Evaluation of Bridge Scour Research: Pier Scour Processes and Predictions (2011)

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1 EXECUTIVE SUMMARY Introduction This report evaluates the current state of knowledge regarding bridge-pier scour, assesses leading methods for reliable design estimates of scour depth, proposes a structured methodology for scour-depth estimation for design purposes, and indicates pier-scour aspects in need of further research. It focuses particularly on research information obtained since 1990, showing that this information provides considerable new insights that compel the need to change the design method currently recommended by the principal authoritative design guides (notably FHWA’s HEC-181, and AASHTO2 ) and used widely by bridge-engineering practitioners. Additionally, it indicates that several important aspects of pier scour processes remain inadequately understood and not yet incorporated into design methods. Problem Statement Pier scour is among the more complex and challenging water flow and boundary erosion phenomenon to understand, let alone formulate. This is true for all types of piers, though pier shape complexities and bridge-site complications, such as woody debris accumulation, readily complicate scour depth estimation. The erosive flow field is a class of junction flow (i.e., flow at the junction of a structural form and a base plane), a notably three-dimensional, unsteady flow field marked by interacting turbulence structures. Additionally, the foundation material’s capacity to resist erosion typically is uncertain. The numerous complexities associated with pier scour have caused it to persist as an active topic of research since 1990. Though the addition of new data may be helpful in defining the influences of parameters affecting pier scour, there is a sense that a form of stock-taking is needed to assess the current state of knowledge regarding pier scour, and determine whether the present design method in HEC-18 adequately reflects current knowledge. Accordingly, there is a need for an incisive evaluation of the overall results of bridge-scour research published since 1990, especially in the context of determining how present design methods should be updated, revised, replaced, or confirmed as being suitably accurate. Objectives The evaluation’s principal objectives are as follow: 1. Complete a detailed literature review of pier-scour processes and prediction, concentrating especially on research conducted or published after 1990. The review cites some publications prior to 1990, to give a sense of the temporal development of knowledge about pier scour, and because many of the design methods use data and concepts developed prior to 1990; 1 Federal Highway Administration, Hydrologic Engineering Circular 18, ”Evaluating Scour at Bridges,” by Richardson and Davis (2001) 2 AASHTO ~ Association of American State Highway and Transportation Officials

2 2. Summarize the state of knowledge on bridge-pier scour processes, doing so in a way that explains how variations in the flow, sediment, and geometrical variables (thereby the main design parameters) influence scour. Also discuss how scour is affected by channel geomorphology, boundary material (sediment, soil, rock), the proximity of bridge components, and the accumulation of woody debris or ice; 3. Delineate proven relationships between scour depth and the various parameters influencing scour at bridge piers; 4. Evaluate the technical adequacy, strengths and limitations of the leading existing methods to predict scour. An important consideration is whether the commonly used methods for scour estimation adequately reflect current understanding of scour processes. Discuss the uncertainties associated with the leading methods for scour-depth prediction, and address any unresolved issues associated with the methods; 5. Propose specific research findings for adoption in AASHTO policies and procedures. Also, document the ranges of applicability and limitation of research findings proposed for adoption into AASHTO policies and procedures; and, 6. Recommend research needed (field studies, laboratory studies, and numerical simulations) to fill gaps where research results are not yet conclusive enough for adoption by AASHTO and wide-scale use by practitioners. Evaluation Considerations Several key considerations guide the evaluation: 1. The flow field and the potential maximum scour depth, at a pier scale, change in accordance with three variables – effective pier width, undisturbed approach flow depth, y, and erodibility of the boundary material in which the pier is sited. Of these variables, effective pier width and flow depth are of prime importance, because they determine the overall structure of the flow field. Effective pier width, a*, embodies pier form and alignment relative to approach flow. For non- cohesive boundary material (silts, sand, gravel), material erodibility is expressible in terms of a representative particle diameter. 2. To understand pier-scour processes and develop reliable relationships for design estimation of scour depth, it is necessary to comprehend the main flow-field features driving scour, and how the features may adjust in importance with varying pier sizes and shapes, and flow conditions. The flow field differs substantially for the narrow-pier and wide-pier categories of pier scour, with the transition-pier category being intermediate between them. The report defines these categories approximately as • Narrow piers (y/a* > 1.4), scour typically deepest at the pier face • Transition piers (0.2 < y/a* < 1.4), scour depth deepest at pier face, and is influenced by y/a* • Wide piers (y/a* < 0.2), for which scour typically is deepest at the pier flank. It is relatively rare for bridge piers to be in this category

3 3. Because considerable uncertainty attends flow conditions and boundary material at bridge waterways, design prudence requires estimation of a potential maximum scour depth, rather than scour-depth prediction. Potential maximum scour depth is the greatest scour depth attainable for a given pier flow field, and can be determined using the primary variables named in item 2. Lesser scour depths result as additional variables are considered, but the uncertainties associated with the variables diminish the estimation reliability. Besides the uncertainties related to foundation material erodibility, the temporal development of scour entails significant uncertainty. Prediction (not design estimation) of scour depth for most pier sites usually involves a high level of uncertainty. Several factors alter pier flow fields and complicate design estimation of pier scour. Factors affecting pier flow field include flow influences exerted by increased complexity of pier geometry, adjoining bridge components (abutment or submerged bridge deck), debris or ice accumulation, and channel morphology. Additionally, flow conditions commonly vary with time, as the dominant scour events occur during the passage of flood flows. Factors affecting boundary erosion include uncertain erosion characteristics of material (clay, rock), layering of boundary material, and protective vegetation. Approach to Scour-Depth Estimation The large number of parameters that may influence pier scour at a bridge site makes it infeasible for a single method to provide reliable design estimates of pier scour depth. A structured, or graduated, approach is needed, whereby the varying levels of pier shape or site complexity are matched with existing practical methods for scour-depth estimation. This approach, though mentioned in HEC-18 and a few other publications on scour-depth estimation, requires stronger expression in design guides. The present report approaches scour depth estimation in terms of a hierarchy of pier shape and site complexity levels: 1. Simple, single-column pier forms; 2. Common pier forms comprising a more complex geometry; 3. Common pier forms in complex situations (e.g., debris accumulation); and, 4. Uncommon or Complex pier forms and situations. This approach is not entirely new. It is usual for difficult or complex pier circumstances to receive additional design attention; for instance, large piers of complex configuration, and piers in complicated waterway sites. Conclusions The evaluation’s main conclusions are: 1. The literature review conducted for the evaluation shows that, since 1990, substantial advances have been made in understanding pier-scour processes, and predicting scour depth. In particular, the following aspects of pier scour have advanced:

4 i. The roles of variables and parameters defining pier scour processes; ii. The leading predictive methods for scour-depth prediction better reflect parameter influences; iii. Knowledge regarding pier scour in clay and weak rock; iv. Insight into pier-site complications caused by debris and ice, and by interaction with bridge components (abutment and bridge deck); and, v. The capacity of numerical modeling to reveal the three-dimensional and unsteady flow field at piers in ways that laboratory work heretofore has been unable to provide. These advances address research problems identified in NCHRP Project 24-8, Scour at Bridge Foundations: Research Needs, published in 1996. They also address aspects of pier scour not envisioned for NCHRP 24-8, such as the roles of turbulence structures within the pier flow field, three-dimensional numerical modeling, and scaling issues in the conduct of hydraulic models of pier scour. 2. The current state of knowledge on pier-scour processes is such that the ensuing aspects of pier scour remain insufficiently understood: i. The pier flow field, especially how it systematically changes with variations of the two primary length scales (effective pier width and flow depth). In other words, more work is needed to define the systematic changes in the flow fields associated with the narrow- , transition-, and wide-pier categories of pier scour; ii. Scour of boundary materials whose erosion characteristics are inadequately understood (some soils, rock). However, existing reliable data indicate that scour depths in cohesive soils and weak rock do not exceed those in non- cohesive material; iii. Quantification of factors further complicating pier flow field (notably debris or ice accumulation, proximity of bridge components, channel morphology) and erodibility of foundation material (especially layered material where the surface layer protects lower layers); and, iv. Temporal development of pier-scour depth. 3. The evaluation outlines the well-understood relationships between scour depth and significant parameters. A group of primary parameters are identified that define the structure and geometric scale of the pier flow field, and therefore determine the potential maximum scour depth. The secondary parameters characterize scour-depth sensitivities within the geometric scale limit, and normally lead to scour depths less than the potential maximum scour depth. The values of the secondary parameters are subject to considerable uncertainty at pier sites. The primary parameters are –

5 y/a, which indicates the geometric scale of the pier flow field in terms of approach-flow depth, y, and pier width, a a/D, which relates the length scales of pier width and median diameter of bed particle, D Ω, a/b, and θ , which define pier face shape, aspect ratio of pier cross- section (face width/ pier length), and approach flow alignment to pier, respectively. These parameters may be merged with pier width, a, to form the compound variable a* = effective pier width. It can be useful to express the two length-scale parameters as y/a* and a*/D The evaluation also explains the limiting extents to which parameter influences can be isolated from each other. Some variables exert multiple influences. Consequently, a limit is soon reached with the approach of formulating a predictive method based on a semi-empirical assembly of parameter influences. 4. An important conclusion drawn from the evaluation is the need to transition from the present semi-empirical method for design estimation of scour depth used in HEC-183 (Richardson and Davis 2001) to a new method better reflecting the relationships between the primary variables and the potential maximum scour depth at a pier. A new method is recommended, the semi-empirical, Sheppard- Melville method as advanced during NCHRP Project 24-32 (Sheppard et al. 2011). In terms of estimating a potential maximum scour depth, related to the scale of the pier flow field, the Sheppard-Melville method simplifies to * tanh5.2* 4.0              = a y a sy (1) This equation can be used for piers founded in sediment or cohesive soil. The designer can use the full Sheppard-Melville method if wishing to account for the influences of a*/D and V/Vc, where V is the mean approach flow velocity and Vc is the critical approach flow velocity for entrainment of bed sediment. The designer, however, must recognize the uncertainties introduced with consideration of these parameters, and with attempting to account for the temporal development of scour. The Sheppard-Melville method better reflects flow-field changes and thereby scour processes, as expressed in terms of the parameters of primary importance, and therefore is more readily extended to the transition- and wide-pier categories; i.e., it more expressly includes the length scales, y/a* and a*/D, and includes the flow intensity parameter, V/Vc 3 And adopted in AASHTO policies and procedures . Additionally, it is the method developed (under NCHRP 24-32) in an effort to account for flow-field adaptation to variations of

6 y/a and a/D. A useful aspect of the Sheppard-Melville method is that it can be simplified to reflect potential maximum scour depth associated with the three pier flow-field categories (narrow, transition, and wide). A weakness of the Richardson and Davis (2001) method is that it does not. Full use of the Sheppard-Melville method for predicting scour depth presently requires a modicum of further research regarding prediction of scour depth during live-bed scour at piers in the transition-pier category, and to a certain extent in the wide-pier category. The Richardson and Davis (2001) method has been in use for the past several decades, and has been refined over time. It is well calibrated for estimating scour depth for piers in the narrow-pier category of scour, and into the transition-pier category. Its scour depth estimates for these applications concur reasonably well with those obtained using the Sheppard-Melville method. Nonetheless, the method is shown not to reflect scour processes as well as does the Sheppard- Melville method. 5. Due to the limits inherent in semi-empirical formulations of pier-scour depth, the evaluation proposes the use of a structured methodology for design estimation of pier-scour depth. The methodology, summarized in Table 1, addresses four levels of pier-site complexity. As pier-site complexity increases a graduated shift occurs from design reliance on a semi-empirical method (Sheppard-Melville) to hydraulic modeling possibly aided by numerical modeling. Table 1 Summary of proposed structured design methodology Pier Design Complexity (Pier Form and/or Pier Site) Design Method i. Simple single-column pier forms (narrow- and transition-pier categories) Semi-empirical method: Transition from current HEC-18 method (Richardson and Davis 2001) to Eq. (7.1) as simplified from Sheppard et al. (2011), NCHRP Project 24-32) ii. Common pier forms (piers with multiple components; e.g., column, pile cap, pile group) Semi-empirical method with effective pier-shape and alignment factors Consider hydraulic model to validate scour-depth estimate iii. Common pier forms in complicating situations (debris or ice accumulation, abutment proximity, bridge-deck Empirical method combined with procedure to address scour contribution of site complication

7 submergence) Consider hydraulic model to validate scour-depth estimate iv. Complex or unusual pier situations (where reliable method or information on parameter influences does not exist; e.g., scour for wide-pier category) Hydraulic modeling, possibly aided by numerical modeling An approximate scour-depth estimate may be obtained using an empirical method suitably developed for the wide-pier category The methodology enables the designer to account for the scour processes, yet also recognize the limits of existing semi-empirical methods for scour-depth estimation. The leading semi-empirical methods (Sheppard-Melville, Richardson and Davis) for scour-depth prediction were developed using data for simple pier forms, and adapted for common pier forms. The accuracy of the methods reduces as pier form complexity increases. 6. The following specific proposals are made with respect to the updating of HEC- 18 and the AASHTO’s manuals: i. Adopt the structured design methodology described in Chapter 7; ii. After the completion of further research, replace the Richardson and Davis (2001) method with Eq. (1) above, and have the option to use the Sheppard- Melville method. The former method inadequately reflects scour processes, though is well-adapted empirically to scour data. The latter method better reflects scour processes. A modicum of further research is needed to complete the development of the Sheppard-Melville method. The present evaluation has occurred at a transitional period when it is recognized that the present design method for simple and common pier forms should be replaced, but a satisfactory replacement method is not fully completed. The present version of HEC-18 should not be updated until after completion of the small amount of additional research associated with the full development of the Sheppard-Melville method. 7. Prioritized research needs are given for design methodology and scour processes, respectively. In a similar manner as used for NCHRP 24-8, the priorities are assigned as critical, high, medium, and low. For design methodology development, two research topics are of critical priority: i. Estimation of potential maximum scour depth for piers in the changeover range from transition- to wide-pier categories, especially for live-bed conditions; and, ii. A reliable method for estimating potential maximum scour at piers in the wide-pier category.

8 For understanding scour processes, no research topics are identified as critical priority, though several are of high priority, all of which concern improved understanding of how site complications affect pier flow field: i. Debris accumulations; ii. Flow field at common pier shapes with multiple components (notably, column, pile cap, piles); iii. Flow field interaction with bridge components, such as a bridge deck or abutment; and, iv. Flow field interaction with channel features. In terms of longer-range research development, a transition underway is recent advances in experimental and numerical techniques used to investigate bridge scour processes that can capture the dynamics of the main turbulent coherent structures (e.g., horseshoe vortex system at the base of the pier, eddies shed in the separated shear layers, large-scale rollers in the wake behind the pier) affecting pier scour. The recent advances include Particle Image Velocimetry and Laser Doppler Velocimetry techniques for experimental studies; and, Large Eddy Simulation, and Detached Eddy Simulation, computer-simulation investigations using fully three-dimensional non-hydrostatic methods. These experimental and computational approaches promise new insight into understanding the fundamental physics of the flow and sediment transport processes at bridge piers and can lead to the development of more accurate relationships to predict local scour depth.