Tributary Channels Research Articles

Initiation and Runout of Post‐Seismic Debris Flows: Insights From the 2015 Gorkha Earthquake

AbstractPost‐seismic debris flows are an important hazard following large earthquakes, propagating destruction downstream from hillslopes where coseismic landslides occur and extending damage for years after shaking stops. Data sets of post‐seismic debris flows are necessary to predict initiation and runout characteristics but are presently scarce. We used satellite imagery supplemented by field observations to compile an inventory of >1,000 debris flows associated with the 2015 Gorkha Earthquake in Nepal. We identified two distinct debris flow types: (1) Material from a coseismic landslide was remobilized in a steep channel during a later monsoon; and (2) a new post‐seismic hillslope failure occurred in saturated conditions and became fluidized and channelized. Runout distance was constrained by channel confluences and may be related to confluence geometry. Unstable landslide debris was largely flushed from steep channels during the first monsoon following the earthquake, and the rate of new hillslope failures tailed off over a few years.

Flow dynamics at channel confluences: few observations from a sub-tropical plateau fringe river of Eastern India

River confluences are that critical interfaces where significant changes in channel hydrological processes occurs. How and why these changes occur are the elementary questions for scaling-up our understanding of the dynamic hydrological processes occurring at the catchment scale. Confluences of major watersheds of the Chotonagpur plateau fringe fans (eastern counterpart) unlike other of the Gangetic flood plain are attributed by immensely fluctuating gauge level under the circumstances of high-intensity seasonally skewed rainfall. To understand this regional character and in view of the importance of channel confluences, the manuscript is hereby inclined to document the hydrological dynamism at 11 tributary junction sites along 86.4-km stretched river Bakreshwar of Eastern India. From the field survey of the said 11 confluence junctions constituting 7 channel reaches and 33 cross sections in total, it has been affirmed that the tributary junctions represent locations of sudden change in the longitudinal flow rhythm of the main river. The abrupt changes in channel flow due to tributary influxes of water at the confluences have a fair relation with size and length of the tributary rivers. However, tributary basin runoff volume relative to the main river basin is found to be the most influential factor to regulate post-confluence increase in velocity and discharge. The study reported here is a fundamental work and hopefully stimulate ideas for fruitful and coherent scientific inquiry on river confluences and its management particularly in Indian circumstances.

Effect of channel tributaries on the evolution of submarine channel confluences (Espírito Santo Basin, SE Brazil)

AbstractConfluences are geomorphologic features fed by distinct channel tributaries that record the contribution of multiple sediment sources. They are key features of both fluvial and submarine channels in geomorphologic and sedimentologic terms. Here, we use high-quality three-dimensional seismic data from SE Brazil to document the response of a submarine channel confluence to turbidity currents originating from a tributary. The studied channel system consists of a west tributary, an east tributary, and a postconfluence channel, with the last two comprising the main channel at present. Downstream from the confluence, changes in planform morphology and architecture were found due to the effect of turbidity currents sourced from the west tributary channel. A channel bend in the main channel curved toward the west when it was first formed but later curved toward the east, and so remains until the present day. This process led to the migration of the confluence point ∼500 m to the east, and changed the bed morphology from discordant (where the beds of tributaries and main channels meet at an unequal depth) to concordant (where the beds of tributaries and main channels meet at approximately the same depth). In addition to the channel bend near the confluence, two other bends further downstream recorded significant changes with time, increasing channel sinuosity from 1.11 to 1.72. These three channel bends near the confluence accumulated a large volume of sediment at their inner banks, generating depositional bars. Multiple channel forms within the depositional bars indicate the occurrence of large-scale lateral migration near the confluence. Hence, turbidity currents from the west tributary are shown to influence the submarine channel by promoting lateral channel migration, confluence migration, increases in channel sinuosity, and the formation of large depositional bars. These variations near the confluence reveal a change in tributary activity and a shift in sediment sources from east to west on the continental shelf. Such a shift suggests variations in sedimentary processes on the continental shelf probably due to avulsions on Doce River Delta.

Distribution of phosphorus in bed sediment at confluences responding to hydrodynamics

Knowledge of the distribution of contaminants at channel confluences is critical for understanding their transport and dwelling at river networks where flow confluences constitute the key nodes. Flume experiments were conducted to study the distribution of phosphorus adsorption in surface sediment under complex hydrodynamics at channel confluences. The influence of the discharge ratio of the tributary discharge to the main stream discharge was also investigated. The results showed that the horizontal velocity and vertical velocity of flow are the dominant factors affecting the distribution of phosphorus adsorption. Flows with a larger horizontal velocity are not beneficial to phosphorus adsorption on sediment and thus low- and high-adsorption strips of phosphorus occur in the maximum velocity zone and the flow separation zone, respectively. Downwelling flows are beneficial to phosphorus adsorption on sediment as they can enhance phosphorus exchange between overlying water and the bed sediment. A small flow penetration with a small discharge ratio causes a decrease in vertical velocity in the separation zone as well as the strong intrusion of a helical cell into the bottom of this zone, which results in an increase in horizontal velocity near the bed and consequently the phosphorus adsorption is largely restricted.

Trace element and isotopic features of carbonates from the mud volcanic ejecta from the Kura depression (Azerbaijan)

A collection of veins and dispersed Ca carbonates from the mud volcanic ejecta from Azerbaijan has been studied. The appearance of calcite veins healing cracks are divers. Isotopic characteristics of the majority of Ca carbonates are the follows: δ13С (from -4 to +2‰) and δ18О (from +20 to +26‰) and are typical of marine carbonate sediments. The fragments of vein calcite enriched in 12C (δ13С from -49.2 to -6‰) were found in the mud masses of a few mud volcanoes and might be connected with the dormant methane seeps, occurring at the high level of Caspian Sea. The dispersed Ca carbonates from mud masses have δ13С from -6.1 to +11.9‰, and δ18О from +23.8 to +32.7‰. Studied carbonates have no geochemical links with the mud volcanic waters of current emission. The aqueous fluids are characterized by high НСО3- contents (up to 8 g/l) and average δ13С(TDIC) and δ18О(Н2О) values as high as δ13С = +20.0‰ and δ18О = +4.0‰. Both isotopic and REE signatures of vein calcites allows one to connect them with post-depositional interaction between formational waters and carbonate sedimentary matter. On the base of these data mud volcanic feeder channels have been considered as isolated hydrodynamical systems, avoiding the connection with the aquafers of host sedimentary strata.

Quantitative analysis of the macro-geomorphic evolution of Buyuan Basin, China

Buyuan River, the largest tributary within the Chinese Lancang-Mekong River region downstream of the Jinghong Dam, plays a crucial role in river function and ecosystem service of the Lancang-Mekong River. The geomorphic evolution of a basin exerts a key control on riverine sediment input and transport. In this study, the geomorphic characteristics of Buyuan Basin are analyzed using morphological parameters, hydrodynamic parameters and the stream power river incision model. The results show that: 1) The slight north-south difference of channel density is most likely due to lithology and independent of tectonic activity and climate. 2) The weak tectonic activity and the low hypsometric integral (HI) value suggest that the macroscopic landform condition limits erosion and sediment production. 3) The logarithmic longitudinal profile of the main channel defends that the upstream sediments generated by erosion are easily deposited in the downstream channel, rather than being transported directly into the Lancang-Mekong River. 4) Approximately 74% of the reaches have annual average stream power less than 500 W·m−1. The narrow variation ranges of stream power in 50% of the river channel indicate relatively stable hydrodynamic environment. 5) Stream erosion and tectonic activity make the longitudinal profiles of the main channel and most tributary channels unstable. The wide range (between 22.01 and 45.58 with θ=0.43) of steepness index (ksn) of longitudinal profiles implies differential uplift in the basin.

A Cretaceous carbonate delta drift in the Montagna della Maiella, Italy

AbstractThe Upper Cretaceous (Campanian–Maastrichtian) bioclastic wedge of the Orfento Formation in the Montagna della Maiella, Italy, is compared to newly discovered contourite drifts in the Maldives. Like the drift deposits in the Maldives, the Orfento Formation fills a channel and builds a Miocene delta‐shaped and mounded sedimentary body in the basin that is similar in size to the approximately 350 km2 large coarse‐grained bioclastic Miocene delta drifts in the Maldives. The composition of the bioclastic wedge of the Orfento Formation is also exclusively bioclastic debris sourced from the shallow‐water areas and reworked clasts of the Orfento Formation itself. In the near mud‐free succession, age‐diagnostic fossils are sparse. The depositional textures vary from wackestone to float‐rudstone and breccia/conglomerates, but rocks with grainstone and rudstone textures are the most common facies. In the channel, lensoid convex‐upward breccias, cross‐cutting channelized beds and thick grainstone lobes with abundant scours indicate alternating erosion and deposition from a high‐energy current. In the basin, the mounded sedimentary body contains lobes with a divergent progradational geometry. The lobes are built by decametre thick composite megabeds consisting of sigmoidal clinoforms that typically have a channelized topset, a grainy foreset and a fine‐grained bottomset with abundant irregular angular clasts. Up to 30 m thick channels filled with intraformational breccias and coarse grainstones pinch out downslope between the megabeds. In the distal portion of the wedge, stacked grainstone beds with foresets and reworked intraclasts document continuous sediment reworking and migration. The bioclastic wedge of the Orfento Formation has been variously interpreted as a succession of sea‐level controlled slope deposits, a shoaling shoreface complex, or a carbonate tidal delta. Current‐controlled delta drifts in the Maldives, however, offer a new interpretation because of their similarity in architecture and composition. These similarities include: (i) a feeder channel opening into the basin; (ii) an excavation moat at the exit of the channel; (iii) an overall mounded geometry with an apex that is in shallower water depth than the source channel; (iv) progradation of stacked lobes; (v) channels that pinch out in a basinward direction; and (vi) smaller channelized intervals that are arranged in a radial pattern. As a result, the Upper Cretaceous (Campanian–Maastrichtian) bioclastic wedge of the Orfento Formation in the Montagna della Maiella, Italy, is here interpreted as a carbonate delta drift.

Submarine channel network evolution above an extensive mass-transport complex: A 3D seismic case study from the Niger delta continental slope

Abstract A submarine channel network, named Abalama Channel System (ACS), has been recognised in the subsurface of the Niger Delta continental slope. It overlies a mass-transport complex (MTC) and consists of six channel segments, delimited by five avulsion points and one confluence point. High-resolution 3D seismic data are used to investigate the development of the ACS and to describe the interaction between the channels and the underlying MTC. The MTC mainly consists of highly disaggregated materials (MTC matrixes) and in plan-view has a very complex fingered geometry, characterised by the presence of erosional remnants (remnant blocks). The different character of the MTC matrixes compared to that of the remnant blocks likely resulted in a bathymetry characterised by negative and positive relief, which provided the initial confinement for the channels of the ACS. In areas where the MTC-induced confinement was weak or decreased abruptly, channels tended to develop higher sinuosity, increasing channels instability and ultimately causing avulsions. Three ideal categories of submarine channel avulsions are observed. Type 1 is characterised by parent and avulsion channel having similar size and maturity; Type 2 is characterised by a large, high-maturity parent channel and a small, low-maturity avulsion channel; Type 3 emphasizes the larger scale and higher maturity of the avulsion channel compared to the parent channel. In the distal part of the study area, topography related to mud diapirs provided lateral confinement that captured flows avulsed at different times resulting in a channel confluence phenomenon. Submarine channel network evolution recorded by avulsion and confluence points represents an important research theme in deep-water sedimentology, as it controls the final distribution of sediments and the extension of sands in the whole deep-water depositional system; hence this study can be used to guide hydrocarbon exploration in analogue systems.

The Aptian Gura Râului conglomerates (Southern Carpathians): remains of an extended subaqueous gravity flow deposit from the eastern flank of the Getic Nappe

The Getic Nappe belongs to the Central Dacides or Dacia Mega Unit. Gravity flow deposits were generated by the Lower Cretaceous overthrust and uplift of this unit over the External Dacides. In the Piatra Craiului Syncline (part of the Getic Nappe), such detrital deposits are represented by the Aptian conglomerates and the uppermost Albian-Cenomanian conglomerates. Part of the infill of the syncline consists of a thick pile of NW–SE oriented conglomerates. Sedimentological study documents the presence of an association which consists of three distinct facies types, represented by: massive conglomerates; alternating conglomerates, sandstones and microconglomerates; and fining- and coarsening-upwards conglomerates. These deposits were accumulated through a series of debris flows, and hyperconcentrated or concentrated flows. The entire assemblage represents a complex routing system within submarine channels, an interpretation made by interpreting facies associations, palaeoflow directions and clast fabrics. A large part of this succession was removed by post-Albian erosion. Thus, the interpretation of the depositional system could be as a series of feeder channels or the submarine sector of a fan-delta system representing a foreland basin fill.

A Thermodynamic Model of Chemical Remagnetization Based on the Example of the Girvas Paleovolcano in the Onega Structure of the Carelian Craton

The Girvas paleovolcano is a structurally complex volcanic complex of Jatulian age. Apparently, it is a shield volcano, which was probably one of the feeder channels of the vast lava field of the western Onega Region within the Girvas volcanic zone. Despite the fact that the Girvas paleovolcano is only partially exposed, the flow structure is well preserved in rocks, which allows one to reconstruct the direction of the lava flow. Within the Girvas paleovolcano, a zone of post-volcanic hydrothermal alterations in volcanic rocks (mainly, tourmalization and silicification in pockets and veins, as well as subsequent epidotization, sulfidization, chloritization and albitization) is distinguished. The secondary alteration zones are confined to fault zones, while their spatial–temporal correlation is still unclear. The paleoreconstruction of the geological structure showed that there were two main stages of the development of the Girvas paleovolcano: (1) pneumatolysis due to magmatic gases released from gabbro–dolerite sills, (2) heating and circulation of exogenous waters with formation of near-ore propylites. Based on the proposed scheme, a thermodynamic model was developed.

Hydraulic Features of Flow through Local Non-Submerged Rigid Vegetation in the Y-Shaped Confluence Channel

A laboratory measurement with acoustic Doppler velocimeter (ADV) was used to investigate the flow through a Y-shaped confluence channel partially covered with rigid vegetation on its inner bank. In this study, the flow velocities in cases with and without vegetation were measured by the ADV in a Y-shaped confluence channel. The results clearly showed that the existence of non-submerged rigid plants has changed the internal flow structure. The velocity in the non-vegetated area is greater than in the vegetated area. There is a large exchange of mass and momentum between the vegetated and non-vegetated areas. In addition, due to the presence of vegetation, the high-velocity area moved rapidly to the middle of the non-vegetated area in the vicinity of tributaries, and the secondary flow phenomenon disappeared. The presence of vegetation made the flow in non-vegetated areas more intense. The turbulent kinetic energy of the non-vegetated area was smaller than that of the vegetated area.

Effect of side slope of main channels on formation and penetration of scour hole in confluences

AbstractIn an open‐channel confluence, deep scour holes and depositional point bars are usually formed due to high bed‐shear stresses and secondary circulations. In the present study, presuming the effectiveness of channel geometry on the flow dynamics at the confluence, some variables including different side slope angles of the main channel (θ = 45°, 60°, 75°, 90°), lateral to downstream discharge ratio (Qr), and downstream densimetric Froude number (Frg3) were experimentally studied under clear‐water condition for the confluence angle α = 90°. According to the results, the increase of θ led to a greater penetration of scour hole across the main and tributary channels, whereas a little scour development was observed along those channels. Meanwhile, an increase in Qr and Frg3 caused further scouring, but their effects on the dimensions of scour hole diminished with increase of θ. Thus, with increase of Qr from 0.194 to 0.552, the mean penetration rate of scour hole to all directions for θ = 45°, 60°, 75°, and 90° was obtained 42.8%, 32.4%, 25%, and 20.5%, respectively. In addition, considering the effect of θ, Qr, and Frg3, some empirical relationships were obtained for estimating the penetration length of scour hole. The derived relationships show that Frg3 plays more important role on the dimensions of scour hole than θ.

Aspects of the extension of forty exploitation of bulk reservoirs for irrigation and hydropower purposes

The article is devoted to solving the problems of reducing the useful volume of bulk reservoirs, which is an urgent task of water management. The development of the design allows to reduce the volume of incoming sediment into the reservoir thicket is the main goal of the present work. The analysis of materials of field studies and a review of existing scientific works was the basis of the research method. The results of field studies on the study of channel processes in the downstream of the feeder channel of a low-pressure reservoir and the dynamics of their siltation. It has been established that the reservoirs accumulate only 0.3-10% of the annual river flow and the degree of annual silting of the capacity of such reservoirs is on average 0.5-2.0%. After 25-50 years, reservoirs may lose half of their usable capacity and after 40-80 years, a complete failure of operation is possible. To slow down this process in the basins of the self-leveling reservoirs, a new structure of structures developed, which ensures an even flow of water in the supply channel and maximum sedimentation of sediments in the channel, which prevents them from entering the reservoir bowl.

Study of indigenous freshwater fish diversity of Bankura (West Bengal), India with special reference to Clarias batrachus

A study was conducted to determine the ichthyofaunal diversity in Bankura district of West Bengal. The area is traversed by major and minor rivers, feeder channels, numerous ponds, bills, reservoirs which have made this drought-trodden district as the highest producer of aquatic products within the state. Field survey was conducted in randomly selected local markets covering twenty-two blocks of Bankura. Fish sampling was carried out using various conventional fishing gears. The entire region harbours a wide variety of aquatic fishes. A total number of 92 indigenous fish species belonging to 30 families were identified during this study. The Cyprinidae family dominated the population with its 36 varieties followed by Channidae, Siluridae and Bagridae. Sonamukhi block situated in Shali basin; Sarenga and Raipur of Kangsabati basin; Dwarkeshwar and seven bundhs- enriched Bishnupur were the major habitats of small indigenous fishes. This paper also denotes presence of 12 globally endemic freshwater fish species viz. Hypophthalmichthys molitrix, Cyprinus carpio, Labeo nandina, Tor khudree, Chitala chitala, Bagarius bagarius, Wallago attu, Ompok pabda, Ailia coila, Anguilla bengalensis, Parambassis lala, Oreochromis mossambicus. The substitution of native Clarias batrachus with invasive African catfish C. gariepinus was observed in various local markets. Some traders even promote illicit farming of this banned species for their own profit. As a consequence, the indigenous, nutritionally-enriched C. batrachus is becoming more endangered.

A sedimentary model of mountainous rivers with application to Alamutrud and Shahrud rivers, N Qazvin, Iran

The most important depositional processes in continental sedimentary basins occur in fluvial systems. In this study, sedimentology of Shahrud River and its eastern tributary (Alamutrud River, N Iran) was studied for the first time. The study area is located in the Southern-Central Alborz structural zone which is mostly composed of Eocene volcanics and Miocene terrigenous sedimentary rocks. The channel trend is controlled by the regional structural trends. Grain size analysis was performed on 110 sediment samples. Water discharge and velocity was measured on 274 points of the river cross section. A total of 182 bedload and suspended load sampling were also performed in 9 hydrometric gauging stations. The results show > 70% of the bedload and suspended load consist of sand and silt, respectively. Mineralogic composition of gravel, sand, and mud fraction of the sediments indicates a dominant volcanic origin. Two gravelly facies (Gmm, Gcm), five sand facies (Sm, Sh, St, Sp, Sl), and one mud facies (Fm) were recognized. These facies have been formed by six architectural elements: channel (CH), gravity flow deposits (SG), gravel bars and bed forms (GB), sand bed form (SB), laminated sand sheet (LS), and fine grain overbank deposits (FF). Accordingly, the system is influenced by flood and the depositional model for its upstream is proposed as gravel-bar braided with sediment-gravity flow. The river in midstream and downstream shows characters of shallow gravel-bed braided and gravelly-sandy meandering rivers, respectively. Coarse-grained sediments feeding by the channel tributaries and alluvial fans have produced distinct discontinuities in the downstream fining trend.

Mantle origin of methane in the Black Sea

A new distribution map of gas seeps and mud volcanoes has been compiled of the Black Sea. It has been derived from the published coordinates for ca. 5000 gas seeps and 80 mud volcanoes. An analysis of the stable isotopic composition of methane has been performed for authigenic carbonates and sediments. The δ13C values of carbonates and sediments form 2 distinct tight groups depending of the geological environments. The diagrams of values of δ13C vs. δD and δ13C vs. C1/(C2 + C3) have been applied to assess earlier classifications of methane. The origin of methane from most of samples has turned out to be uncertainly determined. Based on seismic reflection data, the feeder channels of gas releases and mud volcanoes penetrate to the Pre-Mesozoic basement beneath the Polshkov High, Andrusov and Tetyaev Ridges and Sorokin Trough at a depth of up to 12 km. In the Central Black Sea the feeder channel of mud volcano reaches the mantle surface where a depth is 19 km. The highest concentration of gas seeps distribution is observed at the triple junction of the mantle faults in the NW Black Sea. Assessments of different mechanisms have been made for their ability to produce an unprecedented methane concentration in the Black Sea water column. There occurs thermogenic methane in the sediments from gas seepage and mud volcanoes areas. The thermogenic methane results from post genetic alternation of biogenic methane of the Paleogene—Neogene sediments. Biological methane produced from organic matter plays negligibly small role in accumulating the world’s largest quantities of anaerobic methane in the Black Sea. Nonorganic methane is most likely to form the tremendous reservoir of dissolved gas in the Black Sea water column below 150—200 m. The seeming scarcity of abi-otic methane is accounted for its recycling by microbial activity, misclassificating origin and producing by chemical syntheses, with its biological methane δ13C values.

Structural and sedimentological connectivity on a rilled hillslope

Sedimentological connectivity describes the continuity of sediment transport across a topographically heterogenous landscape. Sedimentological connectivity reflects the changes in the balance between flow sediment load and transport capacity and is mainly influenced by the longitudinal (dis)continuity of channels that is associated with channel depressions and confluences. Past studies have focused on structural and sedimentological connectivity at the basin scale, with only a few examining this phenomenon at the hillslope scale. Important questions have yet to be answered, including “How do within-channel barriers and channel confluences affect the flow/sediment balance?” and “At what spatial extent do these influences take place?” This study investigated the structural and sedimentological connectivity within a rill network on a hillslope in Loudon, Tennessee, USA. We used the difference in a ground lidar generated time-series of seven digital elevation models (DEMs), the DEM of Difference (DoD or ΔDEM), to quantify the spatio-temporal dynamics of erosion and deposition on a hillslope for 6 periods from December 2014 to December 2016. We found that later periods had the greatest amount of erosion and net sediment loss (0.59 ± 17 m3). We compared the observed sediment redistribution patterns to a GIS-based index of connectivity (IC). In order to examine the influence of structural (dis) connectivity on sedimentological connectivity, we compared the time series of patterns to the occurrences of rill channel confluences and depressions. We found a spatially consistent relationship between the IC and the observed erosion/deposition pattern. However, sediment detachment can also be driven by gravitational sidewall failure, which is not well described by the IC. The scouring at rill confluences leads to higher erosion in both upslope and downslope sections of a rill channel; the ponding at rill depressions yields upslope deposition and downslope erosion.

Interaction between channel junction and bridge pier on flow characteristics

Considering many bridge piers that destroyed every year due to scouring, flow behavior around these obstructions was one of the interests of engineers in recent years. This phenomenon has usually been studied in straight channels and has rarely studied at channel confluences. To accomplish this, after calibrating SSIIM1 model for a 90° channel junction, the effect of circular bridge pier at two positions after channel confluence (x/w = −1.5 and x/w = −3, x = distance from upstream corner of junction in flow direction, w = main channel width) on flow pattern were conducted. The results indicated that bridge pier caused backwater rise by 4.1% at upstream of channel confluence compared to without bridge pier case. Water surface profile of main channel reduced in two steps. First step was due to reduction of effective width of main channel because of tributary channel inflow and the second one was due to flow contraction near bridge pier while in the absence of bridge pier it only had one step. Also the results showed that longitudinal velocity decreased at upstream sections of bridge pier. At the centerline of the section corresponding to downstream corner of junction for (x/w = −1.5) and (x/w = −3) these reductions were 25.4% and 16.3% respectively.

Growing Forced Bars Determine Nonideal Estuary Planform

The planform of estuaries is often described with an ideal shape, which exponentially converges in landward direction. We show how growing topographically forced nonmigratory (i.e., anchored) bars determine the large‐scale estuary planform, which explains the deviations observed in the planform of natural estuaries filled with bars compared to the ideal planform. Experiments were conducted in a 20‐m long, 3‐m‐wide tilting flume, the Metronome. From a narrow, converging channel a self‐formed estuary developed characterized by multiple channels, braided bars, a meandering ebb channel, and an ebb delta. Bars hardly migrated due to the alternating current, but the bar width increased with increasing estuary width. At locations where the estuary width was narrow, major channel confluences were present, while the zones between the confluences were characterized by a higher braiding index, periodically migrating channels, and a relatively large estuary width. At the seaward boundary, confluences were forced in place by the presence of the ebb tidal delta. Between confluences, bars were topographically forced to be nonmigratory. Diversion of flow around forced midchannel bars caused bank erosion. This resulted in a planform shape with a quasiperiodic widening and narrowing at the scale of forced bars. Observations in natural systems show that major confluence locations can also be caused by inherited geology and human engineering, but otherwise the estuary outline is similarly affected by tidal bars. These observations provide a framework for understanding the evolution of tidal bar patterns and the planform shape of the estuary, which has wide implications for navigation, dredging, and ecology.

캐나다 앨버타분지 하부 백악기 층서와 남부 평원 지역 하부 맨빌층군 석유시스템에 대한 고찰

The Manville Group in the Alberta Basin, Canada, contains large amounts of hydrocarbon resources and has long been a target of petroleum exploration and production. However, there exists no standard stratigraphic classification system that can be applied for the basin-wide underground reservoirs and petroleum systems, mainly due to the limitations that most stratigraphic nomenclatures were defined by information obtained from underground boreholes. In this review, after extensive literature survey we reinterpret the spatio-temporal relationships of the Mannville Group and its correlative strata on the basis of the depositional history and stratigraphic context in the Alberta Plain region. It seems appropriate to distinguish the Deville Formation lying immediately above the sub-Mannville unconformity as a stratigraphic unit separated from the overlying Mannville Group and the underlying Jurassic strata based on its unconformable relationship both above and below and its depositional origin. The Basal Quartz, commonly used unofficially, is a stratigraphic unit referring to the lower Mannville strata beneath the Ostracod Beds. The Bluesky and the Wabiskaw sandstones in the Mannville Group are very similar to the Glauconitic Sandstone in terms of lithology, but their depositional histories are different from the latter. The former sandstones had been deposited during the overall transgressive stages, whereas the latter is a deposit that was formed during the following regressive stage after the deposition of the former sandstones. The main reservoirs in the southern Alberta region are the Basal Quartz sandstones, which represent fluvial channel sandstones that filled the main channels having a N-S orientation and dendritic tributary channels connected to the main channels. The main source rocks are interpreted to be organic matter-rich marine shales of the Jurassic Fernie Group. The Ostracod Beds are very poor in permeability and thus form a regional seal, forming stratigraphic traps in numerous areas.