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1.
LOCAL SCOUR AROUND BRIDGE ABUTMENTS   总被引：2，自引：0，他引：2  
1 INTRODUCTION The protrusion of a bridge abutment or a spur dike into the main channel creates a disturbance and obstruction to the otherwise equilibrium state of sediment transportation in an alluvial river. The flow accelerates and separates at the upstream face of the abutment as it moves past the obstacle, creating a vortex trail that moves downstream in a direction approximately perpendicular to the structure. The result is that the bed around the structure is eroded locally. The l…  相似文献   

2.
1 wrsoooCTIoxThe Yeuow mver crtes a huge amoun of sedimcht and the noods often cause raPid and severeerosinn and dePOsihon. The channl bed of the YelOw mver often exPeriences degIadation in the mainchannel during fioods. In some cases vigorous erosion uP to l0 meters takes place in a shOrt Period oftiIn. Such phenomenon usually occurs in the ndddie reaChs of the Yelow mver and its tributaries suchas the Wdse mVer the Beiluohe mver etc. For examPle, th6 hyPenconcentraed nood in July l…  相似文献   

3.
1 mnCnONLocal scour close to bridge piers and abUtInnts has long been a subect of concem for engineers, sinceit can We total or partial collapse of bridges. Until to the Present, local scour has been assessed, moshy,on the basis of resultS of labOratOry stodis. These sthes were cwhed out for steady flows lashng longenough as to gUarantee the develoPment of equlllbrium scour i.e., the develoPmen of scour holes whosedePth and 8haPe no lOnger significanti evolve with hme.In nta, such long l…  相似文献   

4.
1 INTRODUCTION Scouring in the bend ways leads to deep sections at the toe of the outer bank of the bend. The presenceof secondary currents and the greater depths at the outer bank cause high velocity along the outer bank.The high velocity and shear stres…  相似文献   

5.
The special feature of bend flow leads to scouring of the bed and bank. Various parameters like flow depth, flow velocity or discharge, geometry of bend and characteristics of bed material may affect the scour process. Experiments were carried out to study the effect of some important parameters on bend scour under clear water condition. Experiments were conducted in a 0.6m wide and 0.7m high flume with 90 degree bend. The lateral variations of bed slope were studied. The maximum depth of scour was correlated to densimetric Froude number, relative bend radius and relative depth of flow.  相似文献   

6.
Upstream damming often causes significant downstream geomorphic adjustments. Remarkable channel changes have occurred in the Jingjiang Reach of the Middle Yangtze River, since the onset of the Three Gorges Project (TGP). Therefore, it is important to investigate the variations in different fluvial variables, for better understanding of the channel evolution characteristics as an example of the Jingjiang Reach. Recent geomorphic adjustments in the study reach have been investigated quantitatively, including variations in sediment rating curve, fluvial erosion intensity, channel deformation volume and bankfull channel geometry. These fluvial variables adjusted in varying degrees in response to the altered flow and sediment regime caused by the TGP operation. A focus of this study has been especially on variation in the bankfull channel geometry. Calculated bankfull dimensions at section‐ and reach‐scale indicate that: (i) there were significant bank‐erosion processes in local regions without bank‐protection engineering, with empirical relations being developed to reproduce the variation in bankfull widths at four typical sections; (ii) the variation in the reach‐scale channel geometry occurred mainly in the component of bankfull depth, owing to the construction of large‐scale bank‐revetment works, with the depth increasing from 13.7 m in 2002 to 15.0 m in 2014, and with an increase in the corresponding bankfull area of about 11%; and (iii) the reach‐scale bankfull channel dimensions responded to the previous 5‐year average fluvial erosion intensity during flood seasons at Zhicheng, with higher correlations for the depth and area being obtained when calibrated by the measurements in 2002–2012. Furthermore, these relations developed for the section‐ and reach‐scale bankfull channel geometry were also verified by the observed data in 2013–2014, with encouraging results being obtained. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

7.
The authors present new measurements of maximum (equilibrium) local scour depth at bridge abutments, the new data being analysed in terms of the multiplying factors given in Melville (1992,1997) and Melville and Coleman (2000). The design method presented in these publications is referred to here as the University of Auckland (UoA) Method. The authors' data apply to the following factors:∏f (flow intensity), ∏θ (abutment alignment) and ∏g (approach channel geometry).  相似文献   

8.
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 m3/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. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

9.
I.INTRODUCTIONhiverchannelsaresubjecttocontinuouschangeingeometryduetoillteraCtionbetWeentheflowanderodibleboundaries.Ofconcerntothedesignersofoilpipelinesacrossariver,bridgesandhydraulicworksistheproblemofscourwhichcanunderminetheStructures.Scouratsiteofbridgesandhydraulicworksoccursduetoconstrictedflowandexistenceofbridgepiers.SuchatabOfscouroccursonlyinashortsection,usuallyillthesameorderofthelengthofthehydraulicworksorbridges.Therefore,thispatternofscouriscalledlocalscour.Man}rresea…  相似文献   

10.
This study focused on a spatial and temporal analysis of the active channel and associated floodplain lakes using aerial photographs spanning five decades (1942, 1962, 1985, 1999) over a 140 km long reach of the Sacramento. Planimetric changes were analysed longitudinally and temporally to highlight the spatial structures and their evolution through time. The results underline complex changes and space–time pattern in bank erosion, channel length and active channel width. The bank erosion and also channel lengthening were higher between 1962 and 1985 than in the two periods studied before and after. Active channel width significantly decreased from 1942 to 1999; partly progressively from upstream to downstream with local widening whatever the studied periods. Similarly the floodplain lakes observed before 1942–1962 were significantly different in size and geometry from those which appeared during the most recent period. The creation of lakes is less frequent after the 1940s, with a secondary peak of occurrence during the 1962–1985 period. The interpretation of these changes is complex because of various human pressures acting over different time scales (bank protection, flow diversion, sediment starvation, land‐use changes) and various natural influences (flood sequences through out the period, geological setting). The findings are discussed by comparison with previous work, and highlight the important effect of dam impact on peak flow and sediment starvation modifying longitudinally hydraulic conditions within the channel, but also the increase in riprap protection which induced change in bank erosion, channel planimetry and floodplain lake characters (geometry, frequency of renewal). Variation in flood intensities is also observed as having positive effects on the bank erosion pattern. Secondarily, land‐use changes also controlled bank erosion intensity. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

11.
Large wood (LW) transport can increase greatly during floods, leading to accumulations at river infrastructures. To mitigate the potential flood hazard, racks are a common method to retain LW upstream of endangered settlements or infrastructures. The majority of LW retention racks consist of vertical bars and, therefore, disrupt bedload transport. It can be hypothesized that inclined racks reduce backwater rise and local scour, as wood will block the upper part of the rack, thereby increasing the open flow cross-section below the accumulation. Flume experiments were conducted under clear water conditions to analyse backwater rise and local scour as a function of (1) rack inclination, (2) hydraulic inflow condition, (3) uniform bed material, and (4) LW volume. In addition, the first experiments were performed under live bed scour conditions to study the effect of bedload transport on local scour and backwater rise. Based on the experiments, backwater rise and local scour decrease with decreasing rack angle to the horizontal. LW predominantly accumulated at the upper part of the rack, leading to an open flow cross-section below the accumulation. The effect of rack angle was included in existing design equations for backwater rise and local scour depth. In addition, the first experiments with bedload transport resulted in smaller backwater rise and local scour depth. This study contributes to an enhanced process understanding of wood retention and bedload transport at rack structures and an improved design of LW retention racks. © 2020 John Wiley & Sons, Ltd.  相似文献   

12.
Spur dikes are river training structures that have been extensively used worldwide for towards enhancing flood control and the stability of embankments and riverbanks.However,scour around spur dikes can be a major problem affecting their stability and hydraulic performance.The precise computation of temporal scour depth at spur dikes is very important for the design of economical and safe spur dikes.This study focuses on experimentally assessing the temporal variation of scour depth around a vertical wall spur dike and identifying the parameters,which mostly influence spur dike performance for a channel bed surface comprised of sand-gravel mixtures.In the current study,the authors did physical experiments in a flume based study to obtain new data,aimed at deriving a new predictive model for spur dike scour and comparing its performance to others found in the literature.It was found that the dimensionless temporal scour depth variation increases with an increase in(i)the threshold velocity ratio,(ii)the densimetric Froude number of the bed surface sediment mixture,(iii)the flow shallowness(defined as the ratio of the approach flow depth,y,to the spur dike’s transverse length,l),and(iv)the flow depth-particle size ratio.It is also concluded that the temporal scour depth variation in the sediment mixture is influenced by the non-uniformity of sediment and decreases with an increase in the non-uniformity of the sediment mixture.A new mathematical model is derived for the estimation of temporal scour depths in sand-gravel sediment mixtures.The proposed equation has been calibrated and validated with the experimental data,demonstrating a good predictive capacity for the estimation of temporal scour depth evolution.  相似文献   

13.
The stability of a river bank depends on the balance of forces, motive and resistive, associated with the most critical mechanism of failure. Many mechanisms are possible and the likelihood of failure occurring by any particular one depends on the size, geometry and structure of the bank, the engineering properties of the bank material, the hydraulics of flow in the adjacent channel and climatic conditions. Rivers flowing through alluvial deposits often have a composite structure of cohesionless sand and gravel overlain by cohesive silt/clay. Bank erosion occurs by fluvial entrainment of material from the lower, cohesionless bank at a much higher rate than material from the upper, cohesive bank. This leads to undermining that produces cantilevers of cohesive material. Upper bank retreat takes place predominantly by the failure of these cantilevers. Three mechanisms of failure have been identified: shear, beam and tensile failure. The stability of a cantilever may be analysed using static equilibrium and beam theory, and dimensionless charts for cantilever stability constructed. Application of the charts requires only a few simple measurements of cantilever geometry and soil properties. In this analysis the effects of cracks and fissures in the soil must be taken into account. These cracks seriously weaken the soil and can invalidate a stability analysis by affecting the shape of the failure surface. Following mechanical failure, blocks of soil must be removed from the basal area by fluvial entrainment if rapid undermining and cantilever generation are to continue. Hence, the rate of bank retreat is fluvially controlled, even though the mechanism of failure of the upper bank is not directly fluvial in nature. This cycle of bank erosion: undermining, cantilever failure and fluvial scour of the toe, operates over several flood events and has important implications for river engineering, channel changes, and the movement of sediment through fluvial systems.  相似文献   

14.
Severe bank erosion at lowland rivers in Bangladesh devours vast tract of lands and renders thousands of people homeless at high flood; also,rapid deposition changes bed topographies and seriously redu...  相似文献   

15.
Study of hydraulic structures such as groins and bandal-like structures can provide valuable information on their influences on morphological processes in natural rivers.These structures usually used for bank protection and formation of deep navigation channel can locally create complex flow patterns,reduce flow velocities and also increase the flood levels.Most of the previous studies are focused on structures like groins under non-submerged flow condition.However,the recent demand of nature friendly low cost and sustainable methods for river bank protection and channel formation leads to the necessity of study different type of structures like bandal-like structures.In this context, this study investigates the flow characteristics and sediment transport process influenced by bandal-like structures through laboratory experiments.The experiments were carried out under live-bed scour condition with sediment supplied from the inlet for two submergence(non-submerged and submerged)conditions.The experimental measurements contribute to better understand the mechanism of deposition/erosion process around different type of hydraulic structures.The performance of bandal-like structures considering the erosion around the structures,the deposition near the bank and the formation of deep main channel show promising results compared with conventional structures such as groins(impermeable and permeable ones).  相似文献   

16.
三峡工程运用后长江中游荆江河段河床持续冲刷,局部河段崩岸频发,影响河道内悬沙输移与河床形态调整.本研究采用实测长程河道地形及固定断面地形资料,确定了2002-2018年荆江河段的主要崩岸区域,估算了崩岸土体的泥沙总量,进而定量分析了河岸崩退对河床调整的影响.计算结果表明:荆江段累计河岸崩退体积约为2.0亿m3,约占该河...  相似文献   

17.
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   

18.
Field investigations that help clarify local sedimentary processes involved in the migration of alternate bars as a consequence of flood events are lacking. A simple approach combining scour chains, stratigraphy and frequent bathymetric surveys is proposed to connect the dynamics of free migrating alternate bars present in disconnected channels of large sandy‐gravelly rivers with their sedimentary products and vice versa. The results show that the spatial distribution of bars before a flood partly governs the scour and fill processes and that the sediment transport rates vary significantly on a single cross‐section. This can be due to preferential axes of the migration of the bars determined by their location on the cross‐section, the bank direction and the discharge. The approach allows the reconstruction of local sedimentary processes involved in alternate bar migration by combining maximum scour depths reached during a flood with frequent channel bed topography surveys and post‐flood stratigraphy. It is also possible to distinguish deposited and preserved sediments compared with sediments by‐passed during the flood. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

19.
《国际泥沙研究》2016,(3):244-250
The equilibrium scour depth at uniform single bridge piers depends on a large number of variables,including the pier horizontal cross-section shape and its alignment angle towards the flow direction.The influence of these variables has been studied by only a few researchers,mostly,on the basis of tests that were far from approaching equilibrium.This experimental study aims at revisiting the influence of piers' shape and alignment on local scouring for length-width ratios smaller than or equal to 4,by increasing the experimental evidence.Fifty five long-duration laboratory tests were run under steady,clear-water flow,close to the threshold for initiation of sediment motion.Five pier shapes were considered:circular,rectangular square-nosed,rectangular round-nosed,oblong,and zero-spacing(packed) pile-groups;the tested skew-angles were 0°,30°,45°,60°,and 90°.It was concluded that i) the shape factor can be taken as 1.0,for rectangular round-nosed and oblong cross-section piers,and as 1.2,for rectangular squarenosed and packed pile-group cross-section piers,ii) the shape factor does not vary significantly with the duration of tests,this way confirming the robustness of the shape factors reported to date,iii) the effect of shape is present at skewed piers although the associated coefficients remain in the narrow range of1.0-1.2,and iv) for length-width ratios smaller than 4,the shape factor is of the same order of magnitude as the skew angle factor and should not be neglected.  相似文献   

20.
In this paper a modelling approach is presented to predict local scour under time varying flow conditions. The approach is validated using experimental data of unsteady scour at bed sills. The model is based on a number of hypotheses concerning the characteristics of the flow hydrograph, the temporal evolution of the scour and the geometry of the scour hole. A key assumption is that, at any time, the scour depth evolves at the same rate as in an equivalent steady flow. The assumption is supported by existing evidence of geometrical affinity and similarity of scour holes formed under different steady hydraulic conditions. Experimental data are presented that show the scour hole development downstream of bed sills due to flood hydrographs follow a predictable pattern. Numerical simulations are performed with the same input parameters used in the experimental tests but with no post‐simulation calibration. Comparison between the experimental and model results indicates good correspondence, especially in the rising limb of the flow hydrograph. This suggests that the underlying assumptions used in the modelling approach are appropriate. In principle, the approach is general and can be applied to a wide range of environments (e.g. bed sills, step‐pool systems) in which scouring at rapid bed elevation changes caused by time varying flows occurs, provided appropriate scaling information is available, and the scour response to steady flow conditions can be estimated. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

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