Sediment Transport Processes Research Articles

Using rare earth element tracers to investigate ridge slope erosion process in contour ridge system under extreme rainfall

Extreme rainfall is becoming more frequent and intense, which no doubt increases the risks and uncertainty of water erosion in widely-used contour ridge systems. Ridge slope erosion can increase the probability of contour failure occurrence and then induce contour ridge losing anti-erosion properties. However, there is little understanding of ridge slope erosive process controlled by contour ridge system responding to extreme rainfall. In this study, five rare earth elements were applied in different ridge segments and soil layers to monitor the successive development of ridge slope erosion process under extreme rainfall in contour ridge system. Simulated rainfall experiments at high rainfall intensity (100 mm h−1) lasting for 60 min were conducted in runoff plots with ridge tillage practices characterized by microtopography indices and ridge geometry indices. Results showed that three sub-processes were observed for ridge slope erosion, including inter-rill erosion, headward erosion, and rill erosion. Headward erosion mainly resulted from the break-point at ridge surface and sidewall collapse at ridge toe, and the pouring of ponding water in furrow greatly promoted rill development. Erosion rate was the largest during headward erosion process, and rill erosion contributed the most to eroded source. The dominated erosion process was transport-limited during inter-rill erosion process, and detachment-limited during headward and rill erosion processes. Sediment transport and deposition processes as well as their transformation occurred simultaneously after the development of headward erosion. Eroded sediment was mainly from the low part of ridge, which was transported by raindrop-impacted sheet flow or overflow. These findings provide better responding of agricultural tillage to global climate change and references for the development of physically-based erosion models.

Modeling the effects of interior headland restoration on estuarine sediment transport processes in a marine-dominant estuary

The effects of interior headland restoration on estuarine sediment transport processes were assessed through process-based numerical modeling. Three proposed interior headland restoration scenarios in the Grand Bay estuary (Mississippi/Alabama) were modeled using Delft3D to understand impacts on suspended sediment concentrations, bed level morphology, and sediment fluxes under present-day conditions and a sea level rise (SLR) of 0.5 m, representing a high projection of SLR by the year 2050. Model results showed localized differences in bed levels near the restored features after a year of simulated morphologic change. The restored headland features acted as a sediment source to the immediate surroundings while also providing some non-significant sheltering effect of backshore shoals and marsh shorelines. Sediment fluxes were sensitive to wind directions and the presence of the restored headlands. However, regardless of wind direction, mean sea level, or restoration action, the greatest sediment fluxes were always export fluxes from the estuary, which were further increased with increased sea level. Suspended sediment concentrations were highly influenced by SLR in a non-linear manner. Sediment concentrations both increased and decreased depending on depth under SLR. Furthermore, SLR allowed for the suspension and deposition of sediments on the marsh platform. Overall, the influence of SLR was more impactful to changing sediment dynamics than the influence of the restoration features.

Characteristics of the Sediment Transport Process in Vegetation Hillslopes under Different Flow Rates

Vegetation filter strips (VFSs) have always been an important measure to control agricultural soil erosion, especially in mountainous and hilly areas with more sloping farmland. To investigate the mechanism of the sediment-trapping process by VFSs, a series of tests were conducted with four gradients of flow rate, 7.5–45 L min−1 m−1, and two different sediment concentrations of 40 and 120 g L−1. The whole process of overland flow was monitored, and sediment and particle size samples from the inflow and outflow were collected and measured. The results showed that the changes in sediment concentration did not significantly affect the corresponding coefficients in the power function relationship between overland flow rate and velocity. Using the Reynolds number alone cannot effectively indicate the flow pattern of overland flow on vegetation hillslopes. The peak particle size and linear function were effective in describing the relationship between sediment particle composition and delivery rate during the sediment-trapping process by VFSs. During the sediment-trapping process, the sediment-trapping capacity of VFSs continued to decrease. The increase in sediment discharge was accompanied by a higher proportion of coarse particles. Under the same flow rate conditions, when the sediment concentration was higher, the coarse particles and their proportion also increased faster. Therefore, using only a certain particle size threshold to distinguish suspended and transported sediment may lead to inaccurate estimation of the sediment-trapping performance of VFSs. This study deepened the understanding of the mechanism of water–sediment processes on vegetation hillslopes and promoted the widespread and efficient application of VFSs management technology.

Bridging Computational Stratigraphy and Reservoir Simulation for Geologically Realistic High-Resolution Reservoir Modeling

Summary Computational stratigraphy (CompStrat) is a state-of-the-art Earth-modeling method that captures the key heterogeneities in subsurface reservoirs through modeling of the detailed flow and sediment transportation processes in various depositional environments. The method is fully based on physics and generates high-resolution 3D Earth models that are much more geologically realistic than those generated by traditional Earth-modeling methods. It can accurately predict and preserve those spatially continuous but vertically thin and volumetrically insignificant layers, such as shale layers, thus enabling a much more accurate representation of natural reservoir connectivity. In the past few years, CompStrat has been studied mainly within the Earth science community and has not been broadly applied yet in reservoir simulation research and practices. Our objective is to bridge this gap and allow this frontier technology to offer geologically realistic Earth models for reservoir simulation to better understand how various geological features contribute and control subsurface flow patterns and performance, subsequently leading to better integration among Earth modeling, flow simulation, and more reliable reservoir performance predictions. CompStrat models often have large numbers of cells (hundreds of millions or more). A large proportion of them are related to thin shale layers. These thin cells can often cause convergence difficulties in reservoir simulations. We developed a grid coarsening method to dramatically reduce the cell number and the simulation time with minimal alteration of the overall model connectivity characteristics. The method reduces the cell number by 85% (to 93%) and the simulation time by 94% (to 99%) with limited loss of accuracy for representative examples. Without this method, the simulation may take impractically long time to run for large models with complex multiphase flow dynamics. The successful removal of the computational bottleneck enables the application of this frontier Earth-modeling method in high-fidelity reservoir simulation. It also facilitates a detailed understanding of the connection between geology and flow to offer valuable insight for reservoir modeling, production forecast uncertainty analysis, and history matching. We developed a method to label, evaluate, and rank geological features based on their influence on flow performance, with shale layers being the specific focus. The labeling is performed semiautomatically, and the evaluation and ranking are done efficiently with a reduced-physics solver. The result is statistically consistent across multiple realizations. The fluvial-deltaic CompStrat sector model on which our grid coarsening and shale analysis methods were tested is shared in the Supplementary Materials.

Agricultural activities lead to sediment infilling of wetlandscapes in the Canadian Prairies: Assessment of contributions by tillage, water and wind erosion

Soil erosion and sediment delivery models have been increasingly employed in studies of catchment sediment dynamics in current years. These models are also used to represent the spatial interaction of soil erosion and sediment transport processes, thereby providing spatially-distributed predictions of soil redistribution rates for agricultural landscapes. It has been widely recognized that individual soil erosion processes and their interactions contribute towards total soil erosion; however, quantifying the rates and patterns of soil erosion processes and their interactions within topographically complex landscapes is challenging. Therefore, the objective of this research was to estimate and model the relative contributions of tillage, water and wind erosion towards total soil erosion in the Canadian Prairie provinces of Manitoba and Alberta using 137Cs, a fallout radionuclide tracer, and three well-established models: (i) Tillage Erosion Model (TillEM); (ii) Revised Universal Soil Loss Equation, version 2 (RUSLE2); and (iii) Single-event Wind Erosion Evaluation Program (SWEEP). The findings indicate that the patterns of 137Cs-estimated soil erosion were closely matched with the erosion pattern predicted by TillEM and suggests that tillage erosion dominates the pattern of total soil erosion on the knolls of hummocky landscapes. Additionally, soil particle size variation within the wetland catchments reflected the modeled patterns of water and tillage erosion. Furthermore, our findings confirmed that colour coefficients are useful in identifying spatial heterogeneity of soil within wetland catchments and reflect the patterns of soil loss and gains. These results indicate that water and tillage erosion, and also their interactions, are main erosion processes in the Canadian Prairie Pothole Region (PPR), but that soil movement by tillage practices has been the predominant redistribution process.

In-situ characterisation of fluvial dune morphology and dynamics under limited sediment supply conditions, Seine River, France

Dune dynamics and associated sedimentary fluxes have been estimated over a period of a week in the river Seine characterised by limited sediment supply. Distributed into two corridors, typical medium and small dunes associated with fluvial environments have been characterised from multibeam data acquired on four dates, 23-29 March 2021, after a limited peak flood. Dune morphometric parameters have been quantified on cross-dune 1 m spacing of bed elevation profiles and dune migration rates have been calculated by analysing successive multibeam acquisitions. Evidence of limited sediment supply conditions have been recognised from areas uncovered by mobile sediment in the troughs between medium dunes. Under such conditions, bedload transport associated with dune migration has been estimated using three methods: differential bathymetry, dune tracking and the Van Rijn empirical formula using a correction factor accounting for limited sediment supply conditions. The three methods provide similar bedload fluxes (less than an order of magnitude difference), and the small differences are related to input data uncertainties and the capability of these methods to capture sediment transport processes, such as sediment bypassing dunes. Based on field observations, this paper brings new constrains to the relationships between dune morphological parameters, migration rates and sedimentary fluxes and environmental parameters (sediment grain size, flow velocity and water depth) under limited supply conditions in a real river.

Numerical investigation of scour around the monopile using CFD-DEM coupling method

The risks posed by local scour around offshore monopile foundations are substantial and threaten the structural safety of these installations. It is therefore essential to study the mechanisms of local scour to mitigate these risks. Numerical simulation is an effective tool to help us understand the mechanisms of local scour. However, the accurate modeling of the local scour process remains to be challenging due to the complexity of sediment transportation, in which the continuity threatment of sediment phase is typically used, the discrete behavior of the sediment is hard to be described. Therefore, in this paper, in order to conduct the deepin investigation into the mechanism of the local scour around monopile, a three-dimensional CFD-DEM model was established by coupling the computational fluid dynamic method (CFD) and discrete element method (DEM). To increase the efficiency and accuracy of the CFD-DEM model, the coarse grain method (CGM) was used to decrease the number of required DEM particles in the simulation, and the angle of repose was used to calibrate the contact parameters between DEM particles. During the validation process of the numerical model, it was found that the development of the scour depth and the morphology of scour pit obtained by the CFD-DEM model showed good agreement with the previously published results. The CFD-DEM simulation revealed that seepage-induced vertical drag force plays a crucial role in the initial stage of local scour. Two sediment transportation processes, "Pile toe erosion-Slope avalanche" and "Push-Accumulation-Wash", were observed and the relationship between bedload sediment flux and the Shields number was quantified. In the end of this paper, the critical evaluation of the CFD-DEM method was conducted.

Impact of land use/cover change (LUCC) on sediment connectivity in small watersheds based on a revised index algorithm

Soil erosion is an important cause of global land degradation and other ecological and environmental problems. Revealing the influence mechanism of land use/cover change (LUCC) on sediment yield can provide a scientific basis for efficient comprehensive and decision-making management of watersheds. As an important indicator of sediment yield and transport capacity, sediment connectivity has been widely studied in recent years. However, the existing index of sediment connectivity (IC) ignores the influence of upslope land use/cover on confluence and the corresponding changes in downslope sediment transport. Moreover, a formula describing the upslope and downslope components does not establish an effective correlation with existing soil erosion formulas. Therefore, the index is not closely related to the erosion process and does not reflect the physical process of sediment transport and sediment deposition. In addition, there have been few reports concerning the effect of LUCC on sediment connectivity inside (on-site impact) and outside (off-site impact) the changed patches. Consequently, by based on addition of the revised parameters of the effective confluence area (Ar) and the runoff velocity factor (v) of the sediment delivery distributed (SEDD) model, an improved sediment connectivity index (ICZQ) is proposed. The results indicate that ICZQ has a significant linear relationship with annual runoff depth or sediment yield modulus in small watersheds within 100 km2 in the Loess Plateau, except for feature years containing rainstorm events (daily rainfall exceeding 50 mm). Therefore, the runoff and sediment transport capacity of small watersheds can be characterized by ICZQ under the influence of LUCC and rainfall change. In 1982–2020, driven by LUCC with an enhancement in vegetation (forest and grass), the ICZQ decreased in 90% of the area and decreased by a significant or extremely significant level over 48% of the area. To better understand the effects of LUCC on sediment connectivity, we analyzed on-site and off-site impacts with rainfall remaining unchanged to control the variables. The contribution rates of the on-site and off-site impacts caused by LUCC on sediment connectivity in Lvergou were 61% and 39%, 48% and 52%, respectively, over two typical periods (1985–1990 and 2015–2020). Due to the large proportion that off-site impacts occupied in the sediment connectivity change, the impact of LUCC on sediment connectivity cannot be ignored, especially when the associated scale is relatively small. The present study provides a quantitative method for the optimization of land use/cover patterns, such as vegetation restoration in watersheds along with a reference for further revealing the impact mechanism of erosion and sediment yield in watersheds.

Identification of Sedimentary Environments through Dynamic Image Analysis of the Particle Morphology of Beach Sediments on the East and West Coasts of Hainan Island in South China

Particle morphology is an important feature of sediments that reflects their transport history and depositional environment. In this study, we used dynamic image analysis (DIA) to measure the size and shape of beach sediments on the east and west coasts of Hainan Island in South China Sea. DIA is a fast and accurate method that can capture and analyze a large number of sediment particles in real-time. We extracted morphological descriptors of each particle, such as equivalent diameter, sphericity, aspect ratio and symmetry, and their distributions based on volume and number. We performed multivariate analysis on the particle morphological data, including alpha diversity, statistical analysis and fingerprint techniques. We found that the Shannon index, calculated by the number distribution of sediment particle morphology, can effectively discriminate between the two sites, reflecting different sediment sources, transport processes and depositional conditions. We also established a composite fingerprint based on seven morphological parameters and diversity indices, which can accurately distinguish between aeolian and hydraulic sedimentary environments. Our study demonstrates the potential application of DIA in identifying sedimentary environments and establishing sediment fingerprints. This can help us understand the sediment transport processes and depositional mechanisms in coastal areas.

Modelling shear stress distribution in ice-covered streams

Abstract Distribution of bed shear stress is the critical factor in regulating the meandering of single-thread rivers. However, the impact of ice cover on bed shear stress is largely unknown. In this study, we develop a theoretical model of cross-stream momentum balance to examine the distribution of bed shear stresses in ice-covered meandering rivers. To validate the theoretical model, field surveys were carried out in a river reach of the Red River in Fargo, North Dakota. Data monitoring was completed using an Acoustic Doppler Current Profiler to obtain time-averaged velocity profiles. Our theoretical model indicates that an ice covering develops high-shear zones near both the inner and outer banks, which might exacerbate sediment transport and enhance bank erosion. Velocity measurements confirm the results of the proposed model and demonstrate a clear impact of meandering river banks on velocity profiles and secondary flow patterns under ice cover. Based on our results, we hypothesize that ice cover increases turbulent stresses near banks, which in turn lead to the enhancement of the bed shear stress. Our work provides new insights into the impact of ice cover on bed shear stress distribution, which could play an important role in driving sediment-transport processes and the long-term morphodynamic evolution of meandering rivers seasonally covered by ice.

Assessment of pebble virtual velocities by combining active RFID fixed stations with geophones

AbstractMonitoring bedload transport in rivers is a challenging research domain teeming with technical innovations and methodological developments aimed at improving our knowledge and models of bedload processes at different spatial–temporal scales. Radio frequency identification (RFID) technology has improved sediment tracking, allowing the characterisation of transport processes of individual particles at flood‐event scales. Meanwhile, geophone sensors have enabled the long‐term continuous monitoring of seismic signals that can provide surrogate measures of bedload fluxes at local scales, during flood events and at sediment‐pulses. The combination of these two techniques could allow sediment transport processes to be linked with both flood events and sediment pulses. In this study, we used a combination of active ultra‐high frequency RFID technology and geophone monitoring stations to link the virtual velocity of tracers with seismic activity, hydraulic forcing, and the properties of the tracked particles. Single and multiple regression models show that seismic activity best explained the observed variance (81%) of the virtual velocity of particles, in comparison with discharge (58%) and stream power (63%). Furthermore, when several control variables (seismic activity and particle properties) were combined in an empirical model, the model explained 89% of the variance and allowed quantification of the portions of the variance explained by hydraulic forcing, geophonic activity and tracked particles. These results show the high potential of these combined monitoring techniques for future in‐field experiments to investigate bedload processes at different spatiotemporal scales in rivers of different morphologies.

Quantifying Morphological Changes and Sediment Transport Pathways on Comet 67P/Churyumov‐Gerasimenko

AbstractComets are active geological worlds with primitive surfaces that have been shaped to varying degrees by sublimation‐driven sediment transport and mass wasting processes. Rosetta's rendezvous with comet 67P/Churyumov‐Gerasimenko (67P) in 2014 provided data with the necessary spatial and temporal resolutions to observe many evolutionary processes on micro‐gravity worlds. Rosetta's observations have thus far revealed that many changes to the surface occurred within 67P's smooth terrains, vast sedimentary deposits that blanket a significant fraction of the nucleus. Understanding the global context of these changes, and therefore the sediment transport pathways that govern the evolution of 67P's surface, requires a thorough description of their changing morphologies and an evaluation of existing global‐scale spatial and temporal trends. Accordingly, we present a time‐resolved synthesis of erosion and deposition activity on comet 67P as it passed through its 13 August 2015 perihelion from September 2014 to August 2016. Our mapping results indicate that, around perihelion, sediment is globally redistributed inter‐regionally from 67P's more active south to the north. Equally important, however, are local, topographically influenced sediment transport processes, with large volumes of sediment moving intra‐regionally over sub‐kilometer distances. We also show evidence for regions of near‐zero net erosion/deposition between approximately 30°N–60°N latitude, which may act as terminal sedimentary sinks, with the remobilization of these materials hindered by multiple factors. Our work therefore provides the most complete mapping of sediment transport processes and pathways across 67P, a critical step toward understanding the global landscape evolution of both 67P and other comets.

Morphological adjustments of the Yamuna River in the Himalayan foothills in response to natural and anthropogenic stresses

AbstractRivers can adjust to natural and anthropogenic stresses through changes in flow regime and sediment dynamics. Flow regulation due to dam construction can modify sediment transport processes, causing morphological adjustments and changes in conveyance capacity. In this study, we explored morphological adjustments along a 46 km segment of the Yamuna River in the Himalayan foothills from Dakpathar Barrage to Hathni Kund Barrage using a combination of remote sensing approaches. The cloud computing platform Google Earth Engine (GEE) was used to analyse multi‐temporal collections of Landsat satellite imagery acquired between 1989 and 2021. Active river channels (including water and exposed sediment) were classified using an MNDWI and NDVI thresholding approach from annually resolved temporal composite images. Declassified CORONA satellite imagery (acquired in 1965, 1973 and 1976) was manually digitized to provide longer‐term insights into channel change. Morphological adjustments were assessed using the RivMAP toolbox, providing quantitative information on the rates of bank erosion and accretion, bank line shifts and changes in active channel width. Results showed a substantial narrowing of the active channel after 2013, which coincided with the construction of the Lakhwar‐Vyasi Dam. We calculated a 67% reduction in mean active channel width, narrowing from ~800 m in 1989 to ~250 m in 2021. During the same period, evidence of sand mining in the active channel indicates a substantial increase in mining activities after 2015. The concept of stasis was explored under different flow regulation scenarios to suggest that the river has undergone a state of inactivity, where the river is doing nothing for much longer durations. We suggest that the combined effects of flow regulation and increased mining activities during the same period have altered river morphology, and further stresses due to a combination of human activities can be damaging for the river regime. This study sheds light on the potential implications of multiple stresses acting on the Yamuna River at the same time, which can inform future sustainable river management activities.

Wind-Wave-Current Coupled Modeling of the Effect of Artificial Island on the Coastal Environment

The effect of artificial island on the geomorphologic processes in the coastal area under the coupled hydrodynamics, wave, and sediment transport system is a complicated and multi-scale problem. Studying these dynamic processes will suggest how coastal ecological restoration should be conducted. In this study, a unified, unstructured, gridded coupled hydrodynamics, wave, and sediment transport model and a topographic evolution model were adopted. Based on the field observations of water depth, velocity, suspended sediment concentration, bed sand, and quaternary thickness, a high-spatiotemporal-resolution numerical simulation of the offshore dynamic environment under the disturbance of artificial island was performed, and the accuracy of the calculation was verified. The research showed that the coupling system with an unstructured mesh was able to reproduce the flow and sediment transport processes with acceptable accuracy. The contracted flow zone between the artificial island and the coastline, the runoff and alongshore current from the river, as well as the tidal flow from the ocean, worked together to mold the local complex morphology around the artificial island. The coupled modeling system, supported with parallel computation, can be used to study coastal environments with small-scale wading structures.

Avulsion dynamics determine fluvial fan morphology in a cellular model

Abstract Fluvial fans are large, low-gradient depositional systems that occur in sedimentary basins worldwide. Fluvial fans can represent much of the geologic record of foreland basins, create hazards, and record paleoclimate and tectonic signals. However, we lack an understanding of how fluvial fans grow into the variety of shapes observed around the world. We explored this aspect using a cellular model of foreland basin landscape evolution with rules for sediment transport, river avulsion, and floodplain processes. We tested the hypothesis that avulsion dynamics, namely, avulsion trigger period and abandoned channel dynamics, are a primary control on fluvial fan development. We found that shorter trigger periods lead to rounder planform fluvial fan shapes because, between avulsions, channel aggradation (and thus avulsion setup) propagates shorter distances from the upstream boundary along channel pathways. This prioritizes lateral sediment dispersion, creating shorter, rounder fans, over sediment delivery further into the basin, which would create elongated fans. Modeled fans with abandoned channel attraction (but not repulsion) generated a commonly observed abrupt fan boundary marked by a transition from distributary to tributary channel patterns. While fluvial fans are thought to be linked to climate, they can occur anywhere that rivers aggrade, lose lateral confinement, and preserve alluvial topography. Instead, fluvial fans might be more recognizable in environments that frequently trigger avulsions and preserve abandoned channels that capture future avulsions.

Sedimentary provenance in continental rifts: U–Pb detrital zircon, Nd and Sr isotopes and lithogeochemistry of the Eocene alluvial sandstones of the Resende Basin, SE–Brazil

Multiproxy analysis of detrital minerals is a powerful tool for unravelling the source-to-sink evolution of syn-rift sedimentary basins that represent important depositional settings during major rift events. The Cenozoic Resende half-graben in SE Brazil is a classic example of a syn-rift basin developed under oblique extension, which preserves evidence of tectonic episodes and palaeoclimatic conditions during the opening of the Atlantic. Sediment in the basin was sourced from the surrounding basement rocks, which to the faulted margin consists of older Proterozoic orthogranulites and orthogneisses, whilst to the flexural margin is characterised by Neoproterozoic passive margin paragneisses. Cretaceous intra-plate alkaline stocks crop out at the western edge and in the central-south basement of the basin and represent a distinct provenance fingerprint. This study uses new detrital zircons U–Pb ages combined with previously published elemental and Nd and Sr isotopic data to characterise the sedimentary provenance of Eocene alluvial deposits and to understand how sediment sourcing has changed throughout the evolution of the basin. U–Pb detrital zircon analyses from samples collected across the Resende Basin are presented herein. Samples yield a spread of detrital age spectra from the Archaean to the Cretaceous. Precambrian zircons range from ca. 3228 Ma to ca. 542 Ma. Cretaceous zircons yield ages between ca. 138 Ma and ca. 67 Ma. The sediment source location is interpreted by comparing the yielded detrital zircon spectra with the age spectra of zircons from the basement rocks. The dominance of detrital zircons yielding ages of ca. 1500 Ma and ca. 1000 Ma is interpreted to be sourced from the Neoproterozoic basement rocks to the flexural border of the half-graben. Cretaceous zircons from samples with whole rock εNd40Ma > −10 are found in both the western and eastern portions of the half-graben but not in the central portions. It is interpreted that an along-axis fluvial system was responsible for a small increase in the compositional maturity of the Resende sandstones from west to east, resulting in the progressive dilution of Cretaceous alkaline igneous detritus along the basin axis. Preserved metamorphic rims in zircon grains indicate low transport abrasion, suggesting that Cretaceous zircons in the eastern depocentre could be associated with other small proximal sources. U–Pb zircon analyses and Nd and Sr isotopes provide reliable insights regarding sediment provenance and transport processes, whilst element geochemistry supplies evidence for hydraulic sorting effects. Consequently, the data represents a more robust estimation of the provenance patterns of the Resende Basin, showing chemical and geochronological differences that support the division of the basin into three depocentres (western, central, and eastern) connected by a longitudinal fluvial system sourced mostly by sediments coming from the south-western flexural border. This study highlights the usefulness of using multiple provenance proxies to unravel the complex development of rift basins, assessing multiple source terrains and the variable degrees of mixing in sedimentary basins.

The characteristics and origin of chlorite coats in the Oligocene gas sandstone reservoirs of the Upper Huagang Formation, Xihu Depression, East China Sea Shelf Basin

Clay coats are widespread in the Oligocene braided river channel sandstones in the Xihu Depression on the East China Sea Shelf Basin. High clay coat coverage in braided channel sandstones is typically associated with good reservoir quality. The morphology, origin and distribution of clay coats and their effect on reservoir quality in different braided channel facies are investigated by core description, core analysis and a series of petrographic techniques. The present clay coats consist of two parts, the inner and outer coats. The inner coats are mainly composed of illite and chlorite components. The inner illite coats are transformed from smectitic clay precursors, while the inner chlorite coats may originate from thermally driven recrystallization of Fe-rich clay precursors. The outer clay coats, composed of well-crystallized chlorite, are possibly the result of neoformation directly from Fe- and Mg-rich pore water. The distribution of inner clay coats is controlled by sediment transport and deposition processes. The inner illite coats that are better developed in terms of content and thickness typically form in low energy braided channels due to relatively less transport abrasion of the smectitic clay precursors. The inner chlorite coats are observed exclusively in high energy mud clast-rich braided channels. The abundance of mud clasts and Fe-rich clay precursors are likely derived from the intense erosion of adjacent overbank mudstones, which are enriched in flocculated fluvial Fe oxides and hydroxides. The high coverage outer chlorite coats tend to occur in high energy braided channels with well-developed primary pores, since they provide sufficient growth space for chlorite. High energy braided channels with relatively low ductile component content generally maintain better porosity and permeability, which facilitates the development of the outer chlorite coats, thereby preserving more pore space by inhibiting quartz cementation and promoting subsequent dissolution of unstable silicate minerals, further improving reservoir quality. The results of this study provide new insights into the prediction of chlorite-coated reservoir quality of gas fields in Xihu Depression while also being relevant to braided river sandstones having comparable depositional settings worldwide.

Suspended-sediment transport related to ice-cover conditions during cold and warm winters, Toudaoguai stretch of the Yellow River, Inner Mongolia, China

The presence of winter ice in cold regions changes the water level, flow rate, velocity distribution, and other parameters of the river, which in turn affects the sediment concentration and channel evolution. Based on data obtained from Toudaoguai Hydrological Station from 1959 to 2021, this study examines the characteristics of the ice regime during cold and warm winters and the water and sediment transport processes along the Yellow River in Inner Mongolia in the context of climate change. The Mann–Kendall test and trend analysis were applied to define the years of temperature mutations and their trends, and the temperature mutation point was determined to be the 1987/1988 season. The study considers the effect of climate change on the combination of hydrological and hydraulic conditions. Therefore, trends in suspended sediment transport, ice type formation, water discharge, and storage in different ice flood seasons (November 1 to March 31, from 1998 to 2021) were attained. Based on the cumulative negative air temperature, winters were categorized into three types, warm, normal, and cold (52.2%, 17.4%, and 30.4%, respectively). Strong and weak grades further divide cold and warm winters, and statistical analyses were used to examine the characteristics of ice, water discharge, channel storage, and sediment transport. The duration of open water, freeze-up, ice cover, and breakup periods were calculated, and the relationship between the suspended sediment transport rate and discharge rate in these various ice periods was defined. The obtained relations show that the suspended sediment rate during the ice cover and first drift was smaller than that during the open water and post-breakup conditions. For the ice cover period, the sediment transport rate was on average approximately four times smaller than the freeze-up condition and six times smaller than the open water condition. The reduced sediment transport rate in the freeze-up period can be attributed to the weakened vertical turbulent mixing and increased flow resistance.

Positive Feedback Between the Tidal Flat Variations and Sediment Dynamics: An Example Study in the Macro‐Tidal Turbid Hangzhou Bay

AbstractTidal flat variations (curvature) in macro‐tidal turbid bays highly correlate with sediment dynamics. Interactions between tidal flats and sediment dynamics in macro‐tidal turbid estuaries, like Hangzhou Bay, China, have been studied through a fully validated 3D sediment model by considering high‐turbidity, flocculation, and water‐sediment density coupling. Numerical results show that the circulation and sediment flux in the curvature of Andong tidal flat have similar magnitudes in the along‐estuarine and lateral directions, demonstrating the important impact of the geometry on hydrodynamics and sediment transport processes. Sediment moves toward the southern/northern bank near the bottom/surface level in curvature. Tidally averaged net sediment flux is southward in the curvature. Tidal flat reclamation changes the curvature of the tidal channel, and modulates the circulation and sediment transport, and then feeds back to the tidal flat evolution subsequently. Bottom friction, nonlinear advection, and centrifugal force are dominant forces for inducing the lateral circulation, followed by the Coriolis force and sediment‐induced baroclinic process. Lateral circulation combined with the suspended sediment concentration (SSC) profile controls the distribution of instantaneous and tidally averaged net sediment fluxes in the curvature. The sediment flux positively correlates with the curvature of channel bends. The increased curvature of channel bends enhances the southward sediment fluxes mainly by increasing the effects of the centrifugal force, nonlinear advection, and Coriolis force on the lateral circulation and SSC. This study revealed a two‐way feedback system between tidal flat and sediment dynamics, which contributes to the impact of anthropogenic activities on geomorphological evolution of similar estuaries worldwide.

Balancing Sediment Connectivity and Energy Production via Optimized Reservoir Sediment Management Strategies

AbstractSediment connectivity plays a fundamental role in sustaining ecosystem goods and services in fluvial systems, including hydropower production. Dams alter the natural processes of sediment transport by trapping sediment and reshaping downstream hydrology and geomorphology. Due to these processes' interconnected nature, dams' impacts extend in time and space beyond the dam site to the entire river system. System‐scale approaches to reduce dam impacts commonly only consider dam siting, overlooking the potential of sediment management strategies integrated into the dam operations to offer more flexible solutions for mitigation. Herein, we contribute a sediment routing model (D‐CASCADE) to assess the impacts of reservoirs and their management strategies on river sediment connectivity. D‐CASCADE is applied to the 3S river system, a tributary of the Mekong River, a hotspot of potential dams in the Lower Mekong. We analyze three dam development portfolios. The effect of reservoir management is examined by assessing daily sediment delivery with specific dam release strategies. Model results predict sediment yield to the Mekong to reduce by 31%–60%. Finally, we explore trade‐offs between hydropower generation and sediment connectivity across cascades of multiple reservoirs. Results show that repeated flushing operations during the early wet season could significantly increase sediment delivery with minimal (max 6%) hydropower losses. While poor trade‐offs between sediment and hydropower have been locked‐in in the Mekong, our results highlight the potential of including sediment connectivity models in multi‐objective decision‐making frameworks to devise integrated water and sediment management strategies that mitigate connectivity disruptions while minimizing losses in other sectors.