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1.
For many incised channels, one of the most common strategies is to install some hard structures, such as grade‐control structures (GCSs), in the riverbed to resist further incision. In this study, a series of experiments, including both steady and unsteady flow conditions, were conducted to investigate the scouring process downstream of a GCS. Three distinct phases, including the initial, developing and equilibrium phases, during the evolution of scour holes were identified. In addition, a semi‐empirical method was proposed to predict the equilibrium scour‐hole profile for the scour countermeasure design. In general, the comparisons between the experimental and simulated results are reasonably consistent. As the studies on temporal variation of the scour depth at GCSs caused by floods are limited, the effect of flood hydrograph shapes on the scour downstream of GCSs without upstream sediment supply was also investigated experimentally in this study. Based on the dimensional analysis and the concept of superposition, a methodology is proposed to simulate the time evolution of the maximum scour depth downstream of a GCS for steady flows. Moreover, the proposed scheme predicts reasonably well the temporal variations of the maximum scour depth for unsteady flows with both single and multiple peak. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

2.
A better understanding of bedrock incision mechanisms and processes is essential to the study of long‐term landscape evolution. Yet, little is known about flow dynamics in bedrock rivers, limiting our ability to make realistic predictions of local bedrock incision rates. A recent investigation of flow through bedrock canyons of the Fraser River revealed that plunging flows, defined by the downward‐directed movement of near surface flow toward the channel bed, occur in channels that have low width‐to‐depth ratios. Plunging flows occur into deep scour pools, which are often coincident with lateral constrictions and channel spanning submerged ridges (sills). A phenomenological investigation was undertaken to reproduce the flow fields observed in the Fraser canyons and to explore morphological controls on the occurrence and relative strength of plunging flow in bedrock canyons. Our observations show that the plunging flow structure can be produced along a scour pool entrance slope by accelerating the flow at the canyon entrance either over submerged sills or through lateral constrictions. Plunging flow appears to be a function of convective deceleration into a scour pool which can be enhanced by sill height, the amount of the channel width that is constricted, pool entrance slope, discharge, and a reduction in channel width‐to‐depth ratio. Plunging flow greatly enhances the potential for incision to occur along the channel bed and is an extreme departure from the assumptions of steady, uniform flow in bedrock incision models, highlighting the need for improved formulations that account for fluid flow. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

3.
SCOUR HOLE CHARACTERISTICS BELOW FREE OVERFALL SPILLWAY   总被引：1，自引：0，他引：1  
1 INTRODUCTION Flow through hydraulic structures often issues in the form of jets. The jet velocities are usually high enough to produce sizable, even dangerous scour hole. The extent of the resulting scour depends on the nature of bed material and flow characteristics. The erosion process is quite complex and depends upon the interaction of hydraulic and morphological factors. Scouring may lead to: endangering the stability of the structure by structural failure or increased seepage, end…  相似文献   

4.
Sequences of arti?cial steps are sometimes used to reproduce the natural step–pool morphology of high‐gradient streams. The depth, length and shape of the scour holes in gravel‐bed rivers can be predicted reasonably using recently developed formulae. However, the properties of the scour holes can sometimes be affected by the distance between structures. This effect is called ‘geometrical interference’ and leads to a reduction of the scour hole compared to its potential size. Geometrical interference may occur in sequences of arti?cial steps in high‐gradient torrents, where structures are sometimes built at distances of a few tens of metres apart, but may also apply to natural step–pool systems. In this paper, a series of tests have been conducted to determine the effect of bed sill spacing and sediment grading on the potential erosion by jets forming over the sills. A new formula is derived, applicable to high‐gradient streams (slope > 0·04), which can be applied to the special case of scour holes developed by interfering sills. Sediment size gradation, not accounted for in previous formulae, is found to have a signi?cant effect on the scour dimensions and is included in the new predictive formula. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

5.
In this work, investigation on the development of local scour around an oblong pier in a 180 degree flume bend is presented. Scour hole can cause failure of the bridge especially during the river floods. In this study, the use of oblong collars for reducing the effects of local scour at a bridge pier is presented together with the time aspect of the scour development. Tests were conducted using one oblong pier in positions of 60degree under one flow conditions. The study was conducted using a physical hydraulic model operated under clear-water conditions in cohesionless bed material. In this study, the time development of the local scour around the oblong pier fitted with and without collar plates was studied. Investigated was the effect of size and elevation collar on the time development of scour and its efficacy at preventing scour at a bridge pier. The time development of the scour hole around the model pier with and without a collar installed was compared with similar studies on bridge piers. The results of the model study indicated that the maximum depth of scour is highly dependent on the experimental duration. It was observed that, as the minimum depth of scour occurs for the square collar at width of 3B placed at elevation of 0.1B below the bed and the size of a collar plate increases, the scour decreases. Measuring depth of scouring based on experimental observation, an empirical relation is developed with regression coefficient 95%.  相似文献   

6.
1 INTRODUCTION'LOcal scour around a pier is a result of the interatiOn amongst the pier, the aPproach flow and theerodible bed. The Presence of a pier results in a stagnation pressure build-up in front of the pier and athree-dimensional tUrbulent flow echaracterized by the downward flow ahead of the pier and the so-called horseshoe vortex along the base of the Pier forms itself The flow modifies the erothe bed inthe vicinity of the pier when local scour takes place (Graf and AJhnakar l…  相似文献   

7.
The flow pattern around a cylinder, installed in a scoured channel bed, was experimentally investigated. Detailed measurements of the instantaneous 3D velocities were performed by using an Acoustic Doppler Velocity Profiler (ADVP), from which the profiles of the time-averaged velocities and turbulence stresses were obtained. It is shown that the influence of the cylinder and of the scour hole alters the approach flow; this is essentially confined to the vicinity of the cylinder and to the inside of the scour hole. The horseshoe vortex is measured as a flow reversal inside the scour hole, formed by the downward flow along the cylinder face and the reversed flow at the scour bed.  相似文献   

8.
Confluences with low discharge and momentum ratios, where narrow steep tributaries with high sediment load join a wide low‐gradient main channel that provides the main discharge, are often observed in high mountain regions such as in the upper‐Rhone river catchment in Switzerland. Few existing studies have examined the hydro‐morphodynamics of this type of river confluence while considering sediment discharge in both confluent channels. This paper presents the evolution of the bed morphology and hydrodynamics as observed in an experimental facility with a movable bed. For that purpose, one experiment was carried out in a laboratory confluence with low discharge and momentum ratios, where constant sediment rates were supplied to both flumes. During the experiment, bed topography and water surface elevations were systematically recorded. When the bed topography reached a steady state (so‐called equilibrium) and the outgoing sediment rate approximated the incoming rate, flow velocity was measured at 12 different points distributed throughout the confluence, and the grain size distribution of the bed surface was analyzed. Typical morphodynamic features of discordant confluences such as a bank‐attached bar and a flow deflection zone are identified in this study. Nevertheless, the presence of a marked scour hole in the discordant confluence and distinct flow regimes for the tributary and main channel, differ from results obtained in previous studies. Strong acceleration of the flow along the outer bank of the main channel is responsible for the scour hole. This erosion is facilitated by the sediment discharge into the confluence from the main channel which inhibits bed armoring in this region. The supercritical flow regime observed in the tributary is the hydrodynamic response to the imposed sediment rate in the tributary. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

9.
A critical concern regarding river bed stabilization and river engineering is the short‐term general scour that occurs in a field setting far from a river‐crossing structure or embankment during a typhoon‐induced flood. This study investigated the improvement of existing techniques that have been used to measure river bed scour. One of these techniques is the numbered‐brick column or scour chains method, in which only the maximum general scour depth of river bed is observed. A wireless tracer for monitoring real‐time scour was set‐up with a numbered‐brick column and was employed to collect synchronous data. The proposed method was successfully used to observe both real‐time scour and the maximum depth at flood peak. This observation was conducted at a steep gravel‐bed reach of the Shuideliaw Embankment on the intermittent Choshui River in Central Taiwan during Typhoon Soulik, which occurred in 2013. Future studies must be conducted to complete the development of an automatic real‐time scour and flood monitoring system for use in severe weather and flow conditions; this would facilitate the identification of river bed scour during conditions of unstable flow and the improvement of flood prevention engineering, bridge closure detection and emergency evacuation procedures. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

10.
Experimental study has been carried out under a clear-water scour condition to explore the local scour around semi-elliptical model bridge abutments with armor-layer bed, compared with the local scour process around semi-circular abutment. Two types of model bridge abutments, namely semi-elliptical and semi-circular abutments have been used in this experimental study. The model abutments had a ratio of streamwise length of abutment to the length of abutment transverse to the flow of 2 for semi-circular abutments and 3 for semi-elliptical abutments. In total, 50 Experiments have been designed and conducted under different flow conditions such as bed shear velocities, flow depth, and dimensions of bridge abutment model, as well as grain size of the bed material. Based on these experiments, the scour process around bridge abutments has been assessed. The dependence of the equilibrium scour depth of the scour hole on hydraulic variables has been studied. Empirical equation describing the equilibrium scour depth of the scour hole around bridge abutments has been developed.  相似文献   

11.
《国际泥沙研究》2016,(2):159-163
Roughened horizontal aprons are bed covering scour countermeasures constructed downstream of stilling basins and other places where scour hole may develop. In these cases scour occurs at the edge of the apron which can lead to failure of the apron. In the present study, 24 experimental tests were carried out on four different aprons with (2, 5, 10 and 14.28 mm) roughness heights and two different bed material sizes of 0.8 and 1.4 mm under different flow conditions. The results indicated that as the roughness height of apron increases, a significant reduction in the scour depth occurs.  相似文献   

12.
Bridge pier scour mitigation under steady and unsteady flow conditions   总被引：1，自引：1，他引：0  
Watercourse morphology is affected by local scouring when the flow interferes with anthropic structures. Controlling the scour hole size is of predominant importance to guarantee bridge safety as well as to limit the variations of river morphology. A combined countermeasure against bridge pier scour is proposed and tested in order to reduce the maximum scour depth and deviate it away from the bridge foundation. In the first part of the laboratory campaign, combination of two countermeasures (bed-sill and collar) was evaluated for a circular pier under clear-water and live-bed steady flow conditions. The proposed combined countermeasure exhibited an efficiency of about 64% in terms of scour depth reduction. Afterwards, it was tested in unsteady flow conditions, first for a circular pier, then in the case of a rectangular pier with round nose and tail, two circular in-line piers and two rectangular in-line piers, under a hydrograph with a peak flow velocity slightly above the threshold condition of sediment motion. Results showed that the combined countermeasure had an efficiency of about 63% for a single circular pier; however, higher efficiency (about 75%) was obtained in applications to rectangular pier and two in-line circular or rectangular piers.  相似文献   

13.
Sediment load plays a major role in the morphological evolution of rivers.Therefore,the analysis of the sediment load interaction with hydraulic structures is of main importance in order to enhance the preservation of fish habitats and river morphological characteristics.The present study analyzes the scour mechanisms downstream of a block ramp in live bed conditions,when the sediment supplied by the approaching flow is balanced by the sediment transported out of the scour hole.Experiments were performed in a model flume and the effect of the approaching sediment concentration on the scour geometry was analyzed.It was observed that the scour features depend deeply on the approaching sediment concentration and four main profile configurations were distinguished.The experimental data were analyzed and empirical relationships were developed in order to evaluate the depth and length of a scour hole,the dune height and the distance of the transversal section of maximum dune height from the ramp toe for different hydraulic and geometric conditions.It was also proved that the dynamic equilibrium shape of a scour hole does not depend on the sediment load time history.  相似文献   

14.
Experimental results of the mean flow field and turbulence characteristics for flow in a model channel bend with a mobile sand bed are presented. Acoustic Doppler velocimeters (ADVs) were used to measure the three components of instantaneous velocities at multiple cross sections in a 135° channel bend for two separate experiments at different stages of clear water scour conditions. With measurements at multiple cross sections through the bend it was possible to map the changes in both the spatial distribution of the mean velocity field and the three Reynolds shear stresses. Turbulent stresses are known to contribute to sediment transport and the three‐dimensionality inherent to flow in open channel bends presents a useful case for determining specific relations between three‐dimensional turbulence and sediment entrainment and transport. These measurements will also provide the necessary data for validating numerical simulations of turbulent flow and sediment transport. The results show that the magnitude and distribution of three‐dimensional Reynolds stresses increase through the bend, with streamwise‐cross stream and cross stream‐vertical components exceeding the maximum principal Reynolds stress through the bend. The most intriguing observation is that near‐bed maximum positive streamwise‐cross stream Reynolds stress coincides with the leading edge of the outer bank scour hole (or thalweg), while maximum cross stream‐vertical Reynolds stress (in combination with high negative streamwise‐cross stream Reynolds stress near the bend apex) coincides with the leading edge of the inner bank bar. Maximum Reynolds stress and average turbulent kinetic energy appear to be greater and more localized over the scour hole before final equilibrium scour is reached. This suggests that the turbulent energy in the flow is higher while the channel bed is developing, and both lower turbulent energy and a broader distribution of turbulent stresses near the bed are required for cessation of particle mobilization and transport. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

15.
This study examines the spatial distributions of third‐order moments of velocity fluctuations, the turbulent kinetic energy (TKE) fluxes, and the conditional statistics of Reynolds shear stress across the equilibrium crescentic scour structures generated upstream of short horizontal static cylinders. Detailed velocity data were collected using three‐dimensional (3D) micro‐acoustic Doppler velocimeter (ADV) across and within the equilibrium scour marks. The analysis reveals that the positive and negative values of third‐order moments associated with the level bed surface and the scour holes are directly related to coherent structures. The components of TKE flux are discussed for the near‐bed region of the level bed surface and scour holes in relation to sweep–ejection events. A cumulant‐discard method is applied to the Gram‐Charlier probability distribution of two variables to describe the statistical properties of the term u′w′. The conditional statistics of the Reynolds shear stress show a good agreement with the experimental data. The distribution of the joint probability density function in the near‐bed region changes cyclically along the scour hole depending on the bottom fluid velocity, which implies a change from upward to downward flux of momentum and vice versa. Both the ejection and sweep events at near‐bed points on the level surface are more important than within the scour region; and in contrast, both events are stronger for the scour marks than the level bed surface at the outer layer. Sweeps dominate over ejections for the scour hole induced by smaller diameter and ejections dominate for larger diameter. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

16.
Results of an experimental study on the countermeasure of scour depth at circular piers are presented. Experiments were conducted for pier scour with and without a splitter plate under a steady, uniform clear-water flow condition. The results of pier scour without splitter plate were used as a reference. Different combinations of lengths and thicknesses of splitter plates were tested attaching each of them to a pier at the upstream vertical plane of symmetry. Two different median sediment sizes (d 50 = 0.96 and 1.8 mm) were considered as bed sediment. The experimental results show that the scour depth consistently decreases with an increase in splitter plate length, while the scour depth remains independent of splitter plate thickness. In addition, temporal evolution of scour depth at piers with and without a splitter plate is observed. The best combination is found to be with a splitter plate thickness of b/5 and a length of 2b. Here, b denotes the pier diameter. An empirical formula for the estimation of equilibrium scour depth at piers with splitter plates is obtained from a multiple linear regression analysis of the experimental data. The flow fields for various combinations of circular piers with and without splitter plate including plain bed and equilibrium scour conditions were measured by using an acoustic Doppler velocimeter. The turbulent flow fields for various configurations are investigated by plotting the velocity vectors and the turbulent kinetic energy contours on vertical and horizontal planes. The splitter plate attached to the pier deflects the approach flow and thus weakens the strength of the downflow and the horseshoe vortex, being instrumental in reducing the equilibrium scour depth at piers. The proposed method of pier scour countermeasure is easy to install and cost effective as well.  相似文献   

17.
The effects of different submerged obstacle longitudinal bars with different arrangement densities on the flow profile and morphology of a scour hole were investigated under clear water conditions. Acoustic Doppler velocimetry(ADV) data were applied to plot the vertical distributions of three-dimensional velocities and turbulent contours.The experimental results indicate that arrangement density(also can represent porosity),structural material(flexible or solid),and the sidewall effect are the main factors impacting turbulent kinetic energy and the morphology of scour holes.For flexible vegetation,the maximum turbulent kinetic energy near the bed surface increased with the arrangement density.For the same structure,the depth and the magnitude of the lateral expansion of the scour hole also increased with the arrangement density.The flexible vegetation reduced the depth of the scour hole because of deflection and arrangement density.The larger volumes of scour found in the upstream and middle sections of solid structures compare well to those in flexible vegetation.The deflection of porous flexible vegetation transported the turbulent kinetic energy downstream,reduced the turbulent kinetic energy near the sediment bed,and increased the stability of the bars.Flexible vegetation bars are able to protect the bank and the bed of a river under normal conditions,making them a good alternative design in the management and restoration of rivers.  相似文献   

18.
Headcut formation and migration was sometimes mistaken as the result of overland flow, without realizing that the headcut was formed and being influenced by flow through soil pipes into the headcut. To determine the effects of the soil pipe and flow through a soil pipe on headcut migration in loessic soils, laboratory experiments were conducted under free drainage conditions and conditions of a perched water table. Soil beds with a 3-cm deep initial headcut were formed in a flume with a 1.5-cm diameter soil pipe 15 cm below the bed surface. Overland flow and flow into the soil pipe was applied at a constant rate of 68 and 1 l min−1 at the upper end of the flume. The headcut migration rate and sediment concentrations in both surface (channel) and subsurface (soil pipe) flows were measured with time. The typical response was the formation of a headcut that extended in depth until an equilibrium scour hole was established, at which time the headcut migrated upslope. Pipeflow caused erosion inside the soil pipe at the same time that runoff was causing a scour hole to deepen and migrate. When the headcut extended to the depth of the soil pipe, surface runoff entering the scour hole interacted with flow from the soil pipe also entering the scour hole. This interaction dramatically altered the headcut processes and greatly accelerated the headcut migration rates and sediment concentrations. Conditions in which a perched water table provided seepage into the soil pipe, in addition to pipeflow, increased the sediment concentration by 42% and the headcut migration rate by 47% compared with pipeflow under free drainage conditions. The time that overland flow converged with subsurface flow was advanced under seepage conditions by 2.3 and 5.0 min compared with free drainage conditions. This study confirmed that pipeflow dramatically accelerates headcut migration, especially under conditions of shallow perched water tables, and highlights the importance of understanding these processes in headcut migration processes. © 2020 John Wiley & Sons, Ltd.  相似文献   

19.
Three-dimensional turbulent flow field was measured around an experimental spur dike by using a micro-acoustic Doppler velocimeter.Time fractions of turbulent burst events including outward interaction,ejection,inward interaction,and sweep were analyzed in each quadrant at the neighbor of the dike before and after the formation of scour hole.Over 80%of burst events near the bed have lower-order of magnitudes for both flat and scoured bed surfaces.Ejections and sweeps are prevalent before the local scour was initiated,and then outward interactions are dominant after the scour hole was formed.Conditional Reynolds stresses and high-order moments of turbulent velocities were analyzed along the thalweg.The magnitudes of u’-w’ pair were much larger than those of v’-vv’ pair in the scour zone.Among four burst events,ejections and sweeps are the higher order events contributing to the Reynolds stresses.Since sediment entrainment and transport are closely associated with turbulent bursts near bed,the development of scour hole greatly depends on the higher order event near the bed.  相似文献   

20.
Debris flows can grow greatly in size by entrainment of bed material, enhancing their runout and hazardous impact. Here, we experimentally investigate the effects of debris‐flow composition on the amount and spatial patterns of bed scour and erosion downstream of a fixed to erodible bed transition. The experimental debris flows were observed to entrain bed particles both grain by grain and en masse, and the majority of entrainment was observed to occur during passage of the flow front. The spatial bed scour patterns are highly variable, but large‐scale patterns are largely similar over 22.5–35° channel slopes for debris flows of similar composition. Scour depth is generally largest slightly downstream of the fixed to erodible bed transition, except for clay‐rich debris flows, which cause a relatively uniform scour pattern. The spatial variability in the scour depth decreases with increasing water, gravel (= grain size) and clay fraction. Basal scour depth increases with channel slope, flow velocity, flow depth, discharge and shear stress in our experiments, whereas there is no correlation with grain collisional stress. The strongest correlation is between basal scour and shear stress and discharge. There are substantial differences in the scour caused by different types of debris flows. In general, mean and maximum scour depths become larger with increasing water fraction and grain size, and decrease with increasing clay content. However, the erodibility of coarse‐grained experimental debris flows (gravel fraction = 0.64) is similar on a wide range of channel slopes, flow depths, flow velocities, discharges and shear stresses. This probably relates to the relatively large influence of grain‐collisional stress to the total bed stress in these flows (30–50%). The relative effect of grain‐collisional stress is low in the other experimental debris flows (<5%), causing erosion to be largely controlled by basal shear stress. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

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