Local Boundary Shear Stress Research Articles

Flow characteristics of a partially-covered trapezoidal channel

Partial ice covers alter the hydraulic regime of rivers in cold regions. Border ice is a type of partial ice cover that forms on the river banks and grows toward the centre of the channel to form a full ice cover; thereby impacting freeze-up processes. Many studies have been conducted to understand flow characteristics under open channel and full ice cover conditions; however, only a few studies have focused on the effects of partial ice covers on flow characteristics in a rectangular channel. This study goes even further by investigating stream-wise velocity and shear stress distributions in a trapezoidal channel with simulated border ice. This is the very first study to investigate the effects of different border ice coverage ratios and roughness conditions on flow characteristics in a trapezoidal channel. Experiments were conducted under seven different roughness and coverage conditions. High-resolution velocity data were collected using acoustic Doppler velocimeters within the fully-developed region of the flume. The magnitude and shape of the stream-wise velocity contours were significantly altered by the presence of the partial ice and rough boundaries. Moreover, local boundary shear stress distributions were significantly impacted by the coverage and roughness conditions. Generally, stream-wise velocity and local boundary shear stress increased within the open section and decreased within the covered section. For practical purposes, depth-averaged velocity and discharge distributions were investigated for each experiment. Results from this study will improve the current knowledge of the flow in partially covered channels and can be used for design and modelling purposes.

Variation of Local Friction Factor in an Open Channel Flow

Objectives: This present paper investigated experimentally on the variation of local friction factor over the cross section in channels of different geometries. The dependency of the variation of friction factor with aspect ratio has been well compared with the results of other researchers and natural river data. Methods/Analysis: Experiments have been conducted on trapezoidal and rectangular open channel for different geometry, hydraulic and roughness conditions. The local friction factors along every vertical interface have been carried out. The actual prediction of boundary shear stress and depth averaged velocity distribution have been calculated by finding the changes of these indispensible parameters, from wide range of data collected from previous literatures. Also the experimental depth averaged velocity and boundary shear stress have been carried out for assessing the friction factor. Findings: The global friction factor f g and local friction factors f l were calculated based on the measured depth-averaged velocity and boundary shear stress. The local friction factor does not remain constant across the channel as it depends upon the geometry and roughness parameter. The paper describes the variation of friction factor for both smooth and rough channels under different flow condition. Local boundary shear stress around the wetted perimeter τ b , and depth averaged velocity, U d , data are used to evaluate local friction factor, f l . Applications/Improvement: Local friction factor variation analysis is a very rare factor that few researchers are taken into consideration. But it has a more importance for flow characteristics which is studied in this paper.

Boundary Shear Stress in an Ice-Covered River during Breakup

River ice complicates river hydraulics and morphodynamics by adding a new boundary layer to the top of the flow. This boundary layer affects the velocity distribution throughout the depth due to increased flow resistance, and varies the local boundary shear stress on the bed (lower boundary) by adding new shear stress on the upper boundary (under surface of the ice). Variation of shear stress plays an important role in incipient motion of upper and lower boundary materials: sediment motion and transport are directly affected by local boundary shear stress, as are ice cover thickness, condition, and progression. This paper provides estimates of upper and lower boundary shear stress during stable ice cover and the important stage of ice cover breakup using available methods based on continuous field measurements of velocity profiles obtained with a bottom-mounted acoustic Doppler current profiler in the Nelson River, Canada. Boundary shear stresses varied dynamically with transformation of the ice cover, including the presence and removal of slush ice and formation of the ice jams. Estimated upper boundary shear stress varied between 0.52 and 23.87 N/m2, whereas lower boundary shear stress varied between 1.51 and 2.40 N/m2, with the highest values observed during breakup. These values are compared with estimates from previous laboratory experiments.

Modeling Flow in an Open Channel with Heterogeneous Bed Roughness

AbstractPhysical and numerical simulations of open channel flow over a heterogeneously roughened bed are examined. The velocity field is mapped at four different cross sections by using an acoustic Doppler velocimeter and the boundary shear stress obtained from both the velocity data and application of a Preston tube. These data illustrate that in the current arrangement the roughness elements dominate the physics of the flow and act as a source of vorticity, which is initially manifested as local boundary shear stress and, in turn, affects the secondary flow structures and momentum transfer in the channel. This paper presents the first attempt at using the Shiono–Knight method to model velocity and boundary shear stress distributions in a heterogeneous open channel. Appropriate advice concerning values of calibration parameters and discretization of the cross section is given. The combined physical/numerical approach provides an insight into the changing properties of the flow structure within a nonunifo...

Momentum Balance Method and Estimation of Boundary Shear Stress Distribution

AbstractDetermination of local boundary shear stress is an important topic in hydraulic engineering. When attempting to determine this from a very thin boundary region, it is a difficult one because it requires special skills and instruments to treat the measured data. With the advent of new equipment, like the acoustic Doppler velocity profilers (ADVP), all the velocity profiles in the primary flow region can be simultaneously measured, and it is necessary to develop a new method to estimate the boundary shear stress using the primary flow data. This paper presents such a method called the momentum balance method (MBM). A theoretical relationship between the boundary shear stress and parameters of the primary flow region has been established. The results obtained from MBM agree reasonably well with other methods, indicating that the new method is workable in closed duct flows, and possibly its applicability can be extended to a wide range of boundary conditions.

Bed topography and the development of forced bed surface patches

Channel topography in gravel‐bedded rivers interacts with the local flow and sediment transport fields to produce “forced patches,” which are temporally stable areas on the bed that display similar grain size and sorting. In an effort to enhance our empirical understanding of the mechanisms responsible for the formation of forced patches, we conducted a near‐field scale flume experiment in which a large (55 m long, 2.74 m wide), straight, sediment recirculating flume was provided a constant water discharge and a unimodal sediment mixture ranging in size from 2 to 45 mm. A sequence of alternate bars developed and became essentially fixed in space, producing quasi‐steady state bed topography over which we made measurements of local near‐bed velocity and sediment transport. The bed developed temporally and spatially persistent forced patches with a general pattern of coarse bar tops and fine pools, which we characterized by visual mapping and with the application of an automated image processing procedure to a high‐resolution (1 × 1 mm) digital elevation model of the bed surface. The boundary shear stress field, calculated from velocity measurements and with a quasi‐three‐dimensional hydraulic model (FaSTMECH), displayed substantial variability across the bar unit. Bed surface grain size did not correlate with local boundary shear stress; instead, topographically forced divergences in the boundary shear stress field were matched by divergences in the sediment transport field. The resultant cross‐stream sediment flux was size‐selective that in turn forced a bed surface textural response leading to coarse bar tops and fine pools. Our observations suggest that size‐selective cross‐stream bed load transport is a mechanism responsible for the development of forced bed surface patches in gravel bed channels that have topographically forced heterogeneous flow fields.

Turbulent flow and drag over fixed two‐ and three‐dimensional dunes

Detailed measurements of turbulent flow were obtained over a fixed flat bed, two‐dimensional (2‐D) dunes and four types of three‐dimensional (3‐D) dune morphologies including (1) full width saddles, (2) full width lobes, (3) sinuous crests, and (4) irregular shaped crests. The time‐averaged turbulence structure over the fixed flat bed was dynamically similar to flow over a flat bed with active sediment transport. The flow field over 2‐D dunes conforms with previous observations of flow over mobile and fixed bed forms. Bed form three dimensionality significantly altered the flow field observed over the 2‐D dunes. Lobe‐shaped dune crest lines, curved crests with topographic highs that bowed downstream, enhanced the level of turbulence producing a better defined wake structure and more vigorous mixing in the separation cell than observed over 2‐D dunes. Saddle‐shaped dune crest lines, curved crests with topographic highs that bowed upstream, diminished the level of turbulence suppressing a well‐defined wake structure and mixing in the flow separation cell. Flow over sinuous‐crested bed forms with lobes and saddles was dynamically similar to flow over full width bed forms. Development of in‐line paired saddles and lobes in the downstream direction appears to be controlled by near‐bed velocity gradients, which were larger over saddles and smaller over lobes. These gradients control local boundary shear stress patterns that should promote scour and high transport rates over saddles and deposition on lobes. Two‐dimensional and 3‐D bed forms with the same height and length offered different levels of flow resistance. The flow observations support the idea that saddles decrease flow resistance and lobes increase flow resistance relative to a 2‐D dune. Bed form crests composed of irregularly spaced lobes and saddles suppressed turbulent flow structure development and reduced drag by 20% below levels for 2‐D or sinuous crested dunes. The results suggest that bed form crest shape needs to be accounted for in flow resistance calculations and that a single drag coefficient is inadequate where the bed can evolve through 2‐D, 3‐D, and irregular 3‐D bed form states.

Erosional properties of the sediments in the Petitcodiac River estuary at Moncton, New Brunswick

The Petitcodiac River estuary was modified after the construction of a causeway in 1968 at Moncton, New Brunswick. To address some of the sediment-related issues associated with recent proposed changes to the causeway, it is essential to characterize the sediment properties. Several samples were obtained in 2003 and 2004 at eight sites from the upper 5 km of the existing estuary to determine sediment properties including particle size, bulk density, in situ water content, and organic content. These properties were determined from cores extracted from the field and reconstituted cores prepared in the laboratory. Facilities to evaluate the erosion rates of relatively undisturbed sediment cores from the field were constructed; four box cores were tested in a 10 m flume to determine erosion rates with depth. In most cases, erosion rates increased with an increase in the applied boundary shear stress and were strongly dependent on sediment properties and the depositional history of the bed.Key words: cohesive sediments, sediment transport, local boundary shear stress, erosion rates.

Study on resistance coefficient in compound channels

This paper presents a further study of the Manning and Darcy-Weisbach resistance coefficients, as they play a significant role in assessing the cross-sectional mean velocity, conveyance capacity and determining the lateral distribution of depth mean velocity and local boundary shear stress in compound channels. The relationships between the local, zonal and overall resistance coefficients, and a wide range of geometries and different roughness between the main channel and the flood plain are established by analyzing a vast amount of experimental data from a British Science and Engineering Research Council Flood Channel Facility (SERC-FCF). And the experimental results also show that the overall Darcy-Weisbach resistance coefficient for a compound channel is the function of Reynolds number, but the function relationship is different from that for a single channel. By comparing and analyzing the conventional methods with the experimental data to predict composite roughness in compound channels, it is found that these methods are not suitable for compound channels. Moreover, the reason why the conventional methods can not assess correctly the conveyance capacity of compound channels is also analyzed in this paper.

Boundary shear stress distributions in trapezoidal channels

This paper deals with the boundary shear stress distributions in trapezoidal open channels. The direction of the transport of surplus energy is discussed, which leads to the assumption that the surplus energy within any unit volume of fluid in 3-D flow will be transferred towards the nearest boundary to be dissipated. Based on this concept, the flow cross-sectional area in trapezoidal open channels is divided into various elements according to the shape of cross-section, aspect ratio and roughness distribution. Analytical equations for the local and mean boundary shear stress along the wetted perimeter are presented. The formulae and the experimental data available in literature are in good agreement.

Variability of bed mobility in natural, gravel‐bed channels and adjustments to sediment load at local and reach scales

Local variations in boundary shear stress acting on bed‐surface particles control patterns of bed load transport and channel evolution during varying stream discharges. At the reach scale a channel adjusts to imposed water and sediment supply through mutual interactions among channel form, local grain size, and local flow dynamics that govern bed mobility. In order to explore these adjustments, we used a numerical flow model to examine relations between model‐predicted local boundary shear stress (тj( and measured surface particle size (D50) at bank‐full discharge in six gravel‐bed, alternate‐bar channels with widely differing annual sediment yields. Values of тj and D50 were poorly correlated such that small areas conveyed large proportions of the total bed load, especially in sediment‐poor channels with low mobility. Sediment‐rich channels had greater areas of full mobility; sediment‐poor channels had greater areas of partial mobility; and both types had significant areas that were essentially immobile. Two reach‐mean mobility parameters (Shields stress and Q*) correlated reasonably well with sediment supply. Values which can be practicably obtained from carefully measured mean hydraulic variables and particle size would provide first‐order assessments of bed mobility that would broadly distinguish the channels in this study according to their sediment yield and bed mobility.

Entrainment of riparian gravel and cobbles in an alluvial reach of a regulated canyon river

Many canyon rivers have channels and riparian zones composed of alluvial materials and these reaches, dominated by fluvial processes, are sensitive to alterations in streamflow regime. Prior to reservoir construction in the mid-1960s, banks and bars in alluvial reaches of the Gunnison River in the Black Canyon National Monument, Colorado, USA, periodically were reworked and cleared of riparian vegetation by mainstem floods. Recent interest in maintaining near-natural conditions in the Black Canyon using reservoir releases has created a need to estimate sediment-entraining discharges for a variety of geomorphic surfaces composed of sediment ranging in size from gravel to small boulders. Sediment entrainment potential was studied at eight cross-sections in an alluvial reach of the Gunnison River in the Black Canyon in 1994 and 1995. A one-dimensional water-surface profile model was used to estimate water-surface elevations, flow depths, and hydraulic conditions on selected alluvial surfaces for discharges ranging from 57 to 570 m3/s. Onsite observations before and after a flood of 270 m3/s confirmed sediment entrainment on several surfaces inundated by the flood. Selective entrainment of all but the largest particle sizes on the surface occurred at some locations. Physical evidence of sediment entrainment, or absence of sediment entrainment, on inundated surfaces generally was consistent with critical shear stresses estimated with a dimensionless critical shear stress of 0.030. Sediment-entrainment potential over a range of discharges was summarized by the ratio of the local boundary shear stress to the critical shear stress for d50, given hydraulic geometry and sediment-size characteristics. Differing entrainment potential for similar geomorphic surfaces indicates that estimation of minimum streamflow requirements based on sediment mobility is site-specific and that there is no unique streamflow that will initiate movement of d50 at every geomorphically similar location in the Black Canyon. Copyright © 2000 John Wiley & Sons, Ltd.

Estimating Local Bed Shear Stress from Velocity Observations

Replicate velocity observations using conventional equipment under typical field conditions are used to evaluate the precision of different methods for estimating local boundary shear stress from velocity measurements. The bed shear velocity u* can be estimated within 3% using the depth‐averaged velocity in the vertically averaged logarithmic velocity profile. To be accurate, this method is limited to relatively simple flow geometries which may be expected to have the appropriate velocity structure. Estimates of u* made using a single near‐bed velocity observation are less precise by a factor of 3 because of the larger uncertainty associated with a single observation. Accuracy of this method requires appropriate flow conditions only near the bed, so it may be applied in a wider range of flow conditions, including spatially variable flow. Estimates of u* from the slope of the near‐bed velocity profile are the least precise and require the most restrictive flow conditions for accuracy but offer the advantage that they may be made without independent knowledge of the bed roughness.

Convective accelerations and boundary shear stress over a Channel Bar

Alternate bars are important features in alluvial channels as they determine flow and transport patterns. They appear fundamental to selection of meander wavelengths and the geometry of bends. Bend flow has been studied extensively: far less study has been made of flow over alternate bars. Field results from Solfatara Creek, a 5.2‐m‐wide, 0.2–0.7‐m‐deep gravel bed channel where flow exits an upstream bend and shoals over a bar in a straight reach, are used to examine patterns of flow and the fluid forces determining the flow field. Large cross‐sectional area changes, tied primarily to variation in depth, force large stream‐wise accelerations and substantial cross‐stream flow off the central bar. The topographically driven downstream and cross‐stream accelerations are sufficiently large that their influence upon the balance of forces is of the same order as the pressure gradient and the boundary shear stress. The importance of convective accelerations in the downstream flow equation in this straight reach concurs with bend flow results, but the similar importance of convective accelerations in the cross‐stream equation contrasts with results from bend flow. While part of the difference may be attributed to the lower stage conditions herein, in the absence of significant curvature change the cross‐stream force balance depends upon the flow going over and around the bar. Local boundary shear stress estimated from the law‐of‐the‐wall and a roughness algorithm decreases out of the upstream bend, increases over the bar top to values approaching the threshold for motion, and then decreases in deeper flow. Strong bed surface coarsening maintains the topography in a stress field that would otherwise lead to planation of the bar top and filling of the deeper regions.

Boundary Shear Stress and Roughness Over Mobile Alluvial Beds

The resistance to flow in the turbulent rough-flow range depends primarily upon the size, shape, and arrangement of the granular material making up the boundary. We have estimated the hydraulic roughness of mobile alluvial surfaces by inverting sediment-transport formulas to solve for the local boundary shear stress required to predict the observed sediment flux and size. Inserting this shear stress value and a near-bed velocity measurement into the law of the wall yields the roughness scale, \Iz\N\do defined as the height above the bed where velocity goes to zero. If the roughness is related to the coarse fraction of the bedload, such as \ID\N\d8\d4, then \Iz\N\do=0.1\ID\N\d8\d4. This roughness, obtained from mobile, naturally packed, and heterogeneous-in-size beds is three times greater than that predicted by the Nikuradse formula developed from nearly uniform and smoothly-packed surfaces. We detect no variation in roughness with transport stage, implying that the large static and slowly moving grains determine flow resistance and that momentum extraction by saltating grains is minor. Application of this simple roughness algorithm allows convenient and accurate calculation of the local boundary shear stress.

Water conveyance in channels with semicircular cross sections

In this experimental study, measurements concerning uniform flow in open channels, with semicircular cross sections and smooth or rough boundaries, are presented, analysed and discussed. Based on local velocity and boundary shear stress measurements, an effort is made to interpret the momentum and energy distribution, the internal boundary shear force and discharge distributions, the maximum boundary shear stress distributions as well as the Manning's n and friction factor's f variation, obtained for various groups of smooth or rough open channels used herein.

Spatially averaged flow over a wavy surface

An understanding of the mechanics of nonuniform flow is important in a variety of ecological and geophysical fluid mechanical problems. Moreover, the ability to predict local boundary shear stress on an uneven bed is essential in erosion and sediment transport problems. In order to elucidate the important fluid mechanical phenomena active over natural quasi-two-dimensional bed forms a series of detailed flow measurements were made above 60- to 100-m-long 1- to 3-m-high dunes in the Columbia River. In this paper, velocity profiles obtained by averaging these flow data along lines of constant distance above the riverbed are examined, and it is shown that they can be constructed from well-known uniform flow results used in conjunction with a hypothesis about the structure of internal boundary layers. This approach permits skin friction as well as total boundary shear stress, averaged over one wavelength of the bed form, to be determined from spatially averaged velocity profiles and, conversely, provides a mechanism whereby zero-order velocity profiles can be constructed for two-dimensional nonuniform channel flows. Corrections for changes in the bottom roughness parameter caused by bed load transport and for flow stratification induced by suspended load transport are derived and applied in order to make the results consistent with the measured spatially averaged shear stress field.

Turbulent flow in wavy pipes

A primarily experimental investigation was undertaken to determine the internal structure of steady, quasi-uniform, non-separated, axisymmetric flows in circular pipes with sinusoidal wall profiles. The quantities measured include radial and longitudinal distributions of mean velocity, pressure, and total head; the Reynolds shear stress and all three components of turbulence velocity; and boundary shear stress and pressure. Two different wall-wave steepnesses were investigated, and a constant Reynolds number of 1·13 × 105 (based on the average pipe diameter) was maintained in most experiments. The boundary shear stress was found to be shifted upstream relative to the boundary wave, whereas the wall pressure is shifted slightly downstream. The turbulence measurements revealed that there is a central core extending over some 60% of the pipe radius in which the turbulence quantities are constant along the pipe. Near the boundary, however, the turbulence velocities and stress vary periodically along the boundary waves. The longitudinal component of mean velocity was found to be distributed radially according to the power law, but with an exponent that varies along each wave; a simple analytical model is constructed to predict the variation of the exponent. It was not found possible to relate the local boundary shear stress to just the local flow characteristics, since the convective or ‘history’ effects play a significant role in its determination. An empirical formula is derived relating the local boundary shear stress to the local velocity distribution and the first two derivatives of the boundary profile.