Average Suspended Sediment Concentration Research Articles

Impact of seasonal variations in fronts on suspended sediments transport off the coastal area of Fujian Province

The Zhejiang-Fujian (Zhe-Min) coastal muddy area plays a crucial role in facilitating sediment exchange through a cross-front. The mud depocenter off the Zhe-Min coastal area is a source of suspended sediment that can be transported to the continental shelf of the East China Sea (ECS). Although the front of the inner shelf of the ECS has been extensively reported, the cross-front material transport off the coastal area of Zhe-Min in summer has not been well studied, especially using measured data. To reveal how the front controls the transport of suspended sediment, this study focuses on the impact of fronts on the dispersion of suspended sediment off the coastal area of Fujian Province in different seasons. The results indicate that the front acts as a barrier, inhibiting the dispersion of suspended sediment into the sea. The high-concentration suspended sediment is mainly found to the northwest of the front, with an average SSC of 8.5 mg/L in winter and 3.1 mg/L in summer. The suspended sediment concentration (SSC) follows a V-shaped distribution along the cross-isobath transects, with lower SSC observed at the 50-m isobath compared with the shallow water area and the deep water area. The SSC at the front was the lowest, with an average concentration of 2.3 mg/L in winter and 1.9 mg/L in summer. The front is crucial for the development of the Zhe-Min coastal muddy area. The winter monsoon is strong, resulting in a sufficient supply of suspended sediments in the muddy area and a high transport flux of suspended sediments in the nearshore. The front hinders the dispersion of high-concentrated sediment from the nearshore to the offshore, resulting in the deposition of fine-grained sediments in the nearshore and the formation of an inner shelf muddy sedimentary zone. The findings of this study will help improve our understanding of the sediment source-to-sink processes in the ECS and land–sea interactions.

Modeling sediment concentrations and loads for two small agricultural watersheds in Prince-Edward-Island (Canada): present conditions and a future scenario

The degradation of soils and its detrimental consequences on aquatic environments is an important research topic in agricultural regions such as Prince Edward Island (PEI, Canada). Enhanced information related to suspended sediments in watercourses can serve as an effective decision-making tool in agricultural land management. This study aims to compare flow, suspended sediment concentrations (SSC), and loads using the Soil and Water Assessment Tool (SWAT) in two watersheds in Prince Edward Island (PEI). The final investigations will focus on the potential variations in hydrological and sedimentary values in the future using a relatively pessimistic climate change scenario. Finally, the projected sediment concentrations and loads will be analyzed, considering their potential impacts on ecosystems. Water level and turbidity were recorded using two water level loggers and two optical backscatter sensors (OBS) deployed in the Tuplin Creek and Spring Valley watersheds. These instruments continuously recorded suspended sediments and flow data from June 2021 to September 2022. The data were used to manually calibrate the hydrological and suspended sediment models. The understanding of sediment loads and the benefits of proposed changes to agricultural practices can be tested with the SWAT model, as it incorporates a land use index that varies spatially and temporally. Calibration and validation of both the hydrological and sediment models were satisfactory, with Kling-Gupta Efficiency coefficients varying between 0.51 and 0.73 and Nash-Sutcliffe coefficients varying between 0.61 and 0.73 respectively, indicating successful simulation of both variables in an agricultural context in spite of relatively short calibration and validation periods. Under the selected climate change scenario (RCP 8.5), daily flows and suspended sediment concentrations were simulated until 2,100, showing a slight increase in the average suspended sediment concentration (CSS). For Tuplin Creek, extremely high sediment peaks (>1,500 mg/L) could become significantly more frequent, potentially causing more frequent and severe ecosystem disturbances according to the simulations.

Event-scale suspended sediment fluxes in a dryland environment valley-bottom wetland: Implications for downstream sediment and phosphorous fluxes

Suspended fine sediment in rivers and small streams contributes to fluvial pollution loads because of its association with anthropogenically introduced heavy metals, pesticides / herbicides, and nutrients, including phosphorous (P). By acting as sinks for suspended sediment, wetlands located along drainage lines have considerable potential in reducing downstream sediment and nutrient fluxes. The aim of this research was to document downstream sediment fluxes within a single valley-bottom wetland to establish the effectiveness of this common wetland type in retaining P attached to sediment. The study was accomplished by conducting a topographic survey (dGPS and 0.5 m Lidar-derived DEM) with surface and sub-surface sediment sampling to establish long-term (decades to centuries) processes of erosion and deposition within the wetland, as well as consider storage of P. This was paired with event-based suspended sediment sampling at eight positions within the wetland over a wet season, as well as time-averaged suspended sediment sampling at a seasonal-scale at the wetland inlet and outlet. Sediment samples were characterised in terms of organic content, total phosphorous and particle size, which were determined using loss-on-ignition, ICP-AES following aqua regia extraction, and a combination of dry sieving and settling, respectively.Over the wet season, average suspended sediment concentrations increased to a maximum value of 0.26 g.L−1 and then declined, potentially because of the exhaustion of locally available sediment. The total P concentration of incoming sediment was 2174 mg/kg compared to 800 mg/kg at the wetland outlet. However, when considering the seasonal flux for the single year sampled, it was found that the wetland was a net downstream contributor of sediment (11 g of sample were retained at the inlet, relative to 19 g at the outlet). Despite this, the wetland retained ∼37 % of the total P supplied to it from upstream sources, with the remainder passing through the wetland as a result of sediment reworking in reforming channels. Thus, while wetlands may be effective at retaining some sediment and nutrients, they are not an alternative to more formal water treatment works.

Monitoring Suspended Sediment Transport in the Lower Yellow River using Landsat Observations

The spatiotemporal variations in suspended sediment concentration (SSC) in the lower reaches of the Yellow River exhibit significant variability and are influenced by reservoir operations. Understanding the spatiotemporal distribution characteristics of SSC in water holds crucial implications for environmental protection and reservoir operation management. Based on daily-scale SSC monitoring data from four hydrological stations in the lower Yellow River, this study established an SSC remote sensing model applicable to Landsat series satellite data. The independent variable of the model, Rrs(NIR)/(Rrs(G) + Rrs(R) + Rrs(SWIR)), demonstrated sensitivity to water bodies with different SSC values. Distinctive spatiotemporal characteristics in sediment transport were observed across the lower Yellow River. Spatially, the SSC values in the Sanmenxia and Xiaolangdi reservoirs were notably lower than those in other river sections, averaging 1008.42 ± 602.83 mg/L and 1177.89 ± 627.95 mg/L, respectively. Over time, the majority of the river sections (96%) exhibited decreasing trends in SSC during 1984–2022, particularly in the downstream Xiaolangdi reservoir, with average SSC values of 4265.58 ± 1101.77 mg/L in the 1980s and 1840.80 ± 2255.15 mg/L in the 2020s. Seasonal variations in SSC were prominent, with higher summer concentrations, averaging 5536.43 ± 2188.77 mg/L (2020s summer) and 814.11 ± 158.27 mg/L (2020s winter). Reductions in SSC during 1984–2022 primarily occurred in summer, weakening its seasonal variability in the lower Yellow River. Water discharge emerged as a critical factor influencing suspended sediment transport, with SSC increasing in high-water-flow months. Following the construction of the Xiaolangdi reservoir, the relationship between SSC and water discharge at different stations underwent notable alterations. This study enhances our understanding of the spatiotemporal dynamics of suspended sediment transport in the lower Yellow River, providing valuable insights for utilizing long-term Landsat series data in the dynamic monitoring of river sediment transport.

Environmental control of the present-day sediment export along the extratropical Andes

Understanding how the environmental factors that determine erosion operate is essential to know the past and future evolution of the Earth and adequately manage natural resources. In this work, the main controlling factors of the current sediment export in the western side of the extratropical Andes (20°-55°S) are investigated using multidecadal series of specific suspended sediment yields from 42 rivers, as well as land cover, climatic, cryospheric, topographic, seismological and geological data. Through an automatic selection of Generalized Additive Models based on predictability and complexity, it was found that the combined effects of extreme runoff, glacier cover and channel steepness explain ~90 % of the spatial variability of sediment export. The runoff effect varies from positive for hyper-arid and semi-arid settings to slightly negative for moderate and wet settings, probably because of an associated increase in vegetation coupled with a recurrence and magnitude of floods that becomes greater than the time it takes for the basins to generate sediment. Cryosphere and channel steepness influences were also observed on the average suspended sediment concentration, which is less associated with water availability and more associated with erodibility and sediment availability.

Hydrosedimentary behavior of a field combining surface drains and tile drains

In agricultural fields, soil erosion is usually associated to surface runoff. However, in drained fields, tile drains are additional pathways for soil particles. Few studies have quantified erosion in drained fields and they mainly focused on tile drained fields, without considering fields combining surface and tile drainage. These studies show high inter-site and inter-annual variabilities that highlight the need for more annual quantifications of erosion in drained fields. Consequently, it is still difficult to quantify the relative importance of the factors affecting soil erosion in drained fields. In addition to a comprehensive review of the existing studies of erosion through drainage, this study aims at quantifying surface and subsurface erosion in a 5 ha cultivated field combining surface and tile drainage. Suspended sediments (SS) and water fluxes have been monitored during two years (2019–2020 and 2020–2021) at a high temporal resolution (1 min), both at the outlet of the tile drain network and at the outlet of the surface drainage rill. SS yield was 0.49 t ha−1 and 1.08 t ha−1 in 2019–2020 and 2020–2021, respectively. During 2019–2020 and 2020–2021, tile drainage contribution to the total runoff was 46% and 21%, respectively and its contribution to SS yield was 9% and 11%, respectively. High temporal resolution measurements of runoff and SS concentrations showed the suspended sediment concentration in the surface drain runoff only increase by 17% from the first to the second study year. Conversely, for tile drainage, average suspended sediment concentrations increased by 260%. These variations and the increase of surface runoff rate suggest a shift in water and sediment connectivity at the field scale. Cropping practices could have generated a slumping of the soil surface during the second year and, at the same time, the development of a macropore network in the subsoil. Cropping practices induced changes of surface horizon characteristics and their impact on the hydrosedimentary behavior of drained soils need to be further studied. This study confirms previous results concerning the temporal dynamics of SS exports in a drained context under temperate climate and adds a new quantification of hydrosedimentary fluxes in a surface and tile drained field separating surface drains and tile drains contributions.

Comparative analysis of discharge and sediment flux from two contiguous glacierized basins of Central Himalaya, India

In the present study, suspended sediment load (SSL), sediment yield and erosion rates in Pindari Glacier basin (PGB) and Kafni Glacier basin (KGB) have been estimated using daily discharge and suspended sediment concentration (SSC) data for three ablation seasons (2017-2019). For this, one meteorological observatory and two gauging sites have been established at Dwali (confluence point), and water samples have been collected twice in a day for high flow period (July to September) and daily for lean period (May, June and October). An area-velocity method and stage-discharge relationship has been established to convert water level into discharge (m3 s-1). For estimating SSC (mg/l), collected water samples have been filtered, dried, analysed and confirmed with an automatic suspended solid indicator. Further, SSL, sediment yield and erosion rates have been computed using SSC data. The results reveal that mean annual discharge in PGB (35.06 m3s-1) has been found approximately 1.7 times higher than KGB (20.47 m3s-1). The average SSC and SSL in PGB have been observed about 396.07mg/l and 1928.34 tonnes, and in KGB, it is about 359.67mg/l and 1040.26 tonnes, respectively. The SSC and SSL have followed the pattern of discharge. A significant correlation of SSC and SSL has been found with discharge in both the glacierized basins (p < 0.01). Interestingly, average annual sediment yield in PGB (3196.53 t/km2/yr) and KGB (3087.23 t/km2/yr) have been found almost identical. Likewise, the erosion rates in PGB and KGB have been witnessed about 1.18 and 1.14mm/yr, respectively. Sediment yield and erosion rates in PGB and KGB have been found in correspondence with other basins of Central Himalaya. These findings will be beneficial for engineers and water resource managers in the management of water resources and hydropower projects in high-altitude areas and in the planning and designing of water structures (dams, reservoirs etc.) in downstream areas.

Pristine levels of suspended sediment in large German river channels during the Anthropocene?

Abstract. Suspended sediment is an integral part of riverine transport and functioning that has been strongly altered during the Anthropocene due to the overwhelming human pressure on soils, sediments, and the water cycle. Understanding the controls of changing suspended sediment in rivers is therefore vital for effective management strategies. Here we present results from a trend analysis of suspended sediments covering 62 monitoring stations along the German waterways (catchment sizes range between 2000 and 160 000 km2) with more than 440 000 water samples taken between 1990 and 2010. Based on daily monitoring of suspended sediment concentration (SSC), we found significant declines in mean annual SSC and annual suspended sediment loads (SSLs) at 49 of 62 monitoring stations totaling −0.92 mg L−1 yr−1. At some stations decreases during the 20 years represent up to 50 % of the long-term average SSC. Significant decreases in SSC are associated with declining SSL despite an increase in sheet and rill erosion by almost 150 % derived from modeling due to changes in land use and management as well as rainfall erosivity. The contemporary suspended sediment loads of the Rhine at the German–Dutch border are approaching the natural base level of ∼1 Mt yr−1, which was reached by the Rhine during the mid-Holocene when the suspended sediment load was adjusted to the Holocene climatic conditions and before the onset of increased loads due to human-induced land use changes in the Rhine catchment. At this point we can only speculate regarding potential reasons for a decline in sediment transport in larger rivers despite erosion increase. We argue that increased sediment retention in upstream headwaters is presumably the major reason for declining SSC in the large river channels studied.

What Controls Suspended-Sediment Concentration and Export in Flooded Agricultural Tracts in the Sacramento–San Joaquin Delta?

We investigated wind-wave and suspended-sediment dynamics in Little Holland Tract and Liberty Island, two subsided former agricultural tracts in the Cache Slough complex in the northern Sacramento-San Joaquin Delta which were restored to tidal shallows to improve habitat. Turbidity, and thus suspended-sediment concentration (SSC), is important to habitat quality because some species of native fishes, including the Delta Smelt, are found preferentially in more turbid waters. Data from October 2015 to August 2016 show that average SSC was greater within Little Holland Tract than in the primary breach that connects the basin to surrounding channels: approximately twice as great at a shallower station farther from the breach and 15% greater at a deeper station closer to the breach. Suspended-sediment concentration within Little Holland Tract was directly related to wave shear stress and inversely related to water depth, based on linear regression. We used measurements of suspended-sediment flux (SSF) through the largest levee breaches to assess whether the enhanced SSC within Little Holland Tract is exported to surrounding waters, thus potentially increasing turbidity over a wider region. Cumulatively, sediment is exported through the Little Holland Tract breaches in winter and imported in summer, consistent with regional patterns in sediment flux, indicating that wind-wave re-suspension within the basin does not control sediment flux from Little Holland Tract on seasonal time-scales. Some sediment was exported during wind-wave events, and results show that sediment export is greater when primary breaches are located downwind of the basin rather than upwind.

Mechanisms of bar adjustments in the Jingjiang Reach of the Yangtze River in response to the operation of the Three Gorges Dam

Dams alter downstream river flow and sediment regimes, causing significant changes in river morphologies. The middle Yangtze River downstream of the Three Gorges Dam (TGD) has experienced rapid erosion in recent years, and the associated morphodynamic changes have negatively impacted the riverbank stability, navigation waterways and ecological functioning. Earlier studies have analyzed recent channel adjustments in the Yangtze River; however, our understanding of changes in the bar morphodynamics remains incomplete. In this study, we collected and analyzed flow and sediment data (1991–2016) and river bathymetry data (1975–2017) and investigated the mechanisms of bar adjustments along the Jingjiang Reach in the post-TGD period. The results indicate that most steady bars with a higher elevation and better vegetation coverage have experienced lateral erosion, while their elevations have remained stable overall. The unvegetated migrating bars with a lower elevation have experienced severe surficial erosion and area shrinkages. A new assessment method of the dominant discharge range on downstream bars is provided after dam closure. The dominant discharge range that determines bar adjustments corresponds to the flow stages between the submersion of migrating bars (∼12,500 m3/s) and the overtopping of vegetated steady bars (∼27,500 m3/s) in the Jingjiang Reach. A quantitative relationship exists between bar erosion in response to changed dominant discharge regimes and sediment decline. The significant bar erosion after the operation of the TGD can be attributed to the increased flow duration and the sharp decline in the suspended sediment concentration (SSC) of the dominant discharge range, and changes in SSCs play a primary role. The bar area will decrease by ∼0.003 km2 when the annual cumulative duration of the dominant discharge increases by one day and by ∼0.234 km2 when the annual average SSC of the dominant discharge decreases by 0.01 kg/m3. Furthermore, vegetation encroachment and colonization play a positive role in stabilizing bar morphologies and limiting surficial erosion, whereas vegetation soley cannot prevent the lateral erosion of steady bars. These findings suggest that multiple controls of discharge, sediment and vegetation on the evolution of fluvial bars and have important implications for river management and ecological evaluation in response to the operation of large dams.

Proglacial river sediment fluxes in the southeastern Tibetan Plateau: Mingyong Glacier in the Upper Mekong River

AbstractGlacial and proglacial erosion are important sediment sources in river basins. The retreat of many glaciers on the Tibetan Plateau has important implications on the supply of fresh water and sediment dynamics for downstream river basins. Despite the importance of water and sediment dynamics at these catchments, existing quantification of suspended sediment fluxes from glacial catchments on the Tibetan Plateau is limited due to poor accessibility and challenging environments. This study presents the results of in‐situ investigations of water discharge and suspended sediment fluxes from the Mingyong Glacier catchment in Yunnan, Southwest China, between August 2013 and July 2017. The results show that the variation in water discharge and suspended sediment was highly seasonal. The variation of average suspended sediment concentration was large—69 ± 45; 119 ± 104; and 94 ± 97 mg/L in 2013, 2015, and 2016, respectively. We estimate that the sediment yield from the Mingyong catchment was highly variable ranging from 1104 t/km2/year in 2013 to 2281 t/km2/year in 2016, with 65%–78% of the total annual sediment load occurring during the summer (June to August). These annual variations in sediment yield can be attributed largely to precipitation patterns and extreme melting events. This study has provided a benchmark dataset that can be used for further works that investigate the impact of climate change on sediment dynamics in glacierized catchments in the Tibetan Plateau. Subsequently, the study helps us to better understand the increasing sediment supply to the Upper Mekong River from glacierized headwater catchments.

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.

Simulation of Sediment Transport in River Ogunpa Using Winxpro Software

The numerical modelling of sediment transport of a river has long been developed. Rivers across Nigeria has interdicted researchers on modelling sediment transport. The environmental problem makes sediment transport study an important area among morphological mechanisms. This research studied the Simulation and modelling of sediment transport in River Ogunpa using WinXspro software. Suspended Sediment concentration was measured from November 2015 to June 2016 across the upstream, midstream and downstream of River Ogunpa using standard methods. The values of manning’s roughness factor used varied from 0.012 – 0.018. Sediment discharges (QS) were computed across the river using Acker and White approach. The average Suspended Sediment Concentration (SSC) across River Ogunpa varied from 2 – 64mg/L. The mathematical relationship for the sediment discharge value was QS = 64625Q + 12922. The estimated Qs varied from 0.00 – 23012kg/s. The variation in measured and validated discharge ranged between 0.95 – 4.19%. Sediment discharge across River Ogunpa was simulated using Winxpro software. The sediment discharge value can be a piece of useful information for dredging operation on River Ogunpa.

Roles of advection and sediment resuspension-settling in the turbidity maximum zone of the Changjiang Estuary, China

Understanding mechanisms of suspended sediment transport in estuaries is critical for predicting estuarine geomorphic evolution, managing navigation and fisheries, and monitoring the health of estuarine ecosystems. Yet monitoring suspended sediment transport in estuaries is challenging due to the influences of both riverine and marine processes, and traditional in situ observations cannot directly differentiate contributions of multiple physical processes (e.g., advection and resuspension/settling). Here we investigated the effects of advection and sediment resuspension/settling on suspended sediment transport within the turbidity maximum zone at the South Passage of the Changjiang Estuary. The Changjiang catchment is notable for its long history of intensive human activities including dam construction, which has caused significant geomorphic changes along the river and its estuary. We conducted synchronized hydrodynamic measurements at three monitoring stations along the river channel and imported time series of current velocity and suspended sediment concentration (SSC) at these stations into a simplified, one-dimensional box model to separate in situ SSC into advection- and resuspension/settling-induced components. Our results show that high SSC consisted of a background SSC of 0.93 kg m−3 maintained by advection that contributes to 97.9% of the total average SSC, and a fluctuating SSC controlled by sediment resuspension and settling. The background SSC was higher than the fluctuating SSC at all times, indicating that sediment advection was stronger than local sediment resuspension in the river channel. We compared time series of SSC components, bed-level change and suspended sediment transport rates in the South Passage with previous observations in the nearby subaqueous delta, and found that the South Passage of the Changjiang Estuary experienced net accretion during summer because of the relatively high background SSC maintained by advection of fluvial sediment. In contrast, the subaqueous delta experienced erosion due to strong local sediment resuspension and weak sediment delivery by advection. Our results highlight the spatial variability in the magnitudes of sediment advection and resuspension/settling as key drivers of geomorphic changes of deltas and estuarine channels in response to sediment reduction caused by upstream dam construction.

Evaluating suspended sediment and turbidity reduction projects in a glacially conditioned catchment through dynamic regression and fluvial process‐based modelling

AbstractElevated turbidity (Tn) and suspended sediment concentrations (SSC) during and following flood events can degrade water supply quality and aquatic ecosystem integrity. Streams draining glacially conditioned mountainous terrain, such as those in the Catskill Mountains of New York State, are particularly susceptible to high levels of Tn and SSC sourced from erosional contact with glacial‐related sediment. This study forwards a novel approach to evaluate the effectiveness of stream restoration best management practices (BMPs) meant to reduce stream Tn and SSC, and demonstrates the approach within the Stony Clove sub‐basin of the Catskills, a water supply source for New York City. The proposed approach is designed to isolate BMP effects from natural trends in Tn and SSC caused by trends in discharge and shifts in average Tn or SSC per unit discharge (Q) following large flood events. We develop Dynamic Linear Models (DLMs) to quantify how Tn‐Q and SSC‐Q relationships change over time at monitoring stations upstream and downstream of BMPs within the Stony Clove and in three other sub‐basins without BMPs, providing observational evidence of BMP effectiveness. A process‐based model, the River Erosion Model, is then developed to simulate natural, hydrology‐driven SSC‐Q dynamics in the Stony Clove sub‐basin (absent of BMP effects). We use DLMs to compare the modelled and observed SSC‐Q dynamics and isolate the influence of the BMPs. Results suggest that observed reductions in SSC and Tn in the Stony Clove sub‐basin have been driven by a combination of declining streamflow and the installed BMPs, confirming the utility of the BMPs for the monitored hydrologic conditions.

Influences of conservation measures on runoff and sediment yield in different intra-event-based flood regimes in the Chabagou watershed

The Loess Plateau in China has suffered severe soil erosion. To control soil erosion, extensive conservation measures aimed at redistributing rainfall, hindering flow velocity and intercepting sediment were implemented on the Loess Plateau. To accurately evaluate the combined effect of conservation measures in the Chabagou watershed, this study classified intra-event-based floods into four regimes via cluster and discriminant analyses. Regime A was characterized by short flood duration and low erosive energy, regime B was characterized by short flood duration and high erosive energy, regime C was characterized by long flood duration and low erosive energy, and regime D was characterized by long flood duration and high erosive energy. The results indicated that peak discharge (qp), runoff depth (H), mean discharge (qm), and runoff erosion power (E) decreased by 75.2%, 56.0%, 68.0% and 89.2%, respectively, in response to conservation measures. Moreover, area-specific sediment yield (SSY), average suspended sediment concentration (SCE), and maximum suspended sediment concentration (MSCE) decreased by 69.2%, 33.3% and 11.9%, respectively, due to conservation measures. The nonlinear regression analysis revealed a power function relationship between SSY and E in both the baseline (1961–1969) and measurement period (1971–1990) in all regimes. Conservation measures reduced sediment yield by not only reducing the runoff amount and soil erosion energy but also transforming the flood regime, for example, transforming a high-sediment-yield regime into a low-sediment-yield regime. Moreover, conservation measures altered the SSY-E relationship in regime A, whereas no obvious difference in regime B or C/D was observed between the measurement period and the baseline period. This study provides a better understanding of the mechanism of runoff regulation and the sediment yield reduction under comprehensive conservation measures in a small watershed on the Chinese Loess Plateau.

Sediment transport, turbidity, and dissolved oxygen responses to annual streambed drawdowns for downstream fish passage in a flood control reservoir

Sediment transport, turbidity, and dissolved oxygen were evaluated during six consecutive water years (2013–2018) of drawdowns of a flood control reservoir in the upper Willamette Valley, Oregon, USA. The drawdowns were conducted to allow volitional passage of endangered juvenile chinook salmon through the dam's regulating outlets by lowering the reservoir elevation to a point where the historical streambed was exposed and transported water and sediment through the reservoir dam. Sediment loads during the drawdown were highest in the first year of monitoring, with a computed value of 40,200 metric tons over a 5-day drawdown, followed by 5 years of lower sediment loads and lower sediment transport rates, suggesting that much of the stored sediment within the reservoir thalweg was transported out of the reservoir in the early years of the consecutive drawdowns. Suspended sediment concentrations (SSC) computed using turbidity and streamflow data resulted in maximum SSC at the onset of the drawdowns, with the highest computed values occurring during the water year 2017 drawdown at 17,500 mg/L (turbidity = 2,990 FNU), and average drawdown SSC values ranging from 654 to 3,950 mg/L for the six years of monitoring. Computed SSC were on the lower range of concentrations that could be harmful to out-migrating juvenile salmon published in other studies. High amounts of particulate organic matter and sand-sized material in drawdown SSC samples affected relations between turbidity and SSC, requiring the use of multiple surrogate regression models over short time frames. Dissolved oxygen minimum values were recorded in two of the monitoring years, with a minimum value of 0.71 and 3.4 mg/L recorded at the onset of the drawdowns in water years 2016 and 2018, respectively. Dissolved oxygen values below 4 mg/L lasted for 1 h, suggesting a rapidly expressed chemical oxygen demand. The response of suspended sediment loads and SSC highlight the site-specific nature of reservoir drawdowns, and the need for evaluation of expected sediment responses for drawdowns being considered at other locations.

Tidal Asymmetry in Ocean-Boundary Flux and In-Estuary Trapping of Suspended Sediment Following Watershed Storms: San Francisco Estuary, California, USA

Suspended-sediment flux at the ocean boundary of the San Francisco Estuary—the Golden Gate—was measured over a tidal cycle following peak watershed runoff from storms to the estuary in two successive years to investigate sediment transport through the estuary. Observations were repeated during low-runoff conditions, for a total of three field campaigns. Boat-based measurements of velocity and acoustic backscatter were used to calculate water and suspended-sediment flux at a location 1 km landward of the Golden Gate. Suspended-sediment concentration (SSC) and salinity data from up-estuary sensors were used to track watershed-sourced sediment plumes through the estuary. Estimates of suspended-sediment load from the watershed and net suspended-sediment flux for one up-estuary subembayment were used to infer in-estuary trapping of sediment. For both post-storm field campaigns, observations at the ocean boundary were conducted on the receding limb of the watershed hydrograph. At the ocean boundary, peak instantaneous suspended-sediment flux was tidally asymmetric and was greater on flood tides than on ebb tides for all three field campaigns, due to higher average SSC in the cross-section on flood tides. Shear-induced sediment resuspension was greater on flood tides and suggests the presence of an erodible pool outside the estuary. The storms in 2016 led to less export of discharge and sediment from the watershed and greater sediment trapping within one up-estuary subembayment compared to that observed in 2017. Results suggest that substantial trapping of watershed sediments occurred during both storm events, likely due to the formation of estuarine turbidity maxima (ETM) at different locations in the estuary. ETM locations were forced nearer the ocean boundary in 2017. Additional measurements and modeling are required to quantify the long-term sediment flux at the Golden Gate.

Remote Sensing of Spatial-Temporal Distribution of Suspended Sediment Concentrationin East Dongting Lake, China

Three Landsat 8 OLI scenes acquired from 2016 to 2017 were used to estimate the suspended sediment concentration (SSC) in East Dongting Lake, China. The results show that SSC in East Dongting Lake is generally low, only the SSC of sediment transport channel connecting South Dongting Lake is significantly high, and that in the west of the lake is relatively small. From 2016 to 2017, the average SSC in the eastern channel was33-120 mg/L, the average SSC in the north branch of the west area was15-43 mg/L, and the average SSC in the south branchof the west area was17-46 mg/L. Under the influence of the dilute water of the Yangtze River in flood season, the overall SSC in the lake area is lower than that in non-flood season, and the average SSCis within 40 mg/L. The variation law of SSC in lake area revealed by remote sensing is consistent with the measured data, which verifies the feasibility of this method and provides a new perspective for dynamic tracking and mastering the distribution law and variation trend of suspended sediment in East Dongting Lake.

Insights on the Impacts of Hydroclimatic Extremes and Anthropogenic Activities on Sediment Yield of a River Basin

Streamflow and sediment flux variations in a mountain river basin directly affect the downstream biodiversity and ecological processes. Precipitation is expected to be one of the main drivers of these variations in the Himalayas. However, such relations have not been explored for the mountain river basin, Nepal. This paper explores the variation in streamflow and sediment flux from 2006 to 2019 in central Nepal’s Kali Gandaki River basin and correlates them to precipitation indices computed from 77 stations across the basin. Nine precipitation indices and four other ratio-based indices are used for comparison. Percentage contributions of maximum 1-day, consecutive 3-day, 5-day and 7-day precipitation to the annual precipitation provide information on the severity of precipitation extremeness. We found that maximum suspended sediment concentration had a significant positive correlation with the maximum consecutive 3-day precipitation. In contrast, average suspended sediment concentration had significant positive correlations with all ratio-based precipitation indices. The existing sediment erosion trend, driven by the amount, intensity, and frequency of extreme precipitation, demands urgency in sediment source management on the Nepal Himalaya’s mountain slopes. The increment in extreme sediment transports partially resulted from anthropogenic interventions, especially landslides triggered by poorly-constructed roads, and the changing nature of extreme precipitation driven by climate variability.