Live-bed Conditions Research Articles

Live-bed scour experiments with 45° wing-wall abutments

Live bed scour experiments were conducted using four types of non-cohesive bed sediments with median diameters d 50 = 0.26 mm, 0.42 mm, 1.06 mm and 1.92 mm and five 45°wing-wall abutments of lengths l = 0.04 m, 0.06 m, 0.08 m, 0.10 m and 0.12 m. The tests were conducted in the range of flow velocity varies from 0.71 to 5 times of the critical velocity. The plotting of non-dimensional scour depths (scour depths/abutment lengths) vs non-dimensional flow intensity (flow intensity/critical velocity) show one peak value of scour depth close to the threshold velocity and thereafter, it reduces and again attain a live-bed maximum in the range of flow intensity 3 to 5 times of the critical velocity. It is also observed that the scour depth increases with the increase in sediment sizes up to threshold value of flow for all sediment sizes and abutment lengths. Further, the scour depth decreases with the increase of non-uniformity under all flow conditions. A design equation is proposed for estimating maximum scour depth at 45° wing-wall abutment under live-bed condition. The calculated values of scour depths using proposed equation, and also with three different live-bed local scour equations, are compared with the measured values of scour depths.

Simulation-Based Approach for Stream Restoration Structure Design: Model Development and Validation

We develop, validate, and demonstrate the potential of Virtual StreamLab (VSL3D), a novel three-dimensional hydromorphodynamics computational model capable of simulating turbulent flow and sediment transport in natural waterways with embedded and arbitrarily complex hydraulic structures under live-bed conditions. The numerical model is based on the curvilinear immersed boundary (CURVIB) approach and can solve the unsteady Reynolds-averaged Navier-Stokes (URANS) equations closed with the k−ω turbulence model in arbitrarily complex waterways with mobile sediment beds. Bed material transport is simulated by solving the nonequilibrium Exner equation for the bed surface elevation coupled with a transport equation for suspended load. Field-scale measurements obtained from experiments carried out in the St. Anthony Falls Laboratory Outdoor StreamLab are employed to validate the predictive capabilities of the numerical model. The VSL3D is used to develop a virtual testing environment of unprecedented resolution and physical realism for designing and optimizing the arrangements and installation of stream restoration rock structures. To our knowledge, the present work is the first systematic attempt to employ unsteady, three-dimensional, hydro-morphodynamics numerical modeling coupled with high-performance computing to develop an engineering framework for designing hydraulic structures for stream restoration applications.

Local Scour around a Model Hydrokinetic Turbine in an Erodible Channel

Laboratory experiments were performed to study the effect of an axial-flow hydrokinetic turbine model on an erodible channel under both clear water and live-bed conditions. Clear water experiments were performed at two scales with a local bed shear stress just below the critical state. Live-bed experiments, performed at small scale, examined the interactions between relatively large-scale bedforms and the flow induced by an axial flow turbine. Spatiotemporal topographic measurements were obtained by sonar and by a state-of-the-art high-resolution scanning system integrated into an automated data acquisition carriage designed and fabricated at the St. Anthony Falls Laboratory at the Univ. of Minnesota. Results indicate that the presence of the turbine rotor increases the local shear stress resulting in accelerated and expanded scour development when compared with typical bridge pier scour mechanisms. The inferred key difference is the alteration of the flow patterns in the rotor wake leading to an accelerated flow region below the bottom tip. The footprint of the rotor is observed in the extension and scaling of the bed surface area impacted by the turbine and consistent with the near-wake region. Temporally averaged bed topography data from live-bed experiments indicate amplified scour depths in the turbine near-wake region as compared with the clear water results despite spatial patterns remaining qualitatively similar.

Time Development of Scour around a Cylinder in Simulated Tidal Currents

A laboratory flume experiment was performed to investigate the time development of scour around a vertical cylinder acting as a scaled model of an offshore wind turbine monopile in tidal currents. The tidal current was simulated by resolving each half-cycle into three time steps, between which flow velocity and depth were varied. Flow direction was reversed between half-cycles, which were otherwise identical. Between them, the three time steps exhibited clear water, transitional, and live-bed conditions. The experiment was run over two full-simulated tidal cycles. The scour hole formed tended to a symmetrical shape after two half-cycles and was both shallower and slower developing than the scour hole in a unidirectional current test carried out in the same flume. This was due mainly to the variable rates of scour caused by the variable flow conditions within each half-cycle, and to a lesser extent to the infilling of the scour hole when the current direction reversed. The lower scour depth recorded in tidal conditions implies that the amount of scour protection required may be less than previous studies suggest.

Scour at Various Hydraulic Structures: Sluice Gates, Submerged Bridges and Low Weirs

Prediction of likely scour depths is an important aspect of the design of hydraulic structures. While detailed guidance is available for the scour design of some structures, eg. bridge foundations, scant information is available for many other structures. Recent, as yet unpublished, research findings for a number of fluvial structure types are presented herein. The research findings span scour downstream from sluice gates, scour in the vicinity of low weirs, together with a particular aspect of scour at bridge foundations that has previously received minimal attention. In each case, new data are presented, together with analyses of the data and simple (preliminary) design relationships are determined from the data. A feature common to bridges and low weirs is the form of the dependence of scour depth on flow intensity for clear-water and live-bed scour conditions. The new data are derived from small-scale laboratory experiments for non-cohesive (alluvial) bed materials; the research findings are limited accordingly.

Wave-Induced Scour at Cylindrical Piles

The dynamics of wave-induced scour at the base of vertical piles are not well defined for some realistic combinations of wave-induced flow and pile diameter. Scour development at marine structures depends strongly on the Keulegan–Carpenter (KC) number, a ratio of flow to structure length scale. Observations and descriptions of scour development in the range 1 < KC < 6 are mostly absent from existing literature. Even though there is little to no vorticity for KC < 6 at a cylindrical pile, a continuum of sediment transport initiated by steady streaming of fluid must exist. Analysis of wave-induced flow and pile characteristics representative of field conditions reveal that sediment mobilization—also referred to as live bed conditions—occurs in the range 1 < KC < 6, even though sediment mobilization is not evident in most laboratory experiments. A comprehensive evaluation of published field and laboratory data for equilibrium wave scour depth at the base of cylindrical piles revealed a marked discontinuity in scour development over the range 1 < KC < 6 that might have been the result of laboratory scaling or capabilities. A new empirical model for estimating wave-induced scour at the base of cylindrical piles was derived by using available laboratory data in such a way as to minimize error over a broad range of flow and pile characteristics. The new equation reduces error by 31% compared with an existing predictive equation and, more important, estimates the equilibrium wave scour depth over a transitional range of KC values not considered previously.

LIVE BED SCOUR AT A SIDE‐WEIR INTERSECTION LOCATED ON AN ALLUVIAL CHANNEL

ABSTRACTThis study aims to obtain both the depth of clear water and live bed scour around side‐weirs located on an alluvial channel for sharp‐crested side‐weirs under subcritical flow conditions. At the downstream end of the side‐weir, a longitudinal bar in the middle of the channel and a scour hole close to the outer bank are formed because of changes in shear stress field and velocities. Dimensionless depth of scour increases with an increase of the dimensionless parameters of flow depth, side‐weir length and sediment size. The dimensionless depth of scour increases almost linearly with the dimensionless parameter of flow velocity for clear water scour, and it decreases for live bed conditions at the straight channel due to the decreasing deviation angle and kinetic energy toward the side‐weirs and bed material coming from upstream of the main channel. Because of the effect of the helicoidal flow created by bends, the dimensionless depth of scour fluctuated with the dimensionless parameter of flow velocity at the maximum value of clear water scour for movable bed conditions at a bend. At the 30° section of a bend, the equilibrium depth of scour is achieved as maximum values due to the deviation on the path of maximum velocity towards the outer bank. Copyright © 2013 John Wiley & Sons, Ltd.

Abutment scour in clear-water and live-bed conditions by GMDH network

In the present study, the Group Method of Data Handling (GMDH) network has been utilized to predict abutments scour depth for both clear-water and live-bed conditions. The GMDH network was developed using a Back Propagation algorithm (BP). Input parameters that were considered as effective variables on abutment scour depth included properties of sediment size, geometry of bridge abutments, and properties of approaching flow. Training and testing performances of the GMDH network were carried out using dimensionless parameters that were collected from the literature. The testing results were compared with those obtained using the Support Vector Machines (SVM) model and the traditional equations. The GMDH network predicted the abutment scour depth with lower error (RMSE (root mean square error) = 0.29 and MAPE (mean absolute percentage of error) = 0.99) and higher (R = 0.98) accuracy than those performed using the SVM model and the traditional equations.

Sensitivity analysis of bridge pier scour depth predictive formulae

Sensitivity analysis is an approach to recognising the behaviour of models and relative importance of causative factors. In this paper, behaviours of six pier scour depth empirical formulae are evaluated on the basis of an analytical method. The sensitivity of predicted scour depth is analysed with respect to the following independent parameters: approach flow depth, riverbed slope and median sediment size. Also their combined influence is studied examining the relative importance of each parameter with respect to the total variation of the maximum scour depth. Results show that: (1) sensitivity significantly depends on flow intensity for most of the selected formulae, whereas for the others it is a constant value or depends on other influencing parameters; (2) different formulae demonstrate various level of sensitivity to the input variables, so that, for a certain error in the input variables, the error in the results may vary consistently; (3) some formulae are very sensitive to the input parameters under some conditions, hence an error in an input variable may be amplified in the output results; and (4) most of the formulae are more sensitive to the variations of the influencing parameters in clear-water than in live-bed conditions.

Prediction of pipeline scour depth in clear-water and live-bed conditions using group method of data handling

In the present study, the Group method of data handling (GMDH) network was utilized to predict the scour depth below pipelines. GMDH network was developed using back propagation. Input parameters that were considered as effective parameters on the scour depth included those of sediment size, geometry of pipeline, and approaching flow characteristics. Training and testing performances of the GMDH networks have been carried out using nondimensional data sets that were collected from the literature. These data sets are related to the two main situations of pipelines scour experiments namely clear-water and live-bed conditions. The testing results of performances were compared with the support vector machines (SVM) and existing empirical equations. The GMDH network indicated that using of back propagation produced lower error of scour depth prediction than those obtained using the SVM and empirical equations. Also, the effects of many input parameters on the scour depth have been investigated.

Live-bed scour downstream of block ramps for low densimetric Froude numbers

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.

Prediction of local scour depth at bridge piers under clear-water and live-bed conditions: comparison of literature formulae and artificial neural networks

The scouring effect of the flowing water around bridge piers may undermine the stability of the structure, leading to extremely high direct and indirect costs and, in extreme cases, the loss of human lives. The use of Artificial Neural Network (ANN) models has been recently proposed in the literature for estimating the maximum scour depth around bridge piers: this study aims at further investigating the potentiality of the ANN approach and, in particular, at analysing the influence of the experimental setting (laboratory or field data) and of the sediment transport mode (clear water or live bed) on the prediction performances. A large database of both field and laboratory observations has been collected from the literature for predicting the maximum local scour depth as a function of a parsimonious set of variables characterizing the flow, the sediments and the pier. Neural networks with an increasing degree of specialization have been implemented – using different subsets of the calibration data in the training phase – and validated over an external validation dataset. The results confirm that the ANN scour depths' predictions outperform the estimates obtained by empirical formulae conventionally used in the literature and in the current engineering practice, and demonstrate the importance of taking into account the differences in the type of available data – laboratory or field data – and the sediment transport mode – clear water or live bed conditions.

Prediction of wave-induced scour depth under submarine pipelines using machine learning approach

The scour around submarine pipelines may influence their stability; therefore scour prediction is a very important issue in submarine pipeline design. Several investigations have been conducted to develop a relationship between wave-induced scour depth under pipelines and the governing parameters. However, existing formulas do not always yield accurate results due to the complexity of the scour phenomenon. Recently, machine learning approaches such as Artificial Neural Networks (ANNs) have been used to increase the accuracy of the scour depth prediction. Nevertheless, they are not as transparent and easy to use as conventional formulas. In this study, the wave-induced scour was studied in both clear water and live bed conditions using the M5’ model tree as a novel soft computing method. The M5’ model is more transparent and can provide understandable formulas. To develop the models, several dimensionless parameter, such as gap to diameter ratio, Keulegan–Carpenter number and Shields number were used. The results show that the M5’ models increase the accuracy of the scour prediction and that the Shields number is very important in the clear water condition. Overall, the results illustrate that the developed formulas could serve as a valuable tool for the prediction of wave-induced scour depth under both live bed and clear water conditions.

Expanding pools morphology in live-bed conditions

Sediment load plays a fundamental role in natural river morphology evolution. Therefore, the correct assessment of the role of the sediment load on natural or anthropic pools morphology downstream of river grade control structures, such as rock chute or block ramps, is of fundamental interest for preserving the fish habitat and the river morphology. This work presents an experimental study on the sediment load influence on rectangular expanding pools downstream of block ramps in live-bed conditions. Several longitudinal and transversal expanding ratios have been tested. Ramp slopes were varied between 0.083 and 0.25. The effect of the pool geometry and the sediment load on hydraulic jump downstream of block ramp as well as scour morphologies and flow patterns have been analyzed. Equations were derived to evaluate the maximum scour hole depth, the longitudinal distance of the section in which it occurs, and the maximum water elevations both in the pool and in the downstream contraction.

Hydraulic Evaluation of W-Weir for River Restoration

Various structural measures have been advocated for river restoration and habitat improvement schemes. The W-weir is one such structure that can be used in mobile bed alluvial rivers to diversify habitat and provide grade control. Laboratory studies have been carried out in a large-scale meandering channel with a mobile bed to investigate their effects on flow and sediment transport processes. A W-weir placed immediately downstream of a riffle section created a strongly three-dimensional flow pattern and high-turbulence zones. Two adjacent scour holes of different depths and substrate are formed under clearwater and live bed conditions. The continuity of sediment transport along the channel was not interrupted by the structure and the upstream afflux is minimal. Overbank flow significantly influenced the action of the weir and the scour hole was shifted closer to the structure. In a relatively tight bend followed by a short crossover reach, the weir may affect bed load transport pathways in the downstream bend. Finally, the study provides insights to guide their design for restoration projects.

Geobag Performance as Scour Countermeasure for Bridge Abutments

This paper presents observations and data from a sequence of laboratory experiments conducted to evaluate geobags as a countermeasure to protect bridge-abutment foundations from failure attributable to scour of the alluvial-river channel in which they are placed. Geobags comprise geotextile cloth bags filled with local sediment or concrete. The experiments focused on the performance of geobags placed as an apron around pile-supported wing-wall abutments retaining erodible embankments, and subject to live-bed and clear-water flow conditions. Though an apron of geobags is shown to substantially reduce or eliminate scour immediately at the abutment, the apron must be formed flexibly of linked geobags. Moreover, a performance concern is that the apron may shift scour to a location flanking or downstream of the apron, and in so doing imperil a nearby pier or riverbank. The experiments indicate the importance of protecting the embankment region beneath and immediately behind the abutment’s pile cap. Live-bed conditions proved to be the more critical for abutment protection, owing to the capacity of dunes to destabilize geobags around the edges of the apron. Design guidelines are given and include using current riprap configurations for sizing and placing geobags.

Scour Around Spherical Bodies and Self-Burial

This paper summarizes the results of an experimental study on scour around spherical bodies and self-burial in steady current and in waves. The equilibrium scour depth below a fixed sphere in steady current for live-bed conditions was found to be S∕D=O(0.3), D being the sphere diameter. The effect of the Shields parameter, θ, and that of the position of the sphere with respect to the bed, e, were investigated. It was found that, under live-bed conditions, the effect of θ is quite weak. It was also found that the effect of e is significant. No scour occurs when e∕D>0.8. It was demonstrated that, in the case of waves, the Keulegan-Carpenter number, KC, is the main parameter that governs the equilibrium scour depth. S∕D is always smaller in waves than in steady currents. Regarding the self-burial, the equilibrium self-burial depth in steady current was found to be e∕D=O(0.5). The self-burial depth in waves is governed by KC, similar to the scour depth for a fixed sphere. e∕D is always smaller in waves than in steady currents. The experiments indicated that the time scale of both the scour process and the self-burial process is a function of θ in the case of steady current, and θ and KC in the case of waves.

Closure to “Design Method of Time-Dependent Local Scour at Circular Bridge Pier” by Md. Faruque Mia and Hiroshi Nago

Pier scour has received considerable attention in the past 20 years. In this closure article, the original researchers (Mia and Nago) respond to the discusser's contention that their original work on bridge pier scour used as main sources research that dates back to more than 10 years while overlooking work that was published recently by two of the discussers (Hager and Oliveto, 2002). In this closure, the authors note that Hager and Oliveto's 2002 research was not available in time to be considered as part of the Mia and Nago research. The authors also consider the differences between their work on clear-water conditions versus the discusser's work for live bed condition. Other topics covered include the exact condition for particle initiation at a bridge pier, the conditional influence of the constriction rate (determined by pier diameter and flume width), and reverse eddies from the wall boundary. The authors also make 2 minor corrections to their original paper.

Protection of bridge piers using a sacrificial sill

This experimental study explores the effectiveness of using a sacrificial sill as a pier scour countermeasure. Experiments were conducted under both clear-water and live-bed conditions. Many interdependent parameters affecting the sill performance were systematically examined. The results show that the sill is efficient in reducing the equilibrium clear-water scour depth, reaching a maximum percentage reduction of pier scour depth, %RDP, of approximately 50% under the most favourable condition. The optimum state occurs under the following conditions: (a) the sill is vertical; (b) the sill height is larger than 0·3 times the depth of flow; (c) the clear distance between the pier and sill is 4·4 times the pier width; and (d) the angle of attack is zero. A large angle of attack or flow skewness is detrimental to the performance of the sill. However, this shortcoming may be overcome by installing a sill with a wedge angle of 270°. The efficiency of the sill decreases under live-bed conditions, with the %RDP reducing to approximately 20% at a velocity ratio, U/Uc = 2·6.

Protection of bridge piers using a sacrificial sill

This experimental study explores the effectiveness of using a sacrificial sill as a pier scour countermeasure. Experiments were conducted under both clear-water and live-bed conditions. Many interdependent parameters affecting the sill performance were systematically examined. The results show that the sill is efficient in reducing the equilibrium clear-water scour depth, reaching a maximum percentage reduction of pier scour depth, %RDP, of approximately 50% under the most favourable condition. The optimum state occurs under the following conditions: (a) the sill is vertical; (b) the sill height is larger than 0·3 times the depth of flow; (c) the clear distance between the pier and sill is 4·4 times the pier width; and (d) the angle of attack is zero. A large angle of attack or flow skewness is detrimental to the performance of the sill. However, this shortcoming may be overcome by installing a sill with a wedge angle of 270°. The efficiency of the sill decreases under live-bed conditions, with the %RDP reducing to approximately 20% at a velocity ratio, U/Uc = 2·6.