Sediment Transport Processes Research Articles

Effect of Land Use Land Cover on the Hydrology of Rwizi River Catchment located in Southwestern Uganda

Changes in land use and land cover usually affect the hydrological processes of river flows, sediment transport, and water quality on a global scale. Hence, the objective of this paper was to investigate the effect of Land Use Land Cover (LULCC) on the Hydrology of Rwizi River Catchment located in South-western Uganda using appropriate standard techniques and procedures including Soil and Water Assessment Tool (SWAT). Model calibration and validation demonstrated good agreement (R² = 0.76-0.86, NSE = 0.75-0.78). LULC analysis revealed significant increases in built-up areas and forestland, with declines in grazing land and wetlands. These changes were driven by socio-economic factors and hydro-climatic influences. The study found strong correlations between LULC types and sediment yield, highlighting the implications for water quality and erosion control. The results show that urban growth raises surface runoff and peak flows as a result of a rise in impervious surfaces, whereas agricultural intensification raises water demand and lowers base flow, especially during dry spells. Deforestation has led to increased sedimentation rates, degrading water quality. These findings underscore the significant anthropogenic and climatic impacts on LULCC dynamics and hydrological processes in the catchment. The study emphasizes the critical need for integrated watershed management strategies to mitigate these impacts and ensure sustainable water management practices. By advancing understanding in hydrological sciences, this research informs policy and management decisions for sustainable development in tropical river basins globally.

Sediment sorting within a relatively wave-exposed and sandy subtidal seagrass (Zostera marina) meadow

Seagrasses impact on sedimentary intertidal and subtidal ecosystems by affecting local hydrodynamics, geomorphology and sediment properties. Their influence on hydrodynamics is to reduce flow velocities in their canopies, and this leads to increased net sedimentation rates and reduction of the grain size. Most investigations of the seagrass-hydrodynamics-sediment feedback system has been carried out over silt and fine-sand beds under tide-dominated conditions, mostly in the intertidal zone. Here, we use sedimentological data from a relatively wave-exposed and subtidal seagrass (Zostera marina) meadow in the Isles of Scilly with a fine-to-medium sand bed and show that the sand within the seagrass meadow is indeed finer than in adjacent, unvegetated regions. However, in contrast to previous studies, this is not due to increased mud/silt content within the seagrass meadow, but an almost 0.1-mm shift in the median sediment size across the sand fraction from 0.25 mm (fine to medium sand) to 0.35 mm (medium sand). The studied seagrass meadow is characterised by a distinct spatial structure comprising of vegetated ‘ridges’ and bare sand ‘valleys’. Even the bare sand valleys within the seagrass meadow are characterised by significantly coarser sand than the adjacent vegetated ridges, providing further confirmation of the efficiency of sediment sorting by wave processes that takes place within seagrass meadows. It is concluded that any numerical modelling involving sediment transport processes associated with seagrass environments must account for variability in the textural characteristics.

New understanding on the interplay of longitudinal and transverse sediment dispersal: Combination of 3D seismic profile and u-pb ages of detrital zircons of Paleogene sedimentary rocks in the northern South China Sea

The northern South China Sea (SCS) experienced a significant transition from a lacustrine to marine environment in the Paleogene, and its deep-water basin of Zhu-II Depression is in particular extensively studied due to its richness in oil and gas resources. However, limited number of boreholes in the deep-water area has long constrained a better understanding on the provenance of the Paleogene strata and the associated sediment transport processes. In this study, detrital zircon U-Pb ages and three-dimensional (3D) seismic-reflection data were systematically employed to investigate the “source-to-sink” pathways and interplay of longitudinal and transverse sediment dispersal. Our results indicate that the Zhu-II Depression sediments of the northern SCS were predominantly derived from the surrounding nearby paleo-uplifts in the early and middle Eocene. A significant provenance shift took place in the late Eocene, when the local paleo-uplift source was replaced by a distant source from the western SCS. Sediments were transported from west to east by the “Kontum-Ying-Qiong River” as a longitudinal dispersal. In the Oligocene, the “Kontum-Ying-Qiong River” delivered large amounts of sediments from Central Vietnam to the eastern part of the northern SCS. Meanwhile, the Pearl River gradually evolved through regional tectonic processes and influenced the deep-water area of Zhu-II Depression as a transverse dispersal. Sediments from both “Kontum-Ying-Qiong” and Pearl Rivers converged and deposited as deep-water deltas in the Zhu-II Depression. This dual provenance system in the northern SCS deep-water area was featured by the interplay between longitudinal and transverse sediment dispersal. It was largely controlled by the tectonic-palaeogeographic pattern inherited from the Mesozoic arc system.

Scale-specific controls of monthly suspended sediment load in a typical inland river

Sediment transport is a multi-scale and non-linear process controlled by multiple variables. Given the intricate nature of sediment transport mechanisms and the overlap of different factors’ effects on it at various time scales, unraveling the relationship between sediment load and its influencing factors is challenging. This study aimed to elucidate the multi-timescale effects of factors on monthly suspended sediment load in a seasonal inland river. In this study, monthly sediment loads measured by the depth-integrating method were obtained from the Tabu River Basin in northern China from 2007 to 2021, alongside data on four controlling factors: runoff, precipitation, water surface evaporation, and the normalized difference vegetation index (NDVI). The results showed that sediment load, runoff, and precipitation showed strong variability during 2007–2021, while water surface evaporation and NDVI exhibited moderate variability. Multivariate empirical mode decomposition (MEMD) decomposed the temporal patterns of sediment load and associated variables into five intrinsic mode functions (IMF) and a residue. IMF1 (4.7 months), IMF2 (9.0 months) and IMF3 (14.2 months) collectively explained 115.88 %, 91.73 %, 91.28 %, 107.69 %, and 106.09 % of the total variability in sediment load, runoff, precipitation, water surface evaporation, and NDVI, respectively. Subsequently, the time-dependent intrinsic correlation (TDIC) was utilized to illustrate the inherent relationships between sediment load and its controlling factors across various time scales. The associations between sediment load and the controlling factors changed significantly with the scale. Runoff was the dominant factor controlling sediment load at IMF1 (4.7 months), IMF2 (9.0 months), IMF3 (14.2 months), and IMF4 (25.1 months). Water surface evaporation controlled the sediment transport process at IMF5 (63.6 months). The prediction of sediment load on the measurement scale, using scale-specific controls analyzed with MEMD (R2 = 0.993), demonstrated superior performance compared to traditional multiple linear regression prediction (R2 = 0.748). This study will provide a theoretical basis for effective prevention and control of soil erosion and watershed management in inland river basins.

Spatial Variation of Asymmetry in Velocity and Sediment Flux along the Artificial Aam Tidal Channel

Tidal flats, crucial for biodiversity and ecosystem services, are facing significant alterations due to human activities such as reclamation. In South Korea, over 65% of tidal flats have been reclaimed since the 1970s, resulting in morphological changes and altered sediment transport dynamics. This study investigates sediment transport processes in the artificial Aam tidal channel, created as part of the megacity development project in Incheon, Korea. Using data from Acoustic Doppler Current Profiler and Vector instruments deployed in 2019 and 2021, we analyzed tidal asymmetry, current velocities, shear stress, and suspended sediment concentration. Our results reveal a pronounced tidal asymmetry influencing sediment transport, with ebb-dominant currents near the channel entrance and flood-dominant currents in the interior. We observed significant sediment deposition in the landward section of the channel, driven by tidal mixing asymmetry and rainfall events. These findings highlight the complex interactions between artificial structures and natural sediment dynamics, informing future coastal development and management strategies.

Investigations into dynamic variation characteristics of near-bottom boundary flows over a long term at strait areas

Dynamic processes at the bottom boundary layers are crucial for various marine applications such as marine energy exploration and environmental protection. This paper explored hydrodynamic characteristics of near-bottom boundary currents based on the continuous measured velocity data at a site near the bottom boundary layer in the Strait of Georgia. Previous studies on the bottom boundary layer encompasses various aspects, including physical, chemical, and biological dimensions. However, the mechanism underlying the hierarchical structure of flow velocity remain poorly understood. Thus, the power spectrum and wavelet analysis were used to analyze the significant period of the averaged velocity and tidal characteristics, and to calculate the thickness of near-bottom boundary layer. The results indicate that the current velocities at the boundary layer tend to flow in south-eastern and north-western directions. There is a significant period of 15 days in August and September. At a depth of 301 m, the vertical velocities are stratified, indicating different velocity layers. The boundary layer thickness varies from 5.23 to 14.74 m, as indicated by the vertical structural characteristics. The structural characteristics of the seabed boundary layer are conducive to deepening our understanding of sediment's incipient motion, transport, and sedimentation process. The findings provide a theoretical foundation for future studies on sediment movement and the numerical simulation analysis of the bottom boundary layer. It is of great significance for the study of wave flow interaction and marine dynamic structure, and can provide scientific support for the development and utilization of the strait area.

Examining temporal trends in heavy metal levels to analyze sediment pollution dynamics in the Saida urban watershed (N-W Algeria).

The study focuses on current pollution in the Saïda basin, a semi-arid region in north-western Algeria. By analyzing sediments, the study provides interesting results on urban pollution and its environmental impact. The research consists of two main phases, each addressing different aspects of pollution. In the first phase, different pollution indicators are used to analyze heavy metals and organic pollutants in urban drainage sediments. The results are compared with sediment quality guidelines, regulatory thresholds, and local and international references. Most of the metallic contaminants exceed the toxicity levels established by the continental crust and sediment quality guidelines, suggesting an anthropogenic origin. In addition, contamination indices show significant accumulation. In this context, the results highlight the importance of accumulation and transport processes in urban sediments. Hydrological parameters significantly influence heavy metal distribution mechanisms. Remarkable variations between copper (Cu) and lead (Pb) suggest a combined or singular source during transport. Conversely, chromium (Cr), nickel (Ni), and iron (Fe) are mainly derived from natural lithological sources. Cadmium (Cd) is associated with anthropogenic sources related to the agricultural use of phosphate fertilizers, whereas zinc (Zn) is mainly derived from physical corrosion processes. In the second phase, a combined descriptive and multivariate statistical analysis examines the mobility and distribution of heavy metals and their relationships with organic matter (OM) over time. Pronounced temporal variations in Cd, Zn, and Cu concentrations are attributed to human activities. Strong correlations exist between OM and cobalt (Co), Cu and Pb, confirming the ability of OM to adsorb these metals under specific geochemical conditions associated with waste disposal. Conversely, Zn, Cd, Cr, and Ni show weak or negative correlations with OM, suggesting diverse sources, including potential agricultural, industrial, and natural origins. The dendrogram confirms the existence of previously identified contaminant groups, suggesting common sources and potential co-occurrence patterns. This analysis highlights the role of the drainage network as a physico-chemical reactor in the mobilization of contaminants. It underlines the importance of sediment interactions in urban pollution processes. Finally, recommendations are proposed to ensure effective pollution control and remediation. PRACTITIONER POINTS: Useful information on pollution and its environmental impact is provided by the analysis of sediments in the urban basin of Saida (NW-Algeria). The results of this study indicate high levels of heavy metals in the sediments, in excess of toxicity limits, and evidence of anthropogenic sources. Temporal variations in metal concentrations indicate the influence of human activities. The study has made it possible to identify the sources, to understand the mobility and distribution, and to control the contamination by heavy metals in the urban sediments. Drainage system serves as a pathway for dispersing contaminants.

Runoff and sediment effect of the soil-water conservation measures in a typical river basin of the Loess Plateau

Soil erosion and sediment transport processes, which are intimately connected with hydrological processes, strongly affect land surface morphology and earth system dynamics. Over the past 60 years, intensive Soil-Water Conservation (SWC) measures implemented on the Loess Plateau have significantly reduced sediment load and concurrently exacerbated runoff declines, while the quantitative evaluation of the effects of SWC measures remains challenging. This study develops a sediment module which simulates the hillslope and in-stream sediment processes, and then incorporates the module into a Geomorphology-Based Ecohydrological Model (GBEHM). Besides, the model also couples with an explicit representation of different types of SWC measures, including both Hillslope SWC (HSWC) and River-network SWC (RSWC) measures. The newly developed hydro-sediment model is then applied to the Kuye River basin, a typical river basin on the Loess Plateau, and used for quantifying the effects of SWC measures on river discharge and sediment load during 1982–2019. Compared with Period 1 (1982–2000), the river discharge decreased by 48.7% in Period 2 (2001–2019), while the sediment load experienced a more substantial decrease of 95.3% between the two periods. Based on the numerical experiment results, HSWC and RSWC contributed to 25.4% (58 million m3/yr) and 28.9% (66 million m3/yr) of the annual runoff decline, as well as 58.1% (36 million t/yr) and 33.9% (21 million t/yr) of the annual sediment load reduction between the two periods, respectively. Besides, HSWC plays an increasingly important role in sediment reduction in recent years due to check dam silting. The model developed in this research is useful to evaluate the multi-faceted effects of existing SWC measures and to help with future SWC planning and policymaking.

Empirical Orthogonal Function Analysis on Long-Term Profile Evolution of Tidal Flats along a Curved Coast in the Qiantang River Estuary, China

Abstract: Tidal flats are dynamic coastal ecosystems continually reshaped by natural processes and human activities. This study investigates the application of Empirical Orthogonal Function (EOF) analysis to the long-term profile evolution of tidal flats along the Jiansan Bend of the Qiantang River Estuary, China. By applying EOF analysis to profiles observed from 1984 to 2023, this study identifies dominant modes of variability and their spatial and temporal characteristics, offering insights into the complex sediment transport and morphological evolution processes. EOF analysis helps unravel the complex interactions between natural and anthropogenic factors shaping tidal flats, with the first three eigenfunctions accounting for over 90% of the observed variance. The first spatial eigenfunction captures the primary trend, while the subsequent two eigenfunctions reveal secondary and tertiary modes of variability. A conceptual model developed in this study elucidates the interplay between hydrodynamic forces and morphological changes, highlighting the rotation and oscillation of tidal flat profiles in response to seasonal variations in hydrological conditions. The findings emphasize the effectiveness of EOF analysis in capturing significant geomorphological processes and underscore its potential in enhancing the understanding of tidal flat dynamics, thereby informing more effective management and conservation strategies for these critical coastal environments.

Human impacts overwhelmed climate as the dominant factor controlling lacustrine organic matter accumulation in Erhai Lake 2000 years ago, Southwest China

Climate and human activity are two important factors in regulating organic matter (OM) accumulation in the lake environment. However, when and how anthropogenic impacts have affected lacustrine OM accumulation in southwest China during the late Holocene have not yet been well defined. Here, a 16.3-kyr n-alkane record derived from Erhai Lake was used to trace OM sources and explore their connections to climate and human activity. The n-alkane distributions indicated that the dominant sediment sources shifted from terrestrial and aquatic plants to algae in the late Holocene. OM accumulation was closely related to catchment soil erosion, sediment transport, and deposition processes regulated by climate conditions before 5.0 cal. kyr B.P., following the patterns that stronger monsoon precipitation favoured more terrestrial and less aquatic OM input, and vice versa. From 5.0 to 2.0 cal. kyr B.P., the synchronous downwards trends in terrestrial OM input and precipitation intensity indicated that climate remained a major driving force for OM accumulation. However, sediment sources experienced large-magnitude and centennial-scale oscillations between allochthonous and autochthonous inputs, reflecting early human impacts appeared and lake ecosystems retained the self-regulated ability to recover from the basin-wide early moderate human disturbances. Afterwards, the increased (decreased) OM contributions from terrestrial (aquatic) plants contradicted the weakening monsoon precipitation since 2.0 cal. kyr B.P., indicating a dominant effect of human activities on OM accumulation. This change was accompanied by highly improved algae productivity and gradually elevated lacustrine trophic status, and the lake ecosystem eventually shifted into another state largely deviating from its climate-driven background due to intensified deforestation and agricultural cultivation. Regional comparison indicated that anthropogenic disturbances have temporal differences in southwest China. This study will further improve our understanding of past climate-human-environment interactions in southwest China.

The response of sediment transport and morphological evolution to storms with different characteristics

Strong wave-current interaction under the impact of storm events can induce a series of complex sedimentary processes of sediment resuspension and transport and morphology changes, significantly changing the topography of coastal zones. However, coastal sedimentary processes during storm events have not been fully understood. In this study, we developed a wave-current-sediment coupled model to investigate the response of dynamical processes to extreme storm events. The model was first validated against the observed data for both storm conditions during the 2007 Typhoon Wipha and fair-weather conditions in 2016 in the Haizhou Bay (HZB) of the Yellow Sea. The simulated results indicated that the longshore sediment transport was dominated originally by tidal effects which were significantly enhanced by wind-induced waves during the passage of the Typhoon Wipha. Storms with different characteristics correspond to two typical sedimentary dynamic response modes based on a series of numerical experiments. The tidal pumping effect (T3 + T4 + T5) and gravitational circulation term (T6) controlled the total storm-induced sediment flux, and T6 played a crucial and special role, typically in the opposite direction of the dominant wind of the storm. The strong wind could lead to the stratification of the water column, causing the down-slope or up-slope cross-shore sediment transport, resulting in coastal seabed erosion/deposition. In addition, the onshore wind was found to have a stronger impact on the sedimentary process. The methodology and findings of this study provide a scientific basis for understanding the response mechanism of sediment transport during storm events in coastal areas.

Functional connectivity related to road linear erosion at rainfall event scale in an agricultural watershed on the Loess Plateau

Intersections of road and channel networks assist in better understanding of runoff and sediment transportation process feedback using the hydrological connectivity concept. However, quantifying functional connectivity and exploring the complex relationship between hydrological connectivity and the processes of runoff and sediment associated with road networks have largely been unexamined by field work. Therefore, our team conducted a 3-year monitoring of hydrological connectivity and road erosion from selected road segments in an agricultural watershed on the Loess Plateau. Rainfall events occurring during 2020–2022 were categorized into three types using K-means clustering analysis: small amounts of low-intensity rainfall (SL-R), large amounts of low-intensity rainfall (LL-R), and large amounts of high-intensity rainfall (LH-R). The monitored roads primarily consist of paved roads and unpaved roads (agricultural roads, abandoned roads, and petroleum roads). In our study, the intersections extracted by the road and channel networks were defined as hydrological connections, and sticks wrapped with clean gauze at the hydrological connections were monitored under variable rainfall conditions for functional connectivity assessment. The runoff heights and sediment traces attached to the sticks are alternative hydrological connectivity indicators. The results revealed an obvious increasing trend in the activation frequency and degree of hydrological connectivity as rainfall types transitioned from SL-R to LL-R and LH-R. Agricultural roads had a more intense response to hydrological connectivity between upslope drainage areas and road surfaces, and petroleum roads performed well in response to hydrological connectivity between road surfaces and down hillslopes. Random forest models indicate that PI30 is the most dominant influencing factor of road erosion and hydrological connectivity between road upslope drainage areas and road surfaces. Road surface erosion rates are the primary determinants of hydrological connectivity between road surfaces and down hillslopes. These findings suggest a close interaction between road erosion and hydrological connectivity driven by these off-site effects. They offer additional evidence concerning runoff and sediment patterns and dynamics in relation to road networks and identify hydrological connectivity and road erosion hotspots. Coordinating measures for hydrological connectivity regulation and road erosion mitigation are essential for minimizing hydrological connectivity and road erosion off-site effects within watersheds.

Sediment dynamic on the tidal flat sheltered by artificial engineering: A case study on eddies

Tidal flats are important ecological and terrestrial resources. With the increasing development of coastal engineering projects, it is important to understand the dynamic response of tidal flats to human activities. Hence, the time series of wave, current, and suspended sediment during the spring tide from April 30, 2013 to May 1, 2013 were recorded by three tripod observation systems placed on the upper, middle, and lower parts of the tidal flats south of a constructed harbor in Wanggang, Jiangsu, China. The results showed that eddies formed south of the reclamations during flood tides. The directions of local tidal currents changed from their original southwest to the present northwest, and the ebb tides also changed but continued to flow northeast. The sediment transport process was basically consistent with the currents. The total net sediment fluxes during the tidal cycle (mean values of two tides) on the mudflat were two orders of magnitude larger than those on the salt marsh. Most of the sediments on both the mudflat and the salt marsh were transported northward alongshore, and a small portion of the sediments was transported seaward on the mudflat but shoreward on the salt marsh. The portion of the tidal flat with an elevation >1 m was wide in the north but narrow in the south, as determined by analysis of LiDAR elevation data, which coincided with the local sediment dynamics. The sediments were transported northward from southern deep waters during flood tides, which assisted in the expansion of the tidal flats in the north. This phenomenon reflected unique local sediment dynamics that were highly constrained by reclamations and Dafeng Harbor in the north as well as seawalls in the west. The tidal flat influenced by eddies can extend up to approximately 10 km south of the Dafeng Harbor. This case study has significant implications for the protection of tidal flats and the construction of coastal artificial engineering projects.

Analysis of sediment sources and sinks based on the RUSLE model and sediment delivery ratio model

AbstractSediment sources and sinks are an effective reflection of the comprehensive management of soil and water conservation in a watershed. However, human interference has made the sediment transport process in watersheds more complex. Research that distinguishes hillslopes and channels to reveal changes in sediment sources and sinks within a watershed and the relationships with key driving factors requires strengthening. In this study, the characteristics of sediment sources and sinks on hillslopes in the Mahuyu watershed, located on the Loess Plateau, were simulated using the Revised Universal Soil Loss Equation model and hillslope sediment delivery ratio model. Furthermore, variations in channel scour and siltation at the event scale were analysed based on the simulated hillslope sediment yield and measured sediment yield at the outlet station. Additionally, the principal hydrological driving factors affecting the sediment yield at the outlet were explored. The results show that 66 sediment yield events occurred in the Mahuyu watershed during the period 2006–2018, and channel sediment yield has emerged as the leading contributor to the watershed sediment yield, accounting for a minimum of 69.8%. There is also a marked decoupling between hillslope sediment yield and watershed sediment yield in the Mahuyu watershed. Furthermore, the maximum daily average streamflow is identified as the critical driving factor responsible for determining the watershed sediment yield, indicated by a coefficient of determination of 0.850. Therefore, we recommend that the future focus of soil and water conservation measures should be shifted from hillslopes to channels.

Estimation of the suspended sediment concentration in ice-covered channels based on the gravitational theory

Rivers in cold high latitude northern regions usually freeze during winter, thereby forming an ice cover on the water surface. The additional ice cover significantly changes the flow characteristics of the rivers and hence affects the process of water and sediment transport. Understanding the vertical profile of suspended sediment concentration (SSC) in ice-covered channel flows is helpful for investigating the sediment deposition and the bed morphology of rivers located in cold regions. To predict the SSC distribution in ice-covered channels, this study is based on the gravitational theory and puts forward a new SSC model by dividing the flow underneath the ice cover into the upper ice layer and the lower bed layer with the maximum streamwise velocity and the zero Reynolds shear stress as the interface. The calculated results of the new SSC model agree well with the collected experimental data, and the accuracy of the prediction results is evaluated by error analysis. Besides, the differences in the governing equations and the prediction accuracy between the gravitational theory and the diffusion theory are discussed, and thus the pros and cons of the proposed SSC model are illustrated. Although the gravitational theory has the controversial point, the validation results and discussions demonstrate that adopting the gravitational theory to estimate the SSC distribution in ice-covered channels is effective. The theoretical approach used herein can provide some guidance for studying the sediment-laden flow of freezing rivers in high-latitude northern regions.

Rapid morphosedimentary development of a shallow subtropical embayment in response to extreme episodic flooding

Variability in supply of terrestrial sediments to the coastal zone impacts sediment transport processes and ecological dependencies. This study investigated surface sediment distribution and transportation within a 2020 km2 shallow embayment following a period of flood induced sediment supply. Extensive field sampling of 221 common sites between 2015 and the 2019 follow-up survey presented, identified significant reduction in fine sediment fractions, evident through a 367 km2 decrease in surface mud coverage. Wind-wave sediment transport processes controlled the spatial distribution of fine sediments, exporting 23 million tonnes of mud from the shallow sedimentation zones. In a broader context, sensitive benthic habitats such as seagrass meadows exhibited signs habitat recolonisation coinciding with areas of reduced mud content. Despite these promising short-term outlooks, the developed fine sediment budget suggests ongoing terrestrial sedimentation will exhaust key sediment sinks within Moreton Bay, effectively reducing the system's ability to buffer future sedimentation events.

Insights into glacial processes from micromorphology of silt-sized sediment

Abstract. Silt-rich meltwater plume deposits (MPDs) analyzed from marine sediment cores have elucidated relationships that are clearly connected, yet difficult to constrain, between subglacial hydrology, ice-marginal landforms, and grounding-zone retreat patterns for several glacial catchments. Few attempts have been made to infer details of subglacial hydrology, such as flow regime, geometry of drainage pathways, and mode(s) of sediment transport through time, from grain-scale characteristics of MPDs. Using sediment samples from MPD, till, and grounding-zone proximal diamicton collected offshore of six modern and relict glacial catchments in both hemispheres, we examine grain shape distributions and microtextures (collectively, grain micromorphology) of the silt fraction to explore whether grains are measurably altered from their subglacial sources via meltwater action. We find that 75 % of all imaged grains (n = 9400) can be described by 25 % of the full range of measured shape morphometrics, indicating grain shape homogenization through widespread and efficient abrasive processes in subglacial environments. Although silt grains from MPDs exhibit edge rounding more often than silt grains from tills, grain surface textures indicative of fluvial transport (e.g., v-shaped percussions) occur in only a modest number of grains. Furthermore, MPD grain surfaces retain several textures consistent with transport beneath glacial ice (e.g., straight or arcuate steps, (sub)linear fractures) in comparable abundances to till grains. Significant grain shape alteration in MPDs compared to their till sources is observed in sediments from glacial regions where (1) high-magnitude, potentially catastrophic meltwater drainage events are inferred from marine sediment records and (2) submarine landforms suggest supraglacial melt contributed to the subglacial hydrological budget. This implies that quantifiable grain shape alteration in MPDs could reflect a combination of high-energy flow of subglacial meltwater, persistent sediment entrainment, and/or long sediment transport distances through subglacial drainage pathways. Integrating grain micromorphology into analysis of MPDs in site-specific studies could therefore aid in distinguishing periods of persistent, well-connected subglacial discharge from periods of sluggish or disorganized drainage. In the wider context of deglacial marine sedimentary and bathymetric records, a grain micromorphological approach may bolster our ability to characterize ice response to subglacial meltwater transmission through time. This work additionally demonstrates that glacial and fluvial surface textures are retained on silt-sized quartz grains in adequate amounts for microtexture analysis, which has heretofore been conducted exclusively on the sand fraction. Therefore, grain microtextures can be examined on silt-rich glaciogenic deposits that contain little to no sand as a means to evaluate sediment transport processes.

Cryospheric and Oceanographic Evolution in the Arctic Makarov Basin Since the Early Pleistocene Revealed by Bulk Mineral Assemblages

AbstractWe present the glacial history of the Makarov Basin (western Arctic Ocean) during the last ∼1.1 Myr, with sediment provenances using the newly refined chronostratigraphy of core ARA03B‐41GC02. According to the principal component analysis of the bulk mineral assemblages, felsic minerals were dominant, and their ratios (K‐feldspar/plagioclase and quartz/feldspars) indicated that sediment supply to the Makarov Basin was mainly from the Siberian margin and partly from northern North America, including the Canadian Arctic. However, their occurrence did not vary significantly between interglacials and glacials due to the mixed sources. In contrast, clinopyroxene and dolomite indicated specific sediment origins from the eastern Siberian margin and northern North America, respectively. The clinopyroxene content followed an eccentricity cycle (∼100‐Kyr) during the early to middle Pleistocene, suggesting that the eustatic sea level changes may have influenced its input from the eastern Siberian margin. The dolomite, transported primarily by icebergs from the Arctic sector of the Laurentide Ice Sheet (LIS) in northern North America, has also followed the same cycle since the early Pleistocene. Thus, the mineralogical signals highlight that sediment origins and transport processes in the Makarov Basin were related to ice sheet evolution at the eccentricity cycle. In addition, dolomite deposition in the Makarov Basin began by the early Pleistocene (∼790 ka), ∼150 ka earlier than in previous North Atlantic records (∼640 ka), indicating the earlier LIS calving in the Arctic sector than in the Atlantic sector.

A Morphological Assessment On The Effects Of Embankments On Sediment Transport In Sandy Estuaries

Many large estuaries around the world are engineered to some degree for both flood protection of coastal communities and maintenance of economically vital shipping routes. Engineering challenges arise due to the highly dynamic morphological nature of estuaries, where the complex processes of sediment transport are driven by an interplay between the turbulent flows of rivers and tides. Naturally, the planforms of sandy estuaries have the tendency to converge landwards exponentially, where deviations from this shape provide alternating space and constrictions where tidal sandbars and channels form (Leuven et al., 2018-a; 2018-b). In the case of embankment by bank protection and dikes, such topographically forced channels can develop deep scour holes that endanger bank stability. Our objective is to study effects of channel and planform dynamics on scour depth variation near protected banks. To this end, we conducted scale experiments of estuaries with completely fixed banks in the tilting tidal flume the Metronome (Kleinhans et al., 2017). We study the effects of these fixed banks on the morphological behaviour of the channel pattern through one experiment, two repeat experiments, and a control without fixed banks. By collection of much longer timeseries than available in nature, we use the experiments to shed light on the topographic forcing and channel dynamics of these systems, and on the repeatability of Metronome experiments.

Holocene sedimentary distribution and morphological characteristics reworked by East Asian monsoon dynamics in the Mekong River shelf, South Vietnam

The distribution of Holocene sediments in the Mekong River shelf area was investigated using shallow seismic and core data. The results show that the shelf depositional stratigraphy since the last glacial period can be classified into four depositional units (U1, U2, U3 and U4). The average thickness of the Holocene deposit in the nearshore area of the Mekong River estuary is approximately 20 m and decreases seaward. In addition to a thickened deposit in the palaeo-river channel, two sedimentary centres have developed, namely, the Mekong River estuary and the Cape Ca Mau area. In contrast to nearshore deposition, the Holocene sedimentation rate rapidly decreased offshore, even in the transgressive systems tract. To understand the driving mechanisms for the variation in Holocene deposition, a three-dimensional numerical model and Geographic Information System(GIS)-based model were applied to reconstruct the oceanographic and sediment transport patterns on the South Vietnam shelf. The simulation results reveal that the Mekong River shelf has undergone significant transformations since the Last Glacial Maximum (LGM). During the LGM, the coastline was situated near the shelf break, and as the sea level rose rapidly, the shoreline retreated. When the sea level reached its peak, the modern Mekong Delta emerged. Furthermore, the distribution of Holocene deposits is controlled by monsoon-influenced seasonal variations in currents, resulting in a triangular delta and narrow strip-like subaqueous clinoform. Our simulation results also show obvious erosional capacity in the offshore area, leading to an evident erosional through to the east of Cape Ca Mau and erosion of Pleistocene sediments in the shelf break area. By combining stratigraphic interpretation and oceanographic simulations, our study provides new insights into sediment transport and erosion processes mediated by ocean currents driven by East Asian monsoons.