Clear-water Scour Conditions Research Articles

Effects on Scour for Different Foundation Geometry of Compound Circular Bridge Piers

This experimental study examines the variation of scour depth with time of Clearwater scour condition around compound circular bridge piers for steady flow conditions. Most of the circular bridge piers are resting on the bigger diameter caissons known as the compound circular bridge piers and are widely used in India for construction of road and railways bridge across the rivers. In past studies, it has been observed that most bridge failure occurs because of scouring due to flowing water around a bridge pier across a river. Most of the past studies were done on the uniform bridge pier and a very few studies have been done so far on scouring around non-uniform bridge piers. Estimation of scour depth is required for the economical and a sound design of bridge pier foundation. In present study, an experimental investigation has been done in a tilting flume for computation of rate of change of depth of scour with time at two different models of compound circular bridge piers by varying the foundation top position with respect to level of bed, i.e., 1. The foundation top at the level of bed, and 2. The foundation top below the level of bed (viz. 10mm, 20mm, 30mm and 40mm) for uniform sediments.

Local Scour of Armor Layer Processes around the Circular Pier in Non-Uniform Gravel Bed

Flume experiments have been carried out under clear water scour conditions to analyze the maximum equilibrium scour depth and scour processes in armored streambeds. A total of 85 experiments have been carried out using different diameters of circular piers and non-uniform gravels. A graphical approach for dimensionless scour depth in equilibrium condition around the circular pier in armored streambeds has been developed. As per this curve, the maximum dimensionless scour depth variation with dimensionless armor particle size depends on the densimetric particle Froude number (Frd50), and the decreasing rate of dimensionless scour depth decreases with the value of Frd50.

The 3-D Morphology Evolution of Spur Dike Scour under Clear-Water Scour Conditions

By changing the alignment angle of spur dike, this study focused on the evolution of scour hole morphology in three alignments under clear-water scour conditions, including the 3-D structure of the scour hole, 2-D profile morphological evolution process and the evolution characteristics of the local bed shear stress. The results show that the plane area and volume of the scour hole both are power functions over time, which is similar to the evolution characteristics of scour depth. Local scour includes three stages: The initial stage, development stage and balance stage. The local bed shear stress presents successively: τb > τc, τb = τc and τb < τc. Based on this characteristic, the evolution mechanism between scour hole morphology and the local bed shear stress is further clarified. Furthermore, although the alignment of the spur dike significantly affects the longitudinal and vertical dimension erosion rates of the scour hole, the scour hole morphology is not only relatively constant but also has a specific proportion, and the evolution process is orderly in the whole process of evolution. To the scouring equilibrium state, the length of the scour hole on the upstream and downstream of the spur dike is approximately in line with the golden section feature. The related results provide technical support for scour parameter design and scour hole protection of spur dike in flood period.

Protecting bridge piers against local scour using a flow-diversion structure

Previous studies indicate that local scour is a leading cause of most waterway bridge failures during flood events. Using armouring countermeasures such as riprap and gabions is a conventional way of reducing scour around bridge piers, but is very costly and time consuming. As an alternative, a flow-diversion structure is proposed that has a triangular prismatic shape with dimensions much smaller than the actual pier and should be installed upstream of the pier. To assess its performance, experiments were conducted under clear-water scour conditions. After achieving equilibrium bed conditions, the bed profile was measured and the maximum scour depth and volume of the scour hole were determined for each experimental test. The results indicated that the clear distance between the pier and the countermeasure to achieve the maximum reduction in local scour was 1·5 times the pier diameter. For this condition, the proposed countermeasure reduced the maximum scour depth by 38% and the volume of the scour hole was decreased by around 61%. To determine the influence of the countermeasure on flow field around the pier, three-dimensional velocity components were measured at grid points using a micro-acoustic Doppler velocity meter. Analysis of the results indicated that the proposed structure could change both the magnitude and direction of the velocity components upstream of the pier and consequently induce a significant reduction in local scour depth and volume around the pier.

Geometric Characteristics of Spur Dike Scour under Clear-Water Scour Conditions

Factors such as flow and sediment characteristics affecting the spur dike’s local depth of erosion have yielded considerable research results, but there is little discussion of the geometry of the spur dike’s local scour holes. This study focuses on the spatial characteristics of the geometry of local scour holes in straight-wall spur dikes. The discussion shows that the spur dike arrangement clearly changes the plane geometry of the scour hole. The maximum scour depth has a power function relationship with the area of the scour hole and the scour hole-volume. Moreover, the ratio of the product of the maximum scour depth and the plane area of the scour hole to the scour hole-volume is a fixed constant. The average slope of upstream of the scour hole and along the axis direction of the spur dike is slightly larger than the angle of repose of the sediment, the slope distribution of the scour holes profiles presents an inverted “U” type, and its profile morphology and slope distribution have geometric similarity. This distribution also reflected that, the interaction between the downward flow and the horseshoe vortex inside the scour hole leads to the formation of a cusp.

Estimation of maximum scour depths at upstream of front and rear piers for two in-line circular columns

Previous investigations indicate that scour around bridge piers is one of the most important factors for the failure of waterway bridges. Hence, it is essential to determine the accurate scour depth around the bridge piers. Most of the previous studies were based on scour around a single pier; however, in practice, new bridges are usually wide and then piers comprise two circular piers aligned in the flow direction that together support the loading of the structure. In this study, the effect on maximum scour depth of the spacing between two piers aligned in the flow direction was investigated experimentally under clear water scour conditions. The results show that the maximum scour depth at upstream of the front pier occurs when the spacing between the two piers is 2.5 times the diameter of the pier. Two semi empirical equations have been developed to predict the maximum scour depth at upstream of both front and rear piers as a function of the spacing between the piers, in terms of a pier-spacing factor. If the new equations for the pier-spacing factor are used with some of the existing equations for scour at a single pier, the predicted scouring depths are in good agreement with observed results. The S/M equation exhibited the best performance among the various equations tested and was recommended for use in prediction of the equilibrium scour depth. The findings of this study can be used to facilitate the positioning of piers when scouring is a design concern.

Experimental investigation of clear-water temporal scour variation around bridge pier in gravel

Variation of temporal scour depth around circular piers under clear-water scour condition was studied experimentally for a variability of configuration studies, including different size of circular piers. A total of 96 experiments were carried out with more than 800 data sets of temporal scour depth in non-uniform gravel bed. The experimental data were used to calculate the temporal scour depth at different planes i.e. 0°, 90°, 180° and 270°, 0° plane indicates the maximum and 180° plane indicates minimum scour zones. Some of the previous methods of estimating temporal scour depth at 0° plane are studied in this study. Three temporal scour depths equations for 0°, 90°, 270° and 180° planes have been proposed in this study. The predictions of the temporal scour depth improved after the revised methods of Kothyari et al. (J Hydraul Eng 133(11):1229–1240, 2007). Results of the present proposed equation for 0° plane were compared with latest available relationships in literature and found in good agreements with observed values.

Quantification of spatial lag effect on sediment transport around a hydraulic structure using Eulerian–Lagrangian model

The spatial lag effect on sediment transport around a hydraulic structure is quantified through numerical simulation using a Eulerian–Lagrangian model. The Eulerian–Lagrangian model uses a deterministic approach of bed load motion and stochastic approach of Einstein's concept, which allows for a reasonable quantification of both non-equilibrium and equilibrium bed load transport rates. Numerical simulations are conducted for local scour around a spur dyke and a weir-type structure. As pointed out in previous experimental studies on the one-dimensional problem, Eulerian–Lagrangian simulation reveals an unsaturated bed load in a scour hole around a hydraulic structure. Furthermore, an oversaturated bed load is also found in the deposition region, particularly behind the structure. The validity of the linear adaptation equation is discussed using numerical results, revealing the possibility that the linear adaptation equation does not appropriately represent the existence of the strong spatial lag effect. The present study finds there are two modes in the response of the adaptation length to the unsaturated bed load, which depend on the clear-water scour condition or the live-bed scour condition. Findings of this study will contribute to multi-dimensional sediment transport simulations that consider the spatial lag effect.

Three-dimensional velocity measurements around bridge piers in gravel bed

ABSTRACTThe experimental results of time average velocity components measured around circular pier models during transient scour stage using acoustic Doppler velocimeter are shown for flow pattern and turbulence characteristics. Totally, four experiments were performed under clear water scour conditions in a model of gravel bed stream. Four circular pier models of diameter 6.6, 8.4, 11.5, and 13.5 cm were used for this study. Detailed controlled measurements on velocity components, and turbulence intensities near the pier and in scour hole at 0° and 180° plane are shown. Flow structure around a pier model in the presence of a scoured region was compared with the flow structure similarly noticed around all pier model runs by utilizing the observations taken at 0° and 180° plane from flow axis. Size of the primary vortex at 0° plane with largest diameter pier model in place (R4 run) is found to be maximum and was approximately 61% larger than that for smallest diameter pier model in place (R1 run). The time-averaged velocity components of turbulence intensities plots at 0° and 180° planes are also presented around each pier.

Large-eddy simulation of a hydrokinetic turbine mounted on an erodible bed

Marine and hydrokinetic (MHK) energy from free-flowing waves, tides and currents comprises an important source of clean and renewable energy. The development of technologies for harnessing MHK energy and assessing its environmental effects is still at early stage. In this work, we develop a computational model for simulating hydrokinetic turbines with bed-load sediment transport under clear-water scour conditions. Turbulent flow is simulated using large-eddy simulation (LES). The bed surface elevation is simulated by solving the non-equilibrium continuity equation, the so-called Exner-Polya equation. The turbine blades are parameterized as actuator lines. The developed model is applied to simulate the flow past an axial-flow hydrokinetic turbine mounted on an erodible bed in an open channel. A good agreement with measurements is obtained. The effects of turbine operating conditions and sediment particle sizes on the bed-load sediment transport are then examined. Finally the turbine wake characteristics are investigated for a rigid flat bed and the eroded bed under different turbine operating conditions.

Hydrodynamics of Flow Obstructed by Inline and Eccentrically-Arranged Circular Piers on a Horizontal Bed Surface

An attempt has been made to study and analyse the hydrodynamic effects of turbulent flow including its pattern and characteristics around three identical circular shaped piers. Experimentation was conducted under clear-water scour condition. Three pier groups were placed with inline and eccentric arrangement in tandem manner on a flume. Two piers were positioned inline along the flow direction and third pier was placed eccentrically in the middle of two inline piers having a transverse distance equal to three times of pier width. Three dimensional velocities were gauged at a horizontal plane located at 4% depth above the zero bed level. The readings were captured in the Cartesian coordinate-system to compute the time-mean longitudinal, transversal and vertical velocities. The hydrodynamic consequences of three piers arrangement near the horizontal bed were investigated. The contour profile and variations of time aggregated mean velocity components, kinetic energy and turbulence intensities are measured or determined and analysed. Velocity vectors and absolute velocity at horizontal plane obtained from velocity measuring areas are utilised to explain further hydrodynamic flow features. The measured depth for scour at equilibrium state for the eccentric middle pier is observed higher than the inline front and inline rear circular piers. The three pier group positioning and the interfering between the circular piers act an important function in flow characteristics and for creating and forming the larger scoured depth around the eccentric middle pier.

HYDRODYNAMIC BEHAVIOR OF FLOW PAST THREE TYPICALLY ARRANGED CIRCULAR PIERS ON DIFFERENT HORIZONTAL PLANES

In this paper an attempt has been made to study the hydrodynamic behavior of flow in terms of three-dimensional turbulent flow patterns and characteristics around three identical circular shaped piers of 7 cm diameter at two horizontal planes. Three pier groups were placed with inline and eccentric arrangement in a staggered manner on a flume. The experiment was conducted under clear water scour conditions. In this experiment two inline piers were placed along the flow, and a third pier was placed eccentrically in the middle of the two inline piers with a transverse spacing. The hydrodynamic effects of the three-pier arrangement were investigated near the bed and near the middle of the flow depth, in order to show the flow behavior at the area close and far away from the sediment bed, respectively. All over the scoured zone at two horizontal planes, the contour profiles and distributions of the time-averaged longitudinal, transverse, and vertical velocity components, turbulence kinetic energy, and turbulence intensities are determined and analyzed. Velocity vectors and absolute velocity obtained from velocity fields are used to show further hydrodynamic flow features. The scour depth of the eccentric circular pier was observed to be higher than the inline front and inline rear circular piers. The three-pier arrangement and the interference between the circular piers play an important role in the flow characteristics and in the creation and formation of the greatest scour depth at the eccentric middle pier.

Experimental investigation of local scour around circular bridge piers under steady state flow conditions

Local scour around bridge piers is one of the main causes of bridge failures. In this study, scour depths and scour hole geometries around circular bridge piers were investigated under clear water scour conditions for various flow rates. The experiments were carried out in a rectangular flume 80 cm wide, 18.6 m long and 75 cm high by using uniform sediment with a median diameter of 1.63 mm and geometric standard deviation of 1.3. The flow rates and approach flow depths were measured with an electromagnetic flow meter and ultrasonic level sensors, respectively. A new empirical equation for the determination of scour depth is proposed based on the experimental findings of the present study and on experimental data available in the literature. The experimental results were also compared with those calculated using several empirical equations developed by previous studies. The proposed new equation to predict scour depth around a circular bridge pier was evaluated in terms of determination coefficient (R2) and scatter index (SI). The Fisher (f)test was also applied to test the statistical significance of the proposed equation.

Reducing local scour at bridge piers using an upstream subsidiary triangular pillar

The objective of this experimental study was to examine the effect of an upstream subsidiary triangular pillar on the development of local scour at a single circular bridge pier. The triangular pillar was placed such that its apex faced the approach flow and its base length was equal to the pier diameter (D) in all experiments. The experiments were conducted to investigate the effect of the spacing between the base of the subsidiary pillar and the bridge pier (S) and the apex angle (α) of the triangular pillar on the dimensions of the scour hole and its temporal development at different Froude numbers. The experiments were conducted in a rectangular flume under clear-water scour conditions at Froude number ranging between 0.1 and 0.3. Relative spacing (S/D) of 0, 0.5, 1, 1.5, 2, and 3, as well as apex angle (α) of 60°, 90° and 120° were tested. The results show that the maximum scour depth at the pier could be minimized using three different combinations of spacing and apex angle. The highest reduction in the maximum scour depth (dsm) of approximately 28 % was possible using S/D of 3 and α of 90° compared to the pier-alone case. Overall the performance of the 90° apex angle pillar surpassed the other two angles.

Scour depth estimation using an equation based on wind tunnel experiments

Scour is the result of degradation and aggradation by wind or moving fluid in the front and back of a pole standing in sand, respectively, and is often observed at the bottom of bridge piers in rivers. In this study, we propose a method of estimating the scour depth around a cylindrical structure standing in sand. The relationships among the depth of the scour, the aspect ratio of the structure (= height/diameter), the fluid velocity, and the sand properties (particle size and density) were determined experimentally using a wind tunnel. The experiments were carried out under clear-water scour conditions. In the experiments, the aspect ratio of the cylindrical structure, the fluid velocity, and the sand particle size were varied systematically. The diameters of the structure were 20, 40, and 60 mm, and the aspect ratio was varied from 0.25 to 3.0. Sand particles of four sizes (200, 275, 475, and 600 μm) were used in the experiment, and the velocity was varied from 4 to 11 m/s. The depth and radius of the scour were measured. As a result, we have developed an equation for estimating the scour depth that uses the aspect ratio, fluid velocity, and sand particle size as parameters.

Physical model study for mitigating local scour downstream of clear over-fall weirs

Abstract Local scouring action downstream of clear over-fall weirs associated with three-dimensional turbulent flow field is a common morphological problem in mobile channel bed. The objective of this study was to assess and quantify the impact of hydraulic structural measures (double lines water jets) on minimizing the scour-hole dimensions downstream of a Fayoum type weir. Ninety runs were conducted considering flow Froude number, different jet discharges and locations. The experimental program was performed under the clear-water scour conditions for a quasi-equilibrium scour hole. A reference case of flat floor without water jets was included in order to evaluate the influence of using the proposed measure. The experiments’ results confirm the acceptability and the effectiveness of the proposed measure (floor water jets) in enhancing the flow hydraulic conditions and the local scour dimensions downstream of hydraulic structures.

Live-Bed Scour at Submerged Weirs

AbstractWeirs or bed sills are low-head hydraulic structures used for bed stabilization, raising upstream water level, and reducing flow velocity. During high-flow events, the weir is fully submerged in the river and scouring occurs both upstream and downstream of the weir. For a fully submerged weir, the scour mechanism around the weir is dependent on approach flow intensity (clear-water scour conditions or live-bed scour conditions) and flow regimes (surface-flow regime or impinging-jet regime) over the weir. The fast evolution of underwater mobile topographies and propagating bedforms increase the complexities of the scour process and the difficulties for scour measurement at the submerged weir under live-bed scour conditions. This paper develops a measurement and data-processing technique for the study of scour at submerged weirs under extreme measurement environments and investigates the scour process both upstream and downstream of submerged weirs under live-bed scour conditions. The experiments are...

Scour hole characteristics of two unequal size bridge piers in tandem arrangement

The work reported herein is concerned with a carefully controlled extensive experimental investigation of the effect of mutual interference of a group of two bridge piers of unequal sizes placed in tandem arrangement at varied pier spacing under steady, uniform flow clear water scour conditions on scour hole characteristics. The data on various characteristics of scour holes developed around the piers are collected and analyzed. Present study reveals considerable mutual interference effect on scour hole characteristics which may lead to bridge failure, if pier group effect is ignored and bridge piers are designed merely as an isolated pier.

Experimental Study of Local Scour around Circular pier Fitted with Collar

Scour is defined as the erosion of streambed sediment around an obstruction in a flow field. Bridge pier scouring is a significant problem in the safety estimation of bridges. In this study, the use of rectangular collar for reducing the effect of local scour at a circular pier in a laboratory flume was presented. The study was conducted using 6.6 m laboratory flume in length and 0.4 m in width. Experiments were conducted for various sizes of rectangular collars fitted at the bed elevation on the scour depth at the circular pier with clear-water scour condition. The time development of scour depth at the circular pier with and without a collar of circular installed was conducted. In collars observed that size of a collar plate increases, the scour decreases. Results showed that the maximum reduction in scour depth equal to 100% (no scour hole). Dimensional analysis technique was used, and from the experimental data an empirical formula was derived. It was found that the predicated scour depth from the formula performs well as compared to the observed scour depth.

Multi-lens pier scour monitoring and scour depth prediction

Severe pier scour is usually the major cause of bridge failures. It is therefore crucial to develop various scour monitoring techniques and prediction models for real-time warning of bridge safety. In the present study, a multi-lens monitoring system for pier scour under laboratory conditions has been developed. By utilising a plurality of lenses, this system is capable of tracking scour images and obtaining real-time scour-depth variation through a series of image recognition processes. Laboratory experiments under unsteady flow conditions were carried out to validate the proposed monitoring method. Then, three time-dependent scour prediction models were employed for simulation and comparison with the measured data. In order to improve the scour prediction results, with the real-time scour monitoring data, a data assimilation scheme is proposed and applied to the scour model under clear-water scour conditions. The result shows that the accuracy of scour prediction for a lead time of 3 h can be improved significantly.