Evaluation of Scour Depth around Bridge Piers with Various Geometrical Shapes

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1 Evaluation of Scour Depth around Bridge Piers with Various Geometrical Shapes Dr. P. D. Dahe * Department of Civil Engineering, SGGSIE&T, Vishnupuri, Nanded (Maharashtra) S. B. Kharode Department of Civil Engineering, SGGSIE&T, Vishnupuri, Nanded (Maharashtra) Abstract Bridges are one of the most important structures which must be designed and protected from failure by the effect of scour. Simulation studies for flow around different hydraulic structures have been successfully carried out in the past. This approach can also be employed for estimating the effect of scour around bridge piers. In this study Hydrologic Engineering Center River Analysis System (HEC-RAS) software is used to evaluate local scour around bridge piers of various shapes. It was found that the HEC-RAS model is a good tool which employs the CSU as a default equation to estimate the scour depth for bridge piers. It is found that the computed scour depth around sharp nose shaped pier has better value whereas the square shaped pier has maximum value of scour depth. Circular pier, rounded nose, and pier with group of cylinders have identical values of scour depth. Keywords Scour, HEC-RAS, CSU equation, pier shape. I. INTRODUCTION Bridges at river crossings are critical structures that require a substantial investment and have an important role in economic development. The major damage to these bridges occurs during floods caused by various reasons. The main reason is the local scour at bridge foundations, namely piers and abutments. The scour can be defined as the lowering of the level of the riverbed by water erosion which may lead to expose the foundations of a bridge [1]. The history of bridge failures shows that most of incidents occurred due to design which did not adequately account for the capacity of alluvial rivers to erode, or scour the channel beds and banks. Images of undermined piers, undercut abutments, and washed-out bridge approaches have haunted bridge designers and engineers. Investigators have used several ways and developed several equations, equipment and experiments for estimation of the depth and pattern of scour around piers. In this study the hydraulic modelling for conducting simulation experiments was performed using the one-dimensional U.S. Army Corps of Engineers Hydrologic Engineering Centre River Analysis System, HEC-RAS (version 5.0, 2014) to evaluate the maximum scour depth around bridge piers. II. TESTING FOR PROGRAMMING CSU equation is often used to find out the scour depth around bridge pier. This equation was developed on the basis of hydraulic data. In the present study, HEC-RAS model is used to estimate the bridge scour around the bridge pier. Apparently, more realistic results are obtained by HEC-RAS model as it considers more related factors as angle of attack, bed conditions, shape of piers, flow concentrations, flow depth and Froude number as against discharge intensity. The HEC-RAS model is tested and verified with the help of input data from a previous case study. All the input data used for testing was taken from the case study of Al-kufa Bridge [7]. This case study includes the field measurement value and HEC-RAS value of scour depth. By using this input data testing and verification is carried out in this study as shown below. Al-Kufa Bridge- Data from case study- K 1 = 0.9, D 50 = 0.19mm, D 95 = 0.58mm, V =0.13m/s, y = 2.58m, a = 2m, L = 12.70m Scour depth from field measurement = 0.74m Scour depth calculated in case study by HEC-RAS = 0.91m. Scour depth calculated in this study using HEC-RAS = 0.89m (as shown below) , IJIRAE- All Rights Reserved Page -41

2 Fig.1 Hydraulic Data Pier Scour III. SCOUR MODELING IN HEC-RAS PROGRAM As per the available literature, such studies are not conducted using HEC-RAS. Moreover, there is no evidence of consideration of geometrical shapes for scour depth studies around bridge piers using other simulation models. This study investigates the scour depth around various shapes of bridge piers using simulation experiments in HEC-RAS. A hydraulic model was developed using HEC-RAS [Version 5.0, (2014)]. The HEC-RAS model allows performing simulation of steady and unsteady flow evaluation in single or networked channels. It is an integrated system of software, designed for interactive use in multitasking, multi-user network environment. The system is comprised of a graphical user interface (GUI), separate hydraulic analysis components, data storage and management graphics and reporting facilities (HEC-RAS, 2014). Figure (1) shows the interface of HEC-RAS model. Fig. 2 HEC-RAS Interface To apply a bridge for flow hydraulic model and to find magnitude of maximum scour depth around bridge piers, the file types required are as follows- 1) Physical description for the stream reach and the bridge is necessary in the geometry file. 2) All the flow inputs and reach boundary conditions needed for the flow analysis should be described in the flow file , IJIRAE- All Rights Reserved Page -42

3 The data required to represent the bridge model are roadway data, data for piers and sloping abutment data. For flow hydraulic model information on flow regime, boundary conditions and discharge are required. From these input data, the model is applied using HEC-RAS one dimensional analysis to calculate the maximum scour around bridge piers. All the input data required to run a bridge HEC-RAS model are presented in the following sections. IV. THE GEOMETRIC DATA The first step to develop HEC-RAS model is to create a HEC-RAS geometric file. The basic geometric data consists of establishing how the various river reaches are connected (River System Schematic); cross section data; reach lengths and stream junction information. These data were provided to the model through the menu of cross section geometrical data as shown in figure (3). The bridge location on the river is specified and its details have been entered to the model using bridge geometrical data menu, as shown in figure (4). Fig. 3 Input Menu of Cross Section Data , IJIRAE- All Rights Reserved Page -43

4 Input menu of Bridge Geometrical data Fig. 4 Input Menu of Bridge Data V. THE RIVER SYSTEM SCHEMATICS he schematic defines how various river reaches are connected. The river system schematic is developed by drawing and connecting the various reaches of the system within the geometric data editor. This is essential as the first input to the HEC-RAS model. The model of river system schematic is single reach for this study. A schematic diagram of the river system is drawn as shown in Figure (4). The reach is drawn from upstream to downstream (in the flow direction), and the HEC-RAS model assumes this to be true. VI. CROSS SECTION GEOMETRY For the analysis the flow in river stream the boundary geometry is specified in terms of ground cross sections. The measured distance between any two cross sections is referred to as reach length. Cross sections should be perpendicular to the anticipated flow lines. The cross section is described by entering the stations and their elevations (x-y) data from left to right. When numbering River Station Identifiers are used assuming that higher numbers are on the upstream and the lower numbers are on the downstream within a reach. All of the required information is displayed on the cross section data editor; figure (5) shows some of the cross sections at upstream and downstream , IJIRAE- All Rights Reserved Page -44

5 Fig. 5 Some Cross Sections VII. BRIDGE GEOMETRY All the necessary data related to the bridge is included in bridge geometry: The location of the bridge is indicated by entering new river station for the bridge in the reach. The distance between the upstream cross section and bridge is included in the roadway data, the width of bridge in direction of flow and the thickness of bridge is described by entering the stations and elevations of bridge (x-y) data from left to right. Pier data include the distance from the left to the centerline of each pier and the width and height of the pier is represented by elevations. These data are entered to the model through the menu of bridge data as shown in figure (6). Fig. 6 Bridge Data Menu , IJIRAE- All Rights Reserved Page -45

6 VIII. FLOW DATA To perform water surface profile calculations it is required to enter the flow data. Details on flow regime, boundary conditions, discharge are included in flow data. The flow data is entered in model is as shown in figure (7) below. Fig. 7 Input Menu of Flow Data IX. SCOUR AROUND BRIDGE PIER The data required to be entered for bridge are D 50 size fraction for the bed material which is equal to (1.28mm), the correction factor of pier nose shape (K 1 = 0.9), the correction factor of angle of attack for flow hitting the pier (K 2 = 1) which is determined by entering the angle of attack, correction factor for bed condition (K 3 = 1.1), correction for bed material (K 4 = 1), velocity of flow is 0.25m/s and flow depth = 4.33m. The result of scour depth at pier can be computed from tab of HEC-RAS which appears at figure (8). The scour for this case of sharp nose shape pier is equal to (1.66m). Figure (9) shows the scour depth at square shape pier equal to 2.03m. The scour depth around circular shape, round nose shape and group of cylinders shaped piers are shown in the figure (10, 11, and 12), which is identical for all these piers and is equal to 1.85m. Fig. 8 Scour at Sharp Nose Shape Pier , IJIRAE- All Rights Reserved Page -46

7 Fig. 9 Scour at Square Nose Shape Pier Fig. 10 Scour at Circular Pier Fig. 11 Scour at Round Nose Shape Pier , IJIRAE- All Rights Reserved Page -47

8 Fig. 12 Scour at Group of Cylinders X. DISCUSSION OF RESULTS AND CONCLUSIONS The simulation experiments for different shapes of bridge piers for estimating the depth of scour were successfully conducted using HEC-RAS in this study. The HEC-RAS model was tested using available data and results from study of Al-kufa bridge [7]. The maximum scour depth obtained is 2.03m which is around square nose shape pier, while lowest scour depth obtained around sharp nose shape pier is equal to 1.66m. The scour depth around circular, group of cylinders and rounded nose shape piers is identical which is equal to 1.85m. Hence the sharp nose shape pier is suitable for the particular hydraulic conditions mentioned in this study. The close agreement between computed and measured scour depth shown in testing and verifications indicate that HEC-RAS model is a good tool for predicting scour depth around bridge piers. HEC-RAS model can be used to estimate the scour depth around bridge piers with various geometrical shapes without changing the geometrical data for simulations. Such studies can be used as a guide to select the shape of bridge pier before construction based upon the estimates of the scour depth. REFERENCES 1. B.W. Melville, S.E. Coleman, Bridge Scour. Water Resources Publications, LLC, Colorado, U.S.A, E.V. Richardson, S.R. Davis, Evaluating Scour at Bridges, fourth edition, Federal Highway Administration Hydraulic Engineering Circular No.18, FHWA NHI , G.W. Brunner, HEC-RAS River Analysis System User s Manual Version California: US Army Corps of Engineers Institute For Water Resource, Hydrological Engineering Center (HEC), M. Fatema, Evaluating Various Factors in Calculation of Scour Depth Around Bridge Pier Using HEC-RAS Software, CSU2001, Froehlich Equation, Journal of Civil Engineering and Urbanism, Vol. 3, Issue 6 : , R. Pasiok, R. Stilger-Szydło, Sediment Particles and Turbulent Flow Simulation around Bridge Piers, Institute of Geotechnics and Hydrotechnics, Wroclaw University of Technology, 27, Wroclaw, Poland, S. Abdelaziz, M.D. Bui, P. Rutschmann, Numerical investigation of Flow and Sediment Transport around a Circular Bridge Pier 34th Hydrology and Water Resources Symposium, Sydney, Australia, July S. Khassaf, Modeling of Local Scour around Al-kufa Bridge Piers, International Journal of Advanced Research, Vol. 1, Issue 8, S. Muttucumaru, Does the Critical Shields Stress for Sediment Transport Depend on Channel-bed Slope, 34th Hydrology and Water Resources Symposium, Sydney, Australia, Nov S.M. Singh, Flow Field and Scouring around Cylindrical Structure in Channel Bed, 2 nd International Conference on Advances in Civil, Structural & Environmental Engineering, W. Wu, W. Rodi, T. Wenka, 3D Numerical Modeling of Flow and Sediment Transport in Open Channels, J. Hydraul. Eng., 126(1), 4 15, , IJIRAE- All Rights Reserved Page -48