a key to reduce the eutrophication impacts and to implement mitigation measures. The objectives of this study were to identify dominant sediment runoff processes in cultivated grain-dominated catchments in a cold climate. We assessed continuous high-resolution turbidity data, temporal and spatial catchment properties and agricultural management data to describe and get a better understanding of the cause-relationship of sediment transfer in two small agricultural dominated catchments in southern Norway. The concentration-discharge pattern, index of connectivity and agricultural activities were considered with the wider aim to establish a link between field and catchment scale. The results showed that the dominant concentration-discharge pattern was a clockwise concentration-discharge (c-q) hysteresis in both catchments indicating that areas close to or in the stream gave the highest contribution to turbidity. The main driver for turbidity was discharge, though soil water storage capacity, rain intensity and former discharge events also played a role. Intensity of soil tillage and index of connectivity (likelihood of water and particles to be transported to the stream) impacted the c-q hysteresis index. Little vegetation cover and high intensity of soil tillage led to a high hysteresis index, which indicates a quick increase in turbidity following increased discharge. Other links between agricultural management and in stream data were difficult to interpret. The findings of this study provide information about discharge, field operations and vegetational status as drivers for turbidity and about the spatial distribution of sediment sources in two agricultural catchments in a cold climate. The understanding of sediment runoff processes is important, when implementing management actions to combat agricultural emissions to water most efficiently.

The Potential Use of Osl Properties of Quartz in Investigating Fluvial Processes on the Catchment of River Mureş, Romania

Abstract To understand the functioning of fluvial systems it is important to investigate dynamics of sediment transport and the source of sediments. In case of reconstructing past processes these studies must be accompanied by the numerical dating of sediment samples. In this respect optically stimulated luminescence is a widely used technique, by which the time of sediment deposition can be directly dated. Recently, in various fluvial environments it has been shown that certain luminescence properties of minerals, and especially that of quartz, can be applied as indicators of fluvial erosion and/or sediment provenance. These properties are residual luminescence (or residual dose) and luminescence sensitivity of quartz grains. However, the values of the parameters above are affected by various factors, the importance of which is under debate. The present study therefore aims to assess these factors along a ~560 km long reach of River Mureş (Maros) a relatively large river with a compound surface lithology on its catchment. The research focused on the sandy fraction of modern sediments, collected from the main river and from three tributaries alike. This way not only longitudinal downstream changes, but the influence of tributaries could also be studied. Based on the data, both investigated parameters show a great variation, which can be attributed to the lithological differences of subcatchments and geomorphological drivers, such as erosional activity and potential number of sedimentary cycles, and human activity. However, relationships are not entirely clear and are influenced by the maximum grain size of the samples investigated, and the recycling of previously laid deposits with different properties. Still, when performing detailed dating studies, and tracing sediments from certain parts of the catchment luminescence properties can be a useful tool in the future.

The Influence of Slipface Angle on Fluvial Dune Growth

AbstractDunes dominate the bed of sandy rivers and they respond to flow by changing shape and size, modifying flow, and sediment transport dynamics of rivers. Our understanding of and ability to predict dune adaptation, particularly dune growth and decay, remain incomplete. Here, we investigate dune growth from an initial flatbed in a laboratory setting by continuously mapping the 3D bed topography using a line laser scanner combined with a 3D camera. High‐resolution profiles of flow velocity and sediment concentration providing both bedload and suspended sediment fluxes were obtained by deploying Acoustic Concentration and Velocity Profiler technology. Our analysis reveals that the magnitude of the dune slipface angle, which determines flow separation and controls turbulence production, adjusts to the imposed flow at time scales similar to the evolution of dune height and length. The initiation of a flow separation zone intensifies through scour, and results in acceleration of the dune growth. Gradients in sediment transport and the rate of dune growth are inherently linked to spatial variations in slipface angles. During dune growth, the slipface angle evolves differently than the ratio of dune height to length, which immediately reaches its equilibrium value after dune initiation.

Rapidly Migrating Secondary Bedforms Can Persist on the Lee of Slowly Migrating Primary River Dunes

AbstractTrains of secondary bedforms, superimposed on primary dunes, have been observed in rivers worldwide. To date, it has remained unclear how these secondary bedforms affect the sediment transport dynamics in a fluvial system. In this study, a field campaign was conducted to acquire a data set with a high spatial and temporal resolution, enabling to track secondary bedform migration. Results show that two distinct scales of bedforms are actively migrating. In parts of the study area, superimposed bedforms fully dissipate at the lee slope of the underlying primary dune, suggesting that the secondary bedforms contribute to the migration of the primary dune. In other parts, however, superimposed bedforms persist on the lee slope and in the trough of the primary dune. The steepness of the primary lee slope seems to control this. We calculated the sediment transport rates associated with both primary and secondary bedform migration, based on dune tracking. The sediment transport associated with secondary bedform migration is of the same order of magnitude, and even exceeds the transport associated with primary dune migration. This implies that secondary bedforms play a crucial role in the transport of bedload sediment, and cannot be ignored in the quantification of sediment transport based on dune tracking.

Effects of large wood (LW) blockage on bedload connectivity in the presence of a hydraulic structure

Large wood (LW) commonly forms log jams, influencing bed composition and biota via complex flow-sediment-wood interactions. A LW blockage, often induced by river-crossing infrastructure, can strongly alter channel hydraulics and sediment transport patterns, thus inducing a ‘forced’ channel morphology. For studying the deposition of bedload gravel fractions in the presence of a rigid LW jam, a novel experimental setup is presented. Using a mobile bed stream table, the effects of a channel-spanning blockage (a ‘dam jam’), resting at a bridge pier, on sediment transport and scour dynamics are examined. Changes in channel morphology were measured using Structure from Motion (SfM) photogrammetry and manually obtained bedload measurements. For the experiment without LW accumulation in place, natural bedload transport was observed over the entire channel length - about 20% of fed colour gravel was recovered from the sediment trap at the outlet, while a total bedload interruption occurred in presence of the LW accumulation, resulting in trapping all fed gravel material upstream of the obstructed cross-section. The volumetric deviation between SfM and manual measurements was less than 0,4%. The porosity characteristics of the LW accumulation were analysed using volumetric meshing techniques. With our experiments, we demonstrated that a relatively high log jam porosity has significant impact on channel morphology. By interrupting the hydraulic energy gradient, the movement of coarse sediment transport is damped, forcing deposition and initiating the development of a sediment wedge upstream of the infrastructure. Our quantitative results provide new insights into how LW can force channel morphology (particularly sediment storage) through the alteration of channel hydraulics. We further elaborate on how novel laboratory-based SfM techniques can be used to enhance the current understanding of complex flow-sediment-wood interaction processes at engineered in-stream structures.

Hidrogeomorfologia em confluência fluvial obtusa, Guarapuava, Paraná

As misturas sólidas e líquidas no curso fluvial são intensificadas em áreas de confluência de rios condicionadas principalmente pela abertura angular da junção. O conhecimento da dinâmica hidrossedimentológica em junções de leito rochoso com diferentes aberturas angulares é fundamental para entendimento do ajustamento morfológico nas confluências e subsidiar ações de preservação e conservação dos ecossistemas fluviais. Nesse sentido, o presente trabalho apresenta os resultados de um estudo hidrogeomorfológico sobre confluência fluvial com ângulo de junção obtuso (>100º), em rio de planalto com leito rochoso, na Formação Serra Geral. A pesquisa avaliou segmento fluvial a montante e jusante do rio das Pedras e no tributário rio das Pombas, em Guarapuava, Paraná, por meio de observações e mensurações das características morfológicas e hidráulicas largura, profundidade do talvegue, declividade do leito e vazão em margens plenas. O ajustamento morfológico em confluências obtusas é peculiar, uma vez que o ângulo de junção fluvial condiciona uma dinâmica específica de fluxo, erosão, transporte e deposição de sedimentos, resultado principalmente de sua associação com a natureza geológica do leito fluvial e os tipos de uso e manejo da terra a montante da confluência. Hydrogeomorphology of an obtuse fluvial confluence, Guarapuava, Paraná A B S T R A C TSolid and liquid mixtures in river courses intensify in areas of river confluence, conditioned mainly by the angular opening of the junction. Knowledge of hydrosedimentological dynamics in bedrock junctions with different angular openings is essential for understanding morphological adjustment at confluences and for supporting actions for the preservation and conservation of river ecosystems. For this reason, this article presents the results of a hydrogeomorphologic study on a river confluence with an obtuse junction angle (>100º), in a plateau bedrock river, in the Serra Geral Formation. The research evaluated a fluvial segment upstream and downstream of the Pedras River and in the Pombas River tributary, in Guarapuava, Paraná, through observations and measurements of morphological and hydraulic characteristics, width, talweg depth, bed declivity and bankfull flow. Morphological adjustment in obtuse confluences is peculiar in that the fluvial junction angle conditions specific flow, erosion, sediment transport and deposition dynamics, mainly resulting from its association with the geological nature of the river bed and types of land use and management upstream of the confluence. Keywords: Fluvial channel stability, bedrock, fluvial morphology.

Multifractal features of the particle-size distribution of suspended sediment in the Three Gorges Reservoir, China

The Three Gorges Reservoir (TGR) in China is the largest hydroelectric project in the world, but the threat of sediment affecting ecological sustainability of the reservoir is a topic of concern. Sediment particle-size distribution (PSD) is informative in understanding sediment transport dynamics and biochemical functions. It is, therefore, important to quantitatively characterize the distribution of sediment particles. In the current study, fractal theory is applied to determine the PSD of suspended sediment in the TGR. The results show that the volumetric fractal dimension (Dv) exhibits a significant seasonal difference (p < 0.05), reflecting sediment source and hydrodynamic sorting control the granularity of suspended sediment in the TGR. More specifically, suspended sediment particles are coarser in the wet season than in the dry season for the Yangtze River, and the opposite is true for the Ruxi River, an important tributary. The generalized dimension spectrum, D(q)–q, and multifractal singularity spectrum, f[α(q)]–α(q), were calculated for each suspended sediment sample. Thereafter, the parameters, D(0), D(1), D(2), α(0), Δα(q), and Δf[α(q)], were determined to characterize the PSD. As a result, the coarser suspended sediment during the wet season is characterised by a more complex PSD pattern, with a wider range of particle sizes, greater heterogeneity, and greater homogeneity of distribution over the measurement interval. However, the multifractal structure of the PSD of suspended sediment is more complex during the dry season than during the wet season, with higher local dispersion and variability. The findings of the current study highlight that multifractal analysis provides important insight for understanding the PSD of suspended sediment in the TGR.

Sediment Transport Dynamics in the Upper Nara Canal Off-taking from Sukkur Barrage of Indus River

Sediment material transported by the Indus River has two origins, the catchment and its wetted perimeter, whereas the amount of materials transported from the river itself depends on variables such as the flow type and the sediment load. The annual sediment load transported in the Indus River ranged from 270 to 600 million tons (MT) before the building of dams and barrages. The average sediment load is 0.715 MT/day or approximately 260 MT/ year after the construction of hydraulic structures. The average particle size, D50, of the bedload is approximately 0.125mm at the Sukkur barrage [1]. In this study, research was conducted to evaluate sediment problems in the Nara Canal, take-off from the left pocket of the Sukkur Barrage on the Indus River. The collected data from the left pocket, the bed, and suspended material in upper Nara and its off-taking canals were analyzed to assess sediment transport dynamics of upper Nara canal at various locations and its off-taking canals. The bed material at RD 553+600 shows a minimum size of 0.07 and a maximum of 0.7mm with 54% fine-sand mixed with 46% medium-sand in 2003. Fine-sand increased to an average of 75% while medium sand diminished to 25% at RD 553+600 in Nara canal and at RD 595+000 in Jamrao complex in 2013. The suspended load with low and high flows during 2012 exhibits that in all the cases the sand proportion was increasing in the Upper Nara canal system flowing to the Jamrao complex. The sediment concentration value indicates that there is the deposition of sand at the upper Nara canal and the Jamrao complex causing a discharge reduction in the Nara Canal system.

Coupling Sediment Transport Dynamics with Sediment and Discharge Sources in a Glacial Andean Basin

Suspended and bedload transport dynamics on rivers draining glacierized basins depend on complex processes of runoff generation together with the degree of sediment connectivity and coupling at the basin scale. This paper presents a recent dataset of sediment transport in the Estero Morales, a 27 km2 glacier-fed basin in Chile where suspended sediment concentration (SSC) and bedload (BL) fluxes have been continuously monitored during two ablation seasons (2014–2015 and 2015–1016). The relationship between discharge and SSC depends on the origin of runoff, which is higher during glacier melting, although the hysteresis index reveals that sediment sources are closer to the outlet during snowmelt. As for suspended sediment transport, bedload availability and yield depend on the origin of runoff. Bedload yield and bedload transport efficiency are higher during the glacier melting period in the first ablations season due to a high coupling to the proglacial area after the snowmelt period. Instead, on the second ablation seasons the peak of bedload yield and bedload transport efficiency occur in the snowmelt period, due to a better coupling of the lower part of the basin caused by a longer permanency of snow. Differences in volumes of transported sediments between the two seasons reveal contrasting mechanisms in the coupling dynamic of the sediment cascade, due to progressive changes of type and location of the main sources of runoff and sediments in this glacierized basin. The paper highlights the importance of studying these trends, as with retreating glaciers basins are likely producing less sediments after the “peak flow”, with long-term consequences on the ecology and geomorphology of rivers downstream.

Seasonal hydrological and suspended sediment transport dynamics and their future modelling in the Orwell Glacier proglacial stream, Signy Island, Antarctica

AbstractClimate change in the Antarctic over the past 50+ years has caused contraction of ice and snow cover, longer melt seasons and intensified glacier melting. These changes affect erosion and sediment redistribution processes that are vital to our understanding of terrestrial and freshwater ecosystems and sediment input to oceans. This 79 day study of the Orwell Glacier meltwater stream on Signy Island (5 December 2019–21 February 2020) used 5 min recordings of turbidity, stream discharge (Q) and air temperature (AT), supplemented by 454 water samples from which suspended sediment concentration (SSC) was gravimetrically determined, to calculate daily suspended sediment loads (SSLs). Qmean was 47.8 ± 3.5 l s-1, SSCmean was 71.0 ± 15.9 mg l-1 and daily SSLmean was 75 ± 8 kg day-1 with a suspended sediment yield of 43.6 t km-2 yr-1. A multiple regression model predicted SSLs reliably (multiple r = 0.95, r2 = 0.91, n = 79) and, when run with ATmean + 1°C (expected on Signy Island by 2060) and ATmean + 2°C (expected by 2100) scenarios, the model predicted 7% and 13% increases in SSLs, respectively. The SSLs estimated in this study are low when compared with others from around the world.

Characteristics of sedimentation and channel adjustment linked to the Three Gorges Reservoir

Construction of large dams is attractive because of their great benefits in flood control, hydropower generation, water resources utilization, navigation improvement, etc. However, dam construction may bring some negative impacts on sediment transport and channel dynamics adjustments. Due to the effects of recent water and soil conservation projects, sediment retention in the newly constructed large upstream reservoirs, and other factors, the sedimentation in the Three Gorges Reservoir (TGR) is quite different from the amount previously predicted in the demonstration stage. Consequently, based on the measured data, characteristics of sedimentation and the related channel deformation in the TGR were analyzed. The results imply that sediment transport tended to be reduced after the Three Gorges Project (TGP). Sedimentation slowed dramatically after 2013 and indicated obvious seasonal characteristics. Due to the rising water level in the TGR in the flood season, the yearly sediment export ratio (Eratio) was prone to decrease. The water level near the dam site should be reasonably regulated according to the flow discharge to improve the sediment delivery capacity and reduce sedimentation in the TGR, and to try to avoid situations where the flood retention time is close to 444 h. The depositional belt was discontinuous in the TGR and was mainly distributed in the broad reaches, and only slight erosion or deposition occurred in the gorge reaches. Sedimentation in the broad and gorge reaches accounted for 93.8% and 6.2% of the total sedimentation, respectively. The estuarine reach located in the fluctuating backwater area experienced alternate erosion-deposition, with a slight accumulative deposition in the curved reach. Sedimentation mainly occurred in the perennial backwater area. The insight gained in this study can be conducive to directly understanding of large reservoir sedimentation and mechanism of channel adjustment in the reservoir region in the main channel of large river.

Hydro‐meteorological drivers influencing suspended sediment transport and yield in a semi‐arid mountainous basin

AbstractUnderstanding erosion and sediment transport dynamics is key for sustainable water resources management. The objective of this research was to assess suspended sediment responses, identifying relationships with hydro‐meteorological drivers, sediment sources and connectivity in a mountainous semi‐arid basin. With this purpose, suspended sediment concentrations of 12 events were measured by automatic sampling and classified based on six descriptors of hydro‐meteorological drivers, their related rating curves and hysteresis loop behaviour, and distribution of precipitation, snowfall and snowmelt. Comparison with sediment transport and yield along the river, previously reported, was also performed. The results allowed us to distinguish up to three types of events with different erosive responses, comparable with other Mediterranean environments and snowmelt‐driven basins. Events classified as type I and II were closely linked to snow processes. Those classified as type III presented higher sediment yield and negligible influence of snow processes. The analysis of rating curves and hysteresis loops confirmed the identified transport responses. Type I and II events, with clockwise behaviour, were related to in‐channel sediment storage. Type III events, with predominantly counter‐clockwise behaviour, were related to hillslope erosion processes of gullying and landslides. The sediment connectivity analysed highlights the role of vegetation cover and land uses, and its sensitivity in high mountainous environments, which fits with the observed sediment transport at the study site. The volume of sediment trapped in the bottomset area of a reservoir downstream was related to type I events (more frequent and of greater magnitude). However, spatial distribution and frequency of type III events are likely to become more relevant under future climate scenarios due to their high sensitivity to hydro‐meteorological drivers. These results highlight the vulnerability of high mountainous areas on erosive dynamics and justify long‐term monitoring as hotspots of future challenge for water resource management. © 2020 John Wiley &amp; Sons, Ltd.

Current asymmetry and its implications for morphological changes in Vembanad Lake, the second largest wetland ecosystem in India

Flow dynamics are a key part of the physical processes that drive the sediment transport in an aquatic system, affecting depositional processes and leading to landform evolution. The current asymmetry (CA) noted in Vembanad Lake (VL), India, have a particularly strong influence on the sediment budget and thus shape its geomorphology. In the present study the Coastal Ocean Model - Finite Volume Coastal Ocean Model (FVCOM) was used to compute the lateral current asymmetries and their implications for the long-term morphological changes in VL during different tidal phases. Model results were utilised to calculate the CA using two criteria: (1) Peak flood/ebb velocities with duration, and (2) Asymmetry Index (AIDV). AIDV values suggest the prevalence of ebb dominance over the entire system except at Cherai and Vallarpadam. However, several isolated pockets were noted on the right/eastern bank (shallow depth) of the system where the peak flood velocity exceeded the ebb, favouring accretion.Contrary to the concept of ebb asymmetry, mean velocity and duration asymmetries were prevalent in the asymmetry index (AIDV); hence we adopted peak flood/ebb velocity and its duration asymmetries to improve our understanding of the sediment transport dynamics. Typical flood asymmetry was identified on the right bank and at many of the lateral stations in VL, which switched to the modified ebb asymmetry during substantial river discharge. Analysis of CA enables understanding of the process that influences selective morphological changes, as explained by the model results. This conclusion was well corroborated by results obtained over the past three decades (1973–2008) from analysis of Landsat satellite imagery. Flood/ebb dominance in VL is explicitly substantiated with regions of accretion/erosion derived from satellite imageries. A holistic approach encompassing modelling, observations, remote sensing, and GIS techniques were used to address the erosion-accretion process in VL.