Channel Bar Research Articles

Channel bar development and bankline migration of the Lower Padma River of Bangladesh

Bankline migration and erosion of banks are very common phenomenon in the Lower Padma River (LPR) of Bangladesh. Therefore, the current research focus on studies addressing issues about channel bar development; the interrelationship among number of bars, bars area, river width, braiding, and sinuosity indices; and its relation to bankline shifting using remote sensing (RS) and GIS techniques. Multi-dated Landsat images of 1977, 1989, 1999, 2014, and 2017 were taken into consideration for the research. The results reveal that the river area and the width of the river have been gradually increased at a rate of about 7.81 km2 and 0.083 km per year, respectively, over the past 40 years. Contrary, the water area has been gradually decreased at a rate of about 1.66 km2 year−1. This is because the channel bar area of the LPR was gradually increasing as a result of bar formation, which is about 9.47 km2 year−1. About 192.73 km2 and 327.70 km2 lands eroded on the left and right banks, respectively, at a rate of 4.82 km2 and 8.19 km2 year−1. In the past, the LPR course was less braided. But now it has become more braided due to the formation of many new bars within the river course. It is assumed that these factors intensified the bank erosion. Moreover, erosion rates have not always been the same on both banks and affected different parts of both banks of the river due to the different braiding intensities. The area of river, where more bars are present, creates more braiding, more sinuous, and finally causes severe erosion. Additionally, the bank sediments of the LPR are dominantly silty sand (loosely compact) and easily subjected to erosion. Moreover during monsoon huge water flows from the upstream naturally exert an additional effect on the bank. In summary, these random phenomena have changed the morphology of the LPR.

Sedimentology of Shahejie Formation, Bohai Bay Basin: a case study of Es1 member in Nanpu Sag

The Es1 sandstone is a fruitful hydrocarbon zone of Eocene Shahejie Formation. Therefore, the present study comprises the subsurface facies analysis established on the description of seven wells. Methods include core observation, thin section petrography, scanning electron microscope (SEM), cathodoluminescence (CL), and X-ray diffraction to evaluate the microfacies, lithofacies, and rock characteristics. Two microfacies of lithofacies have been established as sandstone microfacies and mudstone microfacies on the basis of grain size. Among them, sandstone is further divided into four sub-microfacies on the basis of microscopic studies, rock fragments and mineral composition, which are arkose sandstone, lithic arkose, feldspathic litharenite, and litharenite. These microfacies consisting quartz, feldspar as abundant detrital grains followed by rock fragments and micas, whereas calcite, ferro-calcite, quartz overgrowth, and clay are the main cement. The Es1 sandstone consists of good primary porosity and permeability as well as good secondary porosity caused by fracturing and dissolution of unstable minerals that enhance the reservoir quality. Whereas cementation and compaction reduce the reservoir characteristics. Studied formation divided into five lithofacies including conglomerate sandstone lithofacies, medium to coarse-grain sandstone lithofacies, fine sandstone lithofacies, siltstone lithofacies, and mudstone lithofacies. Core observation, sedimentary structures, facies sequences of the well log, and sedimentary composition reveal that Es1 deposited into the lacustrine delta fan depositional environment. Rock grain size indicates that Es1 contains mixtures of grains including pebbles, sand-size as well as minor silt and clay size. Grain size varies from conglomerate to clay size. The Es1 sandstone was deposited in braided river (conglomerates sandstone), channel bar, lacustrine, fluvial channel (massive bedding, cross-bedding, and graded bedding sandstone), deltaic, and flood-plain (siltstone and mudstone) sedimentary environment. It is concluded that higher transport energies in channels deposited coarse-grain sediments in shallowest part of the lake setting. However, the energy decreases towards greater depth which deposited mudstone in central part of the lake.

Depositional characteristics of fluvial facies in gentle slope zone of lacustrine rift basins: The third member of the Dongying Formation in the Wen’an Slope of Baxian Sag, Bohai Bay Basin, China

We have integrated core, well logs, and seismic data to analyze fluvial types and reconstruct sediment dispersal patterns of the Dongying Formation in the Wen’an Slope, Bohai Bay Basin (China). We analyzed four meandering-braided fluvial successions from the Dongying Formation in the Wen’an Slope. The results indicate that the fluvial systems are dominantly northeast–southwest and consist of channel, channel bar, and flood plain subfacies, which can be further subdivided into six microfacies including braided channel, meandering channel, sandy channel bar, muddy channel bar, crevasse fan, and floodplain. With a paleoflow direction, [Formula: see text] ranges from 40° to 65°, the river belt width [Formula: see text] is estimated to be 1.47–2.64 km, whereas the main channel width varies from 0.02 to 0.29 km and its thickness/bankfull depth [Formula: see text] ranges from 1.2 to 6.4 m. The differential subsidence of rift basin during fault-depression transition period is the main control on fluvial evolution and sand body distribution. The climatic effect causes changes in river flow. During the flood season, river flow and flow rate are large, and early sediments and sand bodies are reconstructed. Lateral erosion and undercutting are very strong in this period. In the dry season, the river is dominated by weak lateral erosion. Our study also suggests that the development of thick mudstone in the upslope may limit hydrocarbon charging and migration and thick channel and channel bar deposits in the middle and downslope cross cut by faults, which are conduits for migration are likely to be the most potential reservoirs in the Bohai Bay Basin.

Hydrodynamic and geomorphic adjustments of channel bars in the Yichang-Chenglingji Reach of the Middle Yangtze River in response to the Three Gorges Dam operation

The evolution of channel bars in response to upstream damming has significant impacts on channel stability, navigation, and aquatic habitats. Here, the effects of the Three Gorges Dam (TGD) operation on downstream channel bars in the Yichang-Chenglingji Reach (YCR) were comprehensively analyzed using remote sensing images, cross-sectional profiles, and hydrological datasets. The morphodynamic adjustments of channel bars in the YCR were significantly different during the pre- and post-TGD periods (i.e., before and after the construction of the TGD). Specifically, the total area of channel bars did not exhibit any significant trend in the pre-TGD period, but displayed a significant reduction following the construction of the TGD, although the morphodynamic response of each sub-reach was different. The channel bars in the YCR were relatively stable in the pre-TGD period, but became more erodible in the post-TGD period. The length/width ratio (LWR) of the bars showed an overall increase trend during the whole period from 1992 to 2017, not changing before and after the dam construction. The water discharge that led to the greatest channel bars adjustment was 27,000–30,000 m3/s (corresponding to bankfull discharge) in the pre-TGD period and 15,000–18,000 m3/s (corresponding to the medium discharge that can submerge the surface of the bars) in the post-TGD period. In addition, the grain size of non-uniform sediment with the highest replenishment degree gradually reduced downstream; these finer sediments were the main sources of material for the channel bars in the YCR. Quantitative relationships between bars area, the most effective bar-forming discharge and suspended sediment load with the highest replenishment degree, were proposed based on the improved Delayed Response Model (DRM). Results indicate that geomorphic adjustments of channel bars in the YCR are closely related to the previous four-year flow and sediment regimes, implying a delayed response of the fluvial system to damming.

Straight from the source's mouth: Controls on field‐constrained sediment export across the entire active Corinth Rift, central Greece

AbstractThe volume and grain‐size of sediment supplied from catchments fundamentally control basin stratigraphy. Despite their importance, few studies have constrained sediment budgets and grain‐size exported into an active rift at the basin scale. Here, we used the Corinth Rift as a natural laboratory to quantify the controls on sediment export within an active rift. In the field, we measured the hydraulic geometries, surface grain‐sizes of channel bars and full‐weighted grain‐size distributions of river sediment at the mouths of 47 catchments draining the rift (constituting 83% of the areal extent). Results show that the sediment grain‐size increases westward along the southern coast of the Gulf of Corinth, with the coarse‐fraction grain‐sizes (84th percentile of weighted grain‐size distribution) ranging from approximately 19 to 91 mm. We find that the median and coarse‐fraction of the sieved grain‐size distribution are primarily controlled by bedrock lithology, with late Quaternary uplift rates exerting a secondary control. Our results indicate that grain‐size export is primarily controlled by the input grain‐size within the catchment and subsequent abrasion during fluvial transport, both quantities that are sensitive to catchment lithology. We also demonstrate that the median and coarse‐fraction of the grain‐size distribution are predominantly transported in bedload; however, typical sand‐grade particles are transported as suspended load at bankfull conditions, suggesting disparate source‐to‐sink transit timescales for sand and gravel. Finally, we derive both a full Holocene sediment budget and a grain‐size‐specific bedload discharged into the Gulf of Corinth using the grain‐size measurements and previously published estimates of sediment fluxes and volumes. Results show that the bedload sediment budget is primarily comprised (~79%) of pebble to cobble grade (0.475–16 cm). Our results suggest that the grain‐size of sediment export at the rift scale is particularly sensitive to catchment lithology and fluvial mophodynamics, which complicates our ability to make direct inferences of tectonic and palaeoenvironmental forcing from local stratigraphic characteristics.

Channel narrowing by inset floodplain formation of the lower Green River in the Canyonlands region, Utah

AbstractThe lower Green River episodically narrowed between the mid-1930s and present day through deposition of new floodplains within a wider channel that had been established and/or maintained during the early twentieth century pluvial period. Comparison of air photos spanning a 74-yr period (1940–2014) and covering a 61 km study area shows that the channel narrowed by 12% from 138 ± 3.4 m to 122 ± 2.1 m. Stratigraphic and sedimentologic analysis and tree ring dating of a floodplain trench corroborates the air photo analysis and suggests that the initial phase of floodplain formation began by the mid-1930s, approximately the same time that the flow regime decreased in total annual and peak annual flow. Tamarisk, a nonnative shrub, began to establish in the 1930s as well. Narrowing from the 1940s to the mid-1980s was insignificant, because floodplain formation was approximately matched by bank erosion. Air photo analysis demonstrates that the most significant episode of narrowing was underway by the late 1980s, and analysis of the trench shows that floodplain formation had begun in the mid-1980s during a multi-year period of low peak annual flow. Air photo analysis shows that mean channel width decreased by ∼7% between 1993 and 2009. A new phase of narrowing may have begun in 2003, based on evidence in the trench. Comparison of field surveys made in 1998 and 2015 in an 8.5 km reach near Fort Bottom suggests that narrowing continues and demonstrates that new floodplain formation has been a very small proportion of the total annual fine sediment flux of the Green River. Vertical accretion of new floodplains near Fort Bottom averaged 2.4 m between 1998 and 2015 but only accounted for ∼1.5% of the estimated fine sediment flux during that period. Flood control by Flaming Gorge Dam after 1962 significantly influenced flow regime, reducing the magnitude of the annual snowmelt flood and increasing the magnitude of base flows. Though narrowing was initiated by changes in flow regime, native and nonnative riparian vegetation promoted floodplain formation and channel narrowing especially through establishment on channel bars and incipient floodplains during years of small annual floods.

Formation, Migration, and Morphodynamic Alteration of 50 Channel Bars in Darjeeling Himalayan Piedmont Zone, India

Channel bars are common, striking fluvio-geomorphic depositional features of alluvial rivers. The study of this article has aimed to investigate the formation, migration, and morphodynamic alteration of channel bars (n = 50) in gravel braided alluvial rivers in Darjeeling Himalayan Piedmont zone. Dynamics of micro to meso bar deposition is mainly accomplished by the channel gradient, huge upstream landslide, and variation of discharge. Multi-criteria analysis method has been used to explain the variation of principal component analysis (PCA1, PCA2, and PCA3), both monsoon to post-monsoon. Bridge scouring, empirical aggradation, and degradation values are 0.1 to 1.05 m and [Formula: see text]. Migration, re-generation, and degradation of the micro bars with their optimum morphology show changes during the rainy season. In the upper ([Formula: see text]), middle ([Formula: see text]), and lower ([Formula: see text]) piedmont region, the bar dynamic area are 0.12 to 0.71, 0.12 to 2.83, and 0.10 to 5.43 km2, respectively. In total, 58% to 72% stability of the channel bars is observed in the upper piedmont region. The coefficient of determination ( R²) of channel bar area and width shows positive (0.63) relation.

Wpływ cofki zbiornika zaporowego na koryto rzeki górskiej – perspektywy badań = The impact of dam-reservoir backwater on mountain river channel – research perspectives

Dam reservoir construction is one of the most important factors shaping river-valley morphology in the Anthropocene. While a large number (>58,000) of these constructions are in operation all over the world, we remain quite ignorant of what happens upstream of them (in so called backwater zone), especially for the case of gravel-bed rivers. Existing studies have shown that adjustments of the gravel-bed river in the backwater zone differ between the initial and long-term adjustments. The initial adjustments (occurring ≈ <20 years following dam construction) are controlled by large floods and in-channel deposition which trigger bi-directional bar↔bank interactions (bank erosion causing bar growth and vice versa) resulting in channel-widening. The long-term adjustments (≈ >20 years following dam construction) are characterized by river sinuosity increa sing and channel planform stabilization resulted from deposition of fine sediment and associated vegetation expansion. The long-term adjustments are controlled by the initial river morphology, which creates accommodation space for the deposition of fine sediment and for the associated expansion of vegetation on channel bars. The multi-thread river in backwater zone is significantly narrowed, its sinuosity increase (phase 1) and the planform is stabilized (phase 2). Whereas, in the case of initially single-thread river only planform stabilization occur (phase 2). This article summarizes recent findings on the backwater effects on gravel-bed channel morphodynamics, suggesting that backwater zones may be treated as hot-spots of human-induced changes in river geomorphology and biogeomorphology.

ФОРМИРОВАНИЕ, ЭВОЛЮЦИЯ И ВРЕМЕННАЯ ТРАНСФОРМАЦИЯ ПАРАЛЛЕЛЬНО-РУКАВНЫХ РАЗВЕТВЛЕНИЙ РЕЧНЫХ РУСЕЛ1

The paper provides a comprehensive assessment of channel formation conditions and hydrological-morphological characteristics, and also considers the features of the channel changes regime of parallel-braided channel reaches, being the most complex and diverse in morphodynamics. The general condition for their development is a large width of channels (respectively, the maximum value of the criterion of the flow quasi-uniformity suggested by I.F. Karasev). In such channels, the flow is divided into two branches, between which, in the middle of the channel, occurs accumulation of sediments and formation of mid-channel bars and islands. At the same time, parallel-braided channels are the result of other channel types (non-meandering) development due to certain changes in determining factors: increased water content and sediment runoff, overgrowth of channel bars, as a result of which mid-channel bars turn into islands, and anthropogenic impacts on rivers. On sandy floodplain rivers, parallel-braided reaches are typical for low-stable and unstable channels in the absence of the directed influence from bedrock banks on the flow and when effective water discharge passes over the floodplain edge. The islands in such braided reaches are mostly small. On rivers with incised channels, they are either similar in morphology and the channel changes regime to wide floodplain unstable channels, differing from the latter in large islands with elongated shape, or form sculptural islands and are very stable. In any variety, parallel-braided channels are very individual in their channel changes regime and there are no universal channel control schemes for them.

Long-Term (1986–2018) Evolution of Channel Bars in Response to Combined Effects of Cascade Reservoirs in the Middle Reaches of the Hanjiang River

Channel bars are essential landforms and their evolution is crucial to aquatic and riparian biodiversity, river’s water-sediment process, and economic development. With the development of water conservation facilities and hydropower projects, numerous changes have been taken place in hydrological regimes and morphology. There have been many changes on channel bars in the middle reaches of Hanjiang River due to the combined effects of cascade reservoirs. However, little was known about such dynamics and their linkages to cascade dams across the entire downstream area. Using Landsat remote sensing images from 1986–2018 and the threshold binary Otsu extraction method, this study completed comprehensive monitoring of nine mid-channel bars (DX1–DX7, XZ1, and XZ2), and three shoal group (XZ3–XZ5) dynamics. Results showed that the mid-channel bars’ area in the reach from Danjiangkou to Xiangyang (DX) decreased over the past 33 years, with the exception of DX4, while the total area decreased by 23.19%, this channel bars’ area change was mainly influenced by backwater from the Cuijiaying Reservoir with high water level after 2010 (r = −0.93, p < 0.01). The total channel bar area from Xiangyang to Huangzhuang (XZ) decreased by 16.63% from 1986 to 2018. The total channel bar area in XZ had a strong negative correlation with runoff at Huangzhuang hydrologic station (r = −0.79, p < 0.05), which was partly attributed to upstream precipitation according to the high correlation between runoff and precipitation (R2 = 0.65). In general, the DX section was under equilibrium between scouring and deposition compared to downstream Xiangyang, the bars in DX section were mainly affected by water level, and bars in XZ section during 1986–2018 were complicated because it was upstream eroded and downstream deposited. In addition, vegetation cover, revetments, flood events, sand mining, land use, and over-exploitation may cause channel bar area dynamics. Hence, more continuous investigations are suggested to focus on effects of cascade reservoir operation on hydrological regime, as well as the changing morphology of channel bars in the middle reaches of the Hanjiang River.

Geology, alteration system, and uranium metallogenic potential of Pasha-Ladoga Basin, Russia

The Mesoproterozoic Pasha-Ladoga Basin is located at the margin of the Karelian Craton and Svecofennian Belt in northwest Russia. It is the only basin in Russia that is currently recognized to host unconformity-type uranium mineralization (e.g., Karku deposit). Mineralization is associated with the Priozersk Formation which comprises a basal sequence of immature arkosic sandstones and conglomerates interpreted as high-energy braided stream channel and channel bar facies within an alluvial fan depositional environment. Diagenetic and hydrothermal alteration of the Priozersk sandstones is similar to that observed in unconformity-related uranium deposits of the Athabasca Basin (Canada) and includes three main stages: (1) an early diagenetic assemblage comprising illite-smectite (K-rectorite), calcite, minor chlorite, and kaolinite precipitated at 100–150 °C; (2) a peak diagenetic-hydrothermal assemblage composed of kaolinite-dickite, Fe- and Fe-Mg chlorite, calcite, and sulfides formed at 150–250 °C; (3) late diagenetic-hydrothermal event involving fracture-controlled kaolinite and late hematite. Similar to the Athabasca Basin type uranium mineralization, the Karku deposit exhibits anomalous concentrations of LREE, Nb, Pb, and positive correlations between U and Cu, Zn, Ag, and Pb that is a geochemical signature of unconformity-type uranium mineralization. Based on oxygen and hydrogen stable isotope study, it was found that fluids in equilibrium with early and peak diagenetic clays were consistent with oxidized basinal brines and reduced basement fluids. In contrast to the Athabasca Basin, mineralizing fluids at the Karku area were more reduced and potassic. A new geological model for the Karku deposit is proposed based on petrographic observations and sandstones geochemistry. The proposed model envisages the permeable rocks of paleoregolith and the coarse-grained basal part of the Priozersk Formation as the main pathway for migration of uraniferous fluids responsible for the formation of Karku uranium deposit. From the perspective of the geological setting, the Karku deposit differs from deposits of the Athabasca Basin in that it occurs in a rapidly deposited rift-related basin with associated mafic magmatism, the host sandstone sequence is relatively thin (e.g., Priozersk Formation) and exhibits abrupt lateral facies variations, and the widespread presence of early diagenetic interstitial swelling clays. Based on these sedimentological characteristics, it is considered that the Karku deposit setting precludes optimum conditions to create high tonnage high-grade unconformity-type U deposits as found in the Athabasca basin.

Вагові характеристики та галузі раціонального використання сталевих центрально стиснутих колон

За результатами експериментального проектування центрально стиснутих сталевих колон шести типів виявлені залежності їх маси від висоти й навантаження. За укрупненими розцінками оцінена вартість виготовлення та монтажу колон різних типів. Результати роботи дозволяють орієнтовно встановити металоємність і вартість колон різних типів за відомою висотою й навантаженням та обрати раціональний тип колони.

Gradual Evolution from Fluvial Dominated to Tide Dominated Deltas and Channel Type Transformation: A Case Study of MPE3 Block in the Orinoco Heavy Oil Belt of the Eastern Venezuelan Basin

Based on the data of core description and sporopollen analysis, the gradual evolution of deltas in vertical direction and transition of channel types in the MPE3 block of the eastern Venezuela Basin have been surveyed by seismic phase and well logging facies interpretation. The results show that due to the great sea level rise, the sedimentary system of the Miocene Oficina Formation in the MPE3 block shifted from the distal‐source sandy braided river delta to tide‐affected delta, and eventually to tide‐dominated delta. Vertically, during the early stage of sedimentation of Oficina Formation, the distributary channels of the delta were dominated by braided river channels. While in the later stage, as the tidal effect was gradually intensified, the channel changed from braided channel to meandering channel. On plane, as a result of differential transgression, sedimentary framework and distribution of sand bodies vary across the study area. Compared with the eastern part, the western part has more braided channels, larger channel bars, less developments of distributary bay and higher ratio of sand to mud. Whereas the braided channels in the south are larger than those in the north. It is the first time we pointed out the impact of marine transgression differences on the sedimentary facies distribution and river type transition in the study area. Factors like the structural and paleogeomorphological change, sea level variation, supply of sediments have strong influence on the evolution of sedimentary system and distribution of sandbodies. It is predicted that the major sandbody is more developed in the central south, which can guide the subsequent horizontal well development.

Bar dynamics in a sandy braided river: Insights from sediment numerical simulations

Bar dynamics in sandy braided rivers are keys to better understand the morphological processes of braided rivers and sediment distributions. Limited by insufficient quantitative information and satellite images that only show the larger-scale morphology and do not reveal the sand bars in the deeper channels, most previous studies have qualitatively assessed bar dynamics, but the ways in which the hydraulic parameters of sandy braided rivers control bar dynamics remains poorly understood. Here, we present sediment numerical simulations on sandy braided rivers to investigate the quantitative controls of river parameters on bar dynamics. Three bar dynamic forms (downstream migration, lateral variations and bar tail elongation) are observed. Unit bars are prone to downstream migration following formation, and the migration rate shows a deceasing trend during braided river evolution. This gradually decreasing migration rate is proven to be strongly connected to the decreasing flow velocity of braided channels that confine bar growth. The lateral variation in bar dynamics exhibits narrowing, migration and widening of sectional morphologies. We suggest that braided channel curvature dominantly controls lateral sediment transportation around the bar and thus results in several lateral variation forms of bars. The bar tail elongation difference between the two sides of the bar is associated with bar asymmetry. Strong bar asymmetry can make apparent differences in bar tail elongation.

Investigation of Coherent Flow Turbulence in the Proximity of Mid-Channel Bar

The turbulent flow characteristics in the vicinity of mid-channel bar are studied in this paper. The velocity is measured with the help of the acoustic Doppler velocimetry (ADV). Fractal modelling is done for analysing the fractal nature of bursting events. The conditional Reynolds stress structure was analysed by using the quadrant technique. Decomposition of Reynolds stress help in studying the distribution of stress among the four quadrants. The hole-size concept is utilized for segregating the extreme bursting events from the low intensity events. Variation of quadrants stresses with the hole-size is analysed. The result of hole-size study indicates that the extreme turbulent burst is produced by the mid channel bar. The new parameter inclination ratio is evolved in this research to reflect streambed elevation changes. The results indicate that the high-level turbulence is generated at region near upstream end of mid-channel bar. The presence of mid-channel bar causes the critical changes to the turbulent structure of flow, where high Reynolds stresses are generated at region near upstream region of mid-channel bar. Third order moments are computed for vertical lines in the proximity of mid-channel bar. High value of third order moments for bar condition indicate that the intense turbulent mixing and momentum exchange take place.

“Grain-size bookkeeping,” a new aid for siliciclastic systems with examples from paralic environments

ABSTRACTSedimentary logs form the foundation of many studies of ancient and modern sedimentary successions. In siliciclastic settings in particular, vertical grain-size trends are important records of past depositional processes and environments, so that they are recorded with care and are often central to final interpretations. It is rare for the actual grain size to play a similar role. Yet there is significant value in (i) digitizing sedimentary logs to produce statistical grain-size data and (ii) the deliberate study of grain size, a process described here as “grain-size bookkeeping.” To illustrate this, over 5.9 km of sedimentary logs have been digitized from the Miocene to Pliocene paleo–Orinoco delta, the Cretaceous Ferron Last Chance and Notom deltas, and the Jurassic Ravenscar Group. The digital data reveal how grain size partitions into distinct sedimentary facies, proximal-to-distal changes, changes related to stratigraphy and base level, and the overall grain size of paralic systems. It emerges that fluvio-distributary channels are the coarsest-grained sediment bodies in each of the studied systems. The coarsest material does not reach the shoreline, though the grain sizes of fluvio-distributary channels and shoreline sand bodies overlap, in accordance with the concept that the former feeds the later. By contrast, overbank sediments are relatively fine-grained, suggesting that, with the exception of channel belts, coarse sediment can largely bypass the delta plain. Grain-size changes occur across some key stratigraphic surfaces, but not consistently so. Channels in valleys are, on average, coarser than similar channels in unconfined systems, but, in the presented datasets, valleys do not contain the coarsest channels. The data have also allowed the analysis of down-system fining rates in ancient, sandy fluvio-distributary systems, with grain size being measured to decrease at rates ranging from 0.7 to 7.7 μm/km—values that compare favorably with modern rivers. Such large-scale trends are ornamented by, and link to, smaller-scale spatial changes associated with, for example, channel bars, crevasses, and mouth bars, and an initial dataset of associated fining rates has been collected. In general, very large systems (rivers) have low fining rates because of their great size, whereas the converse is true for small systems, especially if the grain size range is large. Consideration of downstream fining rates has led to the insight that avulsion initiates an unequal race to the shoreline. Suspended very fine sand and silt is likely to reach the shoreline with the avulsion flood waters, but bedload will advance far more slowly, perhaps too slowly to reach the shoreline before the river avulses again. Some avulsions may lead to notable temporal variations in the caliber of sediment supplied to shorelines. As expected, the largest system, the paleo-Orinoco, is the finest grained. The Ferron deltas have catchment areas 12 and 22 times smaller than the Orinoco, and are the coarsest grained. Remarkably, though their catchment areas differ by a factor of two, they have almost identical sand grain-size distributions. The data have also proved powerful in refining paleogeographic reconstructions, in particular suggesting “missing” depositional elements needed to complete local sediment routing systems. Careful tracking of grain size is also beneficial in that it is a key control on permeability, the description of which is crucial to the prediction of subsurface fluid flow.

Sedimentological impact on reservoir quality of Es1 sandstone of Shahejie formation, Nanpu Sag, East China

Comprehensive sedimentological studies regarding different facies types and the factors persuading their development were carried out at Eocene (Es1) member of Shahejie formation of Nanpu Sag. To evaluate the sedimentary facies, lithofacies, and reservoir quality, wireline log analysis, a petrographic thin-section study, SEM, grain size analysis, XRD, and CL analysis were used. The Es1 consists of five sedimentary facies and seven lithofacies. Sandstone is classified on the basis of standard rock classification scheme as feldspathic litharenite and lithic arkose. Quartz grains are abundant detrital constituent and feldspars are subordinate followed by mica, chert, rock fragments, and iron oxide as a minor constituent. Primary intergranular pores, as well as secondary dissolution pores and fracture pores, are also present that enhance the reservoir quality. Grain size varies from conglomerate to clay size, with massive bedding, cross-bedding, and fine-grained ripple laminated sandstone, which predicts that formation is deposited in lacustrine and fluvial channel or channel bar environment. XRD and SEM analysis show various clay minerals that reduced the reservoir characteristics. The present study discloses the deposition and distribution of lacustrine, braided, meandering river delta, and delta front sand bodies of Es1 member as well as provides some support for reservoir quality of the correlative sedimentary system that has great significance for further exploration.

Downstream geomorphic impact of the Three Gorges Dam: With special reference to the channel bars in the Middle Yangtze River

AbstractThe Three Gorges Dam (TGD) has altered downstream flow–sediment regimes and led to significant changes in the morphodynamic processes in the Middle Yangtze River (MYR). However, due to the complexity of this large river, the driving forces and implication of the morphodynamic processes remain insufficiently understood. This study selected two typical meandering and bar‐braided reaches, the Zhicheng (ZC) and Shashi (SS) reach, to examine their responses to the TGD operation. The results showed that in the post‐dam period significant channel erosion occurred with a higher erosion rate in the ZC reach (closer to the TGD) compared with the SS reach. The area of the Guanzhou mid‐channel bar (ZC reach) and the Sanba mid‐channel bar (SS reach) shrank by 30 and 90% from 2003 to 2015, respectively. The increased fluvial erosion intensity due to the reduction in suspended sediment concentration (SSC) drove the shrinkage of the mid‐channel bars, as demonstrated by empirical relationships between bar geometry and fluvial erosion intensity. An increase of 22 days per year in the frequency of post‐dam medium‐to‐high discharges (10 000–25 000 m3 s−1), and associated with the reduction in SSC, jointly led to the greater erosion at the convex (inner) banks than the concave (outer) banks, which has negatively affected the designed navigation channels at the concave banks by decreasing their discharge partitioning ratios. The post‐dam water level at a given high discharge (>25 000 m3 s−1) showed no evident change, but the water level at a given low discharge (<10 000 m3 s−1) decreased. The reduction in water levels at low flows can affect water supply and riverine ecosystems in the MYR. © 2019 John Wiley & Sons, Ltd.

Coarse sediment storage and connectivity and off-highway vehicle use, Board Camp Creek, Arkansas

Upper Board Camp Creek (BCC) in western Arkansas drains the Wolf Pen Gap (WPG) Trail Complex, a designated Off-Highway Vehicle (OHV) area in use since the 1990s. The mixed bedrock-alluvial channel is quite active, with extensive bars and eroding banks present within the higher-order, main-valley channels. This study was conducted to determine the relative magnitudes of sediment storage and production within the channel, and whether geomorphic changes are synchronous with establishment and increasing OHV use levels on the trail system. Fourteen geomorphically active reaches within the main-valley channels, representing the range of conditions of BCC within the WPG complex, were examined in detail. All sites had significant alluvial storage in the form of point, lateral, or mid-channel bars dominated by cobble and gravel. Sediment storage volumes ranged from 140 to nearly 10,000 m3 per kilometer of channel, with a mean of about 3400. Eleven of 14 reaches also had actively eroding banks. Ten reaches (71%) exhibited net sediment storage. Two are possible net sources, and two may be in approximate steady state (storage ≈ erosion). The imbalance between local bank erosion sources and in-channel storage, and the evidence of activity and mobility of most of the bars indicates a connected system, with coarse sediment mobile during banktop flow events, and no evidence of sediment starvation. Finer (<8 mm) sediment from the trail system does not seem to be accumulating in the stream, suggesting that most is either sequestered before reaching BCC Creek, transported downstream, or deposited on floodplains during overbank flow. Many of the channel bars predate the trail complex, and most are active. This suggests that these features constitute mainly transient storage and are an inherent feature of the channel. At only two reaches could geomorphic changes be confidently attributed to the trail system. Like many streams, BCC has an active channel, independent of the WPG trail system. These results highlight the difficulty of attributing fluvial change to specific causes or forcings in active fluvial systems.

Case study of sandbody architecture and quantitative parameters of the far-source sandy braided river: Saertu Oilfield, Daqing, China

Aimed at the braided river sandbodies deposited in the pure oil area of western Saertu Oilfield in the Songliao Basin, Daqing, Placanticle, the sedimentary physical simulation experiments were designed. The characteristics of channel bar microfacies were identified and analyzed by four rounds of experiments. Based on the measurement results, the correlation parameter model of the braided river reservoir architecture was established by describing the geometric occurrence of the structures of the single-phase braided river sandbodies and channel bar. Combined with prototype sedimentation simulation experiment, the internal quantitative architecture of far-source sandy braided river was researched by identifying and analyzing the interface of individual well. The interfaces of the 5th-order, 4th-order and 3rd-order architectures were emphatically analyzed and the inter-well quantitative architecture model was established. The research results showed that the interlayer in the center of channel bar was approximately horizontal, with dip angle of about 1°. In short-axis toward end direction, the dip angle of the architecture interface of accretionary body was 2–4°. Combined with the internal dip angle and occurrence of channel bar sandbodies, the interface of the accretionary body in the central part of channel bar was parallel, with dip angle of no more than 2°. The dip angle of the accretionary body in water flow direction was large, which was 2°–5°. On this basis, the size of braided river, channel bar and accretionary body was calculated quantitatively in the target area, and different orders of braided river sandstone reservoirs were described.