Bed Scour Research Articles

Bed load transport rates during scouring and armoring processes

Dambreak-induced bed scouring may undermine the foundation of bridge piers and other structures, and that destruction can pose a serious threat. Consequently, this paper aims at exploring the mechanisms of scouring and armoring. Firstly, the incipient velocity for nonuniform sediment particles was studied, and a formula was derived based on the angle of repose of nonuniform sediment. The results showed that the mechanism of incipient motion for sand and fine gravel differed from that for coarse gravel and cobbles. Also, comparison between experimental and field data shows that the results from the proposed formula agree well with those observed for all conditions. Secondly, a birth-death, immigration-emigration Markov process was developed to describe the bed load transport rate associated with scouring and armoring. The comparison between experimental data and computed results shows that our model can predict the bed load transport rate, although there may be some limitations, the chief of which is that there are many variables in the model to be determined through experiment. This makes its application in river engineering inconvenient.

Environmental protection stability of river bed and banks using convex, concave, and linear bed sills

River bed scourings are a major environmental problem for fish and aquatic habitat resources. In this study, to prevent river bed and banks from scouring, different types of bed sills including convex, concave and linear patterns were installed in a movable channel bed in a laboratory flume. The bed sills were tested with nine different arrangements and under different flow conditions. To find the most effective bed sill pattern, the scouring depth was measured downstream of the bed sill for a long experimental duration. The scour depth was measured at the middle and at the end of each experimental test for different ratios of the arch radius to the channel width [r/w]. The experimental results indicated that the convex pattern with r/w=0.35 produced minimum bed scouring depth at the center line whereas the concave pattern with r/w=0.23 produced the minimum scour depth at the wall banks. Therefore, the convex pattern was the most effective configuration for prevention of scouring at the center line of the river while the concave pattern was very effective to prevent scouring at the river banks. These findings can be suggested to be used in practical applications.

Mechanism of rock-bed scour due to impinging jet

Scour of rock bed downstream of high dams is mainly caused by the transient forces as a result of the propagation of fluctuating pressures, which is significantly controlled by the rock fracture-structure. The physical mechanism of rock disintegration and scour pool forming are analysed herein by considering the fracture-structure of rock bed. A two-dimensional discrete fracture network is imitated applying the Monte-Carlo method. The model of transient flow is established along with each discrete fracture to simulate numerically the propagation of fluctuating pressure waves. Applying the method of automatic tracing of rock blocks layer by layer, the mechanism of disorganization of fractured rock, both the transient physical and dynamic characteristics of isolated rocks, and the damage process of the rock bed are discussed. Finally, a stability criterion of rock blocks and the evaluation method of equilibrium scour depth are established.

Review of “River Training and Sediment Management with Submerged Vanes” by A. Jacob Odgaard“River Training and Sediment Management with Submerged Vanes”ASCE Press$75.00

Channel stabilization and restoration efforts have increased dramatically across the nation during recent decades with over $1B spent on these endeavors every year since 1990 Bernhardt et al. 2005 . It is estimated however, that at least 50% of these projects fail O’Neil and Fitch 1992 and others may not perform to original expectations. This is due to the complex physical processes governing interaction of turbulence in the water column with sediments in the stream and bank. Consequently, stream restoration today is more of an art than a science and relies heavily on an analog method that emphasizes a prescribed design approach rather than the application of physically based hydraulic engineering principles to attain performance-based criteria Slate et al. 2007 . The use of in-stream, low-flow structures as channel stabilization measures has become a preferred solution of federal, state, and local governmental agencies Johnson et al. 2002 . These measures have gained acceptance because of their potential to enhance aquatic habitat while directing flow away from the banks and dissipating flow energy Kauffman et al. 1997 . Despite their potential for success, these structures suffer from a lack of proven engineering design criteria, which if available, would certainly reduce the risk of failure, increase cost-effectiveness and expand their use. Odgaard attempts to end this uncertainty for one such structure in his recent publication, River Training and Sediment Management with Submerged Vanes. The author recognizes that while there are several existing publications demonstrating the viability of submerged vanes, readily available design guidelines are still lacking. The book is divided into six chapters. In the first chapter some background information about the vanes, together with possible problems that they can be used to mitigate, is provided. It is pointed out that though the idea was originally proposed in 1947 by Potapov and Pyshkin, systematic laboratory research and development efforts toward the improvement of vane design to render it a more effective flow and sediment control measure didn’t start until about 35 years later Odgaard and Kennedy 1983 . Soon after that field testing followed. The theory and development of submerged vanes is discussed in the second chapter in the context of bank erosion and bed scour prevention, water intake sediment protection, and shoaling prevention. Results from labo-

PREDICTION OF SCOUR DEPTH AROUND BRIDGE PILES USING ARTIFICIAL NEURAL NETWORKS

The safe and economical design of bridge piles requires prediction of the maximum expected depths of scour of the stream bed around them. Scour at bridge piles may be defined as a local lowering in the bed elevation around the piles. A study of the local scour at bridge piles groups was experimentally and mathematically investigated. The case of six piles having the same diameter aligned with the flow direction in two rows altering the separation distance between the centerline of the three piles was established. The aim of this study is the investigation of the preferable separation distance between three piles to reduce scour around them to its minimum value. Artificial Neural Network (ANN) prediction models are more efficient in predictions models once they are trained from examples or patterns. These types of ANN models need large amount of data which should be at hand before thinking to develop such models. In this paper, the capability of ANN model to predict the maximum scour depth around bridge piles is investigated.

Effectiveness of check dams in the control of general transitory bed scouring in semiarid catchment areas (South‐East Spain)

AbstractThe semiarid areas of the Spanish Mediterranean region are environments that are severely threatened by desertification. Hydrological‐forestry restoration is a very useful tool to fight these processes in programmes and projects of basin management. Check dams particularly can control bed erosion in ephemeral channels, among other things. The main aim of this study, then, is to evaluate how much check dams influence the degree of transitory general scouring in dry alluvial channels that function only sporadically and as torrents. Two areas were chosen: the Cárcavo and Torrecilla catchments (River Segura basin), both drained by ephemeral gullies and both having undergone hydrological correction projects during the 1970s. Equations based on grain bed movement commencement criteria were applied for floods of different water depths, bearing in mind bed texture changes and slope adjustment brought about by dam construction. The results are contrasted with the critical velocity values, which in turn depend on the flow depth and the cross‐section area before flooding. Finally, a comparison between real and predicted scouring is established for the cases analysed, including bankfull flow. Especially, the usefulness of Neill's method has been tested in relation to net erosion values measured in the field with successful results.

Sutong Bridge – The Longest Cable-Stayed Bridge in the World

Sutong Bridge is the longest cable-stayed bridge in the world with a main span of 1088 m. In this paper, design and construction concepts are briefly presented. Furthermore, key technologies and innovative achievements mainly on piled foundation bearing capacity analysis, river bed scour protection and monitoring, superstructure wind-resistance study, mid-span closure method as well as long cantilever structure construction control are summarized. (A)

Numerical Modeling of Local Scour below a Piggyback Pipeline in Currents

Local scour below a piggyback pipeline in steady currents is investigated numerically. A piggyback pipeline comprises two pipelines that are arranged in the so-called piggyback configuration with the small pipeline being located directly above the large pipeline. The Reynolds-averaged Navier–Stokes equations and the transport equation for suspended sediment concentration are solved using a finite element method. The bed scour profile is determined through solving sediment mass conservation equation. The numerical model is validated against experimental data available in literature on scour below a single pipeline. Computations are carried out for the diameter ratio [the small pipe diameter (d) to the larger one (D) ] of 0.2 and the gap ( G , between the two pipelines) to the large diameter ratio G∕D ranging from 0.0 to 0.5. It is found that the flow and the scour profiles are influenced significantly by the gap ratio.

Modeling on flash flood disaster induced by bed load

Flash floods result from a complex interaction among hydro-meteorological, hydrological, and hydraulic processes across various spatial and temporal scales. Sichuan Province suffers flash floods frequently owing to mountain weather and topography. A flash flood and gravel bed load transport are two key relative problems in mountain river engineering. Bed materials are often encountered in alternate scouring and deposition in mountain fluvial processes during a flash flood. In this circumstance, CRS-1 bed load numerical model jointly with scale physical model is employed to predict water level and gravel bed scour and deposition for design of flood control dykes and flash flood disaster mitigation. A case study on the mechanism of a flash flood disaster induced by bed load transport for a hydropower station in Sichuan Province is conducted. Finally, suggestions to protect the hydropower station are proposed.

Model Studies for Conceptual Design of Low Flow Weir across Mahaweli Ganga

Water extraction at Meewathura Intake has become difficult or impossible without the assistance of submersible pumps during the dry seasons since the river water level drops below the bottom level of the lowest opening of the Intake. A new intake is also planned to be located on the left bank around 300 m downstream of Meewathura Intake to extract an additional 32,000 m 3 / d a y to expand Kandy South Water Supply Scheme. In order to ensure a total of 41,000 m 3 / d a y from a new intake and from the existing intake, it is envisaged that a weir across the Mahaweli is required to raise the river water level during the dry weather and to extract silt free water. The analysis also shows that weir construction is a better proposal compared to other methods available. Construction of such weir across Mahaweli Ganga would cause flood level rise in the upstream and change the river bed and river bank geometry in the area s u r r o u n d i n g the weir, particularly downstream of weir. To investigate these effects both Mathematical (MIKE 11) and Physical model studies were conducted. Mathematical Model was used to optimize weir dimension and to predict water level rise due to weir construction. Physical Model was used to optimize weir and intake layout, to investigate river bed scouring and bank erosion pattern, and to determine erosion protection measures. The study concludes that modeling techniques could be applied very effectively for such and similar studies.

Erosion of sediments beneath an oscillating cylinder

The depth of erosion of cohesionless sediments underneath horizontal circular cylinders that are oscillated periodically in still water was of primary interest in this study. In this investigation anthracite was used for the sediments. First the formation of vortex ripples was studied by oscillating a box containing anthracite on its own in still water without the cylinder. The geometrical characteristics of the ripples obtained compared well with those of other investigations. In the experimental range studied, the low density of sediments had no explicit influence on the features of the vortex ripples. The box filled with anthracite and the cylinder were oscillated together to perform periodic motion and the depth of erosion beneath the cylinder was measured. For the diameters of cylinder tested, the height of the vortex ripples formed on both sides of the cylinder was comparable with the depth of scour underneath the cylinder. The experimental results are categorised as clear water scour and live bed scour. For both cases, the relative scour depth was weakly dependent on the Shields parameter for Keulegan–Carpenter numbers less than about 30. For Keulegan–Carpenter numbers greater than about 30, it increased with the increasing value of the Shields parameter.

Reliability considerations in bridge pier scouring

The conventional design of bridge piers against scour uses scour equations which involve number of uncertain flow, sediments and structural parameters. The inherent high uncertainties in these parameters suggest that the reliability of piers must be assessed to ensure desirable safety of bridges against scour. In the present study, a procedure for the reliability assessment of bridge piers, installed in main and flood channels, against scour has been presented. To study the influence of various random variables on piers' reliability sensitivity analysis has been carried out. To incorporate the reliability in the evaluation of safety factor, a simplified relationship between safety factor and reliability index has been proposed. Effects of clear water (flood channel) and live bed scour (main channel) are highlighted on pier reliability. In addition to these, an attempt has also been made to explain the failure of Black mount bridge of New Zealand based on its pier's reliability analysis. Some parametric studies have also been included to obtain the results of practical interest.

In situ jet-testing of the erosional resistance of cohesive streambeds

Fletcher’s Creek is located in an urbanizing basin near Toronto and has a bed and banks composed primarily of cohesive Halton Till. Critical shear stress and an erodibility coefficient for the till were determined using an in situ jet-tester that directs a submerged jet of water perpendicular to the sediment surface. The results from 10 jet-tests indicate that the till has a relatively low critical shear stress and relatively high erodibility coefficient and could be susceptible to bed scour during flood events. Many other streams in southern Ontario have urbanizing watersheds with cohesive till beds that may also be susceptible to erosion.Key words: critical stress, submerged jet, erodibility, cohesive soils.

Influence of the macrotidal environment on the source to sink pathways of suspended flux in the Gulf of Kachchh, India: evidence from the Ocean Colour Monitor (IRS‐P4)

A series of tide‐synchronous satellite images, obtained from the Ocean Colour Monitor (OCM) onboard the Indian Remote Sensing Satellite (IRS‐P4), in situ measurements and hydrodynamic model simulations were used to describe dispersal pathways and sources of total suspended matter (TSM) in the Gulf of Kachchh, a macrotidal system with insignificant freshwater inputs. Strong alongshore currents are prevalent at the mouth that move in (out) clockwise during flood (ebb) and undergo cyclic, dynamic changes with the tidal phases. The Gulf, unlike other regions of the Indian coastline, has dynamic turbidity that is zonal (very high in the outer Gulf and in the creeks of the Gulf), particularly during flooding, and reduced in the central region throughout the entire tidal cycle. From this dynamic turbidity with regard to the prevalent currents, it is deduced that inland amplifying flood currents draw in TSM‐rich offshore waters into the Gulf, and are the major source of turbidity in the outer regions of the Gulf. In the inner Gulf, because of these high‐magnitude currents, scouring of the creek bed leads to resuspension to create higher turbidity. Most of the scoured flux sinks off the mouths of these creeks because of the fanning out of ebbing flows from the narrow creeks into the wider Gulf. The areas under specific influence of different sources of turbidity are delineated schematically.

Variations des efforts hydrodynamiques sur radiers horizontal et affouillé en aval d'un dissipateur d'énergie de type auge

The distribution of the hydrodynamic efforts under solid horizontal or scoured bed downstream of a roller bucket spillway can be represented by two typical distributions: type A and B for pressure and type I and II for pressure fluctuation. However, the scoured bed presents a pressure distribution that is closer to the equilibrium pressure compared with that of the horizontal bed and a much lower pressure fluctuation distribution. This could be due to the bed scour in the vicinity of the bucket that carries an effective value for the height of the bucket lip, which decreases the suction effect, and the fact that the eroded shape of the bed corresponds with reduced pressure efforts, which translates into weaker turbulence and therefore a more stable flow. These results show the advantage of a scoured bed that could be more secure when the bucket lip height of the roller bucket spillway is well chosen.Key words: hydrodynamic efforts, pressure, pressure fluctuation, hydraulic energy dissipation, roller bucket.

Dispersion of channel-sediment contaminants in distributary fluvial systems: Application to fluvial tephra and radionuclide redistribution following a potential volcanic eruption at Yucca Mountain

Predicting the fluvial transport and mixing of channel-sediment contaminants is necessary for assessing and mitigating heavy-metal and nuclear-waste contamination in rivers. The dilution–mixing model is widely used for this purpose in tributary channel systems that transport contaminants as bed-material load without significant overbank sedimentation. Here a more general, three-dimensional (3D) contaminant transport numerical model is developed and tested based on bed scour, turbulent mixing of contaminant material with uncontaminated channel-bed sediments, and re-deposition of the mixture by the cumulative effect of many flood events. First, the model is applied to a synthetic alluvial-fan environment downstream from a localized contaminant source in order to illustrate the model behavior. Second, the model is validated against measured tephra concentrations in channels downstream from the Lathrop Wells scoria cone volcano, a localized source of basaltic tephra to downstream channels otherwise comprised of non-basaltic sediments. Third, the model is applied to the problem of predicting the concentration of radionuclide-bound tephra in channels downstream from the proposed nuclear-waste repository at Yucca Mountain, Nevada, in the event of a volcanic eruption through the repository. Contaminated tephra is mobilized from the landscape in this model using threshold criteria for hillslope gradient and channel stream power. Mobilized contaminated tephra is mixed with uncontaminated channel-bed sediments using the contaminant transport model and deposited in channels of the Fortymile Wash alluvial fan where the residents nearest to the proposed repository live. The results of twenty Monte Carlo simulations of eruption fallout and post-eruption redistribution corresponding to a range of wind conditions and eruption magnitudes provide information on the mean and variability of contaminated tephra concentrations to be expected in channels of the Fortymile Wash alluvial fan in the event of an eruption. Mean tephra concentrations are approximately 1% but vary from nearly zero to as high as 26%, reflecting the combined effects of wind direction, eruption magnitude, and dilution of tephra with uncontaminated channel-bed sediments during transport.

Sediment transport and channel adjustments associated with dam removal: Field observations

This study documents changes in channel geometry, bed level profile, and bed grain size distribution and their relations with the sediment transport at the reach scale, following the removal of a low‐head dam. After the removal, net sediment deposition occurred downstream of the dam, and net erosion occurred in the reservoir, but approximately less than 1% of the sediment stored in the reservoir was transported downstream. No bank erosion was evident either upstream or downstream of the dam. Bed deposition and scouring in the reservoir accounted for a decrease in the bed slope of 30%. The stations downstream of the dam had surface bed material sizes at least 40% finer than preremoval conditions. However, the sediment transport rates downstream of the dam were not significantly different from predam to postdam removal or from an upstream control. Overall, the removal of the dam had only minor effects on the channel adjustment downstream of the dam. A simple analysis linking transport to channel geometry explains this effect.

Modeling and calculation of bed score around large-diameter vertical cylinder under wave action

The results of studying the scour of bed composed of fine and coarse sand around models of large-diameter vertical cylinder under the effect of regular waves are presented. Comparison of the results of experiments with fine and coarse sand showed essential qualitative and quantitative distinctions between the two cases. The results of experiments with coarse sand were used to derive, for the first time, a formula for the estimation of the score depth, including two similarity criteria—the sediment motion factor (SM-factor), and Keulegan-Carpenter number. This formula is applicable to both the engineering analysis of the scour depth of gravel bed or protective riprap by crashed rock or fine stone and the approximate prediction of the scour depth of sand bed around structures under real storm conditions. The conditions for correct modeling of wave scour around structures with large horizontal dimensions are considered and the irrelevance of modeling such scour with the use of fine-sand models is demonstrated.

Effects of floods on fish assemblages in an intermittent prairie stream

Summary1. Floods are major disturbances to stream ecosystems that can kill or displace organisms and modify habitats. Many studies have reported changes in fish assemblages after a single flood, but few studies have evaluated the importance of timing and intensity of floods on long‐term fish assemblage dynamics.2. We used a 10‐year dataset to evaluate the effects of floods on fishes in Kings Creek, an intermittent prairie stream in north‐eastern, Kansas, U.S.A. Samples were collected seasonally at two perennial headwater sites (1995–2005) and one perennial downstream flowing site (1997–2005) allowing us to evaluate the effects of floods at different locations within a watershed. In addition, four surveys during 2003 and 2004 sampled 3–5 km of stream between the long‐term study sites to evaluate the use of intermittent reaches of this stream.3. Because of higher discharge and bed scouring at the downstream site, we predicted that the fish assemblage would have lowered species richness and abundance following floods. In contrast, we expected increased species richness and abundance at headwater sites because floods increase stream connectivity and create the potential for colonisation from downstream reaches.4. Akaike Information Criteria (AIC) was used to select among candidate regression models that predicted species richness and abundance based on Julian date, time since floods, season and physical habitat at each site. At the downstream site, AIC weightings suggested Julian date was the best predictor of fish assemblage structure, but no model explained >16% of the variation in species richness or community structure. Variation explained by Julian date was primarily attributed to a long‐term pattern of declining abundance of common species. At the headwater sites, there was not a single candidate model selected to predict total species abundance and assemblage structure. AIC weightings suggested variation in assemblage structure was associated with either Julian date or local habitat characteristics.5. Fishes rapidly colonised isolated or dry habitats following floods. This was evidenced by the occurrence of fishes in intermittent reaches and the positive association between maximum daily discharge and colonisation events at both headwater sites.6. Our study suggests floods allow dispersal into intermittent habitats with little or no downstream displacement of fishes. Movement of fishes among habitats during flooding highlights the importance of maintaining connectivity of stream networks of low to medium order prairie streams.

Submerged and unsubmerged natural hydraulic jumps in a bedrock step-pool mountain channel

High-resolution digital elevation models (DEMs) for natural submerged and unsubmerged jump regions were tested against the classical hydraulic jump (CHJ) and engineering analogues that have dominated previous geomorphic and engineering research. DEMs were compared for two discharge conditions. The data showed bed and water surface features of the natural jump regions differ significantly from CHJ conditions and engineering analogues with respect to boundary conditions. Roughness elements were highly irregular in frequency, spacing, size, and orientation, and were comprised of form obliquity and bed scour in boulder-bed and pure-bedrock conditions. The data also showed bed and water surface features of natural jump regions have similarities and deviations from CHJ conditions and engineering analogues with respect to hydraulic characteristics. Several variations were observed in the hydraulic characteristics between the ballistic and sloping jets. Revised conceptual models based on idealized corollaries were developed for the natural jump regions based on upstream energy head relative to the downstream tailwater depth and step geometry. The DEMs highlight the importance of recognizing the interrelationship between spatial transcritical flow structures and localized topographic heterogeneities in bedrock channels.