Peak Suspended Sediment Concentration Research Articles

Reservoir operation impacts on streamflow and sediment dynamics in the transboundary river basin, Vietnam

AbstractHuman interventions activities around the world, particularly reservoir operation, have dramatically altered hydrological and sediment regimes in most of the major river basins. In the Mekong River, specifically the Upper Srepok River Basin (USRB) which is a main tributary of the river basin connected to the Mekong Delta's rice bowl and the Tonle Sap Lake's top inland fisheries, there are increasing concerns about the impacts of cascade reservoir operations on downstream streamflow and sediment budgets. Previous studies estimating impacts either relied solely on observed data or did not verify simulations of regulated streamflow. Using a process‐based hydrological model calibrated and validated for both natural and regulated streamflow in the USRB, it was found that the monthly hydrological changes were up to ±20% compared to pre‐dam periods at the most downstream station bordering between Vietnam and Cambodia (i.e., Ban Don station). The basin also experienced a slight decrease (less than 2%) in annual streamflow. Additionally, average and peak suspended sediment concentration decreased significantly in both of the annual and seasonal periods. At the Ban Don station, sediment loads were reduced 140 thousand tons/year (i.e., 15%) compared to pre‐dam period. Most of the changes in streamflow and sediment budgets in the basin were driven by the Buon Tua Srah reservoir, which had the highest degree of regulation in the basin. Therefore, integrated and transboundary water and sediment management, particularly at Buon Tua Srah reservoir, needs to be developed for the sustainability of the river basin.

The Rance tidal power station: Toward a better understanding of sediment dynamics in response to power generation

A three-dimensional coupled hydrodynamic and morphodynamic numerical model was developed to analyze sediment dynamics in the Rance estuary, in response to the tidal power station (TPS) built near the mouth in the 1960s. The Rance estuary is a relatively small low-discharge steep-sided ria, located along the Brittany coast in northern France, with a maximum spring tidal range of 13.5m. Taking advantage of this significant tidal regime, the first and currently the second largest operational tidal power station in the world was built at the estuary’s mouth, with peak output capacity of 240MW. After calibration and validation of the model for present-day conditions, suspended sediment concentration (SSC) and bed level evolution were evaluated at tidal and fortnightly scales for different scenarios, with and without TPS. Peak SSC are reached during spring tides and specifically during the estuary’s infilling (flood) stage where both turbines and sluice gates are open. Unbalanced with sediment transport during ebb, sediments are accumulated in the main channel of the upper estuary. Under natural tidal forcing (i.e., without TPS), simulations show that an estuarine turbidity maximum forms in the upper estuary, and sediment deposition is significant. However, sedimentation rates are two times lower than those observed in the presence of the TPS. A possible alternative for reducing sediment accumulation in the upper estuary would be the opening of sluice gates simultaneously with the turbines during falling tide, to enhance ebb currents that would allow particles transport towards the estuary’s downstream.

Variations in the Suspended Sediment Concentration in Mountain-Type Rivers Flowing Into the Sea in the Past 60 years—Taking Nanliu River in Beibu Gulf as an Example

Suspended sediment in the water body of rivers flowing into the sea is of great significance to the accumulation process in river basins and change pattern of landforms. In particular, small and medium rivers entering the sea in mountainous areas exhibit the characteristics of both mountains and streams, and the suspended sediment concentration (SSC) greatly contributes to the formation and evolution of deltas. However, scholars rarely give attention to the factors influencing changes in the SSC in small- and medium-sized mountainous rivers, and few studies have examined SSC changes in small- and medium-sized mountainous rivers. Here, based on daily SSC and flow data obtained at the Bobai Station and Changle Station, the percentile method and regression analysis method are employed to analyse the changes in SSC from the Nanliu River to the Beibu Gulf and possible influencing factors. The main research results indicate that 1) the SSC in river water bodies from 1965 to 2020 generally reveals a downwards trend, with significant annual variations. Specifically, the overall trend can be divided into three stages: 1) the SSC is the highest from 1965 to 1971, and the mean yearly SSC reaches 0.25 kg/m3; 2) the SSC is relatively high from 1972 to 2006, and the mean yearly SSC reaches 0.16 kg/m3; and 3) the SSC is the lowest from 2007 to 2020, and the mean yearly SSC reaches 0.11 kg/m3 2) High values of the SSC are mainly concentrated in the flood season, and low SSC values mostly occur in the dry season. The monthly average SSC in the flood season from April to September and the dry season from October to March exhibits the characteristics of a decrease in the flood season and an increase in the dry season. Moreover, the peak SSC value in the watershed obviously occurs out of sync with the peak flow value. Generally, the former precedes the latter. 3) The curve of the flow rate-sediment ratio is an irregular clockwise rhombus. Moreover, the impact of tropical cyclones, land cover changes, regional GDP per capita, and engineering construction are crucial reasons for the observed variations in the SSC in the Nanliu River Basin. In this paper, the obtained research results provide an important guiding significance for the planning and management of water and sediment resources in the Nanliu River and offer a reference for hydrological planning of other river basins discharging into the sea.

Impact Of Mozhaysk Dam On The Moscow River Sediment Transport

Sediments are an essential part of the aquatic environment that define its transformation and development. The construction of dams results in severe changes in sediment fluxes. This study aims to assess how the sediment load of the upper Moskva River is affected by the Mozhaysk Dam flow regulation and to estimate its dynamics over the years of the reservoir’s existence. Our analysis of the 1968, 2012 and 2016 detailed field data shows a 20-40% decrease in the proportion of the spring flood in the annual sediment load into the reservoir, which is caused by changes in the streamflow regime of the inflowing rivers. The peak suspended sediment concentrations have decreased 5- to 10-fold, likely due to a significant decline in the watershed’s cultivated land area, which caused a decrease in the erosion rate. In the Moskva River below the dam, the seasonal dynamics of the suspended sediment concentration no longer corresponds to the natural regime. The annual suspended load of the Moskva River below the Mozhaysk Reservoir decreased up to 9-fold. The sediment retention in the reservoir has dropped from 90% to 70-85% and is to some extent restored by an outflow of the particulate organic matter produced in the reservoir. We also described the relationships between water turbidity and suspended sediment concentration of the reservoir’s tributaries, which allow for the first time to estimate the sediment load with higher accuracy than was previously possible.

Linkage between turbulent kinetic energy, waves and suspended sediment concentrations in the nearshore zone

Knowledge about the tradeoffs among turbulent kinetic energy (TKE), waves, and suspended sediment concentrations (SSCs) in the nearshore zone is relevant for understanding beach morphodynamics at different temporal and spatial scales. A field measurement lasting for nearly three tidal cycles was conducted to holistically discern couplings between TKE, waves and SSCs, with further evaluation of the relative significance of TKE and waves on SSCs under various wave conditions over a meso-macro tidal beach, Yintan, to the north of Beibu Gulf, China. The results showed a dramatic increase in wave groupiness intensity from the shoaling wave condition to the breaking wave condition and a clear decrease further into the surf bore condition. The near-bed TKEs under the surf bore condition were an order of magnitude larger than those under the breaking and shoaling wave conditions at the measurement position. The averaged SSCs in the near-bed (approximately 10 cm) under the surf bore condition were 1.5 and 4.5 times larger than those under breaking and shoaling wave conditions, respectively. The variations in relative wave height were a decisive indicator for the differences in TKE intensities among different wave conditions, while the occurrence of peak TKEs at the wave front within the intrawave cycle was associated with flow acceleration regardless of wave conditions. Mean SSCs were well correlated with waves in terms of both incident wave scale and wave group scale, which was limited to the shoaling wave condition, and the occurrence of near-bed intrawave peak SSCs was always related to the offshore wave phase. Further, TKE contributed more effectively to sediment suspension at the wave group scale than at the incident wave scale, especially under broken wave condition. Among the hydrodynamic factors, TKE played the most important role in the variations in SSCs for all wave conditions. Flow acceleration served as the second most important factor under the broken wave condition, while wave group, single wave and advection were equivalent and less important factors for SSCs.

Enhanced hysteresis of suspended sediment transport in response to upstream damming: An example of the middle Yangtze River downstream of the Three Gorges Dam

AbstractThe peak in sediment transport in alluvial rivers generally lags behind the peak in discharge. It is thus not clear how the hysteresis in the sediment/discharge relationship may be impacted by damming, which can fundamentally alter the water and sediment regimes in the downstream reaches of the river. In this study, a total of 500 flood events in the Yichang–Chenglingji Reach (YCR) of the Middle Yangtze River immediately downstream of the Three Gorges Dam (TGD) are analysed to study the impacts of dam operations on the hysteresis of suspended sediment transport. Sediment rating curves, hysteresis patterns, as well as lag times, are investigated to determine the relationship between suspended sediment concentration (SSC) and flow discharge (Q) at different temporal scales, from inter‐annual to individual flood events, for the pre‐ and post‐TGD period from 1992 to 2002 and from 2003 to 2017, respectively. The results showed that the TGD operation decreased the frequency and magnitude of floods. The decrease in peak flow and increase in base flow weakened the flood contribution to the annual discharge by nearly 20%. However, the relative suspended sediment load contribution during flood events was much higher than the discharge contribution, and was little impacted by the dam. At seasonal and monthly scales, more than 80% of the suspended sediment was transported by ~65% of the water discharge in the summer and early autumn. The monthly SSC–Q relationship changed from a figure‐eight to an anti‐clockwise pattern after the construction of the TGD. For single flood events, the TGD operations significantly modified the downstream SSC–Q hysteresis patterns, increasing the frequency of anti‐clockwise loops and the lag time between peak Q and peak SSC. These adjustments were mainly caused by differences in the propagation velocities of flood and sediment waves and the sediment ‘storage–mobilization–depletion’ process, whereas the influence of lateral diversions was small. © 2020 John Wiley & Sons, Ltd.

Suspended sediment dynamics during a period of consecutive explosive cyclogeneses in rural temperate catchments: A case study from small-sized catchments in NW Spain

During the winter of 2013/2014, a sequence of explosive cyclogeneses occurred in NW Spain, causing severe flooding and substantial economic damage. Peak discharges were extremely high in many areas, with return periods in excess of 2–5 years. The heavy flow registered during these episodes provides a unique opportunity to evaluate the sedimentological effects resulting from high-magnitude flow events. Such events are relatively rare and the technical difficulties in sampling with sufficiently high frequency means that few observations on sediment transport can be gathered during the same. In this study, we present detailed hydrological and sedimentological observations across a period of consecutive explosive cyclogeneses from two small-sized catchments (i.e., the Corbeira stream and headwater of the Mero river) located in Galicia (NW Spain), in order to examine the factors controlling suspended sediment (SS) dynamics during high-magnitude flow events. The study sites are representative of the biophysical conditions (e.g. rainfall pattern, land use, vegetation cover and management practice) in NW Spain. The catchments of the Corbeira stream and headwater of the Mero river are similar with respect to climate, land use and management practice. The main differences are the distribution of land use and catchment size (Corbeira: 16 km2 and Mero: 65 km2).Large amounts of sediment were transferred in both catchments in response to flow events. A high variability in SS load during these was also observed in the two catchments, which is attributed to variation in rainfall characteristics, magnitude peak discharge, antecedent conditions controlling sediment availability, and suggests that sediment supply and transport vary widely over time. Higher concentration and variability of SS was observed in the Corbeira catchment. Differences in SS dynamics were also noted between the catchments that may be attributed to differences in the spatial distribution of land use, which influence the location of the erosion source and SS availability, and directly and indirectly affects connectivity between the sediment source area and the stream. In the Corbeira catchment, the SS load was closely linked to the hydrological response and rainfall, whereas in the Mero basin, preceding conditions, i.e. the sediment depletion during the previous events, played an important role. The results demonstrated that peak SS concentrations may coincide with, or lag behind, peak discharge, and factors other than discharge (mainly the location of the sediment source and its connectivity with the drainage network) have a major influence on SS concentrations.

Characteristics and simulation of sediment concentration in the northwestern head of Bohai Bay

The evolution and utilization of estuarine and coastal regions are largely restricted by sediment issues. Waves and currents are the main dynamic driving forces of sediment transport in estuary and coastal areas. The study of wave-current movement and sediment transport is of great significance in both academic research and engineering practice, which has received much attention from many scholars and engineers. The northwestern head of Bohai Bay belongs to a transitional area from silty coast to muddy coast, where the sediment movement is complex. Firstly, based on the field observation data measured with a tripod system and the hydrological data in the northwestern head of Bohai Bay, the characteristics of tidal currents, waves, and suspended sediment concentration (SSC) changes under different hydrological conditions were analyzed. Sediment density distribution was detected by a γ-ray densitometer. Results show that the SSC is mainly influenced by wave-induced sediment suspension: Under light wind conditions the SSC was very low, with the peak value generally less than 0.1 kg/m3; the SSC increased continuously under the gales over 6−7 in Beaufort scale, with sustained wind action. The measured peak SSC at 0.4 m above the seabed was 0.15−0.32 kg/m3, with the average value of 0.08−0.18 kg/m3, which is about 3−6 times the value under light wind conditions. The density at the bed bottom was about 1.4− 1.6 g/cm3, and decreased to 1.02−1.03 g/cm3 at 0.1−0.2 m above the bed; there was rarely little fluid mud in the study area. Secondly, the mechanism of wave-induced SSC was studied from the view of flow structure, shear stress in the bottom boundary layer. The friction velocity was calculated by fitting the datasets of the measured logarithmic velocity profile. Comparison shows that the wave shear stresses (calculated from the bottom velocity profile) is higher than the current shear stress (calculated from the upper velocity profile). Following the Reynolds’ decomposition method, expressions of wave-induced and current-induced shear stress were derived by splitting the variables into a fluctuating component, an averaged component and an oscillatory component. Compared with uniform flow, an additional item of wave streaming leads to a higher wave-induced shear stress. Finally, by analytically solving the sediment diffusion equation, a numerical procedure was proposed for sediment concentration under combined action of waves and currents. Some key approaches for fine sediment simulation were presented, including stratification effects, hindered settling, moving bed roughness, reference concentration and critical shear stress. Under combined wave-current conditions, the combined sediment diffusivity is given by the square sum of the wave-related and current-related diffusivities, and a toe-type distribution of wave-related eddy viscosity was employed. The stratification effects are considered by introducing the turbulence damping coefficient. The approaches of hindered settling for silt were employed. The enhanced roughness due to mobile bed effects was included. Comparison shows that, these approaches can properly simulate the measured wave-induced sediment concentration. This study provides basic data for coastal protection and utilization as well as for scientific study of silty-muddy coast, which is of great significance in practice and theory.

Monsoon driven waves superpose the effect from macro-tidal currents on sediment resuspension and distribution

Sediment transport processes in remote tropical and macro-tidal estuaries are typically understudied due to costly instrumentation requirements, access difficulties and extreme weather conditions. Dry season sediment transport in Darwin Harbour (DH), northern Australia, is governed by a flood dominated tidal asymmetry. However sediment transport processes during active moonson in Darwin Harbour are poorly understood. We monitored turbidity and suspended sediment concentration (SSC) using sea bed moorings at more than seven locations in Darwin Harbour over two (Australian summer monsoon) seasons. We compared our results with sea surface elevation, wind, river discharge, wave height and period, and ocean currents recorded over the same period. We found that the maximum SSC (330 mgL−1) correlated with a peak in significant wave height (>1.3 m) and consistently strong (∼7.5 ms−1) long lasting (25–29 days) north to northwesterly winds. A striking feature of this study was that the maximum SSC coincided with neap tides and unusually low rainfall runoff for the first active monsoon in the study period. This contradicts the common misconception that peak SSC during monsoons should occur during periods of high rainfall runoff. The peak SSC was six times the peak during normal conditions of spring tides, and was the result of significant wave heights associated with northwesterly wind driven ocean swell. Wind driven northwesterly waves therefore superpose tidal dominance for sediment resuspension during the observed active monsoons, which occur regularly (1–4 times per year). Following the active monsoon, flood tidal dominance returns and causes a landward spread of SSC through the process of tidal pumping. We strongly recommend wave modelling be included in future port developments and infrastructure projects in Darwin Harbour.

Suspended sediment dynamics in a well-mixed estuary: The role of high suspended sediment concentration (SSC) from the adjacent sea area

Understanding the sediment transport processes, especially the formation and maintenance of the estuarine turbidity maximum (ETM) in estuaries is essential for dealing with harbour siltation and the related estuarine environmental issues. To gain insight into the mechanism of sediment dynamics and ETM formation in the Yongjiang estuary (YE), which is well-mixed and features abundant suspended sediment sourced from the adjacent sea area, field observations were conducted in June 2015. The flow velocity, salinity, and suspended sediment concentration (SSC) were synchronously measured over a 25-h tidal cycle along the YE and its adjacent sea area. Observations showed that there was abundant suspended sediment exchanging between the YE and its adjacent sea area. In particular, there existed an obvious difference in the single or twin peak SSC signals in different stations during a semi-diurnal tidal cycle, especially between the inner-estuary and outer-estuary regions. Detailed analyses on the processes of sediment transport showed that, in the outer sea area, the tidal-driven advection of suspended sediment was dominant to the temporal distribution of SSC, which was featured by a single peak signal in a semi-diurnal tidal cycle. For the estuarine channel, the landward transport of high-concentration suspended sediment by the flood current from the sea region was the major inducement of peak SSC signals in the lower reach of the YE, while the effect of bed sediment resuspension was mainly responsible for the peak signals during the ebb period. Unlike the formation of ETM in highly stratified or partially mixed estuaries, the ETM in the YE, which is located approximately 7 km upstream from the estuary mouth, was mainly controlled by the effect of tidal asymmetry, together with the contributions made by local resuspension of bed sediment and possible fluid mud. Some other possible factors, such as deflocculation, may also impact the formation and maintenance of the ETM in the YE, but further study is needed in the future.

Impact of river interventions on alluvial channel morphology

Rivers play a significant role for sustainment and development of human race. The number of river interventions is increasing to meet water and energy demand. The present paper discusses impacts of Rengali dam on Brahmani river, India. The first-order impact assessment is carried out with hydrological data analysis between upstream and downstream gauging stations. The second-order impact study integrates seasonal stream power evaluation and temporal variation of Brice Index (BI) and cut-off length at a critical river reach. The flow duration and sediment duration curves at downstream gauging station revealed that, lean season discharge has increased (260–300%) and peak suspended sediment concentration has decreased for downstream reach. The seasonal steam power analysis quantified increasing trend of pre-monsoon stream power (296 W/m to 811 W/m) for the river. The planform alteration study showed initiation of braiding (BI of 0.14 in 1975 to 2.33 in 2016), formation of multiple cut-offs and mid-channel bars predominantly in post-dam period. Two stages of planform responses (1985–2009 and 2009–2016) have been observed for the river after dam closure.

Effects of different land-use on suspended sediment dynamics in Sabah (Malaysian Borneo) – a view at the event and annual timescales

Suspended sediment concentrations (SSC) and the duration of high SSC are important for river ecology and water resource conservation. Using annual and storm-event datasets, this paper explores the hypothesis that key suspended sediment variables increase along a land-use disturbance gradient in hilly terrain in Sabah (Malaysian Borneo). Five small (1.7–4.6 km2) catchments of increasing disturbance history – primary forest, old growth virgin jungle reserve, twice-logged forest, multiple-logged forest and mature oil palm – were instrumented from late 2011 with dataloggers and sensors to record river stage, turbidity and rainfall. The oil palm catchment had 4–12 times greater mean discharge-weighted SSC (587 mg L–1), annual sediment yield (1128 t km–2 y–1), median event peak SSC, and duration of SSC above 1000 mg L–1 than in the other catchments. The multiple-logged catchment (last logged around 2004) has SSC characteristics close to values for primary forest, possibly due to increased ground protection against erosion afforded by low understorey regrowth and/or depletion of erodible sediment by multiple logging episodes. Results demonstrate that in hilly terrain even heavily logged rainforest has high value in safeguarding water quality and reducing erosion, whereas oil palm requires careful land management, especially of road runoff and ground cover.

Hyper-concentrated flow response to aeolian and fluvial interactions from a desert watershed upstream of the Yellow River

Aeolian and fluvial interactions (AFIs) are critical earth-surface processes in arid zones, especially in desert watersheds. The response of hyper-concentrated flows to seasonal alternate AFIs shows very high rates of sediment transport and has important environmental and ecological consequences from local to global scales. Here, we present the aeolian processes-induced hyper-concentrated (AHC) flows that occurred in the Sudalaer desert watershed upstream of the Yellow River, which primarily transport non-cohesive coarse aeolian sand (>0.08mm) and show a peak suspended sediment concentration (SSC) of 1.1 to 1.4×106mgl−3. Our field study and theoretical analysis indicate that non-cohesive coarse aeolian sand downstream in the channel can be entrained from the bed and can be suspended in the turbulent flow by the significant runoff generated upstream with a SSC γ0 value of 0.5×106mgl−3. Severe aeolian processes can provide an abundant coarse sediment supply in the channel, which, once entrained, can also trigger and promote the development of AHC flows. We define, for the first time, the ratio of the weight percentage of coarse sediment to fine sediment C>0.08mm/C<0.05mm as the optimal grain size indicator (OGI) in suspended sediment, indicating that, as the fraction of coarse sediment increases, the significant runoff gradually changes to hyper-concentrated flows and reaches the peak SSC when OGI=3.25. Due to the high frequency of sandstorms and the infrequency of rainstorms, most of the significant rainfall-induced runoff with a certain SSC γ0 can develop into AHC flows and can substantially contribute to the total sediment yield, even leading to deleterious effects on the downstream river system and ecology. Compared with other desert watersheds in semiarid regions, we propose that a SSC γ0 of 500kgm−3 is the threshold criteria for the occurrence of AHC flows in the arid desert watersheds. Comprehensive governing of soil erosion in the upstream gully-dissected slopes is an essential and effective measure for controlling AHC flows.

Extreme sediment pulses during ice breakup, Saint John River, Canada

Highly dynamic flow conditions that often occur during the river ice breakup can cause greatly increased in-stream sediment concentrations and loads, with significant impacts on aquatic ecology and the environment. Sharp waves accompanying the releases of ice jams cause transient but very high flow velocities and shear stresses. However, very limited quantitative information is available on how such ice jam-generated waves (or “javes” for short) influence the transport of suspended sediment even though their effects on it are visually apparent. Suspended sediment concentrations (SSCs), obtained during a 5-year study of ice and sediment processes in the Saint John River (NB), reveal occasional spikes in concentration, consisting of relatively brief upward excursions to nearly order-of-magnitude higher values. Such spikes are concurrent with javes and ice runs. Documented cases of sediment pulses during river ice breakup events in the Saint John River form a unique data set demonstrating temporal and spatial characteristics. Peak SSC values approach 1g/L and approximately coincide with peak jave stages and surface ice concentrations. Pulse durations amount to several hours, and are shorter than those of the concomitant javes. Despite their brevity, pulses can deliver sediment loads that are significant fractions of the total load for the entire breakup event. Measurement of breakup sediment pulses is not feasible under existing monitoring programs. Instead, focused short-term studies could lead to development of predictive capability, linking pulse properties to hydro-climatic variables that drive the breakup event. A strong influence of river discharge has been detected, leading to projected increases in pulse sediment amounts under a changing climate.

Acute impacts of reservoir sediment flushing on fishes in the Yellow River

Dam sediment flushing can reduce reservoir sedimentation and deliver sediments to the downstream rivers. However, deceased fishes are often found in the Yellow River during the period of reservoir sediment flushing. To study the acute impacts of flushing and quantify threshold conditions for fish protection, field surveys and laboratory experiments were carried out during periods of sediment flushing. Field surveys in 2010 showed that dissolved oxygen (DO) decreased rapidly with the increase in suspended sediment concentration (SSC) and that deceased fishes involved 10 species. With the model of the severity of ill effects, the sediment flushing in 2010 was estimated to cause fish mortality of 0–20%. To study the lethal effect on fishes under controlled SSC conditions, laboratory experiments were performed with carp (Cyprinus carpio) exposed in water containing different SSCs. It is concluded that the high sediment concentration (average value of 90788 mg/l), fine particles (grain size < 0.075 mm), a long flushing duration (>2 days), and DO reduction (<2 mg/l) contribute to the deaths of fishes during the flushing period in 2010. To decrease lethal effects, the upper limits of SSC and duration are recommended as the peak SSC value of 55 000 mg/l and average value of 32 000 mg/l for the entire flushing period for flushing operations. The measures of SSC control and refuge construction are suggested for eco-friendly management.

Sediment resuspension in a shallow lake with muddy substrates: St Lucia, South Africa

Wind-driven sediment resuspension affects the physical and biological environment of the water column in shallow estuarine lakes. This study investigated the relationship between wind-driven waves and suspended sediment concentration (SSC) using the 33km2 South Lake basin of Lake St Lucia, South Africa as a case study. Five wave poles measuring significant wave height and turbidity were deployed over an aggregate period of twenty days at distributed locations where sediment substrate compositions varied from muddy to sandy and depths ranged from 0.7m to 2.1m. The resulting turbidity dynamics were used to test a simple depth-averaged model of suspended sediment concentrations. The model performed best in the muddy regions of the lake and was able to simulate the resuspension dynamics more accurately than the settling dynamics. Peak suspended sediment concentration levels were best captured for the deeper muddy locations. The model provides a means to make spatially explicit predictions of suspended sediment concentrations that can be used to understand the forcing mechanisms for primary producer growth and distribution or to improve sediment budget calculations.

Aeolian process-induced hyper-concentrated flow in a desert watershed

Ephemeral desert channels are characterized by very high rates of sediment transport during infrequent flood events. Here we show that aeolian process-induced hyper-concentrated (AHC) flows occur in the Sudalaer desert watershed in the Ordos Plateau of China, which primarily transport 0.08–0.25 mm non-cohesive aeolian sand and have a peak suspended sediment concentration of 1.1–1.4 × 106 mg l−3. Aeolian sand supply and storage in the channel play a crucial role in causing hyper-concentrated flow. Our results indicate that non-cohesive aeolian sand can be entrained from the bed and suspended in the turbulent flow when the channel bed slope exceeds a critical threshold (0.0003). We also show that if the frequency ratio of wind-blown sandstorms to rainstorms Tw/Tp exceeds β(γ − γ0)/α (P/V3) (A/L) (where α is the wind-blown sand transport coefficient, β is the runoff coefficient, γ − γ0 is the increase in suspension concentration caused by addition of aeolian sands, P is the density of rainstorms, V is the wind speed of sandstorms, A is the runoff-generating area, L is the aeolian sand-filled channel length), an AHC flow occurs during the passage of a flood in a desert channel. Since high-frequency aeolian processes provide an adequate quantity of transportable sediment and promote AHC flow, most of the infrequent rainfall-induced floods occurring in arid zones can develop as AHC flows.

Seasonal transfer and net accumulation of fine sediment on a muddy tidal flat: Willapa Bay, Washington

Tidal flats act as natural laboratories in which fundamental sediment-transport processes can be directly related to resulting seabed deposits. These environments represent important repositories for terrestrial particles (including organic carbon) entering marine dispersal systems. Along the coast of the US Pacific Northwest, tides, waves, currents, and storms create year-round energetic environments that evolve on myriad time scales, from semi-diurnal to interannual. In southern Willapa Bay, WA, an extensive tidal flat is accreting at a distance away from local fluvial sources. During winter, freshwater input and the peak suspended-sediment concentration (SSC) are one-to-two orders of magnitude greater than in summer, and wind- and wave-generated shear stresses prevent sediment from accumulating on the tidal flat. Temporary deposits form as a drape across secondary channels off the Bear River Channel. Sedimentary structures from these deposits reveal 15–30cm of physically stratified sediment underlain by a discrete shell-hash layer 2–8cm thick. The presence of relatively uniform excess 210Pb activities in the sediment above the shell hash, and only supported activities below indicate rapid deposition of the surficial sediment. During summer, the distribution of bed shear stresses is not significantly weaker than in winter due to the effect of local basin geometry on fetch, yet the SSC is much less, likely due to reduced fluvial sediment supply and enhanced benthic biological factors. Progressively through the summer, tidal currents and wind waves remove the temporary channel deposits and expose the buried shell-hash layer, and concurrent seabed changes allow the tidal flats to trap this remobilized sediment. Accumulation rates determined by 210Pb analysis for cores collected on the tidal flats show mean accretion at 1.4mm/yr, which can be accounted for by local river sources. The mass of sediment stored in the temporary channel drapes during winter is approximately the same as the annual tidal-flat accumulation in the vicinity of these channels. This agreement suggests a mechanism by which sediment temporarily stored in winter channel deposits is subsequently reworked during summer, and transferred onto the tidal flats. The accumulation rate on the flats approximately matches local sea-level rise, which implies this system accretes over longer time scales in equilibrium with the space provided. The preservation of layered deposits (65–90cm thick) buried 155–230cm beneath the modern surface suggests that the past accommodation space was greater than at present. Coseismic subsidence during Cascadia earthquakes represents a possible mechanism by which vertical space can be created.

Climatic and geologic controls on suspended sediment flux in the Sutlej River Valley, western Himalaya

Abstract. The sediment flux through Himalayan rivers directly impacts water quality and is important for sustaining agriculture as well as maintaining drinking-water and hydropower generation. Despite the recent increase in demand for these resources, little is known about the triggers and sources of extreme sediment flux events, which lower water quality and account for extensive hydropower reservoir filling and turbine abrasion. Here, we present a comprehensive analysis of the spatiotemporal trends in suspended sediment flux based on daily data during the past decade (2001–2009) from four sites along the Sutlej River and from four of its main tributaries. In conjunction with satellite data depicting rainfall and snow cover, air temperature and earthquake records, and field observations, we infer climatic and geologic controls of peak suspended sediment concentration (SSC) events. Our study identifies three key findings: First, peak SSC events (≥ 99th SSC percentile) coincide frequently (57–80%) with heavy rainstorms and account for about 30% of the suspended sediment flux in the semi-arid to arid interior of the orogen. Second, we observe an increase of suspended sediment flux from the Tibetan Plateau to the Himalayan Front at mean annual timescales. This sediment-flux gradient suggests that averaged, modern erosion in the western Himalaya is most pronounced at frontal regions, which are characterized by high monsoonal rainfall and thick soil cover. Third, in seven of eight catchments, we find an anticlockwise hysteresis loop of annual sediment flux variations with respect to river discharge, which appears to be related to enhanced glacial sediment evacuation during late summer. Our analysis emphasizes the importance of unconsolidated sediments in the high-elevation sector that can easily be mobilized by hydrometeorological events and higher glacial-meltwater contributions. In future climate change scenarios, including continuous glacial retreat and more frequent monsoonal rainstorms across the Himalaya, we expect an increase in peak SSC events, which will decrease the water quality and impact hydropower generation.

The Intra-Annual Variability of Discharge, Sediment Load and Chemical Flux from the Monitoring: The Yukon River, Alaska

The covered-ice breakup in subarctic to arctic rivers in the early snowmelt season often gives any damage to instruments monitoring physical and chemical factors of water. The serious condition has brought few time series data during the snowmelt runoff except the river stage or discharge. In this study, the contribution of snowmelt runoff to the discharge and sediment load is quantified by monitoring water turbidity and temperature at the lowest gauging station of U. S. Geological Survey in the Yukon River, Alaska, for more than 3 years (June 2006 to September 2009). The turbidity was recorded by a self-recording turbidimeter with a sensor of infrared-ray back-scattering type, of which the window is cleaned by a wiper just before a measurement. The turbidity time series, coupled with frequent river water sampling at mid-channel, produce time series of suspended sediment (SS) concentration, particulate organic carbon (POC) concentration and particulate organic nitrogen (PON) concentration (mg?L–1) by using the high correlation (R2 = 0.747 to 0.790; P 11 to 2.01 × 1011 m3), 8.7% - 22.5% of the annual sediment load (3.94 × 107 to 5.08 × 107 ton), 11.6% - 23.7% of the annual POC flux (4.05 × 105 to 4.77 × 105 ton), and 10.3% - 24.5% of the annual PON flux (2.80 × 104 to 3.44 × 104 ton). In the snowmelt season, the peak suspended sediment concentration preceded the peak discharge by a few days. This probably results from the fluvial sediment erosion in the river channels.