Sediment Deposition Patterns Research Articles

Morphological and geological responses of barrier estuaries to dam-opening systems

River discharge depending on the dam gate operation method is an important factor underlying changes in estuarine sedimentation dynamics, but comprehensive studies are limited by data collection constraints. The hydrological operation of Nakdong Dam, designed to prevent seawater intrusion by opening during ebb tides and heavy rains and closing during flood tides, presents an ideal setting for such investigations. This study focused on the Nakdong River Estuary (NRE), utilizing aerial photography, bathymetry data, and sedimentation rate data to determine alterations in sediment transport and deposition patterns within the two tributary estuaries. Spatial discrepancies in sedimentation patterns across the eastern and western regions of the NRE were particularly evident in the spatial variations of textural characteristics and sedimentation rates. An enhanced ebb tidal prism resulting from dam gate opening in the eastern region caused the development of erosional topography and a rough depositional environment, forming a landward migrating (transgressive) barrier island system. Conversely, the predominantly closed dam gates in the western region maintained a consistent deposition pattern due to a regular tidal prism, facilitating the formation of sedimentary topography and fine sediment deposition. Consequently, barrier islands followed a seaward building (prograding) system. Therefore, the results suggested a role of modern dam gate operation as a major sedimentation mechanism, providing valuable insights for the sustainable management of estuarine sediment environments.

Research on sedimentation characteristics of the end bend water extraction project of cascade reservoir backwater

Abstract The operation mode of upstream and downstream cascade power stations significantly impacts the river bed in the water intake section, leading to complex sediment deposition characteristics in the curved river section at the reservoir’s end. In this research, a standard physical model for bed load and suspended load at a scale of λl=λh=100 was conducted, combined with the operation mode of upstream and downstream power stations, to investigate the sediment deposition patterns in the intake section and to study the sediment deposition characteristics in the intake area. The experimental results reveal that bed load sediment predominantly accumulates on the right bank (convex bank) of the river, with no bed load sediment accumulation at the mushroom head layout of the water intake project on the left bank (concave bank). When the operation of upstream and downstream power stations is uncoordinated, suspended sediment settles in the water intake section, with no sediment accumulation in the local area of the mushroom head of the intake due to the disturbance effect. Safety sensitivity analysis indicates that the safety of the water intake project can be ensured.

Influence of Marine Currents, Waves, and Shipping Traffic on Sulina Channel Fairway at the Mouth of the Black Sea

This study comprehensively explores the intricate hydrodynamic and geomorphological processes that affect the Sulina Channel and bar area. It employs advanced hydrodynamic, wave, and sediment transport models to simulate the influence of marine currents, waves, and shipping traffic on sediment transport and deposition patterns, providing valuable insights for maintaining navigable conditions in the Sulina Channel. It is shown that sediment deposition is highly dynamic, particularly in the Sulina bar area, where rapid sediment recolonization occurs within one to two months after dredging. The simulation indicates that vessels with drafts of 11.5 m cause notable erosion. In comparison, drafts of 7 m have a minimal impact on sediment transport, emphasizing the importance of managing vessel drafts to mitigate sediment disturbances. This research highlights and quantifies the siltation phenomenon from the Black Sea to the mouth of the Sulina Channel, effectively addressing the challenges posed by natural and anthropogenic factors to ensure the Channel’s sustainability and operational efficiency.

Analyzing sedimentation patterns in the Naumure Multipurpose Project (NMP) reservoir using 1D HEC-RAS modeling

Naumure Multipurpose Project (NMP) featuring a 169 m high Concrete Face Rock Filled Dam (CFRD) is the proposed reservoir project in the West Rapti River with an installed capacity of 218.34 MW. Most of the rivers in Nepal carry significant sediment loads that will consequently catalyze reservoir sedimentation. This phenomenon prevails as the primary factor in reducing reservoirs useful life, making ineffective for both flood control and hydroelectricity generation. Ultimately, such process of sedimentation has adverse impacts on projects economic feasibility and long-term sustainability. Therefore, the objective of this research was to examine the expected sediment deposition pattern in the NMP reservoir throughout its operational period by employing 1D HEC-RAS model to simulate the sedimentation process. The model was simulated for 10, 20, 30, 40 and 50 years. Yang’s equation as a sediment transport function, Active layer as a bed sorting method and Toffaleti as a fall velocity method were best suited for the river reach. The delta deposition was formed between 11 km and 22 km upstream of the dam region in the Jhimruk river, with the sediment deposition depth reaching peaks of about 23 m, 38 m, 39 m, 41 m and 49 m in 10, 20, 30, 40 and 50 years, respectively. Similarly, the delta deposition was formed between 13 km and 33 km upstream of dam region in the Madi river, with the sediment deposition depth reaching peaks of about 47 m, 62 m, 60 m, 68 m and 75 m in 10, 20, 30, 40 and 50 years respectively. Headcutting of delta deposition occurred between 20 and 30 year due to high flood during low stage of reservoir. Furthermore, the study revealed that about 6.22%, 11.61%, 15.94%, 22.96% and 25.65% of the storage capacity of NMP reservoir will be depleted in 10, 20, 30, 40 and 50 years respectively.

Evolution and deposition patterns of turbidity currents under complex vegetation canopies

Understanding the flow-vegetation-sediment feedback mechanism is critical for ecological restoration of gently sloped landscapes. Numerous simulation or laboratory experiments on turbidity currents over vegetation have revealed the important roles of vegetation canopy morphology and structure in this feedback mechanism. However, there is still a lack of analysis on the related physical processes. This study investigated the evolutionary characteristics and sediment transport and deposition patterns of turbidity currents with different sediment size distribution in vegetation patches of different canopy morphologies through lock-exchange experiments. The roles of vegetation leaf morphology and canopy structure were explored by varying the leaf morphology and vegetation density with turbidity currents prepared using sediment samples configured with different particle sizes, which are more compatible with natural conditions. The kinetic characteristics and sediment distribution of the turbidity currents over vegetation were simultaneously analyzed, providing a new perspective for updating the methodology in laboratory experiments. The major findings of this study can be summarized as follows: 1) the general spatiotemporal evolution law of turbidity currents is applicable to complex vegetation canopy morphology; 2) canopy structure dominated by leaf morphology is an important influencing factor that needs to be taken into account in the research on vegetation resistance; and 3) the correlation between turbidity current momentum and suspended sediment transport decreases under complex conditions, and coarse-grained suspended sediment tend to limit the movement of turbidity currents and major deposition distance of the sediment. These findings can provide theoretical support for conservation of floodplain soil and water and restoration of watershed ecology.

Comprehensive risk assessment of microplastics in tidal channel sediments in amazonian mangroves (northern Brazil)

Pollution by microplastics (MPs) in mangroves is a growing concern, given its potential ecological and human health impacts. The characteristics of microplastic pollution and a risk assessment of MPs in the Amazon region's coastal sediments are still insufficient, and information about MP pollution in the benthic component of the mangrove ecosystem is lacking. We analyzed MP concentrations in the surface sediment of 9 stations in three tidal channels along the Ajuruteua Peninsula connected to the Caeté River estuary, aiming to assess the hazard level on the environment based on the Pollution Load Index (PLI). Raman and Fourier transform infrared spectroscopy determined the MP's chemical composition. The results showed that the abundance of sediment MPs ranged from 100 to 1200 items kg−1, with an average of 433 ± 261.6 items kg−1. The MPs were mainly composed of transparent and blue fragments and fibers, ranging in size from 100 to 5000 μm. Six types of polymers were identified, including alkyd varnish (AV), resin dispersion (RD), chlorinated polyethylene (CPE), polyethylene-polypropylene (PE-PP), low-density polyethylene (LDPE), and hostaperm blue (HB). Hydrodynamic processes within estuaries and tidal channels play a crucial role in explaining the concentrations found, as circulation determines the pattern of sediment deposition and the particles adhered to it. PLI risk assessment showed that all sampling sites were at hazard level I: a low level of contamination in the mangrove sediments. However, a more comprehensive and systematic monitoring campaign is needed to expand our knowledge about pollution and contamination by MPs in Amazon mangrove areas.

Cretaceous–Paleocene extension at the southwestern continental margin of India and opening of the Laccadive basin: constraints from geophysical data

Abstract. Previous geophysical investigations of the western continental margin of India (WCMI) confirm the two-phase breakup history of the margin with the first breakup taking place between India and Madagascar that created the Mascarene Basin in the Late Cretaceous and the second breakup event in Early Paleocene with Seychelles separating from India. Despite numerous geoscientific studies along the WCMI, the opening of the Laccadive basin, situated along the southern part of the margin, remains poorly constrained. In this study, we evaluate the multi-channel seismic reflection and gravity anomalies at the margin to identify the early rift signatures in conjunction with the magnetic anomaly identifications in the Mascarene Basin. The analysis led to the identification of two trends of extensional structures, a NNW–SSE-oriented structure over the Laccadive Ridge north of Tellicherry Arch, interpreted to result from ENE–WSW extension, and a SSW–NNE-oriented structure in the Laccadive basin region towards the south, interpreted to result from NW–SE extension. Previous plate reconstruction models of the Mascarene Basin using marine magnetic lineations suggest that the ENE–WSW extension observed over the Laccadive Ridge could be related to the India–Madagascar separation. We associate the pattern of sediment deposition and the presence of a Paleocene trap volcanics, linked with the NW–SE grabens observed in the Laccadive basin region, to the extension between the Laccadive Ridge and the western coast of India after the separation of Madagascar from India. We further propose that the anticlockwise rotation of India and the passage of the Réunion plume have facilitated the opening of the Laccadive basin.

Assessing the 3D distribution of soil organic carbon by integrating predictions of water and tillage erosion into a digital soil mapping-approach: a case study for silt loam cropland (Belgium)

Although agricultural intensification has generally increased crop yields, it also resulted in a range of environmental issues. These include increased erosion rates and declined soil organic carbon (SOC) stocks. In order to improve our understanding on how erosion impacts the overall SOC storage capacity of croplands, this study analyses the 3D distribution of SOC as a function of water and tillage erosion in a conventionally ploughed field in the Belgian silt loam region. We present a novel methodological framework to integrate the output of an advanced erosion model as a co-variate within a Digital Soil Mapping (DSM) approach with the objective to create detailed SOC maps. More precisely, we combined (i) the Water and Tillage Erosion Model and Sediment Delivery Model (WaTEM/SEDEM), simulating spatial patterns of soil erosion and sediment deposition due to water and tillage erosions, with (ii) a SOC sampling campaign, resulting in a SOC spatial distribution model that considers both types of erosion. The results show that, as compared to plateaus, SOC stocks are nearly half as large along eroding slopes (i.e. convex slope positions affected by tillage erosion and steep slopes affected by water erosion). Yet, they are up to twice as large in areas characterized by sediment deposition (i.e. concave positions due to tillage erosion and foot slope positions due to water erosion). Our results further show that tillage erosion has a significant influence on the SOC stocks in the top 0.7 m and in particular on the top 0.4 m. The influence of water erosion is less strong but mostly significant along the entire depth profile. Overall, this work demonstrates the relevance of considering different erosion processes when aiming to predict spatial patterns of SOC. Considering the top 1 m and a WaTEM/SEDEM application at a resolution of 10 m our 3D SOC modelling approach obtained a coefficient of determination (R2) of 0.62, a relative root mean square error (RRMSE) of 30.5 % and a relative mean absolute error (RMAE) of 26.0 %. While future work may likely lead to further improvements e.g. a more detailed SOC sampling network along the foot slopes and thalwegs in order to obtain a more spatially detailed prediction of the area characterized by depositions due to water erosion, we demonstrate the great potential of existing erosion and deposition models in doing so.

Unveiling coastal dynamics: Investigating beach sediment distribution patterns along the Lithuanian Baltic Sea coast through heavy mineral analysis

Effective coastal management requires an understanding of sediment dynamics. This study examines the distribution of heavy minerals (HMs) in two distinct coastal regions: the mainland and spit, each with unique sediment deposition patterns. The spit accumulates sediment through alongshore sediment transport (AST), while the mainland experiences erosive processes and significant human impacts. The hydrodynamic behaviour and weathering resistance of HMs enables source identification. This has helped to reveal differences between the two regions, with apatite being characteristic of spit sediments and garnet of mainland beaches. Coastal processes, such as erosion, sediment transport, and accumulation, significantly shape the distribution of the HMs, with garnets dominating erosive shores and amphiboles dominating accumulative shores. Furthermore, the study found that human-made coastal infrastructure also affects the composition of HMs and the sediment distribution through AST. This research has implications beyond the local level, providing global insights into the complexities of coastal dynamics. The study emphasises the importance of using HM analysis in conjunction with grain size analysis to understand the distribution of beach sediment composition. It also highlights the significant role of coastal processes, including anthropogenic impacts such as jetty construction or deepening port entrance canals on AST.

Pattern Analysis of Periodic Sediment Elevation Data: A Decision Support Framework for Deposited Sediment Management in Coastal Environment

This research explores sediment deposition patterns in coastal areas by employing synthetic data representing wandering set and recurrent patterns, utilizing Fourier transformation and power-law analysis to characterize frequency content and scaling behavior. The study employs fundamental models to interpret observed patterns and investigates the relevance of Poincaré's theorem in understanding recurrent patterns. Leveraging Python-based computation systems and open-source satellite data, the findings reveal contrasting behaviors in power-law exponents between scenarios, with wandering set patterns exhibiting scale-free characteristics and recurrent patterns displaying structured frequency distributions. This holistic approach advances scientific knowledge on sediment dynamics, informing evidence-based decision-making for effective sediment management and coastal resilience planning. Through its methodology and insights, the research contributes to understanding sediment deposition processes, elucidating underlying mechanisms driving coastal sedimentation, and providing practical approaches for monitoring and studying sediment dynamics in coastal environments, ultimately enhancing strategies for coastal management and conservation.

Exploring Ecological, Morphological, and Environmental Controls on Coastal Foredune Evolution at Annual Scales Using a Process-Based Model

Coastal communities commonly rely upon foredunes as the first line of defense against sea-level rise and storms, thus requiring management guidance to optimize their protective services. Here, we use the AeoLiS model to simulate wind-driven accretion and wave-driven erosion patterns on foredunes with different morphologies and ecological properties under modern-day conditions. Additional sets of model runs mimic potential future climate changes to inform how both morphological and ecological properties may have differing contributions to net dune changes under evolving environmental forcing. This exploratory study, applied to represent the morphological, environmental, and ecological conditions of the northern Outer Banks, North Carolina, USA, finds that dunes experiencing minimal wave collision have similar net volumetric growth rates regardless of beach morphology, though the location and density of vegetation influence sediment deposition patterns across the dune profile. The model indicates that high-density, uniform planting strategies trap sediment close to the dune toe, whereas low-density plantings may allow for accretion across a broader extent of the dune face. The initial beach and dune shape generally plays a larger role in annual-scale dune evolution than vegetation cover. For steeper beach slopes and/or low dune toe elevations, the model generally predicts wave-driven dune erosion at the annual scale.

Effects of vegetation density on flow, mass exchange and sediment transport in lateral cavities

Large-Eddy Simulations (LES) were used to investigate the hydrodynamics and mass transfer between the flow in the main channel and a vegetated lateral cavity. Fourteen vegetation densities (0 to 10.65 %) were tested, revealing two distinct hydrodynamic patterns. For cavities with low vegetation density (a < 3.99 %), one primary gyre in contact with the interface between main channel and cavity with high velocity was formed; the thickness of the mixing layer grew longitudinally along the interface, and regions with high vorticity and turbulence kinetic energy appeared at the interface and inside the cavity. For cavities with high vegetation density (a > 3.99 %), two gyres in contact with the interface with low velocity were formed, the thickness of the mixing layer did not grow, and the vorticity and turbulence kinetic energy were low inside the cavity. The mass transport presented the same threshold value as the hydrodynamics (a = 3.99 %). For cavities with low vegetation density, a fast mass transfer occurred through the interface between the main channel and cavity and inside the cavity, while the opposite was observed for cavities with high vegetation density. Finally, the modelled hydrodynamics was used to infer possible sediment deposition patterns and flow resistance.

Influence of check dams on water and sediment processes and deposition patterns under simulated rainfall conditions

The Loess Plateau in China has the highest soil erosion and water loss rates in the world. Therefore, check dams, which are more useful engineering measures, have been established to improve soil and water conservation in small watershed. To determine check dam configuration pattern influence on runoff, sediment yield, erosion and deposition pattern in watershed, we conducted a rainfall simulation experiment using the qiao gully small watershed as our normal-scale model. We analysed the runoff and sediment yield characteristics and deposition patterns under four types of check dam configurations: no dams constructed, dam in first order stream channel, dam in second order stream channel, and dams in both channels. Our findings revealed that the runoff and sediment concentrations increased with time, regardless of check dam configuration. The significant effects of check dams on the evolution of storm runoff processes in a small watershed. Compared to no dams, double dams best inhibited runoff reduction and lowered sediment deposition, while construction of one dam in the primary channel was more effective than one dam in the secondary channel. In summary, check dam construction reduced channel erosion, rill erosion, and bank slope collapse at the channel head, while raising the dam-controlled watershed base level. Our findings provide a reference for designing check dams in optimal configurations on plateaus such as the Loess Plateau. The results of this study are of great practical significance in revealing the process of storm runoff and identifying key parts of soil erosion–deposition in the watershed.

Understanding the linkages between land-use transitions and soil erosion/sediment deposition: A case study of the transboundary Sio-Malaba-Malakisi watershed in Kenya and Uganda

Soil erosion consequent of unsustainable land-use practices represents one of the world's most pressing environmental problems affecting many watersheds. An assessment of sediment sources and delivery paths is essential for the development of site-specific soil management strategies for curbing environmental degradation. The present study employed a physically-based lumped approach to estimate soil erosion and sediment deposition patterns across the transboundary Sio-Malaba-Malakisi (SMM) watershed. The Unit Stream Power-based Erosion Deposition (USPED) model was adopted for the study, and integrated with physically-based parameters derived in a Geographical Information System (GIS). A land-use map for 2017 was used for simulating the reference soil erosion and deposition status of the basin. Furthermore, the impacts of two land-use scenarios representing options for cropland expansion (Scenario-I), and land reclamation (Scenario-II), were evaluated for the period 2047, corresponding to a thirty-year period used in assessing catchment management planning for SMM. The annual soil erosion ranged between 0 and 460 t. ha−1yr−1, with a mean erosion rate of 160 t.ha−1yr−1 for the baseline condition. Deposition zones were mainly concentrated along stream banks and floodplain in low-lying sub-catchments. The mean soil erosion rate increased by 61 % and 16 % for scenarios I and II respectively. Overall, the modelled erosion/deposition values were correlated with available sediment data for the basin, realizing a coefficient determination (R2) value of 0.74. These results could serve as a benchmark against which future trends in land-use changes can be compared in the region and hence support the development of regional soil erosion conservation strategies.

Impact аssessment of the September 2022 disaster in the area of Karavelovo, Bogdan, and Slatina villages, Karlovo Municipality

On September 2nd, 2022, an intense and prolonged rainfall event caused severe flooding, landslides, debris flows, and groundwater rise, resulting in substantial damage, particularly in the northern slopes of Sredna Gora Mountain in Central Bulgaria. Various methodologies, including field analysis, remote sensing, and GIS tools, were employed to analyze the disaster’s causes and field conditions. Active sediment sources, deposition patterns, and mapping techniques were used to document the disaster’s impact. The results showed that the disaster resulted from the formation of numerous landslides, and rapid hydrological response led to riverine flooding, high-speed flows, and sediment deposition. An important contribution of this study is the generated landslide inventory map, that can be used for mountain road reconstruction and the planning of mitigation measures.

Incised-valley filling sedimentation in a small river valley of a wave-dominated, embayed coast in response to Holocene sea level rise, Yeongil Bay, Southeastern Korea

Incised-valley infill deposits are closely related to sea level changes during the post-glacial period of the late Quaternary and are an important topic in sea level history. In Asia, there are many large river basins that receive large amounts of sediment from the land, and the infilling deposits of incised valley systems have been well studied. Yeongil Bay is a semi-enclosed, funnel-shaped topographic feature on the east coast of the Korean Peninsula, located in the southwestern part of the East Sea (Sea of Japan). The Hyeongsan River, a small river flowing into Yeongil Bay, is flows perpendicular to the East sea. During the last glacial maximum (LGM), when the sea level was >120–130 m lower than at present in the Korea Strait, inflowing river processes are thought to have formed incised valleys. In this study, the sediment depositional patterns of incised valley fill since the LGM were examined by analyzing the sedimentary facies, optically stimulated luminescence (OSL) dates, and radiocarbon dates by accelerated mass spectrometry (AMS) of three sediment cores in Yeongil Bay. The infilling sequence consists of sediments from channel lag (>12 ka) and meandering river (12–10 ka), wave-dominated estuary (12–7.8 ka), and wave-dominated delta (7.8–0 ka) systems in ascending order. The barrier appear only in the outer part of Yeongil Bay. The barrier emerged from eroding estuary systems at 8.5 ka. Millennial-scale isochrons based on OSL and radiocarbon dates showed that fluvial and estuary systems were deposited in retrogradation and delta systems were deposited in progradation since LGM. The modern delta of the Asian large rivers were initiated by a decreasing rate of sea level rise during the mid-Holocene, and consideration of this is important for RSL. This study emphasizes the importance of accommodation capacity as much as sediment discharge as a factor in the timing of delta initiation. This study provides important insights into the incised-valley infill deposits and past environmental conditions, and may help to gain a better understanding of how coastal environments have changed with sea level changes.

Modeling the Morphological Responses of the Yellow River Delta to the Water‐Sediment Regulation Scheme: The Role of Impulsive River Floods and Density‐Driven Flows

AbstractMorphological evolution of river deltas depends to a large extent on river discharges, which are usually highly unsteady due to natural hydrological cycles and anthropogenic regulations. However, it is unclear that how and to what extent the discharge fluctuations influence the delta morphology. In this study, we focus on the morphological response of the Yellow River Delta to the Water‐Sediment Regulation Schemes, which generate impulsive floods and deliver high sediment load within a short time. Tracking the fate of fluvial sediment released by 10 historical events reveals that 51.3%, 19.7%, and 17.8% of fluvial sediment are deposited in the delta front, the subaerial delta and the prodelta, respectively. Hypopycnal and hyperpycnal flows occur alternately during different phases of the regulation schemes, and the latter contribute to 32% of the deposition volume with 10% of the duration. To explore the effects of river discharge schematizations on delta evolution, numerical experiments are conducted to compare models forced by unsteady and constant discharges, with the latter being a common practice in delta morphology modeling. We show that the constant‐discharge simplification fails to capture the highly‐depositional hyperpycnal flows, leading to dramatic underestimation of river mouth depositions. In addition, we demonstrate that including the 3D flow‐sediment interactions are critical in reproducing the correct plume structures, bed surface sediment compositions and deposition patterns. Our study highlights the importance of proper river discharge schematizations and full consideration of flow‐sediment interactions in modeling deltas affected by episodic river floods and high suspended sediment concentrations.

Bio-physical determinants of sediment accumulation on an offshore coral reef: A snapshot study

Sediments are found on all coral reefs around the globe. However, the amount of sediment in different reservoirs, and the rates at which sediments move between reservoirs, can shape the biological functioning of coral reefs. Unfortunately, relatively few studies have examined reef sediment dynamics, and associated bio-physical drivers, simultaneously over matching spatial and temporal scales. This has led to a partial understanding of how sediments and living reef systems are connected, especially on clear-water offshore reefs. To address this problem, four sediment reservoirs/sedimentary processes and three bio-physical drivers were quantified across seven different reef habitats/depths at Lizard Island, an exposed mid-shelf reef on the Great Barrier Reef. Even in this clear-water reef location a substantial load of suspended sediment passed over the reef; a load theoretically capable of replacing the entire standing stock of on-reef turf sediments in just 8 h. However, quantification of actual sediment deposition suggested that just 2 % of this passing sediment settled on the reef. The data also revealed marked spatial incongruence in sediment deposition (sediment trap data) and accumulation (TurfPod data) across the reef profile, with the flat and back reef emerging as key areas of both deposition and accumulation. By contrast, the shallow windward reef crest was an area of deposition but had a limited capacity for sediment accumulation. These cross-reef patterns related to wave energy and reef geomorphology, with low sediment accumulation on the ecologically important reef crest aligning with substantial wave energy. These findings reveal a disconnect between patterns of sediment deposition and accumulation on the benthos, with the ‘post-settlement’ fate of sediments dependent on local hydrodynamic conditions. From an ecological perspective, the data suggests key contextual constraints (wave energy and reef geomorphology) may predispose some reefs or reef areas to high-load turf sediment regimes.

Numerical modeling of local scour of non-uniform graded sediment for two arrangements of pile groups

A three-dimensional (3D) non-hydrostatic numerical model is established to investigate local scour around four aligned circular piles in uniform and non-uniform sediment mixtures and to provide information for improving scour countermeasures design. In the current study, unsteady Reynolds averaged Navier–Stokes (URANS) equations along with a Re-normalization Group (RNG) k–ε model were applied to simulate the flow field. A non-uniform sediment transport model was applied to estimate the bedload transport. The simulations lasted up to 8 h. The laboratory results of a reference single pile case in uniform and non-uniform sediment were utilized to validate the simulation results of bed elevation changes and time evolution of scour depth and reflected a good agreement. The influence of pile spacing for two arrangements of pile groups, one column (tandem) and two columns (non-tandem) of four aligned piles, on spatial/temporal variations of the bed surface in both sediment bed configurations are discussed in detail. The results reveal the significant effects of the pile spacing on the flow field and patterns of scour hole formation and sediment deposition. In general, lower pile spacing results in more scour upstream of the front pile(s) and shifts deposition more downstream due to the existence of higher energy in the wake region of the piles for both sediment types. Side scouring dominates for non-uniform sediment around each pile due to flow deviating because of the presence of relatively larger particles in front of the pile(s). For non-uniform sediment, deposition begins to develop at the exit area of the scour hole and enhances boundary resistance to scour. The knowledge gained from this study is suitable for the accurate determination of riprap dimensions around the front pile(s) and rear piles in a pile group.

Impacts of Tectonic Subsidence on Basin Depth and Delta Lobe Building

AbstractChannel avulsions on river deltas are the primary means to distribute sediment and build land at the coastline. Many studies have detailed how avulsions generate delta lobes, whereby multiple lobes amalgamate to form a fan‐shaped deposit. These studies often assume a steady subsidence and uniform basin depth. In nature, however, lobe building is disrupted by variable subsidence, and progradation of lobes into basins with variable depth: conditions that are prevalent for tectonically active areas. Herein, we explore sediment dispersal and deposition patterns across scales using measurements of delta and basin morphology compiled from field surveys and remote sensing, collected over 150 years, from the Selenga Delta (Baikal Rift Zone), Russia. Tectonic subsidence events, associated with earthquakes on normal faults crossing the delta, displace portions of the topset several meters below mean lake level. This allogenic process increases regional river gradient and triggers lobe‐switching avulsions. The timescale for these episodes is shorter than the predicted autogenic lobe avulsion timescale. During quiescent periods between subsidence events, channel‐scale avulsions occur relatively frequently because of in‐channel sediment aggradation, dispersing sediment to regional lows of the delta. Avulsion settings for the Selenga Delta preserve discrete stratal packages that could contain predominately deep channels. Exploring the interplay between tectonic subsidence and sediment accumulation patterns will improve interpretations of stratigraphy from active margins and basin models.