Suspended Sediment Data Research Articles

The Ballast Effect of Terrigenous Lithogenic Particles From Rivers and Its Influence on POC Fluxes in the Ocean

AbstractLithogenic materials such as terrigenous lithogenic particles (TLP) can efficiently promote the formation and sinking of mineral‐associated marine organic matter, acting as important ballast and potentially playing an important role in the global carbon cycle. To assess the influence of TLP on fluxes of particulate organic carbon (POC) and other biogeochemical cycles, we construct TLP forcing fields based on global riverine suspended sediment data and then apply them to the Community Earth System Model, version 2 (CESM2) modified with the TLP ballasting effect term. Simulations forced by different concentrations of TLP transported in the surface ocean or along the bottom of continental shelves and slopes are conducted. When the TLP transports seaward along the bottom, simulated POC fluxes at 100 and 2,000 m decrease about 11% and 19%, respectively, for the global ocean, and about 9% and 12%, respectively, for the oceanic regions of continental margins. The initial abiotic ballast processes triggered by TLP input increase POC fluxes, causing additional removal and burial of dissolved iron in continental margins. This further enhances the accumulation of macronutrients in the upwelling regions and their advection transport to neighboring subtropical gyres, thus altering regional productivity when simulations reach quasi‐equilibrium. When consider the impacts of TLP in simulations, the simulated POC flux exhibits an increase in subtropical gyres but a decrease in tropical Pacific and mid‐high latitude regions. The present work highlights the importance of TLP in global biogeochemical cycles, suggesting that the amount of carbon sequestration might be overestimated without TLP in models.

Suspended Sediment Source and Transport Mechanisms in a Himalayan River

The process of estimating sediment load has been a daunting issue in hydraulics and the water resource field. Several methods exist for predicting the sediment load in a catchment or river, but the majority of these methods are empirical and depend on the specific location where they are used. Understanding the underlying mechanism of sediment generation and its transport in connection with precipitation, topography, and subsurface conditions to characterize its process is helpful for determining the sediment load in a river. For this purpose, we analyzed the daily suspended sediment data measured for 8 years at the headworks of the Kabeli A hydropower project in the Kabeli River, which originates from the Himalayan region. The analyses show that the suspended sediment concentration (SSC) varies in an orderly manner over time and asynchronously between seasons with respect to the river discharge. Clockwise hysteresis is observed in the yearly plots between the SSC and river discharge. The hysteresis becomes narrower when compared with the direct runoff obtained from a digital filtering algorithm and, even more so with the direct runoff from the hydrological model SWAT. The analysis shows that the sediment concentration is controlled not only by the total discharge in the river but also by the contribution of ground water to the river discharge, indicating that the total discharge alone cannot reflect the seasonal variation in SSC. It is inferred that the river is supply-limited and the hillslope is transport-limited with respect to sediment sources. The SWAT model suggests that the base flow contribution to the total river discharge is 78%. Here, we present a method for constructing the suspended sediment rating curve by comparing the direct runoff with the sediment concentration. The deduced sediment rating curve captures 84.51% of the total sediment load over the study period in the Kabeli River. This method may potentially be used in similar catchments with supply-limited rivers and transport-limited hillslopes.

Controls on Sediment Transport From a Glacierized Catchment in the Swiss Alps Established Through Inverse Modeling of Geomorphic Processes

AbstractAs climate warms, hydrology and geomorphology in glacierized catchments are evolving, changing sediment export from these catchments, thus impacting downstream ecosystems and communities. Currently, much uncertainty persists regarding interactions among geomorphic processes that evacuate sediment from glacierized catchments. Here, we present a catchment‐scale numerical model of subglacial and proglacial sediment transport with debris meltout processes. We apply the model in a Monte Carlo framework to suspended sediment data collected over 2014–2020 from the Fieschertal catchment in the Swiss Alps, assessing possible combinations of geomorphic processes responsible for suspended sediment discharge. The ensemble of model outputs quantifies the interaction of different geomorphic processes, including the trade‐off between bedrock erosion and sediment previously stored in the catchment. Model runs suggest that, at some periods, up to 20% of the sediment leaving the glacier is deposited in the proglacial area relative to the catchment's total sediment discharge. This shows that the model captures the proglacial area change from sediment source to sediment sink in different hydrological and glaciological conditions. Furthermore, the findings highlight the impact of glacier retreat on the catchment's sediment dynamics, which both reduces the proglacial area's slope and introduces additional sediment to the proglacial area through debris meltout. The model outputs and the parameters quantify the interaction among geomorphic processes, yielding key insights into the drivers of sediment transport in alpine regions. The implications of these interactions are discussed in the context of interpreting processes responsible for controlling erosion rates from glacierized regions and modeling sediment transport in glacierized catchments.

Riverine Microplastic Quantification: A Novel Approach Integrating Satellite Images, Neural Network, and Suspended Sediment Data as a Proxy

Rivers transport terrestrial microplastics (MP) to the marine system, demanding cost-effective and frequent monitoring, which is attainable through remote sensing. This study aims to develop and test microplastic concentration (MPC) models directly by satellite images and indirectly through suspended sediment concentration (SSC) as a proxy employing a neural network algorithm. These models relied upon high spatial (26 sites) and temporal (198 samples) SSC and MPC data in the Tisza River, along with optical and active sensor reflectance/backscattering. A feedforward MLP neural network was used to calibrate and validate the direct models employing k-fold cross-validation (five data folds) and the Optuna library for hyperparameter optimization. The spatiotemporal generalization capability of the developed models was assessed under various hydrological scenarios. The findings revealed that hydrology fundamentally influences the SSC and MPC. The indirect estimation method of MPC using SSC as a proxy demonstrated higher accuracy (R2 = 0.17–0.88) than the direct method (R2 = 0–0.2), due to the limitations of satellite sensors to directly estimate the very low MPCs in rivers. However, the estimation accuracy of the indirect method varied with lower accuracy (R2 = 0.17, RMSE = 12.9 item/m3 and MAE = 9.4 item/m3) during low stages and very high (R2 = 0.88, RMSE = 7.8 item/m3 and MAE = 10.8 item/m3) during floods. The worst estimates were achieved based on Sentinel-1. Although the accuracy of the MPC models is moderate, it still has practical applicability, especially during floods and employing proxy models. This study is one of the very initial attempts towards MPC quantification, thus more studies incorporating denser spatiotemporal data, additional water quality parameters, and surface roughness data are warranted to improve the estimation accuracy.

Sediment sink-source transitions in the middle and lower reaches of the Yangtze River estuary

Human activities have severely altered the Yangtze River (Changjiang) Estuary’s pattern, beach-trough shape, and delta in recent decades. Existing research has studied the sediment “source–sink” relationship between the Yangtze River mainstream and estuary using gauged suspended sediment data. This method overestimated the decline in the suspended sediment concentration in the estuary, as riverbed scouring or deposition have not filtered out amounts related to sand mining or waterway dredging. This study took the Yangtze River Basin as the research subject and used spot observed runoff, sediment, and riverbed topography data gathered between 1980 and 2020 to analyze the sediment trapping effect of the Three Gorges Reservoir (TGR) and study the “source-sink” relationship of suspended sediment transport in the middle-lower reaches of the Yangtze River. The Yangtze Estuary Delta is dominated by scouring, which shows an increasing trend. During the operation of the TGR (2003–2020), the suspended sediment load entering the Yangtze Estuary from the Yangtze River mainstream decreased by 68% compared to the amount prior to operation commencing (1960–2002). However, studies have found that there are still large amounts of bottom sand with riverbed load movement characteristics after the operation of the Three Gorges Project and that riverbed souring of the downstream dam has increased the degree of deposition recharge. The amount of sediment in the Yangtze Estuary decreased between 2003 and 2018 compared with the period 1981 to 2002, with an annual decrease of 16.9%. The cumulative deposition of the Yichang–Datong and Datong–Xuliujing reaches from 1981–2002 was 3.13×108 m3 and 1.60×108 m3, respectively, and cumulative scouring from 2002–2018 was 28.52×108 m3 and 16.01×108 m3. This study concluded that even if the suspended sediment load entering the estuary decreased significantly after the operation of the TGR, the riverbed in the middle-lower reaches of the Yangtze River has gradually transformed from the “sink” of sediment deposition to the “source” of sediment supply.

The Importance of Storm Surge for Sediment Delivery to Microtidal Marshes

AbstractStorm surge has the potential to significantly increase suspended sediment flux to microtidal marshes. However, the overall effects of storm surge on microtidal marsh deposition have not been well quantified, with most modeling studies focusing on regular (astronomical) tidal flooding. Here we applied the Delft3D model to a microtidal bay‐marsh complex in Hog Bay, Virginia to quantify the contributions of storm surge to marsh deposition. We validated the model using spatially distributed hydrodynamic and suspended sediment data collected from the site and ran model simulations under different storm surge conditions with/without storm‐driven water level changes. Our results show that episodic storm surge events occurred 5% of the time at our study site, but contributed 40% of marsh deposition during 2009–2020. Our simulations illustrate that while wind‐driven waves control sediment resuspension on tidal flats, marsh deposition during storms was largely determined by tidal inundation associated with storm‐driven water levels. A moderate storm surge event can double sediment flux to most marshes around the bay and deliver more sediment to the marsh interior compared to simulations that include wind waves but not storm surge variations in water levels. Simulations of bay and marsh response to different storm surge events with varying magnitude of storm surge intensity reveal that total marsh deposition around the bay increased linearly with storm surge intensity, suggesting that future changes to storm magnitude and/or frequency would have significant implications for sediment supply to marshes at our study site.

Wave attenuation potential, sediment properties and mangrove growth dynamics data over Guyana's intertidal mudflats: assessing the potential of mangrove restoration works

Abstract. Coastal mangroves, thriving at the interface between land and sea, provide robust flood risk reduction. Projected increases in the frequency and magnitude of climate impact drivers such as sea level rise and wind and wave climatology reinforce the need to optimize the design and functionality of coastal protection works to increase resilience. Doing so effectively requires a sound understanding of the local coastal system. However, data availability particularly at muddy coasts remains a pronounced problem. As such, this paper captures a unique dataset for the Guyana coastline and focuses on relations between vegetation (mangrove) density, wave attenuation rates and sediment characteristics. These processes were studied along a cross-shore transect with mangroves fringing the coastline of Guyana. The data are publicly available at the 4TU Centre for Research Data (4TU.ResearchData) via https://doi.org/10.4121/c.5715269 (Best et al., 2022) where the collection Advancing Resilience Measures for Vegetated Coastline (ARM4VEG), Guyana, comprises of six key datasets. Suspended sediment concentrations typically exceeded 1 g L−1 with a maximum of 60 g L−1, implying that we measured merely fluid-mud conditions across a 1 m depth. Time series of wind waves and fluid-mud density variations, recorded simultaneously with tide elevation and suspended sediment data, indicate that wave–fluid-mud interactions in the nearshore may be largely responsible for the accumulation of fine, muddy sediment along the coast. Sediment properties reveal a consolidated underlying bed layer. Vegetation coverage densities in the Avicennia-dominated forest were determined across the vertical with maximum values over the first 20 cm from the bed due to the roots and pneumatophores. Generalized total wave attenuation rates in the forest and along the mudflat were between 0.002–0.0032 m−1 and 0.0003–0.0004 m−1 respectively. Both the mangroves and the mudflats have a high wave-damping capacity. The wave attenuation in the mangroves is presumably dominated by energy losses due to vegetation drag, since wave attenuation due to bottom friction and viscous dissipation on the bare mudflats is significantly lower than wave dissipation inside the mangrove vegetation. Data collected corroborate the coastal defence function of mangroves by quantifying their contribution to wave attenuation and sediment trapping. The explicit linking of these properties to vegetation structure facilitates modelling studies investigating the mechanisms determining the coastal defence capacities of mangroves.

Extreme rainstorms drive exceptional organic carbon export from forested humid-tropical rivers in Puerto Rico

Extreme rainfall events in the humid-tropical Luquillo Mountains, Puerto Rico export the bulk of suspended sediment and particulate organic carbon. Using 25 years of river carbon and suspended sediment data, which targeted hurricanes and other large rainstorms, we estimated biogenic particulate organic carbon yields of 65 ± 16 tC km−2 yr−1 for the Icacos and 17.7 ± 5.1 tC km−2 yr−1 for the Mameyes rivers. These granitic and volcaniclastic catchments function as substantial atmospheric carbon-dioxide sinks, largely through export of river biogenic particulate organic carbon during extreme rainstorms. Compared to other regions, these high biogenic particulate organic carbon yields are accompanied by lower suspended sediment yields. Accordingly, particulate organic carbon export from these catchments is underpredicted by previous yield relationships, which are derived mainly from catchments with easily erodible sedimentary rocks. Therefore, rivers that drain petrogenic-carbon-poor bedrock require separate accounting to estimate their contributions to the geological carbon cycle.

Impacts of agricultural expansion on floodplain water and sediment budgets in the Mekong River

In this paper, we address the impact of agricultural expansion on hydrological patterns of water and sediment budget in one of the largest floodplains along the Cambodian Mekong since the 1980s, using field (water level, discharge, sediment, rainfall, and groundwater level) and remote sensing (land use, surface suspended sediment) data, and numerical simulations. Specifically, while both the surface suspended sediment concentration and water level in the Mekong River around Kampong Cham and Neak Luong decreased, the floodplain seasonal water storage increased. In addition, the rate of sediment input from the river to the floodplain was almost constant throughout the studied period. The investigation of the floodplain’s annual sediment budget, however, reveals a significant drop during the analyzed period, mainly due to the decreased sediment trapping rate (66% in the 1980s to 46% in the 2010s). Currently, a good amount of sediment bypass the floodplain and return back to the river. The observed hydrological patterns in the floodplain could have been triggered by the agricultural expansion that has increased surface erodibility (due to removals of primary vegetation) and lowered the land surface elevation (due to groundwater extraction). Despite the well-known impacts of the hydropower dams on the Mekong Delta hydrological conditions, particularly the reduction of sediment reaching the delta due to sediment trapping by dam reservoirs, our observations point to new and more localized driving factors of sediment deficit in floodplain: agricultural expansion. Finally, we used 2D hydrodynamic simulation (Telemac-2D) to visualize the processes of water routing and sedimentation in the floodplain accounting for land cover change since the 1980s. The floodplain hydrology reported in this paper is an unexplored environmental consequence of agricultural expansion in the lower Mekong. Geomorphologically, this study presents a peculiar case of floodplain showing how agricultural expansion can diminish the role of a floodplain as a sediment sink through decreasing the trapping rate.

Monitoring of Suspended Sediments in a Tropical Forested Landscape With Citizen Science

Catchments are complex systems, which require regular monitoring of hydro-chemical parameters in space and time to provide comprehensive datasets. These are needed to characterize catchment behavior on a local level, make future projections based on models, implement mitigation measures and meet policy targets. However, many developing countries lack a good infrastructure for hydrological monitoring since its establishment is costly and the required resources are often not available. To overcome such challenges in data scarce regions like Kenya, a participatory citizen science approach can be a promising strategy for monitoring water resources. This study evaluates the potential of using a contributory citizen science approach to explore spatiotemporal turbidity and suspended sediment dynamics in the Sondu-Miriu river basin, western Kenya. A group of 19 citizen scientists was trained to monitor turbidity using turbidity tubes and water levels with water level gauges in six nested subcatchments of the Sondu-Miriu river basin. Over the course of the project, a total of 37 citizen scientists participated and contributed to the overall dataset of turbidity. The sampling effort and data contribution varied from year to year and among participants with the majority of the data (72%) originating from 8 (22%) citizen scientists. Comparison between citizen-scientist collected suspended sediment data and measurements from automated stations showed high correlation (R2 > 0.9) which demonstrates that data collected by citizen scientists can be comparable to data collected using expensive monitoring equipment. However, there was reduced precision of the measurements of suspended sediment concentrations at low and high levels attributed largely to the detection limitations of the turbidity tubes and citizen scientists not capturing major sediment export events. Suspended sediment concentrations were significantly higher downstream (109 ± 94 mg L−1), a subcatchment dominated by agriculture and rangeland with low forest vegetation cover, as compared to a subcatchment with high forest cover (50 ± 24.7 mg L−1). This finding indicates that forest cover is a key landscape feature to control suspended sediment concentrations in the region. Future citizen science projects should focus on motivation and engagement strategies and the application of robust methods with improved detection limits and resolution to advance hydrological monitoring.

Quantifying Seasonal Seagrass Effects on Flow and Sediment Dynamics in a Back‐Barrier Bay

AbstractSeagrass growth and senescence exert a strong influence on flow structure and sediment transport processes in coastal environments. However, most previous studies of seasonal seagrass effects either focused on small‐scale field measurements or did not fully resolve the synergistic effects of flow‐wave‐vegetation‐sediment interaction at a meadow scale. In this study, we applied a coupled Delft3D‐FLOW and SWAN model that included effects of seagrass on flow, waves, and sediment resuspension in a shallow coastal bay to quantify seasonal seagrass impacts on bay dynamics. The model was extensively validated using seasonal field hydrodynamic and suspended sediment data within a seagrass meadow and a nearby unvegetated site. Our results show that seagrass meadows significantly attenuated flow (60%) and waves (20%) and reduced suspended sediment concentration (85%) during summer when its density reached a maximum. Probability density distributions of combined wave‐current bed shear stress within the seagrass meadow indicate that significant reductions in sediment resuspension during summer were mainly caused by flow retardation rather than wave attenuation. Although low‐density seagrass in winter resulted in much smaller reductions in flow and waves compared with summer meadows, small changes in winter seagrass density resulted in large differences in the magnitude of attenuation of flow and shear stress. Similarly, while high seagrass densities effectively trapped sediment during summer, small changes in winter density resulted in strong changes in net sediment flux into/out of the meadow. At our study site, low seagrass densities provided significant reductions in wintertime sediment loss compared to losses associated with completely unvegetated conditions.

Multivariate event time series analysis using hydrological and suspended sediment data

Hydrological storm events are a primary driver for transporting water quality constituents such as suspended sediments and nutrients. Analyzing the concentration (C) of these water quality constituents in response to river discharge (Q), particularly when monitored at high temporal resolution during a hydrological event, helps to characterize the dynamics and flux of such constituents. A conventional approach to storm event analysis is to reduce C-Q time series to two-dimensional (2-D) hysteresis loops and analyze these 2-D patterns. While informative, this hysteresis loop approach has limitations because projecting the C-Q time series onto a 2-D plane obscures detail (e.g., temporal variation) associated with the C-Q relationships. In this paper, we address this limitation using a multivariate event time series (METS) clustering approach that is validated using synthetically generated event times series. The METS clustering is then applied to river discharge and suspended sediment data (acquired through turbidity-based monitoring) from six watersheds in the Lake Champlain Basin located in the northeastern United States, and results in identifying four common types of hydrological water quality events. Statistical analysis on the events partitioned by both methods (METS clustering and 2-D hysteresis classification) helped identify hydrometeorlogical features of common event types. In addition, the METS and hysteresis analysis were simultaneously applied to a regional Vermont dataset to highlight the complimentary nature of using them in tandem for hydrological event analysis.

Factors controlling discharge-suspended sediment hysteresis in karst basins, southwest China: Implications for sediment management

Assessment of river sediment sources, transport, and controlling factors in karst regions is critical to the development of effective soil erosion and sediment management strategies. This 2-year study used a hysteresis index of discharge and suspended sediment data, combined with the basin's inherent geological characteristics, to assess sediment sources and hydrological connectivity at six sites in an agricultural karst watershed of southwest China. Principal component analysis was further used to describe sediment control by rainfall (antecedent precipitation, precipitation duration, amount, and intensity) and stream hydrology (maximum and total discharge, flood intensity, and runoff coefficient). Results indicate that the annual sediment yield (0.8–6.6 Mg km2 a−1) is low and sediment transport in karst areas is dependent on continuous rainfall. Ground substance component (GSC) and geomorphology characteristics determine the hydrology connectivity and sediment availability. In a sub-basin (CQ) marked by a peak cluster depression with good vegetation, sediment mainly comes from the paddy land, and the surface stream shows clockwise hysteresis. During storms, surface water enters the underground system from sinkholes, resulting in Figure-8 hysteresis with multiple sediment peaks. In another predominantly agricultural sub-basin (HTP), the thick layers of paddy and loess soil surrounding the river channel provide an accessible sediment source, resulting in clockwise hysteresis. The surface outlet of the watershed (HZ) is affected by an upstream reservoir, but bank erosion and sediment activation are evident during storms. The subsurface outlet (MSK) of the watershed shows more anti-clockwise hysteresis due to the activation of the conduit network in the wet season, surface water draining into the underground river and the distant sediment sources. This study shows that discharge-suspended sediment concentration hysteresis analysis can provide a reference for soil and water conservation decision-making in karst areas.

Entrainment and suspension of sand and gravel

Abstract. The entrainment and suspension of sand and gravel are important for the evolution of rivers, deltas, coastal areas, and submarine fans. The prediction of a vertical profile of suspended sediment concentration typically consists of assessing (1) the concentration near the bed using an entrainment relation and (2) the upward vertical distribution of sediment in the water column. Considerable uncertainty exists in regard to both of these steps, especially the near-bed concentration. Most entrainment relations have been tested against limited grain-size-specific data, and no relations have been evaluated for gravel suspension, which can be important in bedrock and mountain rivers. To address these issues, we compiled a database with suspended sediment data from natural rivers and flume experiments, taking advantage of the increasing availability of high-resolution grain size measurements. We evaluated 12 dimensionless parameters that may determine entrainment and suspension relations and applied multivariate regression analysis. A best-fit two-parameter equation (r2=0.79) shows that near-bed entrainment, evaluated at 10 % of the flow depth, decreases with the ratio of settling velocity to skin-friction shear velocity (wsi/u∗skin), as in previous relations, and increases with Froude number (Fr), possibly due to its role in determining bedload-layer concentrations. We used the Rouse equation to predict concentration upward from the reference level and evaluated the coefficient βi, which accounts for differences in the turbulent diffusivity of sediment from the parabolic eddy viscosity model used in the Rouse derivation. The best-fit relation for βi (r2=0.40) indicates greater relative sediment diffusivities for rivers with greater flow resistance, possibly due to bedform-induced turbulence, and larger wsi/u∗skin; the latter dependence is nonlinear and therefore different from standard Rouse theory. In addition, we used empirical relations for gravel saltation to show that our relation for near-bed concentration also provides good predictions for coarse-grained sediment. The new relations extend the calibrated parameter space over a wider range in sediment sizes and flow conditions compared to previous work and result in 95 % of concentration data throughout the water column predicted within a factor of 9.

Analyzing the streamflow-sediment links of three major river basins in India using multifractal theory

This study investigated the cross-correlation between daily streamflow and total suspended sediment (TSS) data of 65 gauging stations located in three major river basins in a multifractal perspective. The novel Multifractal Cross Correlation Analysis (MFCCA) method of cross correlation studies is used to analyze the streamflow-sediment links of Krishna, Godavari and Mahanadi basins. The results showed that for the records of a particular station, the joint persistence of streamflow and TSS is approximately the mean of the persistence of individual series. The streamflow displayed higher persistence than TSS in 60 % of the stations while in majority of stations of Godavari basin the trend was opposite. The annual cross correlation is higher than overall cross correlation in majority of stations but at these time scales strength of their association differs with river basin.

A first-order geochemical budget for suspended sediment discharge to the Bay of Bengal from the Ganges-Brahmaputra river system

The Ganges-Brahmaputra-Meghna (G-B) river system transports >1 × 109 t/yr of sediment, with an estimated 0.7 × 109 t/yr reaching the Bay of Bengal (BoB). This discharge represents a major input of sediment and associated elements to the global ocean, but quantification of the sediment-element mass reaching the BoB has yet to be fully explored. Published geochemical and suspended sediment data are used to calculate a first-order budget for the modern sediment supply of geochemical elements to the BoB. River profile bulk sediment-element concentrations are calculated based on suspended sediment and element measurements taken in the Ganges and Brahmaputra rivers. A Monte Carlo analysis is applied to account for variable sediment and geochemical contributions from each river. Results show that on average, the G-B system contributes ~5% of the global riverine discharge of solid-phase elements from sediment to the oceans. G-B sediments transport >10% of the global element supply of Hf and Zr. For others, like As and Cu, contributions from the G-B are <5%. Results also show that sediment reaching the BoB is relatively enriched in Hf, Zr, Th, REEs, Sn, and Bi, and majorly depleted in Na and Sr compared to UCC elemental concentrations. While limited by data availability and necessary simplifying assumptions, this study nevertheless provides a reasonable first-order budget for the modern discharge of solid-phase elements to the BoB. Insights from this work are significant for understanding the role of the G-B river system in global elemental cycling, and for providing a basis of comparison for future sediment-element discharge in light of rapid environmental change taking place in the region.

Response of nephelometric turbidity to hydrodynamic particle size of fine suspended sediment

Turbidity is used as a surrogate for suspended sediment concentration (SSC), and as a regulatory tool for indicating land use disturbance and environmental protection. Turbidity relates linearly to suspended material, however, can show non-linear responses to particulate organic matter (POM), concomitant with changes in particle size distribution (PSD). In the paper the influence of ultra-fine particulate matter (UFPM) on specific turbidity and its association with POM in suspended sediment are shown for alpine rivers in the Southern Alps of New Zealand. The approach was two-fold: a field-based investigation of the relations between SSC, POM, and turbidity sampled during event flow; and experimental work on hydrodynamic particle size effects on SSC, POM, PSD, and turbidity. Specific turbidity changes over event flow and are sensitive to increasing proportional amounts of sand, UFPM, and POM in suspension. Furthermore, the UFPM is the size fraction (<6 μm) where POM increases. The implications of the current study are that the slopes of turbidity-SSC relations are undesirable in locations that may be dominated by cyclic release of POM or distinct pulses of fine-grained material. At locations where the turbidity-SSC slopes approximate 2, the POM proportion is usually <10% of the total suspended load. However, when turbidity-SSC slopes are <1 this is likely caused by high amounts of side-scatter from UFPM concomitant with higher proportions of POM. Thus, the use of turbidity as a proxy for determining SSC may have serious consequences for the measurement of representative suspended sediment data, particularly in locations where POM may be a significant contributor to overall suspended load.

Characterizing braided rivers in two nested watersheds in the Source Region of the Yangtze River on the Qinghai-Tibet Plateau

Large and complex braided channels (>1 km wide) in the Source Region of the Yangtze River (SRYR) on the Qinghai-Tibet Plateau, China, are characterized by a unique alpine environment with very high elevations and glacier sources. Yet, little is known about the morphological complexity and changes of large braided channels owing to the scarcity of hydrological and topographical data. In this study, we provided the first insight into these issues using compiled long-term (several decades) climatic and suspended sediment data, and Landsat images of braided channels in two nested headwater watersheds (i.e., Tuotuo and Tongtian river watersheds) in the SRYR. Analysis of annual and monthly mean temperature, precipitation, and water discharges illustrated the nature of the hydrological regime and their response to glacier melting in the two watersheds. Examination of annual mean suspended sediment loads and concentrations (C) led to two sediment rating curves and generally similar C values between the two watersheds. Among a suite of morphological indices for braided rivers, braiding intensity may be quantified using four different ones. After comparing the indices, we showed that they had similar ability of characterizing braiding patterns and adopted branch count index (BIT3) for further analysis. We found that BIT3 was linearly correlated with active valley width (WR) during both flood and dry seasons for the 17 selected braided reaches distributed within the two nested watersheds. In addition, braiding density, defined as the ratio of BIT3 to WR, remained approximately unchanged irrespective of elevations and locations of these reaches, indicating clearly that morphological structures of braided channels are spatially uniform in the SRYR. Along the main channels of the two nested watersheds over the entire study period, BIT3 always reached the maximum as RW, defined as the ratio of water body to exposed active riverbed area, was around 0.43. Furthermore, RW was always positively correlated with the associated water discharge (Q). Variable degrees of braiding adjustment in response to changes of hydrological regime were represented by hysteresis loops between BIT3 and Q. These results revealed relatively homogeneous morphologic properties of braided channels across the two nested watersheds and provided an important benchmark for future morphodynamic research on braided rivers in the SRYR.

Seasonal, Spring‐Neap, and Tidal Variation in Cohesive Sediment Transport Parameters in Estuarine Shallows

AbstractNumerical models for predicting sediment concentrations and transport rely on parameters such as settling velocity and bed erodibility that describe sediment characteristics, yet these parameters are rarely probed directly. We investigated temporal and spatial variation in sediment parameters in the shallows of San Pablo Bay, CA. Flow, turbulence, and suspended sediment data were measured at sites located at 1 and 2 m below mean lower low water (MLLW) from November 2013 through April 2015, supplemented by monthlong periods in 2011, 2012, and 2016. Maximum current velocities were 0.40–0.47 m s-1 at these depths; the strongest currents decreased to 0.27–0.34 m s-1 during neap periods. Winters 2013–2014 and 2014–2015 experienced strong drought conditions, limiting the potential for seasonal impact on sediment conditions during this experiment. Despite this, the more storm‐influenced site showed clear changes during the winter: the roughness parameter decreased from 10−4 to 10−5 m, from hydrodynamically rough to smooth conditions, and bed erodibility increased by an order of magnitude. Median settling velocity was 2.05·10−4 m s-1; it varied twofold within a tidal cycle, decreasing as current velocity grew during flood and ebb. This tidal control on floc size affected settling velocity on the spring‐neap timescale, possibly driving a spring‐neap oscillation in erodibility. Our findings highlight variation in sediment dynamics that is commonly ignored in numerical models and the need for field observations to ground truth ongoing modeling efforts.

New Approach for Sediment Yield Forecasting with a Two-Phase Feedforward Neuron Network-Particle Swarm Optimization Model Integrated with the Gravitational Search Algorithm

Predicting sediment yield is an important task for decision-makers in environmental monitoring and water management since the benefits of applying non-linear, artificial intelligence (AI) models for optimal prediction can be far reaching in real-life decision support systems. AI-based models are considered to be favorable predictive tools since the nonlinear nature of suspended sediment data series warrants the utilization of nonlinear predictive methods for feature extraction, and for accurate simulation of suspended sediment load. In this study, Artificial Neural Network (ANN) approaches are employed to estimate the monthly sediment load where the two-phase Feed-forward Neuron Network Particle Swarm Optimization Gravitational Search Algorithm (FNN-PSOGSA) is developed, and then evaluated in respect to 3 distinct algorithms: the Adaptive Neuro-Fuzzy Inference System (ANFIS), Feed-forward Neuron Network (FNN) and the single-phase Feed-forward Neuron Network Particle Swarm Optimization (FNN-PSO). The study is carried out using the monthly rainfall, runoff and sediment data spanning a 10 year period (2000–2009) where about 75% of data are used in model training phase, 25% in testing phase. Three statistical performance criteria namely: the mean absolute error (MAE), Nash-Sutcliffe coefficient (NSE) and the Willmott’s Index (WI) and diagnostic plots visualizing the tested results are used to evaluate the performance of four AI-based models. The results reveal that the objective model (the two-phase FNN-PSOGSA model) and the single-phase FNN-PSO model yielded more precise results compared to the other forecast models. This result accorded to an NSE value of 0.612 (for the FNN-PSOGSA model) vs. an NS value of 0.500, 0.331 and 0.244 for the FNN-PSO, FNN and ANFIS models, and WI = 0.832 vs. 0.771, 0.692 and 0.726, respectively The study also demonstrated that the FNN model generated slightly better results than the ANFIS model for the estimation of sediment load data but overall, the two-phase FNN-PSOGSA model outperformed all comparison models. In light of the superior performance, this research advocates that the fully-optimized two-phase FNN-PSOGSA model can be explored as a decision-support tool for monthly sediment load forecasting using the rainfall and runoff values as the predictor datasets.