Variation In Channel Geometry Research Articles

Investigating tidal river dynamics in a longitudinally varying channel geometry

Examining the flow dynamics in a tidal river, particularly downstream, is of great importance to consider the substantial interaction between river runoff and tidal waves. Previous studies have investigated flow dynamics in tidal rivers mainly concentrated on the lateral structure of the estuarine dynamics, with limited consideration of the impact of geometrical variation of the tidal channel, which has an enormous effect along the longitudinal direction. The aim of this study is to offer a comprehensive understanding of the dynamics of a tidal river with longitudinally varying channel geometry. To accomplish the aforementioned aims, this study provides a spatiotemporal analysis of water current and density for a fortnight period of a mesotidal estuarine channel that encompasses both curved and straight channels in the streamwise direction. Moreover, a quantitative analysis of the governing mechanisms for estuarine dynamics was performed to determine the alteration of driving mechanisms due to channel geometry variation. The longitudinal velocity profile indicates the existence of tidal asymmetry where the ebb currents are dominant over flood currents. The asymmetry, which is induced by the interaction between the channel curvature and density gradient, is observed to be more enhanced in the curved channel than in the straight channel. Moreover, the across-channel bathymetry difference between the curved and straight channels leads to different patterns of lateral flow in those channels. Additionally, the longitudinal and lateral velocities averaged for the spring/neap tides reveal that the channel with a higher curvature degree has greater residual circulation. On the other hand, the periodically varying density distribution denotes the existence of tidal straining, which is slightly modified by the variation in geometry. The notable findings of this study are as follows: (i) the variability in channel geometries introduces different responses of tidal straining and tidal stirring; (ii) the river effect (river-induced shear) in the curved channel substantially increases significantly compared to the straight channel over a certain threshold of river runoff. We believe that the findings of this study deepen our understanding of tidal river dynamics in a longitudinally varying channel geometry.

One-dimensional morphodynamic model of channel deformation with the consideration of river regulation works and its application

在冲积河流上，各类河道整治工程的实施一定程度上会影响河床的演变过程.本文通过改进现有的一维水沙数学模型，重点研究护岸及护滩（底）这类限制河床进一步冲刷的整治工程对水沙输移及河床冲淤变形的影响.首先对固定断面的各节点采用特定的代码进行标记，以此区分河漫滩、有或无整治工程的主槽区域.然后对悬沙输移及河床冲淤变形模块进行改进：当断面发生淤积时，其形态调整不受整治工程的影响，淤积量将在整个断面上进行分配；当发生冲刷时，断面形态调整仅发生在未实施工程的位置或者受工程限制但形成了一定厚度淤积层的区域；在实施了整治工程且无法提供沙源的区域，河床冲刷则不会发生.最后长江中游荆江段2016年的模拟结果表明：考虑整治工程情况时，改进模型计算的河道冲刷量偏小且与实测值更为接近；河槽断面形态与实测结果也更为吻合.;A one-dimensional morphodynamic model has been improved to simulate the fluvial processes influenced by large-scale river regulation works, and the focus is paid to investigate the effect of a bank-, shoal- and bed-protection works. In the refined model, different zones of a cross-section are first demarcated by different point codes, including the zones of floodplain and main-channel with or without engineering. Then the modules of sediment transport and bed deformation are modified. When channel deposition occurs, it is not influenced by regulation engineering and the deposition volume will be allocated at a whole cross-section. However, channel scour will only occur at the unprotected bed or the protected bed with a deposition layer newly formed over the previous simulation period; at the protected bed without sediment to be scoured, erosion is not allowed. Then the Jingjiang Reach of the middle Yangtze River was selected as the study reach. Simulated results for the year 2016 show that the channel scours volume calculated using the improved model was smaller and in closer agreement with the measurements that without the consideration of river regulation works. Besides, the variation in channel geometry calculated by the improved model was more consistent with the measured result.

A depth-averaged model for Newtonian fluid flows in shallow microchannels

Pressure-driven flow has been widely used in microfluidic devices to pump fluids and particles through planar microchannels for various applications. The variation in channel geometry (e.g., contraction or expansion) may lead to complex flow phenomena (e.g., recirculations) useful for microfluidic sampling, such as fluid mixing and particle focusing. In this work, we develop a depth-averaged inertial flow model for Newtonian fluids in shallow microchannels based on an asymptotic analysis of the continuity and momentum equations. The validity and accuracy of this two-dimensional model are assessed through comparisons with the experimental measurements and three-dimensional numerical simulations for water flow through contraction–expansion microchannels of varying depths. Our proposed depth-averaged model provides the accuracy of three-dimensional modeling if the channel depth-to-width ratio remains small (specifically, at ∼0.1 or less).

Combustion in a solid fuel scramjet with channel geometry variation due to burnout

Stability of combustion in solid fuel scramjets is contingent upon proper geometry of fuel grain and availability of a flame holder. Flame stabilization is achieved in the recirculation zone developing due to sudden expansion behind the inlet. Previous experimental and theoretical works suggest that there exists an optimum range of length-to-diameter ratios for the flame holder. However, due to fuel burnout the channel and flame holder geometry change in the process of scramjet operation, the regression rate being variable both in time and length. The purpose of this work is to develop a computational model of a solid-fuel scramjet taking into account the geometry variation, as well as to obtain typical flow patterns at the different stages of combustion. The numerical model is based on a system of fully compressible RANS equations with k-epsilon turbulence model. Turbulent combustion is described by the Eddy Dissipation Concept (EDC) model.Stability of combustion in solid fuel scramjets is contingent upon proper geometry of fuel grain and availability of a flame holder. Flame stabilization is achieved in the recirculation zone developing due to sudden expansion behind the inlet. Previous experimental and theoretical works suggest that there exists an optimum range of length-to-diameter ratios for the flame holder. However, due to fuel burnout the channel and flame holder geometry change in the process of scramjet operation, the regression rate being variable both in time and length. The purpose of this work is to develop a computational model of a solid-fuel scramjet taking into account the geometry variation, as well as to obtain typical flow patterns at the different stages of combustion. The numerical model is based on a system of fully compressible RANS equations with k-epsilon turbulence model. Turbulent combustion is described by the Eddy Dissipation Concept (EDC) model.

One-dimensional modelling of channel evolution in an alluvial river with the effect of large-scale regulation engineering

In an alluvial river, various kinds of river regulation engineering are usually constructed along the reach for the stability of channel regime and the safety of flood control, which greatly affects the process of channel evolution. Therefore, a one-dimensional (1D) morphodynamic model has been improved to simulate the fluvial processes influenced by large-scale river regulation works. In the refined model, each node of a cross-section was labeled using a specified point code representing the zones of floodplain and main-channel with or without regulation engineering, and sediment transport and bed deformation in different zones were treated separately. In the zones without regulation engineering, both bed deposition and bed erosion were permitted to occur; in the zones with regulation works, bed deposition was allowed, while bed erosion was not permitted to occur owing to the restriction of those engineering, unless there was a deposition layer newly formed over the previous simulation period. Then the improved model was calibrated and verified using field observations through the application to a 347-km-long alluvial channel located in the Middle Yangtze River in China. The results for the year 2016 show that the channel scour volume without the effect of river regulation works was 9% greater than the measured value (34.43 × 106 t), while the outcome considering this effect (35.81 × 106 t) was in closer agreement with the measurements. Moreover, the variation in channel geometry obtained from the improved model agreed better with the observed data. River regulation works would limit the channel incision, and it is likely to scour the other unprotected bed of a cross-section or the downstream cross-sections as the flow cannot carry enough sediment from the protected bed. Simulated results indicate that part of the main-channel zone at the Jing65 section was protected, but the riverbed continued to incise by about 0.68 m in the protected area when the effect of bed-protection works was not considered in the model, which was not in accord with the observed channel geometry. Therefore, this effect of large-scale river regulation works on channel evolution needs to be considered for a higher simulation accuracy.

Morphodynamic adjustments in the Yichang–Chenglingji Reach of the Middle Yangtze River since the operation of the Three Gorges Project

To investigate the evolution characteristics of the Yichang–Chenglingji Reach (YCR) of the Middle Yangtze River and develop simulation methods, the temporal and spatial morphodynamic adjustments of the YCR were analysed comprehensively on the basis of runoff, sediment load, and cross-sectional profiles data at 220 cross sections from 2002 to 2016. Results showed that significant channel degradation has occurred in the YCR, especially in the low-flow channels, due to a drastic reduction in sediment yield since the operation of the Three Gorges Project (TGP). The variations of upstream flow and sediment regimes would drive equilibrium channels to become increasingly narrow and deep, and the current bankfull width and depth were less than their equilibrium counterparts in the recent period. However, the recent channel evolution was mainly characterized by the prominent channel deepening via a one-way deformation due to the large-scale bank revetments within the YCR, where the reach-scale bankfull depth increased by 1.6 m in the span of 15 years. A semi-empirical method for determining the equilibrium values of channel degradation and aggradation volume at the YCR was proposed on the basis of a comprehensive analysis of the effects of flow, sediment concentration, sediment gradation, and water depth. Moreover, the equilibrium values of the channel width and depth were estimated with the Least Action Principle method. The calculation methods for the equilibrium width, depth, width/depth ratio, and aggradation/degradation volume were combined with the Delayed Response Model (DRM). Then, the methods for calculating the variations in channel geometry and aggradation/degradation volume were derived. Results showed that the proposed methods effectively simulated the channel geometry and cumulative erosion volume in the YCR from 2002 to 2016. In addition, the response of the channel evolution in the YCR was closely related to the previous four-year hydrological conditions, implying that channel evolution may lag behind the variations of flow and sediment discharges.

Theoretical and experimental investigation for micro-channel fabrication using low power CO2 laser

CO2 laser being a localised non-contact type machining process depends on the thermal and mechanical properties of a material to a great extent. The scope of this research is to study the effect of process parameters on channel profile of polymethylmethacrylate (PMMA) by CO2 laser (0-25 W). The parameters selected are beam power and scanning speed, keeping assist gas pressure constant. Using the principle of energy balance and assuming complete evaporation of molten material, an analytical model for the depth of cut is developed by considering Gaussian beam energy distribution in three-dimensional coordinate systems. Experimental results obtained at 5 mm thick plate by varying process parameters were compared with the analytical data showing good convergence with a variation of 1.83%. An attempt is made to analyse the variation of channel geometry as a function of process parameters analytically. The present model was developed with consideration of x, y and z coordinates associated with laser machining process and was validated by experimentation and 2D model available in the literature. It was observed that the proposed model shows good convergence with the experimental values. Hence, the proposed model is found to be suitable for prediction of micro-channel depth with precision.

An evaluation of unsupervised and supervised learning algorithms for clustering landscape types in the United States

ABSTRACTKnowledge of landscape type can inform cartographic generalization of hydrographic features, because landscape characteristics provide an important geographic context that affects variation in channel geometry, flow pattern, and network configuration. Landscape types are characterized by expansive spatial gradients, lacking abrupt changes between adjacent classes; and as having a limited number of outliers that might confound classification. The US Geological Survey (USGS) is exploring methods to automate generalization of features in the National Hydrography Data set (NHD), to associate specific sequences of processing operations and parameters with specific landscape characteristics, thus obviating manual selection of a unique processing strategy for every NHD watershed unit. A chronology of methods to delineate physiographic regions for the United States is described, including a recent maximum likelihood classification based on seven input variables. This research compares unsupervised and supervised algorithms applied to these seven input variables, to evaluate and possibly refine the recent classification. Evaluation metrics for unsupervised methods include the Davies–Bouldin index, the Silhouette index, and the Dunn index as well as quantization and topographic error metrics. Cross validation and misclassification rate analysis are used to evaluate supervised classification methods. The paper reports the comparative analysis and its impact on the selection of landscape regions. The compared solutions show problems in areas of high landscape diversity. There is some indication that additional input variables, additional classes, or more sophisticated methods can refine the existing classification.

Flow Modeling in Symmetrically Narrowing Flood Plains

Flooding situation in river is a complex phenomenon and affects the livelihood and economic condition of the region. The modelling of such flow is primary importance for a river engineers and scientists working in this field. As a result of topography changes along the open channels, designing the converging compound channel is an essential. Fluvial flows are strongly influenced by geometry complexity and large overall uncertainty on every single measurable property, such as velocity distribution on different sectional parameters like width ratio, aspect ratio and hydraulic parameter such as relative depth. The geometry selected for this study is that of a non-prismatic compound channel having converging flood plain. For the research work the parameters, the water depth, incoming discharge of the main channel and floodplains were varied. Continuous variation of channel geometry along the flow path associated with secondary currents makes the depth averaged velocity computation difficult. In this paper the variations of depth average velocities for five different sections in between the convergence region of the converging compound channel are compared by taking two different flow depth conditions. And also shear stress distribution at the walls and on the bed of two stage converging compound channel is calculated by doing experimentation in laboratory as bed shear forces are useful for the study of bed load transfer whereas wall shear forces presents a general view of channel migration pattern. Then the numerical method is applied to analyse the flow conditions in non-prismatic compound channel configurations, the results of calculations show good agreement with the experimental data. As numerical hydraulic models can significantly reduce costs associated with the experimental models, an effort has been made through the present investigation to determine the different flow characteristics of a converging compound channel such as velocity distribution, depth averaged velocity distribution, boundary shear etc. In this paper a complete three dimensional and two-phase CFD model for flow distribution in a converging compound channel is investigated by using Large Eddy Simulation (LES) model for solving the turbulence equation. This study aims to validate CFD simulations of free surface flow or open channel flow by using Volume of Fluid (VOF) method by comparing the data observed in hydraulics laboratory of the National Institute of Technology, Rourkela.

Modelling the depth-averaged velocity in trapezoidal meandering channels

This paper presents a practical method to predict lateral depth-averaged velocity distribution in trapezoidal meandering channels. Flow structure in meandering channels is more complex than straight channels due to three-dimentional nature of flow. Continuous variation of channel geometry along the flow path associated with secondary currents makes the depth-averaged velocity computation difficult. Design methods based on straight-wide channels incorporate large errors while estimating discharge in meandering channel. Hence, based on the present experimental results, a non-linear form of equation involving three parameters for estimating lateral depth-averaged velocity is formulated. The present experimental meandering channel is wide (aspect ratio = b/h > 5) and with high sinuosity of 2.04. The study serves for a better understanding of the flow and velocity patterns in trapezoidal meandering channel.

Optimized Expanding Microchannel Geometry for Flow Boiling

This study discusses the simulation of flow boiling in a microchannel and numerically predicts the effects of channel geometry variation along the flow direction. Experimental studies by Pan and collaborators and suggestions from Mukherjee and Kandlikar have generated interest in expanding the cross section of a microchannel to improve boiling heat transfer. The motivation for this geometry change is discussed, constraints and model selection are reviewed, and Revellin and Thome's critical heat flux criterion is used to bound the simulation, via matlab, of separated flow in a heated channel. The multiphase convective heat-transfer coefficient is extracted from these results using Qu and Mudawar's relationship and is compared to reported experimental values. Expanding channel geometry permits higher heat rates before reaching critical heat flux.

Numerical investigation on the water saturation of proton exchange membrane fuel cells with channel geometry variation

SUMMARY Many factors, such as mole fractions of oxygen and hydrogen, help improve the performance of proton exchange membrane fuel cells. The variation of mole fractions can be achieved by changing the operating pressure and relative humidity of the fuel cells. The changes in operating conditions are directly related to the electrochemical reaction and water generation of the fuel cells. The geometrical shape of the fuel cells also should be considered a factor in predicting performance because this affects the species' reaction speed and distribution. The current study considers four geometrical cell shapes with varied lengths and electrode and gas channel numbers. The variation in inlet pressure is considered in analyzing the current density distribution of the fuel cells and, subsequently, of liquid water generation. A serpentine gas flow channel is assumed, and its two-dimensional arrangement is considered in the different gas channel numbers and its length. Four inlet pressure variations and four geometrical shape variations also are considered in analyzing the fuel cells' current density and water generation distributions. The results obtained from this research can be utilized in identifying the fuel cells' optimal operating pressure and designing their gas channel number and arrangement. Copyright © 2011 John Wiley & Sons, Ltd.

Extending Fluorescence Thermometry to Measuring Wall Surface Temperatures Using Evanescent-Wave Illumination

Cooling microelectronics with heat flux values of hundreds of kW/cm2 over hot spots with typical dimensions well below 1 mm will require new single- and two-phase thermal management technologies with micron-scale addressability. However, experimental studies of thermal transport through micro- and mini-channels report a wide range of Nusselt numbers even in laminar single-phase flows, presumably due in part to variations in channel geometry and surface roughness. These variations make constructing accurate numerical models for what would be otherwise straightforward computational simulations challenging. There is, therefore, a need for experimental techniques that can measure both bulk fluid and wall surface temperatures at micron-scale spatial resolution without disturbing the flow in both heat transfer and microfluidics applications. We report here the evaluation of a nonintrusive technique, fluorescence thermometry (FT), to determine wall surface and bulk fluid temperatures with a spatial resolution of O(10 μm) for water flowing through a heated channel. Fluorescence thermometry is typically used to estimate water temperature fields based on variations in the emission intensity of a fluorophore dissolved in the water. The accuracy of FT can be improved by taking the ratio of the emission signals from two different fluorophores (dual-tracer FT or DFT) to eliminate variations in the signal due to (spatial and temporal) variations in the excitation intensity. In this work, two temperature-sensitive fluorophores, fluorescein and sulforhodamine B, with emission intensities that increase and decrease, respectively, with increasing temperature, are used to further improve the accuracy of the temperature measurements. Water temperature profiles were measured in steady Poiseuille flow at Reynolds numbers of 3.3 and 8.3 through a 1 mm2 heated minichannel. Water temperatures in the bulk flow (i.e., away from the walls) were measured using DFT with an average uncertainty of 0.2 °C at a spatial resolution of 30 μm. Temperatures within the first 0.3 μm next to the wall were measured using evanescent-wave illumination of a single temperature-sensitive fluorophore with an average uncertainty of less than 0.2 °C at a spatial resolution of 10 μm. The results are compared with numerical predictions, which suggest that the water temperatures at an average distance of ∼70 nm from the wall are identical within experimental uncertainty to the wall surface temperature.

High-Temperature Synthesis of CdSe Nanocrystals in a Serpentine Microchannel: Wide Size Tunability Achieved under a Short Residence Time

A three-dimensional serpentine microchannel was applied in the reaction part of a microreactor for the synthesis of CdSe nanocrystals (NCs). The local fluctuation of velocity in the turns created by the continuous variation of channel geometry was demonstrated to be effective to maintain a uniform residence time and monomer concentration for the constrained fluid under fast flow rates. Therefore, an enhanced growth rate for NCs within a narrow size distribution was accomplished with a short residence time. The triple-ligand system (trioctylphosphine oxide−oleic acid−oleylamine) was demonstrated to be powerful to achieve high-quality CdSe NCs with a short reaction time. The superior kinetic control achieved via the variation of precursor concentration and Se/Cd ratio provided effective control of the size of CdSe NCs, which allowed the rapid synthesis (8−10 s) of CdSe NCs in a wide size range with diameters ranging from 2.1 to 4.0 nm, while maintaining excellent color purity and a high photoluminescence (P...

Effect of Channel Geometry Variations on the Performance of a Constrained Microscale-Film Ammonia-Water Bubble Absorber

An experimental study of the absorption of ammonia vapor in a constrained thin film of ammonia-water solution is presented. A large aspect ratio microchannel with one of its walls formed of a porous material is used to constrain the thickness of the liquid film. Experiments are performed at a pressure of 2.5 bar absolute and 4 bar absolute and at a fixed weak solution inlet temperature. Weak solution flow rates are varied from 10 g/min to 30 g/min (corresponding to the weak solution Reynolds number, Re, from 15 to 45), inlet mass concentrations are varied from 0% to 15%, and gas flow rates are varied between 1 g/min and 3 g/min (corresponding to the vapor Re from 160 to 520). Six geometries, including three smooth-bottom-walled channels of differing depths and three channels with structured bottom walls, are considered. Results indicate that, for identical rates of vapor absorption, the overall heat transfer coefficient of the 400 μm absorber is in most cases significantly larger than that of other absorbers. For the 150 μm and 400 μm absorbers, a trade-off between the high overall heat and mass transfer coefficients is achieved for the highest vapor to solution flow rate ratio.

An experimental test of whether bar instability contributes to the formation, periodicity and maintenance of pool–riffle sequences

AbstractPool–riffle sequences (PRSs) are periodic river‐bed morphologies with wavelengths several times the channel width. Causes of PRS formation and maintenance are not clearly understood, which may limit the effectiveness of protection and rehabilitation measures. Some confusion has existed about whether the PRS morphology is the same as or distinct from alternate bars. In this paper we investigate whether the bar instability forming alternate bars also contributes to PRS formation, periodicity and maintenance. This was unclear because bar instability occurs only when the ratio of channel width/depth exceeds a critical value, generally understood to be approximately 10, which is larger than the width‐to‐depth ratio of many PRSs. A mobile‐bed physical model is used to test whether bar instability occurs in channels characteristic of PRS morphology, with low width‐to‐depth ratio, and high relative roughness. The physical model was scaled from a prototype PRS in a gravel and cobble bed river. Alternate bars formed in the model at channel width‐to‐depth ratios as low as 3·8. The wavelength of the alternate bars formed was generally 2·2–5 times channel width, which was similar to the prototype PRS. Therefore, bar instability can occur in virtually all PRSs, and it contributes to the widespread formation of periodic PRS morphology. The model showed that maintenance of the bar height in the prototype PRS also depends on variations in channel width. It is concluded that periodic PRSs are formed and maintained by the interaction between bar instability, and flow deflections associated with variations in channel geometry such as width variation. Resonance between bar instability and three‐dimensional bed forms such as alternate bars and variations in channel geometry. Variations in channel geometry are also important in determining the location and dimensions of individual pools and bars. Copyright © 2008 John Wiley & Sons, Ltd.

Distribution and storage of sediment-associated heavy metals downstream of the remediated Rum Jungle Mine on the East Branch of the Finniss River, Northern Territory, Australia

Instream, overbank and cut riverbank exposures along the East Branch of the Finniss River downstream of Rum Jungle Mine, Northern Territory have been analysed for their total metal concentrations using instrumental neutron activation analysis (INAA). Concentration values for the < 62.5 μm and bulk sample fractions are compared to Australian sediment quality guidelines values ( ANZECC/ARMCANZ, 2000). The results reveal that channel and overbank environments are contaminated with heavy metals, with many samples exceeding the low and high sediment guidelines values. The < 62.5 μm fraction is consistently more contaminated than bulk samples as are instream environments compared to adjacent overbank environments. Metal concentrations are strongly correlated to sediment pH and Fe values, suggesting that these variables are significant in controlling the spatial distribution of sediment-associated metals. The strong positive correlation between sediment-associated metals and Fe is probably related to the process of metal sequestration by Fe (and Mn) oxyhydroxides. The spatial distribution of sediment-associated metals downstream of the Rum Jungle Mine site does not display a simple distance–metal concentration decay pattern. It is suggested that the non-uniform spatial distribution of sediment-associated metals is a function of local, reach-scale variations in channel geometry and geomorphology, which control sediment storage and transfer patterns. In cut riverbank exposures the vertical of metals distribution is non-uniform and probably reflects differential metal mobility. Despite rehabilitation of the mine site in the 1980s, the elevated sediment-associated metal concentrations that remain within the East Branch of the Finniss River system are likely to render the system contaminated for the foreseeable future and limit the potential for the full recovery of aquatic and terrestrial flora and fauna.

Flow behaviour, suspended sediment transport and transmission losses in a small (sub-bank-full) flow event in an Australian desert stream

The behaviour of a discrete sub-bank-full flow event in a small desert stream in western NSW, Australia, is analysed from direct observation and sediment sampling during the flow event and from later channel surveys. The flow event, the result of an isolated afternoon thunderstorm, had a peak discharge of 9 m 3/s at an upstream station. Transmission loss totally consumed the flow over the following 7·6 km. Suspended sediment concentration was highest at the flow front (not the discharge peak) and declined linearly with the log of time since passage of the flow front, regardless of discharge variation. The transmission loss responsible for the waning and eventual cessation of flow occured at a mean rate of 13.2% per km. This is quite rapid, and is more than twice the corresponding figure for bank-full flows estimated by Dunkerley (1992) on the same stream system. It is proposed that transmission losses in ephemeral streams of the kind studied may be minimized in flows near bank-full stage, and be higher in both sub-bank-full and overbank flows. Factors contributing to enhanced flow loss in the sub-bank-full flow studied included abstractions of flow to pools, scour holes and other low points along the channel, and overflow abstractions into channel filaments that did not rejoin the main flow. On the other hand, losses were curtailed by the shallow depth of banks wetted and by extensive mud drapes that were set down over sand bars and other porous channel materials during the flow. Thus, in contrast with the relatively regular pattern of transmission loss inferred from large floods, losses from low flows exhibit marked spatial variability and depend to a considerable extent on streamwise variations in channel geometry, in addition to the depth and porosity of channel perimeter sediments. | The behaviour of a discrete sub-bank-full flow event in a small desert stream in western NSW, Australia, is analysed from direct observation and sediment sampling during the flow event and from later channel surveys. The flow event, the result of an isolated afternoon thunderstorm, had a peak discharge of 9 m 3/s at an upstream station. Transmission loss totally consumed the flow over the following 7.6 km. Suspended sediment concentration was highest at the flow front (not the discharge peak) and declined linearly with the log of time since passage of the flow front, regardless of discharge variation. The transmission loss responsible for the waning and eventual cessation of flow occurred at a mean rate of 13.2% per km. This is quite rapid, and is more than twice the corresponding figure for bank-full flows estimated by Dunkerley (1992) on the same stream system. It is proposed that transmission losses in ephemeral streams of the kind studied may be minimized in flows near bank-full stage, and be higher in both sub-bank-full and overbank flows. Factors contributing to enhanced flow loss in the sub-bank-full flow studied included abstractions of flow to pools, scour holes and other low points along the channel, and overflow abstractions into channel filaments that did not rejoin the main flow. On the other hand, losses were curtailed by the shallow depth of banks wetted and by extensive mud drapes that were set down over sand bars and other porous channel materials during the flow. Thus, in contrast with the relatively regular pattern of transmission loss inferred from large floods, losses from low flows exhibit marked spatial variability and depend to a considerable extent on streamwise variations in channel geometry, in addition to the depth and porosity of channel perimeter sediments.

Abstract: Slope to Basin floor Variations in Channel Geometry and Reservoir Architecture, Brushy Canyon Formation, Permian, West Texas

Abstract: Slope to Basin floor Variations in Channel Geometry and Reservoir Architecture, Brushy Canyon Formation, Permian, West Texas

Aufeis in the Ivishak River, Alaska, mapped from satellite radar interferometry

Aufeis deposits form every year on many rivers on the North Slope of Alaska; and, through repeated episodes of overflow and freezing, they may reach a thickness of more than 3 m. The presence of aufeis in a drainage basin tends to stabilize river discharge in the same way as a glacier does, by providing melt water during hot dry periods. This distribution of aufeis deposits along the rivers is discontinuous because of variations in channel geometry and under-ice water supply, and monitoring their growth during the winter is generally impractical. However, experiments using satellite radar interferometry (SRI) to map changes in the extent of aufeis near the junction of the Ivishak and the Echooka Rivers in the northern Brooks Range during the winter have produced promising results. Interferograms of the area were made from pairs of images acquired by the European Space Agency's First Earth Remote Sensing Satellite in January through March 1994. The results indicate that aufeis deposits can be mapped on interferograms as areas in river valleys in which the radar phases are poorly correlated and radar backscatter values frequently change. The ability of SRI in monitoring subtle winter aufeis processes fills a major gap in the study of aufeis by remote sensing.