Temporal Variation Of Scour Depth Research Articles

Local scour at group of bridge piers founded in gravel bed in staggered arrangement

Local scour around bridge piers is the common cause of undermining of bridge foundation. Many researchers have studied local scour around bridge pier in finer sediment bed. The literature survey reveals a limited information is available on scour around a group of bridge piers with and without collar founded in gravel bed. Keeping this in view, an attempt is made to examine experimentally the efficacy of the collar around a group of three piers placed in a staggered triangular arrangement in the gravel bed of 3 mm effective size. The data on temporal variation of scour depth, scour hole characteristics, and areal extent of scour hole were collected. The collected experimental results infer very prominent information to the bridge engineers. The obtained results on equilibrium scour depth and areal extent of scour help in knowing the extent up to which bridge pier protection can be provided against scour. It is found that at longitudinal pier spacing (Z) equals to 8.45 times the pier diameter (b), the equilibrium scour depth at nose of the piers was maximum than that of at other pier spacing. At pier spacing Z/b > 8.45, the equilibrium scour depth at downstream pier without collar decreases with increasing pier spacing and reaches to minimum at Z/b = 40. At pier spacing 8.45 < Z/b < 10, the equilibrium scour depth at upstream pier without collar decreases because of decreasing effect of reinforcement and this effect disappears at pier spacing Z/b > 10. The present study reveals that bridge piers without collar became free of mutual interference effect on local scour at centre-to-centre (c/c) pier spacing of 60 times the pier diameter. It is worth mentioning that for piers with collar, the effect of mutual interference vanished at c/c longitudinal pier spacing of 20 times the pier diameter. The results show that additional shielding of collar makes the piers free from mutual interference effect at lesser spacing than the pier group without collar. This concludes that the present investigation is very useful when limited space is available between the existing and newly proposed bridges.

Three-dimensional numerical simulation of wave-induced scour around a pile on a sloping beach

A fully-coupled (hydrodynamic and morphologic) numerical model based on the open-source CFD package OpenFOAM is presented and utilized to simulate breaking wave induced scour around a pile on a sloping beach. The hydrodynamic model is based on RANS equations together with k-ω turbulence closure and volume of fluid method for capturing the free surface. These are then coupled with both bed load and suspended load transport descriptions, which drive resultant morphology of the bed. The model is validated against large-scale laboratory experiments involving interaction of breaking solitary waves with a pile and induced flow and scour. The flow features and hydrodynamic characteristics around the pile are described in detail, and the scouring mechanism at different locations of the pile is thoroughly analyzed. The simulated flow field confirms the formation of horseshoe vortex in front of the pile induced by plunging breaking wave. The temporal variation of scour depth at the side of the pile is satisfactorily simulated while those at the offshore and onshore are under-predicted. The scour depth at the side of the pile increases twice during run-up and draw-down. The scour at the onshore location of the pile starts to increase rapidly at the end of wave drawdown.

Influence of Scour Development on Turbulent Flow Field in Front of a Bridge Pier

This study concerns the turbulent flow field influenced by the scour development around a bridge pier. The scour hole evolution as well as the temporal variation of scour depth around the pier were firstly analyzed. Subsequently, the flow fields in front of the pier at different instants during the scour process were measured using particle image velocimetry (PIV). It shows that the scour depth at the pier front exceeds that of the pier side at the later scouring stage. The temporal development of scour depth can be well predicted by a simple practical engineering model based on an exponential function with a change in the two adjustable coefficients. The flow field indicates that with the development of scour hole, the downward flow in front of the pier becomes more prominent, meanwhile the flow becomes more turbulent. The variation tendency for both velocities and turbulence intensities along the streamwise direction in front the pier shows similarity. The Reynolds shear stress generally increases with developing scour hole, and the region with large value enlarges and moves upstream of the scour hole.

Experimental assessment and prediction of temporal scour depth around a spur dike

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.

PHYSICAL MODELLING OF LOCAL SCOUR AROUND BRIDGE PIER

Accurate estimation of local scour around bridge pier is important for economic and safe design of bridges. Due to the complexity of the phenomenon and weak relationship between laboratory and field data, the conventional method such as laboratory-derived equations may fail in determining the accurate scour depth. In the present study, a new method was developed to determine the temporal variation of scour depth and final scour extension around bridge pier. Accordingly, using the rules of physical modelling for rivers, a simple method was established for physical modelling of local scour around bridge pier. To demonstrate the accuracy of the method, a typical prototype was chosen and the method was applied for determining different dimensions of the model. Based on the sediment density scale, low density sediment with ρs=1.05 kg/m3 was considered in the model for scour experiment. The results of the experiment including temporal variation of scour depth, equilibrium time and scour depth and final extension of scour hole were then scaled up to the prototype. Finally, empirical equations were utilized to predict maximum scour depth for the typical prototype. Results showed that the value of equilibrium scour depth from the present method was in the range of empirical equations prediction.

Temporal scour evolution at non-uniform bridge piers

Scour at uniform cylindrical piers has received considerable attention in the past. However, bridge foundations can be subjected to the flow due to single or combined effects of long-term riverbed degradation, lateral shifting of the stream, contraction scour, local scour or other factors. This paper focuses on bed morphology changes due to local scour at piers founded on piles or caissons when the foundation is located at the initial bed level. Experiments were carried out in a 1 m wide 20 m long rectangular channel under clear-water and steady flow conditions. The majority of runs were of long duration in order to guarantee sufficient interaction between the flow and pier foundation and to approach the equilibrium phase. Two nearly uniform bed sediments were considered to assess grain size effects. The experimental data obtained provided new information on the temporal variation of scour depth, the areal extent of the scour hole and the scour volume, which are useful for validation of advanced numerical models. In this paper, scour depths are compared with the HEC-18 approach and agreements appear suitable only at the equilibrium phase. Moreover, scour progress is compared to that of uniform cylindrical piers and foundation effects are emphasised. A generalised scour equation is proposed, accounting for such effects.

Scour hole depth prediction around pile groups: review, comparison of existing methods, and proposition of a new approach

A dataset of 365 laboratory tests for scour hole depth (SHD) around pile groups (PGs) under unidirectional aligned flow is compiled, and the performances of the existing equations are comparatively evaluated on the dataset using several statistical indices. A formulation based on a correction of HEC-18 equation provides the best estimate with a correlation factor of 0.58. The test durations of the considered data ranged between 4 and 389 h. A time factor (K t ) is proposed to take into account the temporal variation of the SHD around different PGs. Among the datasets, 51 long-duration experiments are scrutinized to show the temporal variation of scour depth toward equilibrium state. The time duration for these tests is up to 16 days. The proposed K t factor for PGs has a superior performance compared to existing single-pier time factors. Subsequently, the equilibrium scour depths are calculated by extrapolation of scour depths reported at the end of the experiments using the K t equation. The results showed that only 27–93% of the equilibrium scour depths were obtained at the end of the experimental measurements. Finally, a new equation for prediction of equilibrium SHD around PGs is proposed, which has 10% less prediction error than the existing equations. This comprehensive comparative study is a significant step forward in the correct estimation of current-induced SHD around PG foundations of hydraulic and coastal structures.

Influence of unsteady flow hydrograph shape on local scouring around bridge pier

Accurate estimation of scour depth is needed for economic and safe determination of bridge pier foundation depth. Most design criteria are based on maximum scour depth due to the steady design discharge without flow time limit. In this paper, the results are presented of an experimental investigation on local scouring around a single bridge pier under steady and unsteady flow regimes. For unsteady flow, triangular shape hydrographs with repeatable peaks have been chosen. Results show that both sharp and flat increases to the peak of hydrographs have a negligible effect on scour depth for the same base time. An equation is obtained to compute the scour depth due to a specific triangular hydrograph with specific base time, peak flow characteristics and pier diameter. Also, an equation is introduced to represent temporal variation of scour depth with respect to the steady flow condition.

“Temporal variation of scour around circular bridge piers” by U.C. Kothyari and Ashish Kumar, ISH Journal of Hydraulic Engineering, 16(Suppl 1), 2010

The writers thank the discussers for their interest in the paper. In the present paper, an algorithm is presented to calculate the temporal variation of scour depth around the circular piers under ...

Predicting time-dependent pier scour depth with support vector regression

The temporal variation of local pier scour depth is very complex, especially for cases where the bed comprises a sediment mixture. Many semi-empirical models have been proposed to predict the time-dependent local pier scour depth. In this paper, an alternative approach, the support vector regression method (SVR) is used to estimate the temporal variation of pier-scour depth with non-uniform sediments under clear-water conditions. Based on dimensional analyses, the temporal variation of scour depth was modeled as a function of seven dimensionless input parameters, namely flow shallowness (y/Dp), sediment coarseness (Dp/d50), densimetric Froude number (Fd), the difference between the actual and critical densimetric Froude number (Fd − Fdβ), geometric standard deviation of the sediment particle size distribution (σg), pier Froude number (U/gDp) and one of the following three dimensionless time scales (T1 = t/tR1, T2 = t/tR2 and T3 = t/tR3). The SVR model not only estimates the time-dependent scour depth more accurately than conventional regression models, but also provides results that are consistent with the physics of the scouring process.

Temporal Variation of Scour around Circular Compound Piers

Bridge piers having a nonuniform cross section over their height are termed as compound bridge piers. Mostly circular piers resting on large diameter wells or caissons (termed here as a circular compound bridge pier) are adopted for use in the Indian subcontinent for the road and railway bridges. In the present study, a new mathematical model is proposed for computation of temporal variation of scour depth at circular compound bridge piers. Two series of laboratory experiments were conducted to collect data for model application. In the first series of experiments, the data on temporal variation of scour depth were collected, albeit up to the transient (developing) stage of scour, around the circular uniform piers and circular compound piers for the clear-water scour condition. The second series of experiments was carried out to measure the size and hence the area of the principal vortex of the horseshoe vortex system and to determine the bed shear stress at the upstream face of the scour hole around the circular uniform piers and the circular compound piers. The mathematical model proposed in this paper has the capability to compute the temporal variation of scour depth around circular compound piers for all possible cases of footing position with respect to bed level, i.e., footing above the bed level, footing at the bed level, and footing below the bed level.

Temporal Variation of Scour Depth at Nonuniform Cylindrical Piers

The paper proposes a semiempirical model to estimate the temporal development of scour depth at cylindrical piers with unexposed foundations. A cylindrical pier with a foundation is considered as nonuniform pier. The concept of primary vortex and the principle of volumetric rate of sediment transport are used to develop a methodology to characterize the rate of evolution of the scour hole at nonuniform cylindrical piers. The model also simulates the entire scouring process at nonuniform cylindrical piers having the discontinuous surface located below the initial bed level. The scouring process includes three zones; viz Zone 1 having the scouring phenomenon similar to that of a uniform pier, Zone 2 in which the scour depth remains unchanged with its value equal to the depth of the top level of foundation below the initial bed level while the dimensions of the scour hole increase, and in Zone 3 the geometry pier foundation influences the scouring process. A concept of superposition using an effective pier diameter is proposed to simulate the scouring process in Zone 3. In addition, the laboratory experiments were conducted to utilize the laboratory results for the validation of the model. The simulated results obtained from the proposed model are in good agreement with the present experimental results and also other experimental data. Also, the effect of unsteadiness of flow is incorporated in the model and the results of the model are compared with the experimental data. The model agrees satisfactorily with the experimental data.

TEMPORAL VARIATION OF SCOUR AROUND CIRCULAR BRIDGE PIERS

ABSTRACT Extensive research has been carried out in the field of scour around bridge piers in past, which resulted in development of various mathematical expressions for computation of equilibrium scour depth. The modern trend in scour investigations is to study the temporal variation of scour rather than the equilibrium scour because the equilibrium scour occurs after a very long period of time. In this paper method of Kothyari et al. (1992) for computation of temporal variation of scour around circular pier has been modified with aid of experimental data of present study and data of various investigators. The mathematical model presented herein has potential of application for computation of temporal variation of scour depth around prototype circular bridge during the passage of the flood hydrograph.

A semi-empirical model for clear-water scour evolution at bridge abutments

The computation of temporal variation of clear-water scour is important for the design of bridge foundations. Previous studies implied that a very long flow duration is needed to achieve equilibrium scour at bridge abutments. However, the corresponding durations in the prototype conditions may yield considerably greater values than the time to peak of most of the design floods of small to medium sized catchment basins. Therefore, there is a need to estimate the temporal variation of scour depth. This study deals with development of a semi-empirical model for determining the time-dependent variation of clear-water scour depth at vertical-wall abutments. This approach applies the sediment continuity equation to a scour hole around a vertical-wall abutment. A sediment pickup function was used to formulate the rate of sediment transport from the scour hole. The results of the proposed model were compared with those of empirical models. The model agreed well with the test results.

Time-wise variation of scouring at bridge abutments

Accurate estimation of the maximum possible depth of scour at bridge abutments is important in decision-making for the safe depth of burial of footings. Besides, investigation of the geometric features of scour holes around abutments provides useful information for the degree of scour counter-measure to be implemented against excessive scouring. Experiments have been performed to investigate time-dependent characteristics of scour holes around vertical wall abutments under clear water conditions with uniform bed materials. Temporal variations of scour depth and scour contours were measured. Using this information, an empirical relation was developed for temporal variation of scour depth. Additional relations were also derived for time-dependent volume and surface area of the scour holes around abutments. The findings of this study may provide useful information for preliminary design of abutment footings and placement details of armoring counter-measures, such as riprap.

Temporal scour development at bridge abutments with a collar

This study investigated the reduction of scour around a vertical-wall bridge abutment using rectangular collars for clear-water flow conditions over uniform sediment particles in a laboratory flume. Collars of different sizes and at different elevations were tested to determine the temporal variation of scour depth around the bridge abutment. The development of scour around the abutments with and without a collar for a time period of 6 h was studied, and observed scour depths were compared. Experimental results showed that, in addition to protecting the abutments against erosion, the addition of a collar is effective in reducing the rate of temporal scour development. A comparison of the present results with those from previous studies revealed that the effectiveness of a collar increases with a decrease in the elevation of the collar and an increase in the width of the collar.Key words: bridge abutment, collar, experimentation, hydraulics, scour, temporal variation.

Temporal variation of clear water scour at cylindrical bridge piers

Computation of temporal variation of clear water scour is important for the design of bridge pier footings. Previous studies indicated that very long flow duration was needed to achieve equilibrium scouring situations. However, the corresponding durations in the prototype conditions may yield considerably larger values than time-to-peak of the design flood. Therefore, there is a need to estimate the temporal variation of scour depth. This study deals with the development of a new semiempirical method for temporal variation of clear water scour at cylindrical bridge piers using the sediment continuity approach. A recent sediment pickup function proposed for sloping beds is used to formulate the rate of sediment transport out of the scour hole. Results of the proposed method agree well with experimental results. The findings of the proposed method are also compared with some recent empirical methods.Key words: bridge, pier, scour, clear water, sediment pickup.

Influence of cohesion on scour around bridge piers

Experimental results on temporal variation of scour around circular bridge piers founded in cohesionless and cohesive sediments under steady clear water flows are reported. The difference between scour patterns in cohesionless and cohesive sediments is brought out. Considering the horse shoe vortex to be the prime agent causing scour, a procedure is developed for computing the temporal variation of scour depth in cohesive sediments. Empirical relationships have also been obtained for maximum scour depth around bridge piers in cohesive sediments.

Scour around spur dikes and bridge abutments

Realistic estimation of scour depth around spur dikes and bridge abutments in alluvial rivers is important for safe and economic design of their foundations. Procedures have been developed by previous investigators for determination of design scour depth in steady flows at abutments and spur dikes by making use of the design discharge. However, the time required by the design discharge to scour to its full potential is generally much larger than the time for which it runs. Therefore, computations on temporal variation of scour depth are also important for design purposes. Scour processes at bridge piers, abutments and spur dikes have been found to be similar except that the boundary layer effect induced by the channel wall upstream of the abutment or spur dike causes less scour around these as compared to the case of piers. In the present study, therefore, the concept of an analogous pier is developed. The analogous pier would have the same equilibrium scour depth as the given abutment or spur dike under similar hydraulic conditions. The parameters relating to drag due to flow around abutment/ spur dike and bridge pier have been found to be useful in establishing a relationship for the diameter of the analogous pier. The temporal variation of scour depth and the equilibrium scour depth at the spur dike and the abutment are then computed using pier scour equations with size of the analogous pier being taken as the pier diameter. Results obtained are verified using laboratory data of several investigators for both clear-water and live-bed scour conditions.

Live-bed scour around cylindrical bridge piers

Considering the primary vortex in front of the pier to be the prime agent causing scour at bridge piers, a scheme has been proposed for computation of the temporal variation of scour depth during live-bed condition. An equation has also been developed for the computation of maximum scour depth around circular bridge piers in uniform sediments during live-bed condition. The equation follows logically from the scheme proposed for the computation of the temporal variation of scour. The effect of unsteadiness of flow on scour depth has been also studied.