Bed Material Load Research Articles

A Comprehensive Assessment of Sediment Transport in Greater-Zab River, Iraq

Abstract The requirement for efficient management of natural resources has recently risen. Consequently, the requirements for a hydrological analysis study is a crucial instrument for managing watershed natural resources-means the region of interest must be thoroughly investigated and evaluated. This study employs morphometric analysis to examine the Greater-Zab watershed in the northeastern part of Iraq. The Greater-Zab River serves as the case study because no sediment observations have been made there at all. Prior investigations have been conducted to gather data on the soil composition and anticipated hydraulic parameters at Km. 73.000 of the Basin, specifically upstream of Ninawa Governorate. These investigations were carried out in relation to the design discharge of 330 m3/s. The main goal of this research is to employ various widely-used methodologies globally to assess an alluvial channel’s capacity to carry the entire bed-material load, using the Greater-Zab River Basin as a case study. There were five different formulas that were utilised in order to get an estimate of the bed’s material load. Larsen’s Observational Methodology [LOM], Karim and Kennedy’s Regression Method [KKRM], Engelund and Hansen’s Stream Power Technique [EHSPT], Akers and White Stream Power Technique [AWSPT], and Yang Stream Power Technique with Dimensionless [YSPTD] are the names of these studies. Taking into account Yang’s 1973 ruling, the study found that 1)The anticipated bed-material load at the site under consideration is roughly (205000 m3/Year); 2) Variations in the outcomes of various formulas used to anticipate the amount of sediment in rivers by a certain factor are expected; and 3) It is crucial to measure the amount of sediment load in significant areas, such as those that support property and life in a major metropolis (as in the case study).

Changes in flow and sediment transport caused by cascade hydropower in the upper reaches of Yangtze River and their influence on spawning of Coreius heterodon

The upper reaches of the Yangtze River (YR) constitute the largest group of hydropower cascade projects in the world. The construction of cascade hydropower systems has a significant impact on the flow regimes and sediment transport in rivers, ultimately affecting fish spawning. Previous studies have examined the effects of fluctuating flow and water temperature on spawning; however, insufficient attention has been paid to their effects on sediment transport. This study analysed changes in runoff and sediment load (SL) at the Xiangjiaba hydrologic station (XJB) and Zhutuo hydrologic station (ZT) in the upper reaches of the YR over the past 60 years. Based on the indicators of hydrological regimes and sediment transport, we used the generalized additive models (GAM) to investigate the effects of cascade hydropower operations from 2009 to 2018 on the spawning abundance of Coreius heterodon (C. H) in the upper YR Fish Reserve. Following the operation of cascade hydropower, the SL and sediment concentration decreased by 56.8% and 56.6%, respectively, while the median size of the bed-material load demonstrated an initial thinning trend followed by subsequent coarsening. In addition, the frequency of the high-flow pulse increased by 32.1%, whereas the peak flow and duration of the high-flow pulse decreased by 41.2% and 62.6%, respectively. However, no significant changes were noted in the indicators of the flooding process. The decrease in sediment transport and changes in the flow regime led to a reduction in the spawning abundance of C. H. Reductions in sediment transport and changes in high-flow pulses caused by cascade hydropower can negatively affect C. H spawning, and SL is the most important indicator of impacts on spawning abundance (contribution of 77.7%). These findings are critical for evaluating the long-term impacts of cascade hydropower on fish habitats.

Streamwise and Vertical Dispersal of Tracer Stones in an Equilibrium Bed

AbstractModeling transport, erosion, and deposition of nonuniform sediment over temporal intervals that are short compared to those characterizing channel bed aggradation and degradation remains an open problem due to the complex quantification of the sediment fluxes between the bed material load and the alluvial deposit. Parker, Paola, and Leclair in 2000 proposed a morphodynamic (PPL) framework to overcome this problem. This framework is used here to model the dispersal of a patch of gravel tracers in three different settings, a laboratory flume, a mountain creek, and a braided river. To simplify the problem, (a) the bed slope, bedload transport rate, and bed configuration are assumed to be constant in space and time (equilibrium), (b) sediment entrainment and deposition are modeled with a constant step length formulation, and (c) the PPL framework is implemented in a one‐dimensional (laterally averaged) model. Model validation against laboratory experiments suggests that, as the transport capacity of the flow increases, the maximum elevation‐specific density of sediment entrainment may migrate downward in the deposit. The comparison between model results and field data shows that the equilibrium solution can reasonably capture tracer dispersal. The equilibrium model can also reproduce subdiffusion and superdiffusion of a patch of tracers in the streamwise direction, depending on the magnitude of the short‐term bed level changes. Finally, the average tracer elevation in a cross‐section decreases in time because particles that are buried deep in the deposit are only rarely reentrained into bedload transport.

Estimation of bedload sedimentation rate in a paleo-drowned river-valley

Extensive, high-resolution seismic data were correlated with borehole, land use, and geological data to calculate the mass of genetically different sediment deposits in the Middle Harbour estuary in Sydney, Australia. Middle Harbour is a drowned-river valley located adjacent to the larger Sydney estuary. The estuary follows a well-defined sedimentation model featuring small fluvial bedload delta deposits in the upper reaches of the embayments, a deep, central extensive mud basin overlying transgressive basal accumulations and a large flood-tide delta at the entrance. Deposits of an estimated 5094 t of bedload, 21,143 t of suspended sediment and 5947 t of transgressive basal material located in the estuary provided average sediment accumulation rates of 0.68 t year−1, 1.29 t year−1, and 2.86 t year−1, respectively. These rates, determined from measured accumulations, were surprisingly low and substantially smaller than modelled rates. However, low sediment accumulation rates for suspended material may be due to fine sediment escaping over the top of the marine tidal delta, which effectively traps all bedload material from exiting the Harbour. Results of this study indicate that Holocene bedload sedimentation in Middle Harbour was slow and regular until a rapid increase after urbanisation commenced in the catchment. Most pre-Holocene material was eroded from Middle Harbour during the Last Glacial period with sediment currently present in the estuary having been deposited since sea-level recovery.

Parameter Calibration of Sediment Transport Capacity Formula of the Third Entrance of Xinqiman Reservoir in Tarim River Based on CCHE2D Model

In this study, sediment transport at the third entrance of the Xinqiman Reservoir in the Tarim River was simulated using the CCHE2D model. The flow conditions, the suspended sediment concentration of each grain size, and the boundary grain size of the wash load in the reach were determined. Regressions between the suspended sediment concentration, S, and flow condition, U3/gH, and flow-sediment factor, U3/Rω, were developed, which strongly adhered to the power law and Zhang Ruijin’s formula for the sediment transport capacity. The parameters in Zhang Ruijin’s formula for the target reach were determined. Under the defined flow and sediment conditions, sediment particles ≥0.091 mm and particles ≤0.061 can be regarded as the bed material and wash loads, respectively. Using multivariate linear regression, the sediment transport capacity coefficient (k0) and exponent (m) were determined to be 0.0197 and 0.928, respectively. These results can serve as an important theoretical reference for the calculation of sediment transport at the third entrance of the Xinqiman Reservoir in the Tarim River.

Armor Layer Uniformity and Thickness in Stationary Conditions with Steady Uniform Flow

The continuous movement of riverbed particles due to turbulent flow determines the stability of non-cohesive riverbeds and banks during riverbed and bank erosion and sedimentation. This study emulated the stable channel design by deriving the low maintenance cost of the channel through bed protection by an armor layer. The study investigated the effects of shear stress and grain size uniformity to determine the minimum non-cohesive armor layer thickness for the stability of riverbeds under steady uniform flow conditions. Experiments were conducted with four different discharges, five armor material gradations, and five bed-slope variations in a full-scale flume. We observed and recorded the behaviors of the five gradations of armor materials for given discharges and bed slopes. Eighty data points were recorded and analyzed. The hydraulic analysis of the flow along with the soil mechanics analysis of the armor materials was done. The soil mechanic analysis was particularly focused on the uniformity coefficient of the armor layer, Cu, to derive the armor layer equation. However, for the manageability of the study, we set the limit of the Cu between 3.0 and 6.0. From the viewpoint of non-erodibility, a wider Cu value indicated a thinner armor layer. Variables that govern the armor layer thickness and the layer thickness itself were derived and proposed. The variables, namely Cu, shear stress (t0 and tc), and mean diameter of the bed load and armor materials (Db50 and Da50). Our results show that these variables governed the thickness of the armor layer, and this is expected to contribute to the design of stable natural channels, which can minimize the cost of irrigation canal maintenance and development. Doi: 10.28991/CEJ-2022-08-06-01 Full Text: PDF

A width‐based approach to estimating historical changes in coarse sediment fluxes at river reach and network scales

AbstractKnowledge about historical changes in sediment fluxes in most coarse‐bedded rivers worldwide is extremely limited. In consideration of this deficiency, we developed a width‐based approach to estimating multi‐decade changes in coarse sediment fluxes occurring at reaches of the Po River and 21 of its tributaries in northern Italy. The estimation was based on temporal variations in the reach‐averaged width of the river's active channel, and such width was expressed through a dimensionless index of coarse bed material load (Iq). The index was determined in two periods: 1954–1998 and 1998–2020. Statistically significant relationships were found between temporal variations in Iq occurring in reaches of the Po River and at key locations of each specific reach (i.e., upstream reaches and tributaries). Such evidence of coherent changes in sediment transfer through space and time led us to conclude that Iq variations can be regarded as a reliable proxy for historical changes in sediment transport in a river reach. The application of the approach to the investigation of the Po River catchment provided new insights into the historical changes characterizing coarse sediment fluxes along the river and its major tributaries. From 1954 to 1998, an average decrease in coarse sediment fluxes of about −20% and −30% occurred along the river and the terminal sectors of its tributaries, respectively. The estimations showed that coarse sediment fluxes exhibited a slightly lower decrease in the last two decades, with sediment flux recovery occurring only in some tributaries. The results suggest that a profound change in sediment dynamics and fluxes has occurred, and is likely still ongoing, in the Po River system, despite the decrease in human disturbances (e.g., in‐channel sediment mining) in more recent times.

Finding of Suitable Transportation Rate Formula by Using "Velocity - Height - Distance" for Bed Material Load

This research aims to find a suitable transportation rate formula for bed load. Laboratory experiments were conducted in a flume for monitoring and measuring the transportation of sediment particles (bed load) which move by jumping, rolling, or sliding within the flume. This study included two parts. The first part is experimenting using a flume and tracking the particles' movement by analysing the images taken by the camera. The measurements that have been taken are the amount of accumulated bed load particles at the end of the flume, which are distributed along a certain distance during a specific time, thus obtaining measuring data about the amount of accumulated bed load and values of moving distance and the required time for accumulating the particles at the flume end. The accumulated height of bed load is also measured. These experiments were conducted at different low-flow velocities. The second part includes the expression of a formula for bed load transportation rate, which is the product of multiplying the accumulated height of bed load by the velocity of bed load particles with distance at a certain time that was devised in the first part. Through the proposed method of obtaining the measurements in the first part Analysis of bed load particle velocity was done by using (π-theorem) from the results of experiments (Cv, V, ρ, ρS, µ, ds, L). To derive the formula of accumulated bed load height at flume end along a certain distance using Rayleigh’s method and using the results of the experiment (δ, V, g, ds). Finally, there can be found an expression of the bed load transportation rate formula. Checking was made for this formula, which was compared with other researchers' equations using statically measured. It was found that the derived formula was acceptable to calculate the transportation rate of bed load.

Artificial bifurcation effect on downstream channel dynamics of a large lowland river, the Atchafalaya

AbstractArtificial river bifurcations have been widely created for different purposes. Yet, long‐term dynamics in the bifurcated channels and natural channels downstream of the artificial bifurcation nodes are not well investigated. Herein, we employed an approach combining three decades (1977–2006) of bathymetric survey records, geographic information system (GIS) application, and bed material transport modeling to investigate riverbed dynamics in an artificially bifurcated channel and the downstream natural channel of the Atchafalaya River, the largest distributary of the Mississippi River. Decadal changes in the riverbed volume were determined, and erosion/deposition patterns in these channels were assessed. Hydraulic properties under low, medium, and high flow conditions were quantified to analyze main factors affecting the long‐term trend of channel dynamics. Our study found that flow from the Atchafalaya River into the artificially bifurcated Wax Lake outlet channel (WLOC) steadily increased. Hydraulic gradient, stream power, and shear stress under all flow conditions of the WLOC were greater than those of the Atchafalaya River natural mainstem channel (ARMC) downstream of the artificial bifurcation. On average, 52% of the total bed material loads (BMLs) from the Atchafalaya entered the steeper and straight WLOC with 41% of the river's flow from 1977 to 2006. Despite the higher sediment inflow, the WLOC degraded with an average rate of −0.12 × 105 m3 km−1 yr−1, while the ARMC downstream of the bifurcation aggraded substantially with an average rate of 0.38 × 105 m3 km−1 yr−1 over the three decades. The disproportional flow–sediment ratio and channel erosion‐aggradation patterns below the artificial bifurcation were apparently a reflection of the difference in the river hydraulic properties. These findings imply that lowland alluvial river diversions for introducing sediment to build new land in world's sinking deltas need to develop sustainable solutions in order to avoid causing severe bed scouring in one but excessive deposition in the other channel downstream of the bifurcation.

Coarse sediment dynamics and low-head dams: Monitoring instantaneous bedload transport using a stationary RFID antenna

Coarse sediment transport in fluvial systems serves an important role in determining in-stream physical habitat, spawning potential and benthic community structure. However, despite more than a decade of pressure in Europe to restore stream continuity under the Water Framework Directive (WFD), there have been relatively few empirical studies on how low-head, run-of-river structures (i.e., weirs) disrupt the processes and dynamics of bedload conveyance. In this study we present an investigation into how coarse sediment is transferred through a low-head dam via the real-time monitoring of bedload transport over a weir in southeast Ireland. Critical discharge values for particle entrainment over the structure were derived from the novel use of a stationary RFID antenna, coupled with continuous recording of water levels and sediment captured downstream using pit-style sediment traps. The stationary RFID antenna was installed along a weir crest using both ‘pass-under’ and ‘pass-over’ configurations as a means of detecting the moment bedload tracers moved over the dam crest. Results show that 10% of tracers deployed upstream were detected passing over the weir, while a further 15% that were not detected were recovered downstream. These results indicate bedload material as large as the upstream D70 (i.e., 90 mm) can move over the structure during infrequent high-flow events. However, thorough searches of the seeded area upstream of the dam also suggest that as many as 43% of the total number may have passed downstream, indicating that tracers moved over the weir after the antenna was damaged during a high-flow event, or were missed due to either particle velocity or signal collision. In addition, 30 of the tracers that remained upstream were shown to have either been buried due to the subsequent influx of sediment entering the reservoir, or were reworked though the surface material. Critical discharge values indicate size-selective transport patterns may dominate and a strong correlation between event peak discharge and total bedload captured downstream. These findings provide more evidence that low-head structures may eventually adopt a morphology that allows for the intermittent storage and later export of a channel's bedload downstream as hypothesized by other authors. Building upon these findings and those of other recent field studies, we present a set of possible schematic models that offer a basis for understanding the unique ways low-head dams can continue to disrupt sediment conveyance long after they have reached their functional storage capacity. The limitations of using a stationary RFID antenna and possible recommendations for future studies are discussed.

Alluviation of a side-channel by bed material load. Field measurements and modelling

In this research, we explore the alluviation process of an artificial side-channel built in August 2015 and designed to alleviate bank erosion risks on the main river. The study area is located on the lower Gállego River, a meandering-wandering gravel-bed river draining the Southern Pyrenees. This river reach is still dynamic (i.e. chutes, meander migration and other riverine processes are often observed during floods) despite the presence of three dams in the upper catchment, encroachments and extensive mining during the 1960s–80s that led to severe incision. In this research, we focus on the effects of bankfull-like flood events (Qpeak = 354 m3/s) between February 2016 and February 2017, which alluviated 62% of the side-channel. This research aims at finding the physical mechanisms that explain the alluviation process and at putting them in the general context of the current geomorphic trend of the Gállego River in response to past and ongoing impacts.The causes of the alluviation are studied through collecting and analysing field data, which in turn, are used as input in a novel modelling strategy that includes one-dimensional hydraulic modelling, semi-empirical expressions to obtain the sediment supply entering the side-channel system and a morphodynamic model able to deal with granular sediment mixtures in compound channels. The analyses of the numerical results along with the recent history of impacts and general channel pattern adjustments help us understand about the morphodynamics mechanisms responsible for the alluviation of the side-channel.

Assessing first‐order BQART estimates for ancient source‐to‐sink mass budget calculations

AbstractConstraining the timing and volume of sediment dispersal in an ancient sedimentary system is vital to understand a basin's infill history. One preferred method for a first‐order approximation of ancient sediment load estimates, the BQART model, is based on empirical observations of modern river systems relating basin morphology, topography, climate, run‐off and bedrock characteristics. Despite the popularity of such methods, a comprehensive assessment on the validity of using modern river observations to measure sediment load on geological timescales is lacking. Here, we investigate the uncertainties, sensitivities and practicalities surrounding the use of modern empirical observations in general and the BQART model in particular, to evaluate ancient sediment river loads. Although catchment area and relief are the least constrained parameters in an ancient sedimentary system, the temperature parameter may have an even more significant impact in the range of predicted sediment load estimates using a BQART approach. The applicability of BQART is most suitable for regional to continental scale source‐to‐sink systems that are based on robust paleogeographic and paleoclimatic models of cold (<2°C) or warm temperate (>8°C) climates. One further needs to consider the high amplitude discharge events that can dominate the stratigraphic record which are not captured by historical observations of sediment load over a 30‐year period. In addition, our limited understanding of bedload material transport and an unknown pristine environment in the Anthropocene reduce the reliability of modern sediment load estimates for the ancient. Mass budget estimates in deep time based on empirical relationships of modern river systems can thus provide first‐order estimates within an order of magnitude but need to consider the limitations imposed by extrapolating the modern to the ancient. Here, we present a framework to consider the suitability of the BQART method for ancient source‐to‐sink mass budget analyses.

Grain Size‐Specific Engelund‐Hansen Type Relation for Bed Material Load in Sand‐Bed Rivers, With Application to the Mississippi River

AbstractMany sand‐bed rivers worldwide have been experiencing significant reductions in sediment load over the past several decades. This is one of the causes of river delta drowning worldwide. This problem, however, has not been studied in detail in the context of sediment grain sorting. Considering the good performance of the original Engelund‐Hansen relation (OEH) for uniform sediment, and the fact that bulk grain size‐specific relations for bed material load that allow for sorting are relatively rare in the case of sand‐bed rivers, a grain size‐specific Engelund‐Hansen type relation (SEH) is proposed in this study based on data from a large flume. We embed both the OEH and the SEH in a one‐dimensional river morphodynamic model to simulate the morphodynamic evolution of the middle Mississippi River in response to the upstream cutoff of sediment supply. Simulation results using a single characteristic grain size show that bed material load delivered to the delta reduces only gradually in response to the cutoff of sediment supply, in agreement with previous studies. However, implementing the full grain size distribution of sediment leads to a much faster reduction of bed material load delivered to the delta, because the bed surface coarsens in response to grain sorting. This armoring inhibits the bed degradation that would replenish sediment load along the channel. The bed material load of finer sediment declines more rapidly than that of coarser sediment. The results of this study have practical implications for river delta restoration.

Trend of bed load material and bed load transport in Sungai Kenau for year 2013 until 2020

A study was carried out to determine the trend of bed load material characteristic and bed load transport rate in Sungai Kenau at Sungai Lembing, Kuantan, Pahang between year 2013 until 2020. Sediment is a material that can be moved and deposited to the new location through the river. This study will be focusing in bed load transport as it is the key component to determine the sediment transport rate in the river. Two sampling station have been chosen to measure the bed load transport in year 2013 until to 2020. Schocklitsh and Duboys equation was used to predict sediment transport load and it was compared with measured sample from Helly-Smith equipment. The collected sample was analysed to determine the trend of bed load material and bed load transport in Sungai Kenau for year 2013 until 2020. According to Duboys calculation the highest bed load transport rate is Sample 1 with 0.013220 lb/sec and the lowest is on Sample 5 with 0.002310 lb/sec both of the samples are taken on the same station but on the different day. Meanwhile according to Schocklitsh calculation the highest bed load transport rate is Sample 7 with 3.79374 lb/sec and the lowest is on Sample 5 with 1.98018 lb/sec both of the samples also taken on the same station but on the different day. It can be sure that the higher flow rate of the river the higher bed load transport rate. It can be obtain that according to Wentworth Scale the median grain size in Sungai Kenau were between 0.68 mm to 2.6 mm where the bed material can be classified as coarse sand to the very fine gravel.

The characteristics of sediments in the middle and lower Yangtze River influenced by dam operation

Since the Three Gorges Dam (TGD) was operated, the sediment characteristics in the downstream river have changed significantly. The characteristics of sediments can be partly reflected by the critical diameter to distinguish wash load and bed material load (WAB) and the ratio of wash load to suspended sediment (WTS). However, the WAB and WTS of the Yangtze River(YR) have not been fully investigated. In this study, data of sized suspended sediment and bed material at four stations in the middle and lower YR were collected to calculate the WAB and WTS, using the most-curvature method. The variations were then analyzed. The results suggest that the WAB and WTS of Yichang station varied greatly before the impoundment, while that of other stations were in small fluctuations. After the impoundment, the WAB of Yichang station increased dramatically to 1.856 mm, and the growth rate of the WAB at Jianli and Hankou stations were 1.0% and 4.1%, respectively (compared with WAB in 2002), while that of Datong station was -8.4%. Comparing with the rising WAB, the WTS of Yichang station continued to rise until it was close to 100%. However, the WTS of other stations decreased overall (30%-40% in Jianli and Hankou, and 8% in Datong).

SEDIMENT DYNAMICS AND STABILITY STATUS OF THE KARRA KHOLA, HETAUDA DUN VALLEY, CENTRAL NEPAL SUB-HIMALAYA

The Karra Khola in Hetauda, Siwalik region, originates within the Dun Valley, and contributes the Rapati Nadi in Hetauda Metropolitan city. The stability status of the river is of main concern because of rapid growing of the river corridor and peripheral land areas into suburban city. The river was surveyed for hydraulic parameters, sediment characteristics, and sediment loads. Rate of sediment transport and sediment yields were computed, and competency of the river was evaluated using Shield’s parameters and Reynolds numbers. The results show that the river sediments are sandy gravel to gravelly sands, and are moderately to very poorly sorted. The total sediment yield of the whole basin near the outlet is around 2% of the maximum total sediment yield. The specific stream power (SSP) ranges from 20.98 to 2866.34 Wm-2. The dimensionless boundary shear stress to dimensionless critical shear stress ratio exceeds unity, revealing that the river is competent enough to transport its bed material loads, except in downstream stretch before the river confluences with the Rapati Nadi. The Karra Khola clearly exhibits status of degradation in the upstream stretch, high rates of transportation due to lateral erosion in the midstream stretch, and aggradation in the downstream stretch.

Internal erosion of soil pipes: Sediment rating curves for soil pipes

AbstractThe collapse of soil pipes due to internal erosion can result in fully mature gullies. Few studies have measured the rates of sediment detachment and transport through soil pipes in situ. The objectives of this work were to determine suspended sediment concentration (SSC) in soil pipes as a function of pipeflow rate to develop sediment rating curves (SRC) and measure the bedload transport as a function of cumulative flow per storm event. H‐flumes were installed in seven discontinuous gullies formed by pipe collapse and instrumented for pipe discharge measurements and suspended sediment sampling. The typical response to pipeflow was an initial flush of high concentration of suspended sediment followed by a decrease as pipeflow increased (rising limb of hydrograph). Pipeflows were often so dynamic that it was difficult to consistently capture the initial flush of sediment, resulting in weak to non‐existent SRCs. The falling limb of the hydrograph tended to have a relatively low SSC. Thus, soil pipe SRCs tended to be better represented by hysteretic SRCs, although relationships between SSC and flow rate were poorly represented by SRCs. A power law equation given by SSC = aQb was adopted to represent the SRC relationships. Fitting this equation to data showed a correlation between the offset, a, and the slope, b, with the slope decreasing as the offset increases. Both SRC parameters (a and b) were correlated to the contributing area of the individual pipe. Bedload appeared to be an important contributor to sediment transport, with bedload – expressed as an average event sediment concentration (mg l−1) – decreasing as the volume of the event discharge (m3) increased. A significant portion (11–31%) of the bedload material was gravel and aggregates (>2 mm diameter material). While this work was the first to determine SRCs for soil pipes, refined sampling and measurement techniques are needed. © 2020 John Wiley & Sons, Ltd.

Mud in rivers transported as flocculated and suspended bed material

Riverine transport of silt and clay particles—or mud—builds continental landscapes and dominates the fluxes of sediment and organic carbon across Earth’s surface. Compared with fluxes of sand-sized grains, mud fluxes are difficult to predict. Yet, understanding the fate of muddy river sediment is fundamental to the global carbon cycle, coastal landscape resilience to sea-level rise, river restoration and river–floodplain morphodynamics on Earth and Mars. Mechanistic theories exist for suspended sand transport, but mud in rivers is often thought to constitute washload—sediment with settling velocities so slow that it does not interact with the bed, such that it depends only on upstream supply and is impossible to predict from local hydraulics. To test this hypothesis, we compiled sediment concentration profiles from the literature from eight rivers and used an inversion technique to determine settling rates of suspended mud. We found that mud in rivers is largely flocculated in aggregates that have near-constant settling velocities, independent of grain size, of approximately 0.34 mm s−1, which is 100-fold faster than rates for individual particles. Our findings indicate that flocculated mud is part of suspended bed-material load, not washload, and thus can be physically described by bed-material entrainment theory. Fast settling rates of suspended silt and clay particles suggest that mud in rivers is largely flocculated and part of suspended bed-material load.

Multiband (X, C, L) radar amplitude analysis for a mixed sand- and gravel-bed river in the eastern Central Andes

Synthetic Aperture Radar (SAR) amplitude measurements from spaceborne sensors are sensitive to surface roughness conditions near their radar wavelength. These backscatter signals are often exploited to assess the roughness of plowed agricultural fields and water surfaces, and less so to complex, heterogeneous geological surfaces. The bedload of mixed sand- and gravel-bed rivers can be considered a mixture of smooth (compacted sand) and rough (gravel) surfaces. Here, we assess backscatter gradients over a large high-mountain alluvial river in the eastern Central Andes with aerially exposed sand and gravel bedload using X-band TerraSAR-X/TanDEM-X, C-band Sentinel-1, and L-band ALOS-2 PALSAR-2 radar scenes. In a first step, we present theory and hypotheses regarding radar response to an alluvial channel bed. We test our hypotheses by comparing backscatter responses over vegetation-free endmember surfaces from inside and outside of the active channel-bed area. We then develop methods to extract smoothed backscatter gradients downstream along the channel using kernel density estimates. In a final step, the local variability of sand-dominated patches is analyzed using Fourier frequency analysis, by fitting stretched-exponential and power-law regression models to the 2-D power spectrum of backscatter amplitude. We find a large range in backscatter depending on the heterogeneity of contiguous smooth- and rough-patches of bedload material. The SAR amplitude signal responds primarily to the fraction of smooth-sand bedload, but is further modified by gravel elements. The sensitivity to gravel is more apparent in longer wavelength L-band radar, whereas C- and X-band is sensitive only to sand variability. Because the spatial extent of smooth sand patches in our study area is typically <50 m, only higher resolution sensors (e.g., TerraSAR-X/TanDEM-X) are useful for power spectrum analysis. Our results show the potential for mapping sand-gravel transitions and local geomorphic complexity in alluvial rivers with aerially exposed bedload using SAR amplitude.

Estimating bed material fluxes upstream and downstream of a controlled large bifurcation ‐ the Mississippi‐Atchafalaya River diversion

AbstractBed material transport at river bifurcations is crucial for channel stability and downstream geomorphic dynamics. However, measurements of bed material transport at bifurcations of large alluvial rivers are difficult to make, and standard estimates based on the assumption of proportional partitioning of flow and bedload transport at bifurcations may be erroneous. In this study, we employed a combined approach based on observed topographic change (erosion/deposition) and bed material transport predicted from a one‐dimensional model to investigate bed material fluxes near the engineering‐controlled Mississippi‐Atchafalaya River diversion, which is of great importance to sediment distribution and delivery to Louisiana's coast. Yang's (1973) sediment transport equation was utilized to estimate daily bed material loads upstream, downstream, and through the diversion during 2004–2013. Bathymetric changes in these channels were assessed with single beam data collected in 2004 and 2013. Results show that over the study period, 24% of the Mississippi River flow was diverted into the Atchafalaya River, while the rest remained in the mainstem Mississippi. Upstream of the diversion, the bed material yield was predicted to be 201 million metric tons (MT), of which approximately 35 MT (i.e., 17%) passed through the bifurcation channel to the Atchafalaya River. The findings from this study reveal that in the mainstem Mississippi, the percentage of bed material diversion (83%) is larger than the percentage of flow diversion (76%); Conversely, the diversion channel receives a disproportionate amount of flow (24%) relative to bed material supply (17%). Consequently, severe bed scouring occurred in the controlled Outflow Channel to the Atchafalaya River, while riverbed aggradation progressed in the mainstem Mississippi downstream of the diversion structures, implying reduced flow capacity and potential risk of a high backwater during megafloods. The study demonstrates that Yang's sediment transport equation provides plausible results of bed material fluxes for a highly complicated large river diversion, and that integration of the sediment transport equation with observed morphological changes in riverbed is a valuable approach to investigate sediment dynamics at controlled river bifurcations.