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River cross-section characteristics are not available for many regions of the world, and they are generally expressed as hydraulic geometry (HG) relationships. These relationships usually represent average characteristics of a region, which can generate biased estimates in smaller basins. Therefore, to improve the representation of river channel geomorphological characteristics, the objective of this study was to use a large database of river cross-section geometries to refine HG relationships for five river basins in Brazil. HG relationships were developed for the entire basins and for hydraulically homogeneous regions, obtained through a geographical convenience method. The relationship between the coefficients of the geometric relationships and morphoclimatic characteristics was assessed, showing a trend between precipitation and slope and the coefficients of the depth relationships. The estimated bankfull width and depth values were also compared to observed data and global datasets, showing an improvement in estimates with the more refined HG relationships. [ABSTRACT FROM AUTHOR]

The present study aims to examine the changing hydro-geomorphological properties of the Mayurakshi River located in a plateau fringe region in Jharkhand and West Bengal. The field investigation of 14 cross-sectional sites has been conducted during the post-monsoon season of 2021 to determine important geomorphic parameters such as channel depth, width, bank height, hydraulic radius, and wetted perimeter. Post-processed kinematic survey and electromagnetic current meter tool have been used to measure the river bed profile and channel velocity, respectively, to obtain more accurate locational information. The primary objective of this study is to compare field-observed discharge data with the discharge volume predicted by a few hydraulic and hydrological models in order to determine the models that are best suited for further application in a scientifically accepted and cost-effective manner. The results demonstrate that there is a noticeable rise in flow velocity from upstream (0.22 m/s) to downstream of the river (0.85 m/s), which is also supported by the results of Manning's equation and kinematic wave parameter. The most correct relationship is then presented by minimizing the deviations between the calculated and analytical discharges at three separate stages. The results show that the proposed Manning's technique is quite accurate with the observed data, as determined by the mean absolute percentage error (60.16%), root mean square error (30.53%), and normalized root mean square error (0.05) indices. [ABSTRACT FROM AUTHOR]

This study investigates the finishing characteristics of internal channels with different cross-sectional geometries using free abrasive grains and evaluates the cooling performance of these channels before and after finishing. Three types of channels with circular, triangular, and hexagram cross-sections were designed and fabricated using laser powder bed fusion (L-PBF). A fluid flow in the channel was evaluated using computational fluid dynamics simulations, and the finishing characteristics and cooling performances of the channels were experimentally investigated. The results indicated that the use of free abrasive grains enabled the improvement in the surface quality as well as the cooling performance of the channel. The cross-section of the channel affected the fluid flow in the channel and finishing progress. The initial surface roughness varied with the cross-section of the channel owing to the limitations of L-PBF, and the triangular section had a relatively uniform surface quality throughout the channel compared with the other cross-sections. The cooling time decreased with the surface area of the channel. To obtain the uniform surface quality, the application of a suitable cross-section is needed for the finishing process. The outcomes of this study demonstrate that a triangular-section channel is suitable for improving both surface quality and cooling performance. [ABSTRACT FROM AUTHOR]

Simulating riverbank erosion involves modeling the complex interactions between water flow, sediment transport, and bank stability. The present study employs a novel approach, combining the Self Organizing Map (SOM) algorithm and the Long Short-Term Memory (LSTM) network, to capture the behavior of riverbank erosion. A two-stage training procedure is promoted to enhance predictive accuracy and emphasize the need for preprocessing input data related to hydrological conditions, geomorphological features, and soil properties. Pivotal variables that can characterize changes in channel geometry are designed as the output targets, such as the vertical displacement of the riverbed, the horizontal displacement of the riverbank, and the channel width. The goal of this study is to create an alternative to addressing challenges associated with predicting riverbank erosion by utilizing new training methods of artificial intelligence (AI) models. The proposed method performs well in assessing three output variables, showing low mean relative errors, less than 0.081, and high correlation coefficients above 0.981, along with R-squared values over 0.963. These results highlight the effectiveness of this method in accurately describing cross-sectional changes. The proposed method is designed with practical applications in mind, satisfying the need for methods that are not only accurate but also operationally efficient.

This paper investigates the effects of different channel geometries on the performance of Proton Exchange Membrane Fuel Cells (PEMFCs). The study employs computational fluid dynamics (CFD) coupled with thermal and electrochemical simulations to analyze five channel geometries (cases A to E) of bipolar plates. A thorough study on this topic is not found in the literature and aims to identify designs that optimize performance and align with cost-effective production methods. Among the various studied geometries, case D, featuring a trapezoidal cross-section, exhibited the most favorable performance compared to the others, with a current density value of 2.01 A/cm2 and a maximum temperature of 74.89 °C at 0.3 V, leading to an increase in generated power of 4.46%, compared to base case A. The trapezoidal shape enhanced the contact area with the reacting region, resulting in higher reaction rates and an improved overall performance. However, the study also highlights the relevance of velocity and turbulence, with case B demonstrating an enhanced performance due to its higher velocity, and case E benefiting from localized higher velocity regions and turbulence created by baffles. Case B can increase generated power at its peak by around 3.21%, and case E can improve it by 1.29%, with respect to case A. These findings underscore that contact area has a major impact on the PEMFC performance, but velocity and turbulence also play relevant roles. Additionally, trapezoidal channels can be easily manufactured through sheet metal-forming techniques, aligning well with new market trends of weight and cost reduction on bipolar plates. Fuel and oxygen utilization percentages, 38.14% and 62.96% at 0.3 V, respectively, further confirm the superiority of trapezoidal channels, providing insights into optimizing the PEMFC performance. This exhaustive study contributes valuable information for designing efficient metallic bipolar plates and advancing the development of practical fuel cell technologies. [ABSTRACT FROM AUTHOR]

Abstract Microchannel heat exchangers are heat exchangers with a tube diameter of less than 1 mm. Conventional cooling approaches such as the forced‐air cooling technique fail in high technological compact systems because of the small‐sized surfaces of chips and circuits. In comparison, microchannel heat exchangers are being extensively utilized in compact‐sized devices where a high heat‐transfer medium is required. Moreover, consumers' continued desire for compact products has prompted researchers to study microchannel heat exchangers for their ability to boost the rate of heat transfer that ensures the safety of compact designs. This study presents the evaluation of performance parameters and the manufacturing aspects of microchannel heat exchangers. This study also examines how microchannel heat exchangers are affected by several parameters, including the type of working fluid used, Brownian motion, geometry of the channel, Reynolds number, Nusselt number, Knudsen number, wall resistance of the channel, the effect of gravity, and inlet and outlet arrangement for fluid. Investigating the various geometries for the microchannel indicates that the least pressure drop occurs in square shape cross‐section channels while the highest pressure drop occurs in channels with triangular cross‐sections. Moreover, it has been observed that, with the addition of nanoparticles to the working fluid, the thermal properties of the exchangers as well as the pressure drop increases while at the same time it reduces the boundary layer thickness. In addition, the Reynolds number affects the performance irrespective of the channel geometry. When the fluid is added with nanoparticles, like, Al2O3 and copper oxide (CuO) with different volumetric fractions (φ) of 0%, 0.5%, 1%, 1.5%, and 2%, the performance of the microchannel rises with rising the Reynolds number but conversely when the fluid is used in pure form the performance decreases with the rising value of Reynolds number. In addition, it has been observed that the overall improvement is obtained at φ = 2% and Re = 100 for CuO–water nanofluid. Apart from this, the least heat transfer is recorded at φ = 0.5% and Re = 1.00 for both nanofluids. Moreover, this study concludes that the Nusselt number is independent of the Reynolds number in the regime of laminar flow. It is further evident that as the nanoparticle size reduces, the Nusselt number rises when all the remaining conditions are the same. Apart from this, investigating the inlet/outlet arrangement through finite volume method for D‐, I‐, N‐, S‐, U‐, and V‐type arrangements, it is evident that the V‐type sink has overall greater performance. In terms of gravity, it is observed that it has no effect on the performance of microchannel heat exchangers. This study further illustrates the effects of the fabrication method on the performance of the microchannel heat exchangers.

Microchannel heat exchangers are heat exchangers with a tube diameter of less than 1 mm. Conventional cooling approaches such as the forced‐air cooling technique fail in high technological compact systems because of the small‐sized surfaces of chips and circuits. In comparison, microchannel heat exchangers are being extensively utilized in compact‐sized devices where a high heat‐transfer medium is required. Moreover, consumers' continued desire for compact products has prompted researchers to study microchannel heat exchangers for their ability to boost the rate of heat transfer that ensures the safety of compact designs. This study presents the evaluation of performance parameters and the manufacturing aspects of microchannel heat exchangers. This study also examines how microchannel heat exchangers are affected by several parameters, including the type of working fluid used, Brownian motion, geometry of the channel, Reynolds number, Nusselt number, Knudsen number, wall resistance of the channel, the effect of gravity, and inlet and outlet arrangement for fluid. Investigating the various geometries for the microchannel indicates that the least pressure drop occurs in square shape cross‐section channels while the highest pressure drop occurs in channels with triangular cross‐sections. Moreover, it has been observed that, with the addition of nanoparticles to the working fluid, the thermal properties of the exchangers as well as the pressure drop increases while at the same time it reduces the boundary layer thickness. In addition, the Reynolds number affects the performance irrespective of the channel geometry. When the fluid is added with nanoparticles, like, Al2O3 and copper oxide (CuO) with different volumetric fractions (φ) of 0%, 0.5%, 1%, 1.5%, and 2%, the performance of the microchannel rises with rising the Reynolds number but conversely when the fluid is used in pure form the performance decreases with the rising value of Reynolds number. In addition, it has been observed that the overall improvement is obtained at φ = 2% and Re = 100 for CuO–water nanofluid. Apart from this, the least heat transfer is recorded at φ = 0.5% and Re = 1.00 for both nanofluids. Moreover, this study concludes that the Nusselt number is independent of the Reynolds number in the regime of laminar flow. It is further evident that as the nanoparticle size reduces, the Nusselt number rises when all the remaining conditions are the same. Apart from this, investigating the inlet/outlet arrangement through finite volume method for D‐, I‐, N‐, S‐, U‐, and V‐type arrangements, it is evident that the V‐type sink has overall greater performance. In terms of gravity, it is observed that it has no effect on the performance of microchannel heat exchangers. This study further illustrates the effects of the fabrication method on the performance of the microchannel heat exchangers. [ABSTRACT FROM AUTHOR]

Bedrock river width is an essential geometric parameter relevant to understanding flood hazards and gauging station rating curves, and is critical to stream power incision models and many other landscape evolution models. Obtaining bedrock river width measurements, however, typically requires extensive field campaigns that take place in rugged and steep topography where river access is often physically challenging. Although prior work has turned to measuring channel width from satellite imagery, these data present a snapshot in time, are typically limited to rivers ≥ 10–30 m wide due to the image resolution, and are physically restricted to areas devoid of vegetation. For these reasons, we are generally data limited, and the factors impacting bedrock channel width remain poorly understood. Due to these limitations, researchers often turn to assumptions of width‐scaling relationships with drainage area or discharge to estimate bedrock channel width. Here we present a new method of obtaining bedrock channel width at a desired river discharge through the incorporation of a high‐resolution bare‐earth digital elevation model (DEM) using MATLAB Topotoolbox and the HEC‐RAS river analysis system. We validate this method by comparing modeled results to US Geological Survey (USGS) field measurements at existing gauging stations, as well as field channel measurements. We show that this method can capture general characteristics of discharge rating curves and predict field‐measured channel widths within uncertainty. As high‐resolution DEMs become more available across the United States through the USGS three‐dimensional elevation program (3DEP), the future utility of this method is notable. Through developing and validating a streamlined, open‐source, and freely available workflow of channel width extraction, we hope this method can be applied to future research to improve the quantity of channel width measurements and improve our understanding of bedrock channels. [ABSTRACT FROM AUTHOR]

This paper investigates the effects of different channel geometries on the performance of Proton Exchange Membrane Fuel Cells (PEMFCs). The study employs computational fluid dynamics (CFD) coupled with thermal and electrochemical simulations to analyze five channel geometries (cases A to E) of bipolar plates. A thorough study on this topic is not found in the literature and aims to identify designs that optimize performance and align with cost-effective production methods. Among the various studied geometries, case D, featuring a trapezoidal cross-section, exhibited the most favorable performance compared to the others, with a current density value of 2.01 A/cm2 and a maximum temperature of 74.89 °C at 0.3 V, leading to an increase in generated power of 4.46%, compared to base case A. The trapezoidal shape enhanced the contact area with the reacting region, resulting in higher reaction rates and an improved overall performance. However, the study also highlights the relevance of velocity and turbulence, with case B demonstrating an enhanced performance due to its higher velocity, and case E benefiting from localized higher velocity regions and turbulence created by baffles. Case B can increase generated power at its peak by around 3.21%, and case E can improve it by 1.29%, with respect to case A. These findings underscore that contact area has a major impact on the PEMFC performance, but velocity and turbulence also play relevant roles. Additionally, trapezoidal channels can be easily manufactured through sheet metal-forming techniques, aligning well with new market trends of weight and cost reduction on bipolar plates. Fuel and oxygen utilization percentages, 38.14% and 62.96% at 0.3 V, respectively, further confirm the superiority of trapezoidal channels, providing insights into optimizing the PEMFC performance. This exhaustive study contributes valuable information for designing efficient metallic bipolar plates and advancing the development of practical fuel cell technologies.

Photovoltaic thermal (PVT) systems are solar systems with the primary goal of producing both heat and electricity. The geometrical design of PVT systems is an important aspect of energy production and, consequently, substantial research has been done to investigate the effect of different geometric configurations on the performance of PVT systems. Thus, the present paper provides a comprehensive review of the works that have explored the effects of various geometric channel configurations on the performance of PVT systems in terms of photovoltaic (PV) module temperature, thermal efficiency (ɳTh), electrical efficiency (ɳEI), and overall efficiency (ɳOv). This review paper concluded that the geometry of the channel and flow channel configurations at the backside of the PV module are the most prominent factors to enhance the heat transfer rate. Most studies found that the optimal fluid flow channel configuration is the helical type, which provides good performance due to an increase in the heat transfer rate due to several bend sections that boost the heat transfer by disrupting the thermal boundary layer. Future research directions in this thrust area are also outlined in this review paper. [ABSTRACT FROM AUTHOR]

Since the Three Gorges Reservoir began impoundment in 2003, drastic changes have been occurring in the Huanghuacheng (HHC) Reach, which is about 330 km from the dam. With measured data, we present a comprehensive analysis of sediment deposition and channel adjustment in this reach for the period 2003 to 2018. Results show that the reach witnessed severe sedimentation, and the channel became narrower and shallower with the mainstream shifting from the left to right branch. An empirical model is proposed to calculate the sediment deposition amount. A correlation is also established between sedimentation and reach-scale morphological parameters (width, water depth, flow area, and the ratio of width to depth). [ABSTRACT FROM AUTHOR]

Several studies have focused on the importance of river bathymetry (channel geometry) in hydrodynamic routing along individual reaches. However, its effect on other watershed processes such as infiltration and surface water (SW)‐groundwater (GW) interactions has not been explored across large river networks. Surface and sbsurface processes are interdependent, therefore, errors due to inaccurate representation of one watershed process can cascade across other hydraulic or hydrologic processes. This study hypothesizes that accurate bathymetric representation is not only essential for simulating channel hydrodynamics but also affects subsurface processes by impacting SW‐GW interactions. Moreover, quantifying the effect of bathymetry on surface and subsurface hydrological processes across a river network can facilitate an improved understanding of how bathymetric characteristics affect these processes across large spatial domains. The study tests this hypothesis by developing physically based distributed models capable of bidirectional coupling (SW‐GW) with four configurations with progressively reduced levels of bathymetric representation. A comparison of hydrologic and hydrodynamic outputs shows that changes in channel geometry across the four configurations has a considerable effect on infiltration, lateral seepage, and location of water table across the entire river network. For example, when using bathymetry with inaccurate channel conveyance capacity but accurate channel depth, peak lateral seepage rate exhibited 58% error. The results from this study provide insights into the level of bathymetric detail required for accurately simulating flooding‐related physical processes while also highlighting potential issues with ignoring bathymetry across lower order streams such as spurious backwater flow, inaccurate water table elevations, and incorrect inundation extents. Key Points: The effect of river geometry on subsurface processes, such as infiltration and lateral seepage in large‐scale fluvial modeling is highlightedSubsurface processes in the floodplain are controlled by overall channel characteristics, rather than channel shapeChannel conveyance capacity and longitudinal slope are critical bathymetric controls of subsurface processes in floodplains of river network [ABSTRACT FROM AUTHOR]

Successful management of flooding and erosion hazards on floodplains depends on our ability to predict a river channel's shape and the lifespan during which it will continue to flow. Recent progress has improved our understanding of what sets the lifespan and width of single‐thread channels; the next challenge is to extend this knowledge to braided channels and their interwoven sub‐channels (threads). In this study, we investigate the lifespan and width of braided channel threads in a large experimental data set, coupled with particle‐image velocimetry‐derived measurements of riverbank erosion and accretion. We find that, unlike single‐thread channels, braided channels in the experiment do not exhibit an equilibrium between bank erosion and accretion. Instead, bank erosion outpaces lateral accretion, causing individual threads to widen and infill until they are abandoned. Thread lifespan is limited to the time it takes for threads to triple their width: tripling of the width yields enough bank material to aggrade more than half the channel depth, at which point flow is rerouted to a narrower thread. In consequence the width of active threads is limited to three times their initial width. Threshold channel theory accurately predicts the median thread width, which is roughly double the initial width and two‐thirds the limiting width. The results are consistent with existing field data and suggest that differential bank migration is sufficient to explain why braided channels show greater width variability and higher width‐to‐depth ratios than their single‐thread counterparts. Plain Language Summary: River channels move and change shape over time, which can endanger people and property on adjacent land. To mitigate flooding and erosion risks, it is important we understand what sets a channel's shape and the lifespan during which it will continue to flow with water. Our understanding is particularly limited for braided channels, which form from multiple interwoven sub‐channels called threads. Here, we present findings from an experimental braided channel, and use a new technique to map the migration of riverbanks over dozens of hours from thousands of photographs. We find that, unlike the banks of other channels, braided channel banks migrate at different speeds. For a given thread, water flow erodes sediment from some banks faster than it can deposit sediment on the other banks, causing the thread to grow wider and shallower over time. Threads widen until they triple in width; at this point, the flow is so shallow that the flow abandons course in favor of a new, narrower thread. These findings support observations of thread width in nature, and can explain why braided‐river channels are generally wider, shallower, and shorter lived than rivers formed by a single channel. Key Points: Experimental braided channels show riverbank erosion outpaces accretion, causing channel threads to widen as they migrateThreads persist until enough eroded bank sediment accumulates on the riverbed to reroute flow and abandon the threadThreads initiate with near threshold‐channel geometry, and widen by a factor of three before they are abandoned [ABSTRACT FROM AUTHOR]

• Regenerative cooling effect is assessed in 3D complex channels with multiple bends. • Coupled flow, heat transfer, and simultaneous pyrolysis and coking are modelled. • Heat transfer deterioration occurs in bends and is worsened at high heat fluxes. • A novel self-adaptive flow regulation structure is proposed to mitigate local coking. Regenerative cooling by endothermic hydrocarbon fuels is a promising thermal management method for aero engines, where the fuel needs to flow in complex cooling channel geometries such as cavity-based flame-holders. The understanding of the heat transfer characteristics of regenerative cooling in complex geometries is still quite limited. This study investigates the coupled flow, heat transfer and pyrolytic reaction characteristics of hydrocarbon fuels, as well as the surface carbon deposition, in curved cooling channels with multiple bends and angles using a computational fluid dynamics (CFD) model. A 3D full cavity-shaped parallel cooling channel model is established, and the influence of heat flux, flow direction and flow redistribution are investigated numerically. The results show that significant heat transfer deterioration occurs in the bending section with a 90° turning angle, and is worsened at high heat fluxes. A bi-directional flowing configuration can mitigate the local heat transfer deterioration problem with a decrease of wall peak temperature by 129.5 K under a heat flux of 2 MW/m2 in the inlet section. A novel self-adaptive micro-structure concept, driven by the local fluids pressure difference, is proposed at the deteriorated regimes to redistribute the fluid between adjacent channels, which could improve the cooling performance and reduce the carbon deposition locally. However, a large fluid redistribution amount may lead to an overall increase of the coke deposition, escalating from 320.4 mg to 392.7 mg under a heat flux of 2 MW/m2, due to increased wall temperature resulting from the channel with reduced flow rate. [ABSTRACT FROM AUTHOR]

Channel structure plays an important role on the performance of proton exchange membrane fuel cell (PEMFC). In this study, the channel geometry of a PEMFC is optimized through genetic algorithm to obtain better performance. For the first time, a machine learning method called Bagging Ensemble Regression is employed as the surrogate model to calculate the fitness value of the algorithm, which accelerates the optimization process. First, a three-dimensional PEMFC simulation model is developed as the optimization prototype through CFD technology. Second, the Bagging ensemble model is trained through training data obtained from the CFD model. Then the Bagging ensemble model is integrated into the genetic algorithm to conduct the optimization process. Finally, the optimal model obtained is compared with the optimization prototype in terms of polarization curves, pressure drop, and reactant distribution, and the advantages of using Bagging ensemble model are discussed. Results show that the optimal model has a smaller pressure drop and a more uniform reactant distribution than the basic model at the expense of just a little power density. The presented surrogate model shows high prediction accuracy with only a small amount of training data, which is superior to the commonly used surrogate models. [Display omitted] • Genetic algorithm is employed to optimize the channel geometry of PEMFC. • It is the first approach based on Bagging-SVM Ensemble Regression for PEMFC optimization. • This method has advantages in terms of both training data scale and prediction accuracy. • The optimal geometry shows smaller pressure drop and more uniform reactant distribution. [ABSTRACT FROM AUTHOR]

Flood routing process is constrained by the inflow and sediment conditions and channel geometry, while the influences of the latter is seldom analyzed. Based on the measured hydraulic data and a coupled ID morphological model, the braided reach of the Lower Yellow River was selected as the study section to analyze the influence of adjustment of channel geometry to the flood routing in the case of same magnitude of bankfull area. The results of the analysis of measured hydrological data indicate that when the magnitude of flood peak discharge is slightly changed, the velocity of flood propagation is larger in the 1uuTower m1d deeper cross section with smaller geomorphic coefficient, and the corresponding propagation time is smaller; the rising rate of floodwater level shares a negative correlation with geomorphic coefficient, with the determination coefficient equals to 0.91 ; numerical simulation by ID model shows that keeping the upstream conditions unchm1ged, the flood propagation time of the channel with smaller geomorphic coefficient in 2003 is reduced from 64 h to 21 h, and the flood peak discharge and the deposition volume reduced by 21 %· m1d 34%· respectively, which indicates that the channel with narrower and deeper geometry can reduce the time of flood propagation and relive the channel aggradation as well. [ABSTRACT FROM AUTHOR]

Coastal wetlands are nourished by rivers and periodical tidal flows through complex, interconnected channels. However, in hydrodynamic models, channel dimensions with respect to model grid size and uncertainties in topography preclude the correct propagation of tidal and riverine signals. It is therefore crucial to enhance channel geomorphic connectivity and simplify sub‐channel features based on remotely sensed networks for practical computational applications. Here, we utilize channel networks derived from diverse remote sensing imagery as a baseline to build a ∼10 m resolution hydrodynamic model that covers the Wax Lake Delta and adjacent wetlands (∼360 km2) in coastal Louisiana, USA. In this richly gauged system, intensive calibrations are conducted with 18 synchronous field‐observations of water levels taken in 2016, and discharge data taken in 2021. We modify channel geometry, targeting realism in channel connectivity. The results show that a minimum channel depth of 2 m and a width of four grid elements (approximatively 40 m) are required to enable a realistic tidal propagation in wetland channels. The optimal depth for tidal propagation can be determined by a simplified cost function method that evaluates the competition between flow travel time and alteration of the volume of the channels. The integration of high spatial‐resolution models and remote sensing imagery provides a general framework to improve models performance in salt marshes, mangroves, deltaic wetlands, and tidal flats. Plain Language Summary: In hydrodynamic models, it is common to smooth topographic data to build the numerical grid. However, in coastal wetlands dissected by complex channel networks, this process would prevent an appropriate representation of channels for tidal propagation. This might lead to unreliable calculations of water fluxes between channels and wetlands. To address this issue, we modify channel geometry using remotely sensed data to enhance connectivity in simulations. Channel depth and width are determined by comparing model results to high‐resolution field measurements. We develop a simplified cost function that can determine the optimal channel depth for tidal propagation without running computationally expensive simulations. Our results provide a framework to improve model performance of tidal flows along wetland channels by integrating numerical simulations, a simplified cost function, and remotely sensed channel networks. Key Points: Remote‐sensed channel network can enhance hydrological connectivity in numerical modelsWe provide a simplified cost function method to determine the minimum channel depth in numerical modelsA minimum 2‐m channel depth and a 4‐grid width are required to simulate tidal propagation in wetland channels [ABSTRACT FROM AUTHOR]

Microchannels are effective heat sinks for microelectronic systems. However, it remains unclear what form of channels will be most effective in improving the overall performance of microchannel heat sinks. The effect of channel geometry was studied using computational fluid dynamics to understand the heat transfer and fluid flow characteristics of microchannel heat sinks with rectangular grooves in sidewalls and different shaped ribs in the center core flow. Four different rib configurations are considered, including rectangular, diamond, forward triangular, and elliptic. In order to fully understand the design and operation of microchannel heat sinks, the overall performance of these devices was analyzed and evaluated in detail in terms of Nusselt number, apparent friction factor, and thermal enhancement efficiency. The enhancement mechanism of fluid flow and heat transfer was discussed in order to determine the optimum channel structure. The objective was to optimize channel structure for microchannel heat sinks, thereby enhancing heat transfer and reducing flow resistance and pressure drop. The results indicated that the overall performance can be greatly improved by the combination of grooves and ribs. This combination can make full use of the advantages of ribs to increase flow disturbance and to enhance heat transfer, and the advantages of grooves to increase flow area and to reduce pressure drop. The shape and dimensions of ribs have a significant effect on the overall performance. The overall performance obtained with rectangular ribs is the best at Reynolds numbers less than 500, but lower than that obtained with elliptic ribs at Reynolds numbers greater than 500, and even lower than that obtained with diamond ribs at Reynolds numbers greater than 700. The rectangular grooved channel with rectangular ribs yields the best overall performance with a relative rib width of 0.25 and at a minimum rib-groove distance of 0.1 mm and a Reynolds number of 500.

Certainly, photovoltaics (PV) is regarded as one of the most reliable and competitive energy production alternatives due to a massive decline in prices. Coming years are expected to witness PV systems leading the renewable electricity capacity growth by surpassing 550 GW. However, the efficiency of the enormous number of PV modules being produced and installed remains poor due to overheating. PV systems that are cooled using liquid coolants, popularly known as photovoltaic-thermal (PV/T) is a promising solution, but the inherent thermal conductivity of the conventional coolants being used makes it less attractive. Besides the evolution of next-generation PV modules and efficient designs, a breakthrough was made when nanoparticles successfully dispersed in coolants that led to better heat transfer and energy conversion. In recent years, hybrid nanoparticle-based fluids have helped augment heat transfer through PV panels. In this review, we discussed the evolution of PV/T system; difficulties faced in improving thermal and electrical production, and explain how the design modifications and hybrid nanofluids contribute in enhancing the system performance. In terms of the PV/T efficiency, we specifically highlight the role of hybrid nanofluids and the struggles arise from design to operation through 10 big questions and some possible solutions are also suggested to overcome those issues.

Water electrolysis technology is required to overcome the intermittency of renewable energy sources. Among various water electrolysis methods, the proton exchange membrane water electrolysis (PEMWE) cell has the advantages of a fast response and high current density. However, high capital costs have hindered the commercialization of PEMWE; therefore, it is important to lower the price of bipolar plates, which make PEMWE expensive. In addition, since the flow field inscribed in the bipolar plate significantly influences the performance, it is necessary to design the enhanced pattern. A three-dimensional two-phase flow model was used to analyze the two-phase flow and electrochemical reactions of the PEMWE anode. In order to compare the experimental results with the simulation, experiments were conducted according to the flow rate, and the results were in good agreement. First, as a result of comparing the performance of the channel and PTL (porous transport layer) flow fields, the channel flow field showed better performance than the PTL flow field. For the channel flow field, the higher the ratio of the channel width-to-rib width and the permeability of PTL, the performance got better. In the case of the PTL flow field, with the increased capillary pressure, the performance improved even if the PTL permeability decreased. Next, the direction of gravity affected the performance only when the channel flow field was used, and the X+ and Z+ directions were optimal for the performance. Finally, increasing the inlet flow rate could reduce the difference in performance between the channel and PTL flow fields, but the pressure drop gradually increased.

Equilibrium geometry of single‐thread rivers with fixed width (engineered rivers) is determined with a flow resistance relation and a sediment transport relation, if characteristic discharge, sediment caliber and supply are specified. In self‐formed channels, however, channel width is not imposed, and one more relation is needed to predict equilibrium geometry. Specifying this relation remains an open problem. Here we present a new model that brings together a coherent train of research progress over 35 years to predict equilibrium geometry of single‐thread rivers from the conservation of channel and floodplain material. Predicted channel geometries are comparable with field observations. In response to increasing floodplain width, sand load and grain size, the equilibrium slope increases, bankfull depth and width decrease. As the volume fraction content of mud in the sediment load increases, bankfull width‐to‐depth ratio and slope decrease suggesting that mud load has a strong control on channel patterns and bankfull geometry. Plain Language Summary: Empirical relations expressing balance between sediment erosion and deposition, or linking channel properties such as bankfull discharge, channel width, depth and slope have long been used to model channel geometry owing to a lack of equilibrium solutions based on first principles. Here we present a solution for equilibrium geometry of single‐thread, sand‐bed rivers based on first principles that compatibly predicts measured field channel geometries. Empirical and theoretical formulations are used to compute sediment fluxes and bank migration respectively. The equilibrium solution can be used to predict long term (equilibrium) impacts of changes in hydrology and land use on alluvial rivers, and thus identify management and restoration practices based on how each river tends to evolve in space and time. Key Points: Conservation of floodplain sediment allows us to predict the geometry of self‐formed channels in agreement with field observationsBankfull geometry and discharge of alluvial rivers can be predicted if the hydrologic regime, sediment supply and caliber are specifiedWash‐load plays a prime control on channel geometry of single‐thread rivers [ABSTRACT FROM AUTHOR]

This study aims to achieve a minimum base temperature (or junction temperature) and hence better thermal performance. Trapezoidal curved plate-fin heat sink with dolphin fins and rectangular channel (Model-1) and trapezoidal curved plate-fin heat sink with dolphin fins, cut corner, and rectangular channel (Model-2) were designed and compared with a standard plate-fin heat sink. The effects of fins on the airflow and heat transfer in designed plate-fin heat sinks have been investigated numerically. The numeric results show that the use of fins and small changes in geometry significantly improve the heat transfer rate. Outcomes of the study showed 44–51% and 57–62% convective heat transfer enhancement compared with a standard plate-fin heat sink, without any overall mass augmentation, in Model-1 and Model-2, respectively. The presence of dolphin fins reduces the thermal resistance by up to 30%, which contributes to the overall thermal enhancement of the designed plate-fin heat sinks. Simulation results show that increasing the fins in areas close to the heat source and reducing the non-working areas significantly influence the thermal performance of heat sinks. The results also show that the trapeze plate-fin heat sinks with the different channel-fin geometries are superior to the standard trapeze plate-fin heat sink in thermal performance. [ABSTRACT FROM AUTHOR]

Light is a primary constraint on primary production and drives many ecological processes in stream ecosystems, yet light regimes have received considerably less attention than other factors of the stream environment, such as hydrology or nutrient cycling. Light received by streams can be highly heterogeneous in both space and time resulting from changes in topography, channel characteristics, and riparian vegetation. Both the structure and phenology of riparian vegetation can be important determinants of the seasonality and magnitude of light reaching the stream surface, particularly in smaller forested streams. Despite the importance of riparian phenology on temporal patterns of stream light availability, existing models do not account for the seasonal dynamics of canopies. We developed a dynamic, biophysically based model (StreamLight) that incorporates canopy structure and phenology to predict light reaching the stream surface. We compared StreamLight to an existing model at 21 sites across the USA and found that, across sites, our biophysically based model produced light estimates that were more strongly correlated to observations and reduced the magnitude of errors in comparison to the existing model, particularly for streams that were relatively narrow compared to the height of riparian vegetation. Because smaller streams represent most global stream length, we expect that, in many smaller forested streams, the inclusion of canopy structure and phenology will enhance our ability to predict light regimes. We also used model simulations to examine the importance of controls on stream light environments and found that channel width was the strongest control on light environments. StreamLight represents an important incremental step forward in developing mechanistic models of river network productivity and in linking shifts in terrestrial vegetation structure and phenology to aquatic ecosystem productivity and thermal regimes. [ABSTRACT FROM AUTHOR]

Improving the mechanical durability of proton exchange membrane is important to ensure the longevity of fuel cells. Therefore, the mechanisms of fatigue crack growth of the membrane in fuel cells during humidity cycles are critical issues. In this paper, a cyclic cohesive finite element method is used to predict the fatigue crack growth of the membrane. It is found that humidity amplitude has significant effects on the durability and that the crack under the channel is more prone to grow. Furthermore, both clamping conditions and channel geometry can also greatly affect the durability of the membrane. The squeezing effect in the direction perpendicular to the pre-crack and the bulging effect in the direction parallel to the pre-crack also compete with each other. As a result, the clamping displacement can be favorable to the durability of the membrane when its squeezing effect is stronger than its bulging effect. The results also suggest that deviating the channel or having triangular and semi-elliptical channels can improve the durability of the membrane. [Display omitted] • Fatigue crack growth in the membrane of fuel cells is simulated using cyclic CFEM. • The regions where the crack is prone to grow are clarified. • Larger humidity amplitude leads to larger in-plane tensile stress in the membrane. • Two competing effects of clamping displacement are revealed. • The effects of channel geometry are investigated. [ABSTRACT FROM AUTHOR]

A key element in hydraulic and hydrologic modeling is the specification of representative channel geometry. For continental‐scale modeling, the large amount of high‐resolution data required, as well as the considerable computational effort needed to incorporate such data, has led to simplifying assumptions such as rectangular or trapezoidal channels for river reaches. The National Water Model (NWM) uses a trapezoidal channel representation for 2.7 million river reaches to forecast water discharge for the entire continental United States. This has created uncertainties in when to initiate hydraulic predictions. The aim of this study is to: (1) evaluate the impact of simplified (NWM trapezoidal) channel geometry representation on hydrological model predictions and (2) suggest an improved representation of channel geometry while maintaining parsimony in model input and runtime. The Hydrologic Engineering Center's River Analysis System model was used to simulate hydraulic dynamics in three study sites under varying streamflow conditions (including flooding) with four different geometry representations at each site: NWM (trapezoidal), surveyed, and two proposed generalized geometries. Statistical analyses show that more realistic channel geometry improves simulated Muskingum‐Cunge routing parameters and stage/discharge predictions, indicating potential for geometric improvements to enhance hydrological models like the NWM. Research Impact Statement: Improved representations of channel geometry have potential to substantially improve simulation of continent‐wide hydrology, especially for flood forecasting. [ABSTRACT FROM AUTHOR]

Microchannels are effective heat sinks for microelectronic systems. However, it remains unclear what form of channels will be most effective in improving the overall performance of microchannel heat sinks. The effect of channel geometry was studied using computational fluid dynamics to understand the heat transfer and fluid flow characteristics of microchannel heat sinks with rectangular grooves in sidewalls and different shaped ribs in the center core flow. Four different rib configurations are considered, including rectangular, diamond, forward triangular, and elliptic. In order to fully understand the design and operation of microchannel heat sinks, the overall performance of these devices was analyzed and evaluated in detail in terms of Nusselt number, apparent friction factor, and thermal enhancement efficiency. The enhancement mechanism of fluid flow and heat transfer was discussed in order to determine the optimum channel structure. The objective was to optimize channel structure for microchannel heat sinks, thereby enhancing heat transfer and reducing flow resistance and pressure drop. The results indicated that the overall performance can be greatly improved by the combination of grooves and ribs. This combination can make full use of the advantages of ribs to increase flow disturbance and to enhance heat transfer, and the advantages of grooves to increase flow area and to reduce pressure drop. The shape and dimensions of ribs have a significant effect on the overall performance. The overall performance obtained with rectangular ribs is the best at Reynolds numbers less than 500, but lower than that obtained with elliptic ribs at Reynolds numbers greater than 500, and even lower than that obtained with diamond ribs at Reynolds numbers greater than 700. The rectangular grooved channel with rectangular ribs yields the best overall performance with a relative rib width of 0.25 and at a minimum rib-groove distance of 0.1 mm and a Reynolds number of 500. [ABSTRACT FROM AUTHOR]

Mountainous headwater streams represent a substantial proportion of the global stream network. These small streams may flow episodically, seasonally, or perennially, providing diverse values and services. Given their broad importance and growing pressures on terrestrial and aquatic resources, we must improve our understanding of the drivers of flow permanence to facilitate informed land and water management decisions. We used field observations from >10 cross‐sections in each of 101 non‐fish bearing, headwater streams across four geomorphic provinces in Northern California to quantify flow permanence and network connectivity during the summer low flow period in 2018. At each stream cross‐section, we noted the presence or absence of streamflow and used this information to classify streams as perennial (continuous streamflow in all cross‐sections) or non‐perennial and connected (surface water in the most downstream cross‐section) or disconnected. At each cross‐section, we also quantified channel size (width and depth) and grain size. We coupled field observations with geospatial data of catchment physiography, hydrology, and climate in random forest models to investigate controls of flow permanence and network connectivity. Potential drivers of flow permanence or network connectivity included in our models were channel geometry, grain size, slope, aspect, elevation, annual and seasonal precipitation, air temperature, and topographic wetness index. We found more perennial streams in the Klamath Mountains and Sierra Nevada than in the Cascades and N. Coast regions. Streams in the Klamath were the most connected followed by streams in the N. Coast, Sierra Nevada, and Cascades. The most important variables for predicting flow permanence were channel grain size, winter 2018 precipitation, and drainage area. Comparatively, the most important variables for predicting network connectivity were winter and spring 2018 precipitation, grain size, and bankfull depth. Our study illustrated the complexity of the processes that drive flow permanence and highlighted the uncertainty in projecting the precense of water in streams across diverse regions. [ABSTRACT FROM AUTHOR]

Sediment supply (Qs) is often overlooked in modelling studies of landscape evolution, despite sediment playing a key role in the physical processes that drive erosion and sedimentation in river channels. Here, we show the direct impact of the supply of coarse‐grained, hard sediment on the geometry of bedrock channels from the Rangitikei River, New Zealand. Channels receiving a coarse bedload sediment supply are systematically (up to an order of magnitude) wider than channels with no bedload sediment input for a given discharge. We also present physical model experiments of a bedrock river channel with a fixed water discharge (1.5 l min−1) under different Qs (between 0 and 20 g l−1) that allow the quantification of the role of sediment in setting the width and slope of channels and the distribution of shear stress within channels. The addition of bedload sediment increases the width, slope and width‐to‐depth ratio of the channels, and increasing sediment loads promote emerging complexity in channel morphology and shear stress distributions. Channels with low Qs are characterized by simple in‐channel morphologies with a uniform distribution of shear stress within the channel while channels with high Qs are characterized by dynamic channels with multiple active threads and a non‐uniform distribution of shear stress. We compare bedrock channel geometries from the Rangitikei and the experiments to alluvial channels and demonstrate that the behaviour is similar, with a transition from single‐thread and uniform channels to multiple threads occurring when bedload sediment is present. In the experimental bedrock channels, this threshold Qs is when the input sediment supply exceeds the transport capacity of the channel. Caution is required when using the channel geometry to reconstruct past environmental conditions or to invert for tectonic uplift rates, because multiple configurations of channel geometry can exist for a given discharge, solely due to input Qs. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd [ABSTRACT FROM AUTHOR]

Fires are a common feature of many landscapes, with numerous and complex ecological consequences. In stream ecosystems, fire can strongly influence fluvial geomorphic characteristics and riparian vegetation, which are structural components of stream–riparian ecosystems that contribute to biodiversity and ecosystem function. However, the effects of fire severity on stream–riparian ecosystems in California's Sierra Nevada region (USA) are not well described, yet critical for effectively informing fire management and policy. At 12 stream reaches paired by fire severity (one high-severity burned, one low-severity burned), no significant differences were found in riparian plant community cover and composition or stream geomorphic characteristics 2–15 years following wildfire. In addition, minimal changes in riparian vegetation and stream geomorphic properties were observed in the first summer following the extensive and severe Rim Fire. However, an upstream-to-downstream influence of multiple fire occurrences was observed over the previous 81 years within each catchment on stream geomorphic metrics, including sediment size, embeddedness and channel geometry, at our study reaches. The inconsistent effects of wildfire on stream–riparian vegetation and geomorphic characteristics over space and time may be related to time since fire and precipitation. Riparian vegetation and geomorphic properties of mountain stream reaches that burned with high- or low-severity fire did not differ from each other in this California, USA study. However, the proportion of each catchment burned multiple times over the past ~80 years was correlated with reach-level geomorphic metrics. [ABSTRACT FROM AUTHOR]

The flood discharge capacity of an alluvial river is related to its channel geometry,which also depends on the upstream flow and sediment regime. It is important to analyze the relationship between flood discharge capacity and channel geometry and flow and sediment regime,in order to investigate the channel evolution of the braided reach in the Lower Yellow River. The relationship was developed between bankfull discharge and flow-sediment regime,represented by incoming sediment coefficient and fluvial erosion intensity,and the channel geometry,represented by the pre-flood hydraulic geometry from 1986 to 2015 in the braided reach at section- and reach- scales. Results indicate that:① Serious deposition and main channel shrinkage occurred in the study reach from 1986 to 1999,with the flood discharge capacity declining rapidly,and the main channel became narrower and deeper continuously owing to significant channel degradation after the operation of the Xiaolangdi Reservoir,with the flood discharge capacity recovering gradually from 1999 to 2015;② Empirical formulas (power functions) were developed between section- and reach- scale bankfull discharges and the integrated effects of flow-sediment regime and channel geometry respectively,whose correlations are greater than 0.50,but the correlation coefficient for the reach scale improved by 17% at least,as compared with the section scale;and ③ A higher correlation (0.94) was obtained between the reach-scale bankfull discharge and the previous five-year average fluvial erosion intensity during flood seasons and the post-flood hydraulic geometry in the last year,and the corresponding formula can well reflect the variation in bankfull discharge of this study reach,which also provides a method to analyze the variation in bankfull discharge of other reaches. [ABSTRACT FROM AUTHOR]

Recent progress in remote sensing has snapshotted unprecedented numbers of river planform geometry, providing opportunity to revisit the oversimplified channel shape parameterizations in global hydrologic models. This study leveraged two recent Landsat‐derived global river width databases and created a reach‐level width dataset to measure the validity of model parameterizations at ~1.6 million kilometers of rivers in length. By showing state‐of‐the‐art parameterization schemes only capture 30–40% of the width variance globally, we developed a machine learning (ML) approach surveying 16 environmental covariates, which considerably improved the predictive power (R2 = 0.81 and 0.77 for two testing cases). Beyond the commonly discussed upstream basin conditions, ML revealed that local physiographic factors and human interference are also important covariates for width variability. Finally, we applied the ML model to estimate bankfull river width, creating a new reach‐level dataset for use in global hydrodynamic modeling. Plain Language Summary: Large‐scale river models typically parameterize channel shapes (e.g., bankfull width) based on discharge or drainage area with a power law model or stream order with look‐up tables. These highly simplified representations reflect our limited understanding of spatial variability of channel shapes, leading to great uncertainty in global river modeling. Using the most up‐to‐date global snapshots of channel planform geometry derived from satellite images, this study revisited state‐of‐the‐art channel shape parameterizations used in large‐scale river models. We also used a machine learning approach to improve the predictive power for the width spatial variability. Results from this study can complement the understanding of downstream hydraulic geometry from a data mining viewpoint, which also informs the improvement of channel shape parameterization in models. Key Points: A machine learning approach to predicting global bankfull river width was developedLocal physiographic factors and human interferences are also important covariates for width variabilityEstimates of global reach‐level bankfull river width were provided for use in models [ABSTRACT FROM AUTHOR]

Current estimations of sediment transport at the watershed scale are limited by the difficulty of accurately simulating the sediment transfer along the main stem. The typical approach to simulating watershed sediment transport involves the adoption of hydrologic sediment routing schemes that do not fully capture the contribution and timing of side tributaries, and the inclusion of a simplified channel geometry that does not include its hydraulic feedback. In this paper, we present the results of a coupled hydrologic-hydraulic model of sediment transport applied to a small watershed of Iowa. The model was developed to simulate both the hydrologic network and non-equilibrium sediment transport that occur during a flood. The model results highlight the importance of including side tributaries in order to capture a realistic duration of shear stress that ultimately affects sediment transport. Comparisons with bank erosion measurements indicate that the presented approach is also promising to estimate sediment sources along the main stem. [ABSTRACT FROM AUTHOR]

In the present paper, an economic water benefit design mathematical solution has been derived for trapezoidal open channel section involving pumping which is based on hydraulic and economic considerations. The solution utilizes the derivative optimum of water benefit function and maximizes the benefit gained from the delivered water to reach the maximum water benefit MWB design solution. The solution is applied on a channel example and the results obtained are compared with those obtained by an earlier solution based on the minimum construction cost MC design of the channel only. The channel costs in both solutions are found to be of nearer magnitudes with little deviation. It is not necessary that the channel geometry in both cases must be identical, because the MWB solution depends on several factors such as; water benefit variables, pumping cost parameters and channel length, while the minimum cost MC solution depends on the channel cost parameters only. The MWB design solution is advantageous in that it achieves money gains even the delivered water is of cheap price, while this advantage is not satisfied in the earlier solution. However, the present study introduces an analytical solution for developing comprehensive water benefit and costing. The results of the given channel example involving pumping, demonstrate the simplicity and practicability of the MWB design solution as well as water benefit verification

Improving the desalination performance of membranes is always in the spotlight of scientific research; however, Janus channels with polarized surface charge as nanofiltration membranes are still unexplored. In this work, using molecular dynamics simulations, we demonstrate that Janus graphene oxide (GO) channels with appropriate geometry and surface charge can serve as highly efficient nanofiltration membranes. We observe that the water permeability of symmetric Janus GO channels is significantly superior to that of asymmetric channels without sacrificing much ion rejection, owing to weakened ion blockage and electrostatic effects. Furthermore, in symmetric Janus GO channels, the transport of water and ions is sensitive to the charge polarity of the channel inner surface, which is realized by tuning the ratio of cationic and anionic functionalization. Specifically, with the increase in cationic functionalization, the water flux decreases monotonously, while ion rejection displays an interesting maximum behavior that indicates desalination optimization. Moreover, the trade-off between water permeability and ion rejection suggests that the Janus GO channels have an excellent desalination potential and are highly tunable according to the specific water treatment requirements. Our work sheds light on the key role of channel geometry and charge polarity in the desalination performance of Janus GO channels, which paves the way for the design of novel desalination devices.

We study a one-dimensional Poisson–Nernst–Planck system for ionic flow through a membrane channel. Nonzero but small permanent charge, the major structural quantity of an ion channel, is included in the model. Two cations with the same valences and one anion are included in the model, which provides more rich and complicated correlations/interactions between ions. The cross-section area of the channel is included in the system, and it provides certain information of the geometry of the three-dimensional channel, which is critical for our analysis. Geometric singular perturbation analysis is employed to establish the existence and local uniqueness of solutions to the system for small permanent charges. Treating the permanent charge as a small parameter, through regular perturbation analysis, we are able to derive approximations of the individual fluxes explicitly, and this allows us to study the competition between two cations, which is related to the selectivity phenomena of ion channels. Numerical simulations are performed to provide a more intuitive illustration of our analytical results, and they are consistent.

The friction stir channelling (FSC) process is a technological innovation based on the friction stir fundamentals. To produce friction stirred channel components, a non-consumable tool, similar to that used for friction stir welding (FSW), is used. Friction stir channelling is a single step manufacturing process in which a continuous channel with a specific path is produced in a monolithic metal component. This work discusses the influence of the channel geometry on the bending strength of friction stirred channel aluminium alloy specimens, as well as their fatigue behaviour. Moreover, fatigue analyses using the finite element method were also carried out, considering the channels' geometrical features in order to assess and compare the fatigue lives. The stress intensity factor for different crack lengths was determined using ABAQUS. The fatigue crack growth curve was established according to the Paris Law, and the crack was only allowed to propagate along the materials interface. It was observed that the critical zones are located in the vicinity of the channel corners and found that the fatigue crack propagation period on FSC specimens is very short compared to the crack initiation period. Base and FSW material properties play a major role on fatigue crack propagation simulation. The accuracy of the predicted results increased when considering lower stress amplitudes and by using the FSW material parameters. • A fatigue crack propagation model of friction stirred channels was simulated. • For FSC specimens, the crack propagation period is almost negligible. • FSC specimens exhibit a very low fatigue strength. • The fracture of FS channels occurs in the nugget/TMAZ interface on the AS. [ABSTRACT FROM AUTHOR]

• River bathymetry affects various flood inundation outputs differently. • Cross-sectional area, and not the channel shape, is critical to hydraulic modeling. • Both channel area and thalweg are important for velocity and shear estimates. • Simplified geometries can be used for reaches with small sinuosity, width or depth. River bathymetry, which is vital for accurate hydraulic modeling, is not readily available at large scales because of the logistical difficulties in field surveys and uncertainties associated with remote sensing techniques. Several studies have explored the potential of conceptual models and interpolation algorithms to estimate bathymetry. These models have certain underlying assumptions that limit their accuracy and widespread implementation. This study aims to provide insights into the choice of bathymetric model for different hydraulic applications by analyzing the effect of different bathymetric models on hydraulic modeling under different geomorphologic and flow settings. This study does not aim to reinforce the importance of bathymetry, rather its objective is to identify the bathymetric properties that are critical to accurate estimation of different hydraulic outputs. The study accomplishes its objectives by implementing three different bathymetric models with varying cost and efficiency at four sites with diverse bathymetric characteristics. Five hydraulic outputs, namely wetted cross-sectional area, water surface elevation, inundation extents, velocity and shear are estimated at three characteristic low and high flows and the results from the four sites are grouped together to perform an unbiased and robust evaluation. The performance of these models is evaluated using the best available bathymetric representation generated from detailed field surveys as a benchmark. The results indicate that 1D flood modeling is somewhat insensitive to channel shape as long as the estimated shape reflects the channel storage accurately. Velocity and shear related applications should incorporate bathymetry that represent both the cross-sectional area and channel thalweg accurately. Reaches with low sinuosity (<1.5), or small width and depth can be modeled reasonably using simple geometric shapes, whereas for larger streams, interpolation of surveyed data along the thalweg or complex bathymetric models that account for river anisotropy, are recommended. [ABSTRACT FROM AUTHOR]

Abstract Floodplain storage commonly represents one of the largest sediment fluxes within sediment budgets. In watersheds responding to large scale disturbance, floodplain-channel lateral connectivity may change over time with progression of channel evolution and associated changes in channel geometry. In this study we investigated the effects of channel geometry change on floodplain inundation frequency and flux of suspended sediment (SS) and total phosphorus (TP) to floodplain storage within the 52.2 km2 Walnut Creek watershed (Iowa, USA) through a combination of 25 in-field channel cross section transects, hydraulic modeling (HEC-RAS), and stream gauging station-derived water quality and quantity data. Cross sectional area of the 25 in-field channel cross sections increased by a mean of 17% over the 16 year study period (1998–2014), and field data indicate a general trend of degradation and widening to be present along Walnut Creek's main stem. Estimated stream discharge required to generate lateral overbank flow increased 15%, and floodplain inundation volume decreased by 37% over study duration. Estimated annual fluxes of SS and TP to floodplain storage decreased by 61 and 62% over study duration, respectively. The estimated reductions in flux to floodplain storage have potential to increase watershed export of SS and TP by 9 and 18%, respectively. Increased contributions to SS and TP export may continue as channel evolution progresses and floodplain storage opportunities continue to decline. In addition to loss of storage, higher discharges confined to the channel may have greater stream power, resulting in further enhancement of SS and TP export through accelerated bed and bank erosion. These increased contributions to watershed loads may mask SS and TP reductions achieved through edge of field practices, thus making it critical that stage and progression of channel evolution be taken into consideration when addressing sediment and phosphorus loading at the watershed scale. Graphical abstract Unlabelled Image Highlights • Channel-floodplain connectivity was quantified using field data and hydraulic modeling. • Stream channel cross sectional area increased ~17% over a 16 year period. • Fluxes of sediment and phosphorus to floodplain storage decreased with increased channel conveyance. • Sediment and nutrient budgets should account for the impacts of channel evolution on floodplain storage. [ABSTRACT FROM AUTHOR]

Lateral migration of the Bhagirathi River temporally creates unavoidable geomorphic hazards in West Bengal, India. The Bhagirathi River flows SW for ~67.30 km between the confluence point of Ajay and Jalangi rivers in East Burdwan and Nadia districts of West Bengal. The course of Bhagirathi is notably migration prone and cultivates problematic changes along its course over time. In the study, we have looked into its migration tendency and unpredictability for past 238 years and then predicted the lateral shifting of river centerline using temporal satellite imageries — Landsat-5 (TM) of 1987 (8, December), 1995 (28, January), 2005 (7, January) and LISS-IV satellite imagery (2017, 5 January);SOI Toposheet — 1968-1969 (79A/2, 79A/3, 79A/6 and 79A/7) and Rennell's map of 1779. Other highlights are the quest of fluvial features, oxbow lakes, mid-channel bars, channel migration rate, meander geometry, channel sinuosity in different parts of river course and the parts that experience intensive bank erosion. The entire river course has been subdivided into three segments; viz. reaches A, B and C. Investigation displays that degree of sinuosity decreases from its anterior course (1968) to the existing course (2017). Reach-specific outputs display that reach B is highly sinuous (SI value 1.94 in 2017) and SI increases temporarily, whereas for reaches A and C it decreases with time. The rate of migration is higher in reach B than that in reaches C and A. The study displays a notably decreasing trend of migration in comparison with its previous lateral migration and shows that the migration nature over time is intensively inconsistent and unpredictable except very few portions of the river course. The nature of deposition within the river channel shows an unstable behavior during the entire period of the study. Meander geometry depicts a rapid change of river course innate to meander bends and shows a higher rate of migration by meander loop cut-off rather than lateral migration that reflects the inconsistency, erosion vulnerability and unpredictable nature of migration. The present work offers a valuable source to comprehend channel changes in Bhagirathi River and serve as an efficacious base for river-bank migration and erosion hazard planning and management. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Slug-flow range & Mass Transfer rates are dependent on channel diameter & geometry. • Flow rates of liquid pairs governs mass transfer coefficient. • Enhanced Mass Transfer in Microdomain. • Helical channel- the best option for higher mass transfer & slug-flow range. • Optical microscopy employed to visualize the flow dynamics. Abstract An exhaustive study of various flow patterns that occurs in liquid-liquid flow along with detailed mass transfer study in micro-domain has been presented. Experimental studies on mass transfer have been performed using a hydrocarbon phase (toluene) from which extraction of acetic acid was done using water as a solvent in co-current flow. The same liquid-liquid systems (toluene-acetic acid-water) have been used for flow pattern study. Two channel diameters (0.15 mm and 0.25 mm) have been considered in the study with three different microchannel geometries, i.e. straight channel, helical channel and a gradual 90-degree bend. The microchannels used in this study are developed by the non-lithographic method. The present study unravels various flow patterns and their transitions in such small diameter microchannels with very high enhancement in mass transfer rates. The range of flow rates is varied from 10 to 90 μ L/min and mass transfer co-efficients are obtained in the range of 0.0053 – 0.7952 s−1 for all channel geometries. The Reynold number mixture for all the flowrates have been evaluated and it varies from 7 to 50 which clearly indicates that the flow lies in the laminar regime. The study may find useful application in pharmaceutical and fine-chemical industries. [ABSTRACT FROM AUTHOR]