Combining Individual Scour Components to Determine Total Scour (2018)

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SUMMARY PROJECT DESCRIPTION The objective of this research project is to determine the relationships among individual scour components observed in the same flow event at a bridge, and determine how to combine them to produce realistic estimates of total scour depth for safe and economical design of bridge foundations. The scour components of interest are lateral contraction scour, abutment scour, vertical contraction or pressure scour, and local pier scour. Contraction scour results from acceleration of the flow caused by the constriction of the bridge opening, either due to the bridge embankment narrowing the channel width or to the water level exceeding the height of the bridge forcing submergence and orifice flow. Local abutment and pier scour are the result of several different types of vortex action caused by flow obstruction by the bridge and the turbulence that is generated. Although these scour processes occur simultaneously and interact under some conditions, current scour depth prediction methods for the design of bridge foundations treat them as independent without interaction by adding them together as a conservative assumption. The pressure scour component occurs as the result of submerged orifice flow or bridge overtopping flow which makes its contribution especially important in an era of more extreme flood events. As a result, bridges that were historically designed for free flow may become more vulnerable to pressure scour and a wider array of scour interactions. Furthermore, the degree to which current scour design practices that ignore scour component interactions are overly conservative is unknown. It is within this context that investigation of the degree of interaction of simultaneous scour processes is undertaken in this study. RESEARCH APPROACH The research approach was a combination of experimental and numerical techniques at a more advanced and comprehensive level than has been attempted in most previous bridge scour research projects that focused on only one component of scour under idealized flow conditions. In this study, a total of 85 scour experiments were conducted in flumes at the Georgia Institute of Technology and the University of Auckland. The experiments incorporated different physical scale ratios with movable sediment beds formed in typical prototype compound channel shapes in which a wide, relatively shallow floodplain flow adjoins a deeper, faster moving main channel flow. Several different embankment lengths were studied varying from the case of an abutment with a long setback distance from the main channel to a bankline abutment terminating at the bank of the main channel. For each abutment length, the flow rate was increased along with the tailwater elevation to obtain cases of free flow, submerged orifice flow, and overtopping flow. Clear-water scour experiments, in which there is no sediment transport into the scour hole from upstream, were conducted as well as live-bed scour experiments distinguished by a continuous sediment transport rate through the bridge section from upstream. For each scour experiment, detailed distributions of velocity and turbulence were measured in the approach flow and all the way through the bridge opening using a fixed bed at the beginning of scour. Then the movable bed was installed, and the experiments were run for three to five days as required until equilibrium was reached, after which the bathymetry of the scour hole was measured. 1

2 Numerical modeling of the velocity and flow fields causing scour was accomplished by employing a range of computational fluid dynamics (CFD) models as an integral component of the research plan. The purpose of the CFD models was to gain insight into the coupling between turbulence structures (Rodi et al. 2013, Lyn 2008b) and the scour interaction processes, to augment the experimental observations, and to explore options for estimating the predictor parameters to be used in the proposed scour prediction formulas. At the highest level, large-eddy simulation (LES) was applied to the specific channel and bridge geometry of the experiments at Georgia Tech for a limited number of experiments because of the large demand on computational resources by LES. The three-dimensional, Reynolds-Averaged Navier-Stokes equation (3DRANS) was applied to a series of numerical experiments with varying abutment lengths. Finally, the two-dimensional model, SRH-2D, was implemented to demonstrate its capabilities to compute flow parameters needed in the proposed scour-predictor equations for the experiments, and also at prototype scale for a field example. FINDINGS To provide a framework for predicting maximum scour depth in scour interaction situations, the interactions were divided into four categories based primarily on the independent variables controlling the interactions, but in addition, abutment length and type of flow were also considered. Abutment lengths were designated as long setback abutments (LSA), short setback abutments (SSA) and bankline abutments (BLA) with the deepest scour depth in the floodplain for LSA and in the main channel for SSA and BLA. Types of flow were designated as free flow (F), submerged orifice flow (SO), and overtopping flow (OT). Relationships for predicting total scour were developed for each of the four categories: I. Abutment/Lateral Contraction Scour with or without Vertical Contraction Scour for LSA: combination of local scour around the abutment induced by the turbulent structure of the flow and acceleration of the flow resulting from the width constriction offered by the bridge opening for free flow around LSA, with or without the addition of vertical contraction scour in SO and OT flows; II. Abutment/Lateral Contraction Scour with or without Vertical Contraction Scour for SSA/BLA: same combinations as Category I, but for SSA and BLA; III. Abutment/Lateral Contraction Scour for LSA with pier scour in F, SO, and OT flow; IV. Vertical Contraction Scour and Pier Scour in the floodplain outside the zone of influence of the abutment. In Category I and Category II scour, similar approaches were taken to formulate prediction equations, but separate equations were determined for the case of the LSA and the combined cases of SSA and BLA. The adopted principle was that lateral contraction, vertical contraction, and local abutment scour can be combined and treated by a single equation that focuses on the simultaneous influence of the change in flow distribution caused by the bridge on these three scour components. More specifically, a dimensional analysis determined that the primary

3 independent variables for these scour components are the following:  flow intensity, defined as the ratio of approach flow velocity to its critical value for incipient sediment motion (V1/Vc);  backwater ratio given by the ratio of the approach flow depth to the unconstricted value at the bridge section (Y1/Yo);  unit discharge ratio defined as the mean discharge per unit width in the bridge section to that in the approach flow section (q2/q1). It was shown that these three dimensionless variables also appear in the theoretical equation for idealized long contraction scour. Previously, it has been suggested that the total scour depth can be calculated as an amplification factor multiplied times the theoretical contraction scour depth, but in this study, it was found that a more effective approach was to use a similar mathematical form of the idealized equation but with the parameters determined by regression analysis of the very large data set available as a result of this study. This approach removes the constraint on the equation parameters fixed by the unrealistic assumptions of the idealized contraction scour equation. Furthermore, it preserves the effect of an amplification factor that decreases with increasing unit discharge ratio as local turbulence effects become less influential, but not negligible, in comparison to flow contraction. The effect of the amplification factor is reflected in the slower rate of increase of scour depth with increases in the unit discharge ratio in comparison to the rate fixed by the oversimplified theoretical contraction scour equation. Separate clear-water scour prediction equations were formulated for the LSA, with the scour hole occurring on the floodplain in Category I, while for the BLA and SSA the scour hole developed in the main channel to define Category II. Floodplain flow variables were used as scour predictors in the former case, while scour depth depended on the same variables evaluated in the main channel in the latter case. The proposed Category II equation is of the same form as that for Category I but with a smaller leading coefficient and negligible influence of the backwater ratio because of the larger flow depths in the main channel. Abutments and embankments were protected in both cases by riprap blankets of standard design, and both spill-through and wingwall abutments were tested with no significant influence of abutment shape on the results. It was also found that Category II live-bed scour depth, which is the most common case in the field, could be adequately predicted by the Category II clear-water scour equation. Category III scour introduces the interaction of Category I scour processes with pier scour. This interaction is two-way in terms of the influence of pier scour on abutment/contraction scour and vice versa. Pier location was varied from a position very close to the toe of the abutment to a position in the floodplain well out of the zone of influence of the abutment and contraction scour processes. Both dual rectangular column piers and wall piers were studied with no appreciable difference in the results for maximum scour depth. Partly due to the influence of some portions of the abutment riprap blanket rolling into the scour hole in the early phases of scour, the deepest point of abutment/contraction scour tended to move under the bridge or just outside the bridge. As a result, the pier scour had little effect on the abutment/contraction scour depth. On the other hand, defining pier scour as the maximum scour depth just upstream of the pier, it was found that pier scour was affected by abutment/contraction scour. The interaction was formulated as an

4 excess pier scour caused by the abutment in addition to the calculated isolated pier scour under the same flow conditions. The excess pier scour was found to depend on the same variables as Category I abutment/embankment scour and on the distance of the pier from the abutment. The ratio of excess pier scour to abutment/contraction scour is applied to the Category I scour equation estimate with the ratio determined by the distance of the pier from the abutment. The excess pier scour depth is calculated and then added to the scour depth from the isolated pier scour formula to obtain the combined scour at the pier. The maximum abutment/contraction scour depth is not influenced by the pier regardless of its lateral position, so no pier scour is added to it. Because the main channel in the experiments was not wide enough for a systematic series of pier positions, the case of pier interaction in the main channel in Category II scour utilizes the same excess pier scour ratios as for the long setback abutment but applied to the Category II abutment/contraction scour equation. Category IV scour is the interaction between pier scour and vertical contraction scour in either overtopping or submerged orifice flows. In this particular category, the two scour processes were somewhat independent, at least in a first order analysis. The vertical contraction scour was calculated in terms of the ratio of velocity under the bridge to its critical value in contrast to using the approach flow velocity ratio and the relative vertical geometric contraction ratio. Some previous approaches to calculation of vertical contraction scour alone conform to this method in agreement with isolated vertical contraction scour data collected in this study. The supposition is that velocity under the bridge is a better direct predictor of vertical contraction scour. On the other hand, pier scour in the presence of vertical contraction scour depends primarily on the pier width and approach flow velocity immediately upstream of the bridge. In this case, it was found that the current procedure of adding scour components did not result in excessive overprediction. The CFD simulations provided significant insights into the physics of the flow field and potential interactions of the various scour mechanisms, especially for Category I and Category II. In particular, LES results for six of the laboratory experiments (LSA and SSA with F, SO, and OT flow) uncovered a three-dimensional quantification and visualisation of very complex flow fields. The LES results were validated extensively with experimental data from all six flow cases. A key element in this success was the implementation of a more accurate free surface computation method than ordinarily used in CFD models. As shown by the LES results, scour in free flow coincided with a region of high turbulent kinetic energy (TKE) and increased bed shear stress in the shear zone between the separated flow coming around the abutment and the lower velocity circulation in the separation zone. A significant horizontal circulation cell was set up just downstream of the abutments. In addition, the submerged orifice flow and overtopping flow experienced high shear stress in the vertical flow contraction zone under the bridge across the floodplain. Strong vertical circulation occurred downstream of the bridge for overtopping flows. The 3D Reynolds-Averaged Navier-Stokes (RANS) model reproduced the integral flow quantities needed in the relatively simple scour prediction formulas proposed in this study, but it was unable to match the details of the hydrodynamics of the flow, especially the recirculation zones downstream of the bridge and the TKE distribution across the bridge opening. The 2D model results showed that the required scour parameters related to the flow field could be well predicted as evidenced by comparisons with the experiments and a field example computed at prototype scale.

5 EVALUATION A summary of the recommended scour prediction equations and their uncertainties is given in Table 4-7 on the next page. This table shows that scour interactions produce less scour than predicted by current methods of adding individual scour components in Categories I, II, and III. Overestimation of maximum scour depth by current methods in comparison to measured values ranged from 20% to 45 % at the 95% level of confidence for these three categories. On the other hand, approximately 80% of maximum scour depth estimates using equations developed in this study fell within ±10% of the line of perfect agreement. Using current methods, zero to 15% of the data points fell within ±10% of the line of perfect agreement. For Category IV scour, 100% of the data points fell within ±20% of the line of perfect agreement using the proposed equations. The results of this study are valid for clear-water and live-bed scour in compound channels with the bridge subject to free flow, submerged orifice flow and overtopping flow. In order to define a common and consistent measure of embankment failure, the experimental results were all obtained for the embankment slopes protected by riprap and bordered by a riprap apron which was designed according to current practice for clear-water scour. Riprap size and extent of the apron had to be increased for the higher flow rates in live-bed scour for submerged orifice and overtopping flows. The experimental results, and therefore the scour prediction equations obtained from them, were shown to be valid for four categories of scour interaction experienced by wingwall and spill-through abutments, and wall and dual-column rectangular piers. The experimental results were validated with experimental results from other investigators and by some limited field data that included extreme events. A detailed application procedure with a flow chart and a comprehensive example are given in Chapter 5. Estimated scour depths using current methods are excessively large at the abutments in comparison with the proposed equations. Table 4-7 shows statistically the comparison between existing methods and proposed equations using experimental and field data. The following recommendations are made based on the findings of this study:  Implement the scour interaction equations proposed herein for estimating maximum depth of scour;  Investigate additional properties of turbulent flow features using LES, especially for overtopping flows, and their quantitative connection with scour depths;  Add a sediment transport model to the LES model to estimate scour depth directly;  Adopt the Bureau of Reclamation model SRH-2D or a similar 2D model using k-epsilon turbulence closure and including submerged orifice and overtopping flows as well as free flows for estimating scour prediction parameters;  Extend current riprap protection design procedures for embankments and riprap aprons to include more energetic submerged orifice and overtopping flows;  Commit to a long-term scour monitoring program at bridges to obtain a more robust set of field data for model validation at a wide variety of bridges.

6 Table 4-7. Summary of proposed combined scour equations and prediction errors. Category Model Components Included Applicability % of data falling between ±10% of line of agreement 95% confidence interval of prediction ratio** Current method Proposed model Current method Proposed model I 2/1 1 1 1 2 2/3 1max2 ***363.2         fc f f f fo f fo f V V q q Y Y Y Y (4-8) A, L, V Applicable for spill-through and WWA BLA/LSA 14.5% 75.5% (1.16 to 1.28) (0.97 to 1.04) II 2/1 1 1 1 2max2 **725.1      mc m m m mo m V V q q Y Y (4-10) A, L, V Applicable for spill-through and WWA BLA/SSA 0% 78% (1.38 to 1.47) (0.96 to 1.02) III Abutment and contraction scour not affected by pier scour (Yf2max/Yfo)ab from Eq. (4-8) Same as Type-I P, A, L, V Applicable for spill-through abutment LSA with rectangular and wall piers 0% 74% (1.29 to 1.42) (0.94 to 1.02) Pier scour affected by abutment and contraction scour Eq. (4-12) 5.73;***906.1 1 2/1 1 1 1 2 2/3 1max2                f p fc f f f fo f excessfo f Y L V V q q Y Y Y Y Eq. (4-13) 117.5 OR 3;***283.1 1 1 2/1 1 1 1 2 2/3 1max2             f p f p fc f f f fo f excessfo f Y L Y L V V q q Y Y Y Y piersexcessff dYY )()( max2max2  P, A, L, V Applicable for spill-through abutment LSA with rectangular and wall piers 0% 79% (1.29 to 1.51) (0.94 to 1.06) IV P (CSU or S & M eq) + V (Lyn 2008) : Lp/Yf1>11 Eq. (2-35) or (2-36) + Eq. (2-27) P, V P + V 100%* 100%* - -

Table 4-7, continued Note: * these values are for ±20 percentage from line of agreement, **prediction ratio = mean predicted value/mean observed value Symbols: A= abutment scour, L= lateral contraction scour, V= vertical contraction scour, P= pier scour, WWA = Wingwall abutment, LSA = long setback abutment, I = interactive abutment and contraction scour in floodplain, II = interactive abutment and contraction scour in main channel, III = interactive abutment, contraction, and pier scour in floodplain, IV = interactive pier and vertical contraction scour. 7