Tributary Channels Research Articles

Deposition pattern of stony and muddy debris flow at the confluence area

Debris flow fan affects the river profile and landscape evolution. The propagation of multiple debris flows along a river can cause inundation and breaching risk, which can be exemplified by the Min River after the Wenchuan earthquake, Sichuan province, China. In this work, large flume tests were conducted to examine the interactions between debris flows and water current with the fan geometry, momentum, runout distance, deposited width, the relative water level upstream and dominated stress. The results reveal that stony flow commonly travels at a high speed and forms a long rectangle shape fan, while the muddy flow generally travels at a low speed and forms a fan-shaped depositional area. The stony flow can block a river even when the momentum is close to the water current; the muddy flow can block a river when the momentum is lower than that of water current. In case of complete river damming, the relative water level upstream indicates that the inundation risk from the muddy flow damming river would be higher than the inundation risk of stony flow. The diversion ratio of muddy flow decreases as damming ratio. Comparison of dimensionless numbers reveals that stony flow is dominated by grain collision stress combined with turbulent mixing stress, while the muddy flow is dominated by viscous shear stress over friction stress. The fan geometry, damming ratio, diversion ratio, and the dominated stress all together indicate that stony flow strongly interacts with water current while the muddy flow does not. The results can be helpful for understanding the physical interactions between water current and various debris flows, and debris flow dynamics at the channel confluence area.

Turbulent Flow Structure in a Confluence: Influence of Tributaries Width and Discharge Ratios

River channel confluences are rather important interfaces where intense changes in physical, mixing and sediment transport processes occur. Following an experimental campaign, the main flow mechanisms in confluences and the development of the shear layer formed between the two tributary flows are presented. As the experimental flow cases comprised changes in the flow discharge and channel widths of the tributaries, the influence of width and discharge ratios on the turbulent flow structure and shear layer is also evaluated. Main findings indicate that changes in the difference between momentum ratio in the tributaries have a significant effect on the magnitude and location of flow mechanisms.

Distribution and release of perfluorinated compounds (PFCs) in water-sediment systems: The effect of confluence channels

Perfluorinated compounds (PFCs) released from sediment contribute to pollution in rivers. Considerable research has been conducted on submerged bottom sediment in long, straight channels. However, less attention has been paid to the junction of a plain river network where the hydrodynamic disturbance of confluence dominates. To date, the release mechanism of PFCs in confluence channels has not been revealed quantitatively. In this study, we carried out a series of flume experiments to evaluate the effect of varied discharge ratios of mainstream and tributary waters on the efflux of PFCs from sediment. The results demonstrate that the convergence of tributary water can affect the mainstream water significantly. The two most obvious features are the separation zone and the maximum velocity zone. The release of PFCs in sediment is correlated with the flow velocity. The discharge in the separation zone is low, and the discharge in the maximum velocity zone is high. The release of PFCs in water bodies, including the true soluble phase, colloid phase, and particle phase, increase with an increase to the discharge ratio. Perfluorooctanoic acid (PFOA) is more widely distributed in the true dissolved phase, and perfluorooctane sulfonate (PFOS) dominates in the colloid and particle phases. Our results indicate that the variation of PFCs in sediment is strongly correlated with the hydrodynamic partition. This study provides a scientific basis for regulating PFC transport at river channel confluences.

Concentration profiles of ions and particles under hydrodynamic focusing in Y-shaped square microchannel

Three-dimensional ion and particle concentrations under hydrodynamic focusing in a Y-shaped square microchannel are numerically simulated to clarify the decrease of the ion concentration along the flow direction within the focused particle stream. The simulation model is theoretically governed by the laminar flow and advection–diffusion equations. The governing equations are solved by the finite volume method. The ion and particle concentration distributions at five cross sections after the confluence of the branch channels are analyzed in 30 cases in which the sheath to sample flow rate ratio Qsh/Qsam and the Reynolds number Re are varied as parameters. The results show that the decrease of the cross-sectional average ion concentration along the flow direction within the particle stream overline{c}_{i} is described by the diffusion length during the residence time with a characteristic velocity scale. In addition, the deformation of the particle stream due to inertial effects is described by a scaled Reynolds number that is a function of the flow rate ratio. The simulated particle stream thicknesses are validated by theory and a simple experiment. This paper reveals the relationship between the ion and particle concentrations and the dimensionless parameters for hydrodynamic focusing in the Y-shaped square microchannel under typical conditions.

Urban geomorphology of the Vistula River valley in Warsaw

ABSTRACT Using ALS LIDAR DEM and OpenStreetMap data we visualise in ArcGIS the geomorphic features of a large, lowland river which flows through the area impacted by urbanisation of a big city – the capital of Poland. We present on one map the main geomorphological surfaces and their exact boundaries: valley edge, terrace front and floodplain juxtaposed with buildings and the main transportation corridors. We identify convex aeolian and fluvial landforms: dunes, levees, sandy lobes including crevasse splays, ridges between swales, sandy bars, islands; and concave erosional landforms: floodplain channels, crevasse channels, oxbow lakes, palaeo-meanders, tributary channels, and chute channels. We draw implications for flood management, geo-archaeology geo-heritage conservation. We search for traces of extreme flood events in the Holocene, also on the higher terraces which the river developed by its deposition in the Pleistocene.

Coronary-Cameral Fistula to Pulmonary Artery: An Innocent Bystander?

Coronary-cameral fistula (CCF) is a rare congenital communication between a coronary artery and a cardiac chamber or a great vessel. Most patients are asymptomatic and these lesions are incidentally detected during coronary angiography, with the reported incidence being up to 0.2%. The most frequent draining sites are right ventricle, right atrium, and pulmonary arteries, with less frequent drainage to the left side of the heart. The majority of CCFs are hemodynamically inconsequential and do not require treatment. However, when large, these lesions can cause myocardial ischemia by causing coronary steal or high-output heart failure, and should be treated. Treatment modalities include transcatheter closure with embolic agents (microcoil or gelfoam) and surgical ligation. Choice of therapy is governed by size of the CCF, tortuosity of the feeder channel, size of the communication to prevent embolization, and concomitant coronary artery disease.

КОМПЛЕКСНЫЕ ГЕОЛОГО-ГЕОФИЗИЧЕСКИЕ ИССЛЕДОВАНИЯ ПОДВОДНОГО ВУЛКАНА 7.10 (КУРИЛЬСКАЯ ОСТРОВНАЯ ДУГА)

Submarine volcano 7.10, which is part of the North Iturup group of submarine volcanoes of the Kuril island arc, was studied in 5 voyages of the research vessel Vulkanolog in 1982–1989. Comprehensive studies have shown that the edifice of the 7.10 submarine volcano is composed of rocks of a range from dacites to basalts. The summit and slopes of the volcano are devoid of sediments, and the base is overlain by a sedimentary stratum, thickness of which reaches 800 m. The minimum depth recorded above the top of the volcano is 210 m. In the volcanic edifice, subvertical, northeastern and northwestern feeder channels, as well as peripheral magma chambers at depths of 2.5–3.0 km, were identified. It is assumed that during the terminal eruption, small lava flows poured out in the northeast and southwest directions, while the main lava flow poured out in the southeast direction and reached the base of the volcanic edifice.

Fringe or background: Characterizing deep-water mudstones beyond the basin-floor fan sandstone pinchout

ABSTRACT Mud dominates volumetrically the fraction of sediment delivered and deposited in deep-water environments, and mudstone is a major component of basin-floor successions. However, studies of basin-floor deposits have mainly focused on their proximal sandstone-prone part. A consequent bias therefore remains in the understanding of depositional processes and stratigraphic architecture in mudstone-prone distal settings beyond the sandstone pinchouts of basin-floor fans. This study uses macroscopic and microscopic descriptions of over 500 m of continuous cores from research boreholes from the Permian Skoorsteenberg Formation of the Karoo Basin, South Africa, to document the sedimentology, stratigraphy, and ichnology of a distal mudstone-prone basin-floor succession. Very thin- to thin-bedded mudstones, deposited by low-density turbidity currents, stack to form bedsets bounded by thin packages (< 0.7 m thick) of background mudstones. Genetically related bedsets stack to form bedset packages, which are bounded by thicker (> 0.7 m thick) background mudstones. Stratigraphic correlation between cores suggests that bedsets represent the distal fringes of submarine fan lobe elements and/or lobes, and bedset packages represent the distal fringes of lobe complexes and/or lobe complex sets. The internal stacking pattern of bedsets and bedset packages is highly variable vertically and laterally, which records dominantly autogenic processes (e.g., compensational stacking, avulsion of feeder channels). The background mudstones are characterized by remnant tractional structures and outsize particles, and are interpreted as deposited from low-density turbidity currents and debris flows before intense biogenic reworking. These observations challenge the idea that mud accumulates only from hemipelagic suspension fallout in distal basin-floor environments. Thin background mudstones separating bedsets (< 0.7 m thick) are interpreted to mainly represent autogenically driven lobe abandonment due to up-dip channel avulsion. The thicker background mudstones separating bedset packages (> 0.7 m thick) are interpreted to dominantly mark allogenically driven regional decrease of sand supply to the basin floor. The recognition of sandstone-prone basin-floor fans passing into genetically linked distal fringe mudstones suggests that submarine lobes are at least ∼ 20 km longer than previously estimated. This study provides sedimentological, stratigraphic, and ichnological criteria to differentiate mudstones deposited in different sub-environments in distal deep-water basin-floor settings, with implications for the accurate characterization of basin-floor fan architecture, and their use as archives of paleoenvironmental change.

The Smokescreen of Media during COVID-19 and Its Perpetual Resolutions: A Thick Description

Regarding the information of Coronavirus, the platform of social and mass media played a vital role. As there was little knowledge about COVID-19 at the beginning, numerous fake news, misinformation, and rumors were spread by the digital media across the globe, which made people lose their nerves and panic decisions. To weaken and stop the spreading of misinformation and fake news in the course of the COVID-19 pandemic are requisite because the misinformation and rumors about the pandemic create anxiety, distress, and disturbance among the public, and leads them to several psychological disorders. Instead of viewing social media as a tributary channel, it must be organized to deliver significant data and information. Moreover, it lets people report their questions directly. Several governments have taken drastic steps to hold the pandemic of misinformation, globally; however, several measures are mandatory to avert such communication barriers

Geomorphology and sedimentary processes on the Sloggett Canyon, Northwestern Scotia Sea, Argentina

The southern Argentine Continental Margin hosts the Tierra del Fuego Submarine Canyon System, located in the northwestern Scotia Sea, and formed by 4 main canyons: Valentín, Sloggett, Nueva Canyon, and Wollaston. This region is especially interesting due to its complex tectonic setting and its proximity to the polar regions where intense bottom currents associated with the Argentine Contourite Depositional System take place. Here, we present for the first time the complete high-resolution swath bathymetry, 28 m grid size, of the Sloggett Canyon obtained during the YTEC-GTGM 0 cruise on board the Argentine R/V Austral. This new information allowed us to make a detailed morphosedimentary description of the architectural elements of the canyon and to identify the main sedimentary processes that controlled its formation and evolution together with other oceanographic, climatic, and tectonic ruling factors. The Sloggett Canyon, with a total length of 147 km, starts in the continental shelf with the head area located at 90 m water depth, only 8.5 km away from the coast, and ends at 3690 m water depth joining the abyssal Yahgán Basin. It is deeply carved into the continental slope, with 1600 m as the maximum incision observed in its middle sector. The canyon head area is characterized by two different tributary systems that act as routing conduits for sediment supply from the shelf to the canyon domain. On the western flank, the tributary system is densely spaced with deeply incised V-shaped valleys that suggest the occurrence of active sediment flows that probably evolve into erosive turbidity currents. On the eastern flank, on the contrary, the tributary channels are sparse less incised, and U-shaped, suggesting that they are no longer active. Here, mass wasting processes seem to dominate as evidenced by the presence of slumping and sliding scars. The canyon valley shows two predominant orientations, NW-SE and NE-SW, with sharp bends of approximately 90°. These orientations would have been controlled by the lineaments of the West Scotia Ridge and the Beagle Channel Fault System, respectively. The completely surveyed Sloggett Canyon highlights the complexity of the Tierra del Fuego continental margin, and brings new insight for future multidisciplinary research in this region.

MINERALOGICAL, BIB‐SEM AND PETROPHYSICAL DATA IN SEAL ROCK ANALYSIS: A CASE STUDY FROM THE VIENNA BASIN, AUSTRIA

The Vienna Basin is a major hydrocarbon province with a long exploration history. Within the basin, secondary migration from Upper Jurassic source rocks into stacked Middle Miocene (Badenian) sandstone reservoirs was formerly considered to have occurred almost entirely along major fault zones. However recent exploration data has suggested that in areas where no major faults are present, oil may have migrated vertically through the sandy mudstone intervals separating individual reservoir units, which are therefore imperfectly sealed. In order to investigate possible secondary migration through the semi‐permeable mudstones, this study links variations in gross depositional environment (GDE) to variations in mudstone properties (e.g. mineralogy and pore size distribution). The study focussed on the mudstones which seal reservoir sandstones referred to locally as the “8.TH” and “16.TH” units. The bulk mineralogical composition of 56 mudstone and sandy mudstone (and minor intercalated muddy sandstone) samples from seal layers in 22 wells was studied by X‐ray diffraction analysis, broad ion beam – scanning electron microscopy (BIB‐SEM), mercury intrusion porosimetry (MICP) and N2 adsorption. These data are interpreted in the context of GDE maps of the Vienna Basin which were previously established using seismic and well log data.Results indicate that the gross depositional environment strongly controlled the pore space characteristics of the mudstones. The sandy mudstones in the NW part of the study area were influenced by a complex eastward‐prograding deltaic system which deposited coarse detritus into a major palaeo depression (“Zistersdorf Depression”) located in the centre of the basin. Higher overall porosity and a dominance of larger pore size classes, probably resulting in reduced seal quality, were observed for sandy mudstones from well locations within feeder channels and also from within the Zistersdorf Depression. Similarly, sandy mudstones from locations associated with the long‐term input of coarser sediments in shoreline, coastal and proximal offshore settings in the NW and central parts of the study area are considered to be of lower sealing quality compared to fine‐grained mudstones deposited in distal, open‐marine settings which prevailed in the SE part of the study area throughout the Middle Miocene.In general, pore geometries were influenced by mineralogical composition; quartz‐ and detrital carbonate‐rich samples show equidimensional pores, while more elongated pores (with a higher average aspect ratio) characterize clay‐rich samples. Furthermore, matrix mesopores (2‐50 nm) determined by N2 sorption are more abundant in clay‐rich versus quartz‐rich samples, and show a pronounced positive trend with increasing percentage of illite‐smectite mixed‐layer clay minerals.This study shows that regional‐scale mudstone seals in the Vienna Basin have been influenced by variations in sedimentation associated with lateral variations in gross depositional environment during the Middle Miocene. The observed pore characteristics will serve as input data for future models of secondary migration.

Influence of Erodible Beds on Shallow Water Hydrodynamics during Flood Events

Flooding has become the most common environmental hazard, causing casualties and severe economic losses. Mathematical models are a useful tool for flood control, and current computational resources let us simulate flood events with two-dimensional (2D) approaches. An open question is whether bed erosion must be accounted for when it comes to simulating flood events. In this paper we answer this question through numerical simulations using the 2D depth-averaged shallow-water equations. We analyze the effect of mobile beds on the flow patterns during flood events. We focus on channel confluences where water flow and sediment mobilization have a marked 2D behavior. We validate our numerical simulations with laboratory experiments of erodible beds with satisfactory results. Moreover, our sensitivity analysis indicates that the bed roughness model has a great influence on the simulated erosion and deposition patterns. We simulate the sediment transport and its influence on the water flow in a real river confluence during flood events. Our simulations show that the erosion and deposition processes play an important role on the water depth and flow velocity patterns. Accounting for the mobile bed leads to smoother water depth and velocity fields, as abrupt fields for the non-erodible model emerge from the irregular bed topography. Our study highlights the importance of accounting for erosion in the simulation of flood events, and the impact on the water depth and velocity fields.

The March 2015 catastrophic flood event and its impacts in the city of Copiapó (southern Atacama Desert). An integrated analysis to mitigate future mudflow derived damages

The March 2015 extraordinary hydrometeorological event in the Andes cordillera caused severe floods in the southern Atacama Desert. One of the most affected cities was Copiapó (northern Chile) located downstream of the junction between the Copiapó river and its ephemeral tributary Quebrada Paipote. This work analyses the features of this catastrophic flood and relates them with the identified impacts. A large volume of water mixed with fine sediments overflowed the tributary channel generating a flood that affected 72% of the urban area. The rheological (velocity, density and flow regime) and sedimentary features of the flow reveal the occurrence of massive mudflows that infilled the space available inside the buildings, buried the streets with a sandy mud deposit of more than 30 cm medium thickness and collapsed the sewer network. The post-event survey carried out by the Ministry of Housing and Urban Planning (MINVU) was used for the development of fragility curves that allows modelling the probability of damage. Results indicate that the greatest probability of building damage is generated by the accumulation of sediments instead of by the flow depth. On the other hand, once the very fine grain sediments of the top of the deposit dried up, it increased the concentration of post-event suspension particulate matter, causing a health issue. This work highlights the need to understand mudflow processes and their consequences in arid environments to improve urban planning and mitigate future damages since their impacts strongly affect infrastructures and communities.

Tributary Channel Width Effect on the Flow Behavior in Trapezoidal and Rectangular Channel Confluences

Channel confluences happen commonly in water transport networks and natural rivers. Utilizing a 3D CFD code, a series of numerical simulations were performed using a large eddy simulation turbulence model to investigate the effect of the variations in tributary channel width and the transverse geometrical shape of the main channel on the flow parameters and vertical structure in a T-shape confluence. The code was calibrated using the experimental data from the literature. Flow parameters were considered in ratios of tributary width to the main channel width in trapezoidal and rectangular channels. Results indicate that decreasing the width ratio of the tributary channel to the main channel significantly affects the flow structure in the confluence. Generally, it increases the width and length of the main recirculation zone. It also increases the maximum velocity near the bed, especially in cases with a trapezoidal shape. Besides, it highly affects the structure and formation of the recirculation zone in trapezoidal channels.

Sedimentary architecture of the undeformed Lower Miocene Görendağı Submarine Fan deposits from Neo-Tethys Ocean (Eastern Anatolia-Turkey): Tectonic control on preservation of fan morphology

Submarine fan deposits which constitute the remnants of Neo-Tethys are widely observed throughout the Bitlis–Zagros Suture Belt. Most of these deposits are located in the Arabian-Eurasian collision zone and have been intensively deformed due to ongoing tectonic activity. In this study, the depositional characteristics and preservation processes of the undeformed, Lower Miocene Görendağı Submarine Fan deposits, associated with this zone, are studied with respect to their sedimentological and structural features. These deposits display feeder or distributary channels, overbank, and lobe sediments and classical Bouma Ta–d divisions. They are subdivided into inner, middle, and outer fan, according to sedimentological and morphological properties. Sand-rich submarine fan deposits have 39 m thickness and approximately 2 km of lateral extension.The submarine fan body is bounded to the north and the south by two reverse faults, namely the Özyurt and Göktepe faults. These faults formed an accommodation space where the fan was deposited and also caused the deepening of the basin. The position and activity of these oppositely inclined reverse faults, which form a triangular zone, are the main reason for preservation of the original fan morphology in the Görendağı Submarine Fan. Our results indicate that the Görendağı Submarine Fan represents one of the last sequences deposited before the closure of the southern branch of the Neo-Tethys Ocean and also the most well-preserved.

Study of pollution transport through the river confluences by derivation of an analytical model.

Due to the entrance of pollutants in different branches of the river network, it is essential to study contaminant transport at the river confluences. In the present study, it was attempted to investigate the conservative pollution transport at channel confluence by operating a series of experiments in the laboratory flume. In the designed laboratory model, two branches, with different widths of 45, 25 cm, were intersected and a channel confluence was created. Five entrance discharges and three initial contaminant concentrations, introduced using a linear feeder, were chosen as experimental variables. Conservative tracer of sodium chloride solution was used, and the electrical conductivities were measured at eight locations of the main channel and upstream branches with 2 seconds interval. Junction zone was assumed as a control volume, and by applying mass equilibrium to it, a new mathematical model was extracted. It was observed that there is concentration fluctuation in the falling limbs of the experimental breakthrough curves of the junction zone; however, it was diminished by downstream motion. Moreover, the observed pollution graphs had double peak points which changed to a single point with an increase of distance from the confluence position. Operation of the presented model was investigated by variation of its parameters. It was found that the contaminant residence time parameters of the confluence zone have the most significant influence in the simulation of the analytical model. Additionally, it was observed that the values of Gaussian distribution of the upstream branches could displace the position of pulses of resultant breakthrough curves or can overlap them. Moreover, the model performance was examined using statistical goodness of fit parameters like Nash-Sutcliffe, R2, and mean absolute error (MAE). Their values were calculated as 0.88, 0.91, 66.88 (ppm), respectively.

Experiments on the Morphodynamics of Open Channel Confluences: Implications for the Accumulation of Contaminated Sediments

AbstractContaminated sediments are common in river networks. The flow convergence and particular flow structures in confluences, such as the flow separation zone, may result in greater accumulation of contaminated sediments than in other river locations, but this issue is rarely studied. In addition, the contaminated sediment transport is driven by the particular morphodynamics occurring at confluences. This article describes a novel confluence flume experiment on both morphodynamics and deposition patterns of contaminated sediments as a function of geometric and flow conditions. The initial equilibrium bed geometry was developed from a mobile bed and then fixed, allowing for subsequent flow velocimetry and sediment feeding. Colored sediments of fine gradation, which mimic contaminated sediments, were then fed to the tributary channel. The results suggest that the junction angle primarily determines the confluence bed morphology and sediment transport pattern while the discharge ratio is a secondary factor. It was also observed that most introduced sediments tended to deposit immediately after the cessation of feeding at the stoss of the bar in the flow separation zone. The time history of the transport of the contaminated sediments was also investigated. Sediment that initially deposited at the stoss of the bar eventually moved to the lee of the bar and deposited around the bar downstream of the confluence, demonstrating that the sedimentation pattern evolved to a state similar to the equilibrium bed morphology.

Rotational manipulation of a microscopic object inside a microfluidic channel.

Observations and analyses of a microscopic object are essential processes in various fields such as chemical engineering and life science. Microfluidic techniques with various functions and extensions have often been used for such purposes to investigate the mechanical properties of microscopic objects such as biological cells. One of such extensions proposed in this context is a real-time visual feedback manipulation system, which is composed of a high-speed camera and a piezoelectric actuator with a single-line microfluidic channel. Although the on-chip manipulation system enables us to control the 1 degree-of-freedom position of a target object by the real-time pressure control, it has suffered from unintended changes in the object orientation, which is out of control in the previous system. In this study, we propose and demonstrate a novel shear-flow-based mechanism for the control of the orientation of a target object in addition to the position control in a microchannel to overcome the problem of the unintended rotation. We designed a tributary channel using a three-dimensional hydrodynamic simulation with boundary conditions appropriate for the particle manipulation to apply shear stress to the target particle placed at the junction and succeeded in rotating the particle at an angular velocity of 0.2 rad/s even under the position control in the experiment. The proposed mechanism would be applied to feedback controls of a target object in a microchannel to be in a desired orientation and at a desired position, which could be a universally useful function for various microfluidic platforms.

Flow Topology in the Confluence of an Open Channel with Lateral Drainage Pipe

The purpose of this paper is to develop design guidelines for flood control channel height in the vicinity of the confluence of a submerged drainage pipe and a flood control channel. The water exchange in the confluence of an open channel with a lateral drainage pipe produces unique hydraulic characteristics, ultimately affecting the water surface elevation in the channel. An accurate prediction of the water surface elevation is essential in the successful design of a high-velocity channel. By performing several experiments, and utilizing a numerical model (FLOW-3D), this study investigated the impact of submerged lateral drainage pipe discharges into rectangular open channels on flow topology in the confluence hydrodynamics zone (CHZ). The experiments were conducted in different flume and junction configurations and flow conditions. Moreover, the simulations were performed on actual size channels with different channel, pipe, and junction configurations and flow conditions. The flow topology in the CHZ was found to be highly influenced by the junction angle, as well as the momentum ratios of the channel flow and the pipe flow. The findings of this study were used to develop conservative design curves for channel confluences with lateral drainage pipe inlets. The curves can be used to estimate water surface elevation rise in different channel and pipe configurations with different flow conditions to determine the channel wall heights required to contain flows in the vicinity of laterals.

Integrating Microbial Community Assembly and Fluid Kinetics to Decouple Nitrogen Dynamics in an Urban Channel Confluence.

Understanding the characteristics of biogeochemical processes in urban channel confluences is essential for the evaluation and improvement of water environmental capacity. However, influences of biogeochemical processes in confluence were always overlooked or simply parametrized since the transformation processes controlled by microbial community assembly were hard to quantify. To address this knowledge gap, the present study proposed a novel mathematical modeling system, based on microbial community assembly theory and fluid kinetics, to decouple nitrogen dynamics into flow-induced transport and microorganism-induced transformation processes, and quantified their contributions to nitrogen concentrations. Results revealed that variable selection processes (including hydrodynamic conditions) contributed to significant difference in microbial communities among different hydraulic regions. Variation in microbial communities further shifted transformation processes. Rhodobacterales and Sphingomonadales, which were reported to be vital participants in denitrification process, were enriched in flow separation region, and promoted it as a hotspot for nitrogen removal. In the flow separation region, microorganism-induced transformation processes accounted for 56% of total nitrogen removal, which was significantly higher than that in other regions (12% on average; p < 0.01). Results and findings could provide useful information for the improvement of water environmental capacity.