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138 CHAPTER 9 CONCLUSIONS 9.1 Introduction This chapter presents the evaluationâs main conclusions, which link to the following considerations outlined in Section 1.4: 1. The flow field and the potential maximum scour depth, at a pier scale, change in accordance with three variables â effective pier width, flow depth, and erodibility of the foundation 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 velocity. For non-cohesive foundation material (silts, sand, and gravel) 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. These categories can be defined approximately as ⢠Narrow piers (y/a* ⢠Transition piers (0.2 < y/a > 1.4), scour typically deepest at the pier face * < 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 design scour estimation, that piers are in this category The foregoing categories can be expressed in terms of y/a. Pier-scour literature normally expresses data trends in terms of a dependent variable (notably, flow depth or bed particle diameter) relative to a pierâs constructed width, a. 3. Because considerable uncertainty attends flow 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 associated with 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.
139 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. Factors affecting boundary erosion include uncertain erosion characteristics of material (clay, rock), layering of boundary material, and protective vegetation. Figures 1-2 and 1-3, shown at the reportâs outset, give a sense of the variability of pier sites, and infer the potentially large number of variables involved in pier scour. 9.2 Conclusions The evaluation leads to the following main conclusions regarding the six objectives given in Section 1.2: 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: 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 (Parola et al. 1996), such as scour at wide and skewed long piers, and the scour effects of debris accumulation. 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. However, further significant research has yet to be done in each of these areas. 2. The current state of knowledge on pier-scour processes is such that the knowledge points indicated in Objective 2 are addressed in Chapter 3, with elaboration in the references cited. 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
140 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. Figures 3-4, 3-5, and 3-6 illustrate the flow fields associated with the three categories, respectively; ii. Scour of boundary materials whose erosion characteristics are not adequately understood (some soils, rock). However, existing reliable data indicate that scour depths in cohesive soils and weak rock do not exceed those in cohesionless 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 (Chapter 4) outlines the well-understood relationships between scour depth and significant parameters. Extensive use is made of Melville and Coleman (2000) in delineating the relationships, with more recent information cited from other sources. Notable examples of recent information are with regard to similitude in hydraulic modeling of turbulence structures; scour at large piers; erosion of clay and, to a lesser extent, erosion of rock; and, the purported influences of suspended sediment. A group of primary parameters are identified in Section 4.3. They 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 â y/a, which indicates the geometric scale of the pier flow field in terms of approach-flow depth and pier width (in a vertical cross-sectional plain transverse to the pier, and a plain streamwise to the pier) a/D, which relates the length scales of pier width and median diameter of bed particle â¦, a/b, and θ , which define pier face shape, aspect ratio of pier cross-section (face width/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. 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
141 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 (Chapter 5) is the need to transition from the present semi-empirical method for design estimation of scour depth used in HEC-1810 (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 proposed, the semi-empirical, Sheppard-Melville method as advanced during NCHRP Project 24-32 (Sheppard et al. 2011). 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 . Additionally, it is the method developed (with funding for NCHRP 24-32) in an effort to account for flow-field adaptation to variations of 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 in 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 (outlined in Chapter 8) 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 (NCHRP 24-32) method, as indicated in Appendix A. 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, outlined in Chapter 6, addresses four levels of pier-site complexity (Table 6-1). 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, as indicated in Table 9-1. 10 And adopted in AASHTO policies and procedures
142 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.
143 Table 9-1 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 submergence) Empirical method combined with procedure to address scour contribution of site complication 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 6. The following specific recommendations are made with respect to the updating of AASHTOâs manuals: i. Adopt the structured design methodology described in Chapter 7; and, ii. Transition from the Richardson and Davis (2001) method to the Sheppard- Melville method (Sheppard et al. 2011), given in simplified form as Eq. (1) above. 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.
144 The present version of HEC-18 should recommend the use of the current method (Richardson and Davis, 2001) and the Sheppard-Melvile method. In due course, the latter method should replace the former one. 7. Tables 8-2 and 8-3 list prioritized research needs for design methodology and scour processes, respectively. In a similar manner as used for NCHRP 24-8 (Parola et al. 1996), the priorities are assigned as critical, high, medium, and low (definitions in Table 8-1). 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. 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 (PIV) and Laser Doppler Velocimetry (LDV) based experimental studies and Large Eddy Simulation (LES) and Detached Eddy Simulation (DES) numerical 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.
