Sediment Mobility Research Articles

Influencing Factors of Phosphorus Mobility and Retention in the Sediment of Three Typical Plateau Lakes

The mechanisms driving changes in the stability of phosphorus (P) in sediments under lake ecosystem degradation remain poorly understood. This study investigated the P-binding forms in sediments from three plateau lakes with different trophic states in Yunnan Province, China, aiming to elucidate the responses of sediment P compositions to human activities, lake trophic status, and dissolved organic matter (DOM) characteristics. The results showed that human activity directly contributed to sediment P retention. The trophic type of lake exerted a discernible effect on P mobility in the sediments, as eutrophic algae-type lakes had a higher content of sediment mobile-P. Moreover, the sediment DOM promoted the adsorption of BD-P and NH4Cl-P. Generally, exogenous pollution caused by human activity leads to lake eutrophication and a decline in lake ecosystem stability. This variation was largely influenced by water depth. A decrease in lake ecosystem stability leads to increased P mobility in sediments, which increases the risk of endogenous pollution. The DOM plays an important role in the mobility of sediment P. These insights offer a novel perspective for understanding how lake ecosystem characteristics are related to endogenous P loads in lakes.

The importance of organic matter in controlling the metal variability and mobility in seagrass sediments.

Metal contamination in seagrass beds has been extensively studied in the past decades. Most of earlier studies have focused on reporting metal concentration in different compartments of seagrass ecosystem, with little attention given to the role of sediment organic matter in controlling the metal mobility and bioavailability. This study investigated metal contamination in seagrass sediments in Hainan Island, China and illustrated how various geochemical factors impact the spatial variability of the metal concentrations. Particularly, we explored how organic matter influence metal storage and mobility in the seagrass sediments. The enrichment of metals in these sediments was not only associated with levels of organic matter but also their sources. The organic matter content showed positive correlations with both metal concentrations and acid-extractable fractions of chromium, silver, and cadmium, highlighting its significant role in influencing metal accumulation and mobility in sediments. Overall, this research enhances our understanding of metal cycling within seagrass ecosystems and highlights the vital role of seagrass-derived organic matter in regulating metal dynamics.

Sediment flow connectivity index data for the Apulia region (Italy): An open-source geodatabase and the innovative CONNECTOSED WebGIS platform.

An open-source geodatabase and its associate WebGIS platform (CONNECTOSED) were developed to collect and utilize data for the Sediment Flow Connectivity Index (SfCI) for the Apulia region of southern Italy. Maps depicting sediment mobility and connectivity across the hydrographic basins of the Apulia region were generated and stored in the geodatabase. This geodatabase is organized into folders containing data in TIFF, shapefile, Jpeg and Pdf formats, including input variables (digital elevation model, land cover map, rainfall map, and soil units dataset for each hydrographic basin), classification graphs (ranking of variable values), dimensionless index maps (slope, ruggedness, rainfall, land cover, and soil stability) and key products (maps of sediment mobility, SfCI, and applied SfCI). The geodatabase maintains the mapping methodology underlying the SfCI algorithm by integrating various Earth datasets from multiple sources through ArcMap™, QGIS® and Matlab® software. This approach aligns surface characteristics with driving forces to describe the spatial variability of sediment pathways and identify hotspot areas. The availability of both input and processed data enables the computation and continuous updating of this applied geomorphological indicator, which is useful for assessing susceptibility to rapid Earth surface changes related to multi-hazard exposure. The geodatabase and the CONNECTOSED platform are valuable tools for researchers and stakeholders involved in land monitoring. The geodatabase and the CONNECTOSED platform are essential tools for researchers, policymakers, and stakeholders involved in land monitoring and environmental management. These tools provide open access to extensive datasets and detailed descriptions of surface dynamics, establishing connections between the causes and effects of extreme phenomena, such as floods, landslides, fires, soil pollution. This integration allows users to combine various forms of environmental data, a capability that is vital for enhancing scientific knowledge, supporting the development of insights, and fostering more informed, evidence-based decision-making in land use planning, conservation efforts, and sustainability initiatives.

Efficacy of bentonite/iron-coated sand as a sediment stabilizer in controlling cadmium, lead, zinc, and arsenic release determined using the diffusive gradients in thin film technology.

In this study, we investigated the efficiency of a bentonite/iron-coated sand (B/ICS) stabilizer in reducing the mobility and accumulation of heavy metals (Pb, Cd, Zn, and As) in contaminated sediments. Bentonite is effective in the adsorption of heavy metals, while ICS is effective in the adsorption of As. When combined, the stabilizer can be applied to mixed-contaminated sediments containing both heavy metals and As. The experimental setup involved B/ICS with sediments, followed by the analysis of heavy metal accumulation using diffusive gradients in thin-film resin gels for a period of 28days. The results obtained showed significant decreases in Pb, Cd, and Zn accumulation owing to the use of the stabilizer, particularly when its concentration was 15% of the sediment. Conversely, As accumulation did not show a decreasing trend owing to the low selectivity of the resin gel for As. Regardless, at the end of the 28-day test period, vertical distribution analysis indicated a notable decrease in heavy metal concentrations in the stabilizer layer of the contaminated sediment. These findings indicated that B/ICS is a cost-effective and efficient stabilizer for reducing heavy metal mobility in sediments. Long-term experiments are recommended to further validate these findings and optimize the use of the stabilizer in the field.

Mobility of Sediment and Phosphorus in a Small Stream During Artificial Flood Wave

This study investigates the dynamics of sediment and phosphorus transport in small streams affected by recurrent flood waves. The experiments were carried out in two streams with contrasting conditions: one in an agricultural headwater section, which serves as a sediment source, and the other in a mid-water section influenced by fish ponds. High-frequency data on suspended sediment and phosphorus concentrations were collected during several flood waves to assess how floods affect the transport regime, including the role of small reservoirs. The results showed that even initially clear water can mobilize significant amounts of sediment from the riverbed when discharge increases. Sediment mobilization was strongest at the beginning of the flood wave, with concentrations peaking early and then decreasing. Successive floods without additional sediment input led to a decrease in sediment concentrations, as the stream had already flushed out the available sediments. Hysteresis loops showed that most of the sediment transport occurred during the rising part of the flood wave, indicating that the sediment originated from the nearby streambed areas. Phosphorus was mobilized simultaneously with the sediment, and an almost linear relationship was observed between sediment concentration and total phosphorus (r = 0.979, p < 0.0001). However, only a fraction of the phosphorus was dissolved, so most of the phosphorus was bound to the sediment particles. Soluble reactive phosphorus showed a weaker correlation with sediment concentration (r = 0.272, p = 0.047). This shows how important it is to consider both sediment dynamics and phosphorus mobilization in event-based flood models.

Assisting recolonization of near‐shore seagrasses

Natural recolonization of seagrasses may take decades after disturbances and is particularly challenging in near‐shore environments, where sediment mobility inhibits seagrass establishment. We assisted recolonization in fifteen 4x10 m unvegetated experimental plots in a Mexican Caribbean near‐shore fringe where the seagrasses had died due to massive inundations of holopelagic Sargassum species, and where a containment barrier was placed to avoid future inundations. The applied treatments were: artificial substrate (AS: 90 belowground, artificial, biodegradable, 15x15 cm‐sized substrates, cut from a 0.91 x 0.45m Biodegradable EcoSystem Engineering sheet), transplant (TR: 90 cores, 4.5 cm diameter, with Halodule wrightii), and control (C: no manipulation), each with five replicates. After 6 months, 63% of H. wrightii transplants survived, presenting mean rhizome extension of 7.6 cm, and H. wrightii from natural surrounding patches started to colonize the plots. After approximately 8 months, AS and TR plots already showed higher light conditions and lower fluctuations in sediment levels than the controls. After 14 months, the AS and TR plots reached higher mean (± SE) density (respectively, 4024 ± 620 and 3484 ± 360 shoots/m2) and cover (60.3 ± 3.85 and 53.7 ± 2.84 %), compared to the control plots (2043 ± 381 shoots/m2, 35.3 ± 5.7 % cover). Higher density of H. wrightii likely favored the natural establishment of Thalassia testudinum seedlings, with an average of 2.07 (± 0.15) and 1.87 (± 0.18) seedlings in AS and TR plots, respectively, compared to 0.48 (± 0.23) in the controls. Both techniques accelerated seagrass recolonization, but artificial substrates required less effort and avoided harvesting of donor meadows.

Experimental investigations of colloid-associated metal mobility in mine-impacted wetland sediment.

Metal mining operations can release toxic metals to surrounding environments where site-specific conditions control the movement of contaminants. Colloid-facilitated transport, the transport of contaminants with small, mobile particles, has been recognized as a potential contaminant transport vector in groundwater, but it remains unclear under what conditions it is important and whether neutral, metal-rich mine drainage from legacy mining impacts this transport vector. This work presents a set of laboratory column experiments that study the effect of colloids on metal mobility in saturated, wetland sediment that has been receiving neutral mine drainage for nearly a century, using mixed and single metal input solutions at neutral pH. Results indicate that colloid-facilitated transport is only important when small (<0.01μm) colloids, most likely formed from organic matter, are present. Larger particles were found to be generally immobile, so could aid in the immobilization of metal contaminants. These findings imply that colloid-facilitated transport is an important transport vector in mine-impacted wetland sediment and should be considered when remediating mine sites.

Impact of offshore energy activities on trace elements content and mobility in marine sediments.

The offshore oilfields in the North Sea area are increasingly employed for projects beyond oil production, like carbon capture and storage (CCS). Still, the fossil fuel production from mature fields is significant. It has raised environmental concerns associated with discharging produced waters (PW) and drilling mud into the sea. These discharges, which may be highly saline and contain production chemicals, vary significantly in metals and particulate content. Due to density and release depth, the plume is assumed to sink towards the seafloor. Also, a single oilfield can input up to 7.5 tons of Ba, 675kg of Fe, and 619kg of P into the water column through PW. Therefore, this study investigates the impact of these discharges on seafloor sediments around two Danish oilfields, assesses the mobility of metals within these sediments, and evaluates the environmental status. PW samples were collected at the discharge outlets from the platforms. Sediment cores were taken near the two oil platforms and from control sites. Using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and an optimized BCR sequential extraction, we analyzed the composition and distribution of 24 elements in sediment samples. The results revealed significant differences in total extracted concentrations between sediments near the platforms and those from distant locations and stratigraphically older samples, with relevant levels of Br, Ba, and Sn near the platforms (averaged 14, 27, and 0.1ppb, respectively). Sediment quality indices showed considerable enrichment and geo-accumulation of toxic metals, particularly at one of the platform sites. However, cumulative indices did not display significant pollution anomalies. Therefore, our findings suggest that oil extraction activities may increase the availability of toxic metals in nearby sediments, potentially impacting marine ecosystems.

Integrated geological study in an offshore renewable energy test site: a case from the Basque continental shelf (Bay of Biscay, Spain)

Testing and research centres for offshore renewable energy, exemplified by facilities like BIMEP (Biscay Marine Energy Platform) on the Basque coast of Spain, play a crucial role in driving the energy transition. This study utilises pre-existing data at the facility site, such as high-resolution bathymetry and granulometric information from sediment samples, to conduct a comprehensive geological analysis including both sedimentary and rocky seabed. A litho-structural analysis is presented, including a lithological prediction for the continental shelf, the recognition of the main structures, such as NW-trending folds and predominantly NE-SW oriented fractures, and a detailed fracture analysis. Sedimentary seabeds are analysed through a Seabed Sediment Map, illustrating a granulometry-based NE-SW oriented banded distribution. Bedforms are also studied, they are asymmetric and mainly oriented NE-SW. The Seabed Sediment Map and the bedform analysis reveal the effect of an SE-directed bottom current as the main mechanism controlling sediment mobility. This current matches with the predominant swell from the NW and with the direction of the most energetic waves in the area. This approach could serve as a methodological example, offering a cost-effective means for the preliminary geological characterisation of offshore energy sites, and is crucial for establishing a baseline (‘zero state’) before the deployment. This baseline is essential for evaluating and mitigating the impact of new infrastructure on sediment dynamics, which subsequently affects the overall functioning and health of the marine ecosystem.

Microbial mediated hardening of a Central Mediterranean upper shelf seafloor during the MIS 5.5; a possible post global warming scenario?

Higher sea level and sea surface temperature than at present have been inferred for the Last Interglacial (MIS 5.5, 135–116 ka), making it a good analogue for modeling the climate response of the environment to global warming in the near future. With the aim of predicting the possible evolution of some central Mediterranean shallow sandy seabeds, a MIS 5.5 biocalcarenite was investigated (Gulf of Taranto, Italy). Limited post-depositional diagenesis affected this deposit, which represents an infralittoral sandy seabed, with local vegetation and relatively high energy conditions. Medium-coarse sand sized bioclasts, with a negligible quantity of siliciclastic, compose the sediment that also hosts large shells of mollusks. Micritic cements are widespread, often in continuum with the micritized part of the shells, showing non-isopachous aphanitic and filamentous rims, aphanitic micro-mounds, vacuolar peloidal menisci, and aphanitic pore-filling matrix. All these cements consist of submicrometer anhedral or nanospheroidal crystals of low-Mg calcite, mixed with a smaller amount of irregular plate-like crystals of saponite. Micritic cements are also rich in mineralized filamentous, tubular, and subspherical bacterial bodies. This highlights the occurrence of an epilithic and endolithic microbial community forming a biofilm that stabilized the mobile sediment as consequence of the microbial mediated early cement precipitation. This led the synsedimentary hardening of some parts the mobile sandy substrate, the settlement of sessile taxa - such as Spondylus gaederopus, oysters, serpulids and barnacles, together with endofaunal organisms. Early micritic cementation is common in modern tropical climate, whereas is substantially absent in the modern Mediterranean. Consequently, its presence in the MIS 5.5 deposit confirms warmer sea water temperature compared to today (estimated at ca +2 °C) and suggests that similar hardening of mobile substrates may occur in the near future as a response to global warming.

Vanadium redox speciation in the acid-extractable phase of Krka River estuary surface sediment

This study investigated the redox speciation and mobility of V in the acid-extractable fraction of surface sediments from the Krka River estuary using an optimized IC-UV/Vis analytical method. The separation of V(IV) and V(V) redox species was done using anion-exchange based chromatographic method, while pseudo-total V concentrations were measured using HR ICP-MS analytical instrumentation. Extracted V concentrations from the sediment fraction (pH = 5, HCl) and determined pseudo-total V concentrations were used to calculate the Enrichment Factor (EF) and Risk Assessment Code (RAC), indicating potential anthropogenic influence and environmental risk. A simple PHREEQC model was developed to asses V speciation in the oxic bottom seawater layer simulating possible remobilization of the leached sediment phase. The results of the study show that minor fraction of V is present in the acid-extractable phase across the surface sediment of Krka River estuary. Higher V mobility is mostly observed at locations rich with clay minerals, terrigenous input, and carbonates. Anthropogenic influence was linked to higher enrichment but lower mobility, suggesting binding to less mobile sediment phases (reducible, organic and residual fractions). The predominance of reduced V(IV) species in the acid-extractable sediment fraction indicates a potentially low V toxicity risk in the sediments of Krka River estuary, even in cases of high potential remobilization of V. However, the model predicted complete oxidation of V(IV) to V(V) upon remobilization into the oxic bottom water layer. This highlights the complexity of V behavior in natural estuarine systems, where the toxicity risks of possible V remobilization still remain unclear. Results of this study demonstrate the need for the strengthening efforts in speciation of V in the mobile sediment phase to obtain a cohesive outlook on its potential toxicity and biogeochemical cycling.

Mobilization of phosphorus in sediments of eutrophic lakes induced by elevated sulfate levels

Elevated sulfate levels in eutrophic lakes have been observed to induce the release of endogenous phosphorus. While previous studies have predominantly examined its impact on iron-bound phosphorus (FeP), the influence on organic phosphorus (OP), a crucial active phosphorus component in sediments, remains poorly understood. In this study, mesocosms were established with lactate supplementation and varying sulfate concentrations to explore sulfate reduction and its impacts on phosphorus mobilization in freshwater sediments. Lactate addition induced hypoxia and provided substrates, thereby stimulating sulfate reduction with a decline of sulfate levels, an increase of sulfide concentrations, and fluctuations of sulfate-reducing bacteria. Meanwhile, concentrations of total dissolved phosphorus and phosphate were dramatically promoted during lactate decomposition, with a higher sulfate concentration associated with greater phosphorus elevation, correlating with the decrease of total phosphorus in sediment. The increase in phosphorus of the overlying water was partly attributed to FeP release from the sediment, confirmed by a decrease in its sediment content. FeP release was ascribed to dissimilatory reduction of iron oxides or chemical reduction mediated by sulfides in anoxic sediments, leading to the desorption and subsequent release of phosphorus. Evidences included the proliferation of iron-reducing bacteria, a decrease in Fe(II) concentrations in sediment pore- water, and the continuous accumulation of solid iron sulfides in surface sediments. Furthermore, OP mineralization in sediment also contributed to the increase in phosphorus in water columns, confirmed by a reduction in its content and the abundance of fermentation bacteria in surface sediment. Notably, the decrease in OP content accounted for >80 % of the total phosphorus reduction in surface sediment in the end. Thus, sulfur cycling plays a critical role in iron and phosphorus cycling, significantly stimulating not only the mobilization of FeP but also OP in sediments, with OP mineralization potentially being the primary contributor to endogenous phosphorus release.

Geomorphology and sediment mobility on sand banks: A study of Dogfish Bank, Hecate Strait, Northeast Pacific Ocean

AbstractMovement of sediment along shallow continental shelves is a natural process with wide‐ranging environmental and economic implications, making it of high importance to marine spatial planning efforts in the offshore. Development of marine renewable energy, for instance, requires detailed understanding of the morphodynamics of mobile bedforms to select foundation types and ensure safe installation of infrastructure in shallow shelf environments. This study evaluates geomorphology and sediment mobility of Dogfish Bank (&lt; 20 mbsl) in the Hecate Strait offshore British Columbia, Canada, using hydroacoustic and airborne bathymetric data combined with seismic profiles and grain‐size information. These data reveal current‐swept features ranging from sediment‐depleted lag to sediment‐abundant sand ridges and dunes, with sand ribbons and furrows in‐between. Seismic reflection data show up to 15 m of surficial sand concentrated beneath north‐aligned sand ridges that dominate the bathymetry of northwest Hecate Strait. Sand ribbons (typically understood sediment‐limited features in shallow marine environments) are notably maintained over seabed with comparable sand thickness to adjacent dunes (i.e. sediment‐abundant features), suggesting local spatial variability in hydrodynamics and sediment characteristics (principally grain size) influence expression of mobile bedforms. Repeat mapping between 2008 and 2019 shows dunes and ribbons both migrate northwards, with largest seafloor changes along northeast‐facing lee sides of dunes, matching closely with published models of sediment mobility which suggest northward bedform migration is largely driven by storms. Median total migration distance is 164 m (northward) for dunes (time‐averaged rate of 14.9 m/year). Sand ribbons show less migration (median northward distance of 73 m) and migrate in a depth‐dependent manner. Because sand ribbons are typically flow‐parallel features, their lateral migration likely results from varying current directions and flow acceleration over shallower seabed. Sand ribbon migration should therefore a consideration in studies examining seabed change, particularly when they are formed over unconsolidated sediment.

Autogenic Formation of Bimodal Grain Size Distributions in Rivers and Its Contribution to Gravel‐Sand Transitions

AbstractRiverbeds often fine downstream, with a gravel‐bedded reach, a relatively abrupt gravel‐sand transition (GST), and a sand‐bedded reach. Underlying this behavior, bed grain size distributions are often bimodal, with a relative paucity (gap) around the range 1–5 mm. There is no general morphodynamic model capable of producing the grain size gap and gravel‐sand transition autogenically from a unimodal sediment supply. Here we use a one‐dimensional morphodynamic model including size‐specific bedload and suspended load transport, to show that bimodality readily evolves autogenically even under unimodal sediment feed. A GST forms when we include a floodplain width that abruptly increases at some point. Upstream of the transition, non‐gap gravel ceases to move and gap sediment is preferentially transported. At the transition, non‐gap sand rapidly deposits from suspension, enhancing gap sediment mobility and diluting its presence on the bed.

A mathematical model for bedrock incision in near‐threshold gravel‐bed rivers

AbstractGravel‐bed rivers that incise into bedrock are common worldwide. These systems have many similarities with other alluvial channels: they transport large amounts of sediment and adjust their forms in response to discharge and sediment supply. At the same time, the occurrence of bedrock incision implies behaviour that falls on a spectrum between fully detachment‐limited ‘bedrock channels’ and fully transport‐limited ‘alluvial channels’. Here, we present a mathematical model of river profile evolution that integrates bedrock erosion, gravel transport and the formation of channels whose hydraulic geometry is consistent with that of near‐threshold alluvial channels. We combine theory for five interrelated processes: bedload sediment transport in equilibrium gravel‐bed channels, channel width adjustment to flow and sediment characteristics, abrasion of bedrock by mobile sediment, plucking of bedrock and progressive loss of gravel‐sized sediment due to grain attrition. This model contributes to a growing class of models that seek to capture the dynamics of both bedrock incision and alluvial sediment transport. We demonstrate the model's ability to reproduce expected fluvial features such as inverse power law scaling between slope and area, and width and depth consistent with near‐threshold channel theory, and we discuss the role of sediment characteristics in influencing the mode of channel behaviour, erosional mechanism, channel steepness and profile concavity.

Evidence for littoral convergence and sediment delivery to Mattole submarine canyon, Northern California

At geologic timescales, a significant proportion of the sediment eroded from continents is transferred from rivers to littoral cells, then through submarine canyons to deep ocean basins. Accumulation and occasional discharge of sediment from canyon headwall regions is a likely driver for the sediment gravity flows believed to be responsible for much of this mass transport, but the responsible mechanisms and event timescales in canyon heads are imprecisely defined, largely due to the drowning of canyon heads since the last glaciation and the resulting dearth of modern depositional environments to observe. We investigated a littoral cell adjacent to Mattole submarine canyon in Northern California to better understand the influence of bathymetry and coast shape on sediment trajectories and hypothesize that canyon heads promote littoral sediment accumulation due to bathymetric sheltering of the adjacent coast from waves and convergence of littoral cells exposed to varying wave direction. The uppermost gullies of Mattole Canyon extend to the littoral cell, and a train of sediment waves within the uppermost thalweg of Mattole Canyon suggests that gravity-driven flows of shore-derived sediment may have recently occurred. Mattole Canyon and nearby Mendocino Canyon flow into the deep Mendocino Channel, which has the highest observed Late Holocene sediment accumulation rates in Northern California. We documented the beach slope, grain size, and sediment provenance along 15 km of coast adjacent to Mattole Canyon and conducted numerical wave modeling to infer the dominant direction and magnitude of sediment transport, relative wave sheltering by bathymetry, and estimated sediment mobility in the vicinity of the canyon head over a multi-year period. South of the canyon headwall, in the vicinity of the Mattole River outlet, coarser sediment rich in metasandstone clasts and steeper beach slopes coincide with decadal-scale beach accretion. North of the canyon head, sediments are generally finer, beaches are flatter, clast lithologies include more quartz and mudstone grains, and there is decadal-scale net erosion. Wave modeling suggests the Mattole headwall region is subject to sustained sheltering from waves due to canyon bathymetry, littoral sediment convergence above the canyon head for much of a typical year, and occasional bed entrainment of sediment in the canyon’s uppermost gullies by large waves. Larger winter waves from the northwest likely result in net southerly drift north of the canyon head, with a high likelihood of preferentially transporting fine-grained, more quartz-rich sediment towards the Mattole headwall. Smaller summer swells from the west and south likely results in net northerly transport of metasandstone clast-rich Mattole River-derived sediment towards the canyon head. The imbalance in seasonality in wave conditions, coast shape, and sediment delivery by the Mattole River and other coastal creeks is broadly consistent with the spatial patterns in grain size, mineralogy, slope, and beach width that we observe in the vicinity of the canyon, and the net delivery of sediment to the Mattole Canyon head. An approximate mass balance of sediment flux from coastal streams feeding Mattole River littoral cell suggests that sediment volumes likely accumulate in the canyon headwall region that could supply several density flow events in a typical decade. These findings highlight the importance of the connectivity of the canyon to the nearshore region for the processes and products in canyons and fans. We hypothesize that feedbacks between canyon bathymetry, sediment accumulation, and mass evacuating flows promotes the long-term persistence of canyon systems.

Ecological risk assessment of heavy metals (Pb, Cd, Cr, Ni, Zn, Fe) and metalloid (As) in surface sediment of Anzali International Wetland, Iran

ABSTRACT This study aimed to assess the metal concentration, mobility, bioavailability, ecological risk, and toxicity in the surface sediment of Anzali International Wetland. Contamination and ecological risk were evaluated using parameters such as Global Contamination Factors (GCF), Risk Assessment Code (RAC), Toxic Unit (TU), sum of Toxic Units (ΣTUs), Toxic Risk Index (TRI), ecological risk (ER), Risk Index (RI), and modified Hazard Quotients (MHQ). Heavy metal mean concentrations (mean ± SD) ranked as follows: Zn (95.46 ± 20.71) > Cr (19.79 ± 2.79) > Ni (18.49 ± 5.23) > Pb (13.97 ± 3.20) > As (6.45 ± 1.87) > Cd (0.90 ± 0.34). Cd exhibited high mobility and bioavailability, while Cr, Pb, Zn, Ni, and As mainly existed in residual fractions, indicating low bioavailability and minimal ecological risk. Ecological risk assessment identified “moderate” to “considerable” risk potential for Cd at specific sites. The Risk Index categorized Anzali sediment as having “low” to “moderate” risk potential (150 < RI < 300). Values of RI, ΣTUs, and MHQ suggested low to moderate risk potential and low toxicity in the sediment. Global Contamination Factor (GCF) values indicated high contamination (12.53 to 79.11). Recommendations include installing effective sewage treatment plants and promoting proper agricultural practices to mitigate metal toxicity in Anzali Wetland and also protect human and environmental health.

Fine Sediment in Mixed Sand‐Silt Environments Impacts Bedform Geometry by Altering Sediment Mobility

AbstractGeometric characteristics of subaqueous bedforms, such as height, length and leeside angle, are crucial for determining hydraulic form roughness and interpreting sedimentary records. Traditionally, bedform existence and geometry predictors are primarily based on uniform, cohesionless sediments. However, mixtures of sand, silt and clay are common in deltaic, estuarine, and lowland river environments, where bedforms are ubiquitous. Therefore, we investigate the impact of fine sand and silt in sand‐silt mixtures on bedform geometry, based on laboratory experiments conducted in a recirculating flume. We systematically varied the fraction of sand and silt for different discharges, and utilized an acoustic Doppler velocimeter to measure flow velocity profiles. The final bed geometry was captured using a line laser scanner. Our findings reveal that the response of bedforms to an altered fine sediment percentage is ambiguous, and likely depends on, among others, bimodality‐driven bed mobility and sediment cohesiveness. When fine, non‐cohesive material (fine sand or coarse silt) is mixed with the base material (medium sand), an increased dune height and length is observed, possibly caused by the hiding‐exposure effect, resulting in enhanced mobility of the coarser material. However, weakly cohesive fine silt suppresses dune height and length, possibly caused by reduced sediment mobility. Finally, in the transition from dunes to upper stage plane bed, there are indications that the bed becomes unstable and dune heights vary over time. The composition of the bed material does not significantly impact the hydraulic roughness, but mainly affects roughness via the bed morphology, especially the leeside angle.

Evaluating the Effect of Morphologic Units on Fractional Sediment Mobility and Bedload Transport in a Small Pool‐Riffle Reach

AbstractThis study examines the spatial pattern of fractional sediment mobility and assesses the influence of morphologic units on bedload transport in a small pool‐riffle reach with limited supply. Using a 2D hydraulic model and a subsurface‐based sediment transport model, shear stresses, fractional sediment mobility, and bedload transport were examined for flow events ranging in magnitude between 0.2Qbf and 1.5Qbf. Results reveal that while spatial variations in shear stress decrease as discharge increases, only a small proportion of the bed experiences high transport rates. At the reach scale, riffles are the primary morphological unit contributing to fully mobile sediment for all size fractions. However, at a subunit scale, there is evidence of sediment transport reversal for grains &gt;32 mm at flows near or exceeding bankfull discharge in association with shear stress reversal. These transport reversals are important for maintaining pools despite their infrequent occurrence in the study reach. Sediment transport maps indicate that bed morphology considerably influences sediment transport at low to moderate flows. During these events, the shear stress is sensitive to local bed topography and partial mobility is the dominant transport process. In contrast, variations in bedload transport rates decrease during high flows when the flow is less sensitive to variations in bed topography and the bed becomes fully mobile.

Estimation of bed material transport in gravel‐bed streams using the virtual velocity approach: Insights from the North‐Western Himalayas, India

AbstractThis study focuses on evaluating the sediment mobility and transport patterns in two Himalayan rivers (Aglar and Paligad Rivers) during monsoon and non‐monsoon flows. The virtual velocity approach involving the measurements of the bed proportional mobility (Y), active layer depth (ds), displacement length and virtual velocity of mobilized grains was employed. Both local (0.5 m subsections) and wetted cross‐sectional average parameters were used. While using local parameters the total annual bed material transport was estimated to be 67 100 (±20 400 t) and 18 400 t (±6000 t) in the Aglar and Paligad Rivers, respectively. Of this, nearly 60% of transport occurred during the monsoon and the overall contribution of partial transport (PT) remained low (&lt;6%). However, based on cross‐section average parameters, total transport was estimated to be 42 300 (±15 800 t) and 12 200 t (±4700 t), in Aglar and Paligad, respectively, with nearly 79% and 68% occurring during the monsoon. Moreover, the contribution of PT increased to nearly 18% and 29% for the Aglar and Paligad Rivers, respectively. Additionally, the dependence of PT on Y and full transport on ds results in an abrupt shift in transport rates at the transition from partial to full transport, causing discontinuity in transport curves. Therefore, a unified function was proposed to represent the extent of transport for both partial and full transport, yielding continuous transport curves. These findings are particularly relevant for efficient river management as the region houses several hydropower plants and is highly vulnerable to climate change.