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

Chapter: Chapter 8: Research and Education Issues

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Suggested Citation:"Chapter 8: Research and Education Issues ." National Academies of Sciences, Engineering, and Medicine. 2011. Evaluation of Bridge Scour Research: Pier Scour Processes and Predictions. Washington, DC: The National Academies Press. doi: 10.17226/22886.
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Suggested Citation:"Chapter 8: Research and Education Issues ." National Academies of Sciences, Engineering, and Medicine. 2011. Evaluation of Bridge Scour Research: Pier Scour Processes and Predictions. Washington, DC: The National Academies Press. doi: 10.17226/22886.
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Suggested Citation:"Chapter 8: Research and Education Issues ." National Academies of Sciences, Engineering, and Medicine. 2011. Evaluation of Bridge Scour Research: Pier Scour Processes and Predictions. Washington, DC: The National Academies Press. doi: 10.17226/22886.
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Suggested Citation:"Chapter 8: Research and Education Issues ." National Academies of Sciences, Engineering, and Medicine. 2011. Evaluation of Bridge Scour Research: Pier Scour Processes and Predictions. Washington, DC: The National Academies Press. doi: 10.17226/22886.
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Suggested Citation:"Chapter 8: Research and Education Issues ." National Academies of Sciences, Engineering, and Medicine. 2011. Evaluation of Bridge Scour Research: Pier Scour Processes and Predictions. Washington, DC: The National Academies Press. doi: 10.17226/22886.
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Suggested Citation:"Chapter 8: Research and Education Issues ." National Academies of Sciences, Engineering, and Medicine. 2011. Evaluation of Bridge Scour Research: Pier Scour Processes and Predictions. Washington, DC: The National Academies Press. doi: 10.17226/22886.
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Suggested Citation:"Chapter 8: Research and Education Issues ." National Academies of Sciences, Engineering, and Medicine. 2011. Evaluation of Bridge Scour Research: Pier Scour Processes and Predictions. Washington, DC: The National Academies Press. doi: 10.17226/22886.
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Suggested Citation:"Chapter 8: Research and Education Issues ." National Academies of Sciences, Engineering, and Medicine. 2011. Evaluation of Bridge Scour Research: Pier Scour Processes and Predictions. Washington, DC: The National Academies Press. doi: 10.17226/22886.
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Suggested Citation:"Chapter 8: Research and Education Issues ." National Academies of Sciences, Engineering, and Medicine. 2011. Evaluation of Bridge Scour Research: Pier Scour Processes and Predictions. Washington, DC: The National Academies Press. doi: 10.17226/22886.
×
Page 136
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Suggested Citation:"Chapter 8: Research and Education Issues ." National Academies of Sciences, Engineering, and Medicine. 2011. Evaluation of Bridge Scour Research: Pier Scour Processes and Predictions. Washington, DC: The National Academies Press. doi: 10.17226/22886.
×
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Suggested Citation:"Chapter 8: Research and Education Issues ." National Academies of Sciences, Engineering, and Medicine. 2011. Evaluation of Bridge Scour Research: Pier Scour Processes and Predictions. Washington, DC: The National Academies Press. doi: 10.17226/22886.
×
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Suggested Citation:"Chapter 8: Research and Education Issues ." National Academies of Sciences, Engineering, and Medicine. 2011. Evaluation of Bridge Scour Research: Pier Scour Processes and Predictions. Washington, DC: The National Academies Press. doi: 10.17226/22886.
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126 CHAPTER 8 RESEARCH NEEDS 8.1 Introduction The longevity of pier scour as a research topic reflects the potential complexities of the flow field and erosion processes involved. The flow field is highly three-dimensional, marked by a set of interacting turbulence structures, unsteady, and varies with pier form and size, flow depth and the erodibility of pier foundation material. Additionally, erosion processes evolve as scour progresses. Moreover, a number of bridge waterway features may modify the flow field, and foundation material may be of variable and unknown erodibility. This chapter outlines the research needs required to improve the reliability of design estimates of scour depth, as obtained using the design methodology described in Chapter 6. The research needs are structured in two broad terms: 1. Design issues; and, 2. Better understanding of scour processes in order to address the design issues. There also are research needs related to longer-term improvements in design methodology. These needs, not elaborated here, concern overall developments in computer-based modeling and bridge-site monitoring. In several decades, the advances in numerical modeling conceivably could facilitate the practical estimation of pier-scour depth estimation. They already enable remarkably comprehensive complete insights into pier flow fields. The priority range used for NCHRP 24-8, “Scour at Bridge Foundations: Research Needs” (Parola et al. 1996a), is used herein. The priorities are assigned as critical, high, medium, and low. Table 8-1 defines the thoughts associated with each priority. Low priority research needs are not included here. The research needs identified here relate to those identified in NCHRP Projects 24-32 and 24-08, as recapped in Appendix B. However, distinguishing themes of the present recommendations are an emphasis on the development of a practicable approach to design estimation for potential maximum scour depth at a pier site, and how potential maximum scour depth varies with pier flow field. The ensuing sections briefly elaborate the research needs listed in Tables 8-2 and 8-3 for single-column piers, and more complex pier forms and complicating site factors. 8.2 Research Needs for Single-Column Piers Table 8-2 lists research needs whose resolution would advance the design methodology recommended in Chapter 7 for single-column piers, the simplest pier form. The needs are presented in terms of design issues and scour processes.

127 8.2.1 Design Issues The following research topics aim at design method improvements: 1. Piers in the transition- through wide-pier categories. Though numerous laboratory studies have been conducted of piers in the narrow- to transition-pier categories (0.2 < y/a*), additional research is needed for piers in the transition- though wide-pier categories (y/a* < 1.4), especially regarding three considerations: i. The Sheppard-Melville method simplified as Eq. (7.2) is immediately useful for design estimation of a potential maximum scour depth, but the method’s full form (Eq. (6.8)) requires further development for piers in the transition- pier category and subject to live-bed scour. Data and observations (laboratory and field) for pier scour subject to these conditions are fairly scarce, owing to the lack of laboratory flumes of sufficient cross-sectional area and flow capacity, and to the difficulty of obtaining field data. In particular, the influences of the parameter a*/D need to be determined for live-bed scour over the full range of pier-scour categories; ii. Pier scour for the changeover from the transition- to wide-pier scour categories requires more investigation to extend the Sheppard-Melville method more reliably into the wide-pier category, or ascertain the method’s limit of applicability in this regard; iii. Further to items i and ii, the role of pier alignment (and the commensurate alignment factor used to assess an equivalent pier diameter, a*) should be investigated for piers in the transition- to wide-pier categories. For such piers, the alignment factors may not be valid, because the flow field and scour condition changes from one category to the next; 2. The combination of pier scour and contraction scour, especially for scour of cohesive soil. Though Briaud et al. (2005) in NCHRP 24-15 investigated pier scour in cohesive soil, and subject to constricted flow, in laboratory flume experiments, additional work should examine the overall scour bathymetry of contraction and pier scours; and, 3. Likely occurrence of maximum scour. Eq. (7.2) as recommended provides an estimate for the potential maximum scour depth at a pier site. There is merit in determining a procedure for assessing the uncertainty associated with the occurrence of the potential maximum scour depth at a pier. Topics 1 and 2 align with the objectives of NCHRP Project 24-32, “Scour at Wide Piers and Long Skewed Piers” (Sheppard et al. 2011). Topic 3 aligns with the objectives of NCHRP 24-34, “Risk-Based Approach for Bridge Scour Prediction.”

128 8.2.2 Scour Processes The research topics listed below aim at better understanding of scour processes in order to improve design methods: 1. Flow Field. To understand scour and its depth variations with the principal parameters (i.e., those characterizing pier size and form, flow depth, and the erosion resistance of foundation material), it is necessary to understand how pier flow field varies with the parameters. The ensuing topics require further research: i. Systematic changes in the main flow field features with the primary parameters determining potential maximum scour depth. This research need reflects the inadequate understanding of the pier flow field and how it changes with variations of pier size and (cylindrical8 ) form, flow depth, and bed particle size (or erosion resistance of foundation material). Though many studies illuminate aspects of pier flow field, at present no single study or published document systematically describes these changes. In particular, the flow field variations associated with the three categories of pier flow field need improved definition: a. Pier flow field leading to deepest scour at the leading face of the pier (narrow-pier category of pier scour); b. Pier flow field causing deepest scour at the sides of the pier (wide-pier category of pier scour); and, c. The transitional condition between these categories (as occurs when flow orientation varies at a long skewed pier in a relatively shallow flow). All existing scour equations use curves indicating the effects of pier shape and alignment associated with the narrow-pier scour category. Further research is needed to provide curves for the transition- to wide-pier categories of scour. Achieving this entails knowledge regarding the main flow field features for these categories. ii. Influence of bedforms on pier flow field. Live-bed scour is marked by the presence of bedforms, whose dimensions depend on length scales9 other than pier width. However, pier width determines the length scale of scour, and thereby the presence of bedforms within the scour hole. Two topics need further research: a. Influence of approach-bed bedforms on pier flow field for categories a, b, and c in Item i above; and, b. Influence of scour-zone bedforms on pier flow field. 8 Circular, rectangular, other 9 flow depth (dunes) and bed particle size (ripples)

129 2. Erosion of Foundation Material. Erosion of foundation material at a pier occurs in several ways, depending on the nature of the material (non-cohesive sediment, cohesive soil, rock). Remaining research needs address aspects of flow field entrainment and transport of foundation material: i. Erosion of non-cohesive foundation material by turbulence structures and unsteady flow features in the pier flow field. To understand pier scour, it is necessary to know how boundary material is entrained, transported and deposited by turbulence structures in the pier flow field, for flow field categories i, ii, and iii. In ways still to be defined, the turbulence structures vary in importance in accordance with pier size and (cylindrical) form, flow depth, and bed particle size. The literature on pier scour inadequately addresses the role of the set of influential turbulence structures (including horseshoe vortex system, wake vortices). ii. Erosion of cohesive foundation material. NCHRP Project 24-15, “Pier and Abutment in Cohesive Soils,” yielded considerable insights into scour of cohesive foundation material. In addition to the substantially longer period needed to scour clay, the study showed that clay eroded as fragments or chunks subject to steady and oscillatory flow forces acting in the scour hole. Further research is needed to: a. Determine and document how clay erodes, especially for several types of clay subject to the narrow-, transition-, and wide-pier categories of scour; and, b. Confirm whether or not pier scour in clays does not attain greater potential maximum depths than scour in cohesionless sediments. iii. Erosion of rock foundation material. NCHRP Project 24-29, “Scour at Bridge Foundations on Rock,” now underway with a review of pier scour of rock foundation material, will suggest research needs regarding pier scour in rock foundations. Though the project’s findings have yet to be disseminated, several prominent facets of scour in rock should be noted here: a. Because the unsteady hydrodynamic forces associated with the turbulence structures in the pier flow field play a major part in scour of rock around bridge piers, more information is needed as to how pier scour develops in rock of different erosion characteristics, and how the resulting scour forms relate to pier flow field; and, b. Confirm whether or not pier scour in various rock types does not attain greater potential maximum depths than scour in cohesionless sediments.

130 8.3 Research Needs for Complex Pier Forms and Site Factors Table 8-3 lists research needs whose resolution would advance the design methodology recommended in Chapter 7 for piers whose form is more complex than a single column or which face more complicating site factors; i.e., 1. Piers of common form (column, pile cap, piles, or spread footing); 2. Piers subject to complicating site factors; and, 3. Piers of unusual form or complexity. The research needs are presented in terms of design issues and scour processes. 8.3.1 Design Issues The following research topics aim at design method improvements: 1. Common pier forms. The frequent use of common pier forms suggests there is merit in conducting hydraulic model tests to obtain scour data for such piers. The purposes of the tests would be as follow: i. Develop shape and alignment factors for common pier forms by relating the scour data to scour depth at a cylindrical pier. Adapt the relationship for cylindrical piers (Eq. (7.2)) so that it accurately reflects scour at common pier forms not of simple cylindrical form; and, ii. Obtain a body of data on potential maximum scour depth at common pier forms. The data should be valid for the pier scour and wide-pier scour situations, and enable use of the fully developed Sheppard-Melville method. iii. As for common pier forms, there is merit in determining a procedure for assessing the uncertainty associated with the occurrence of the potential maximum scour depth at a pier. 2. Complicating site factors. The design methodology must accommodate site factors complicating, or contributing uncertainty to, the flow field at a pier, or erodibility of the boundary material. An effort is needed to adapt Eq. (7-2), or empirical relationships using scour data obtained for common pier forms, to account for the complicating factors: i. Determine how scour-depth estimation should account for bridge-deck submergence; ii. Confirm a design guideline regarding pier scour estimation for piers in layered sediment, particularly when the top layer acts as a protective armoring layer; and, iii. Though major progress has been made in understanding and formulating debris-accumulation effects on scour depth (Lagasse et al. 2010), scope exists for extending this progress to encompass debris accumulation at

131 selected common pier forms especially prone to debris accumulation (e.g., pier bents). Topics i and iii align with recommendations stemming from NCHRP Project 24- 26, “Effects of Debris on Pier Scour.” A large debris raft, or ice accumulation, extending across several piers or an entire bridge waterway may modify the pier flow field similarly as would a submerged bridge deck. Item ii relates to channel geomorphology issues as considered in NCHRP Project 24-27(03), “Evaluation of Bridge-Scour Research: Geomorphic Processes and Predictions.” 3. Wide piers and uncommon pier forms. As indicated in Section 7.7, a reliable method has yet to be developed for scour-depth estimation at piers in the wide- pier category. The Sheppard-Melville (Sheppard et al. 2011) and Melville (1997) methods purport to be of use for wide-pier scour, but are based on data from comparatively few laboratory experiments. It is likely that an existing method for abutment scour could be adapted for this purpose. Moreover, as wide-piers are built it will be useful to obtain field data from them. Such data should be compared with data from abutments, sheet-pile coffer dams, and spur dikes built with solid foundations that penetrate to depth within a channel bed. Time is a further complicating factor for some piers. Equilibrium scour depth may not be attained during a single, scouring flow event. Moreover, with time, channel conditions in the vicinity of the bridge waterway may alter. 8.3.2 Scour Processes 1. Flow Field Factors. The following practical aspects of bridge-waterway hydraulics affect the flow field at pier and require further research: i. Flow field at piers of multiple components. Many common pier forms comprise multiple components (e.g., pier column, pile cap, piles or spread footing) that may complicate the pier flow field, and thereby increase the uncertainty of scour depth estimation, in comparison with scour at cylindrical piers. In support of the use of pier-shape factors with the estimation equation, and further laboratory tests on scour at such piers, it is useful to understand how the main flow features at a cylindrical pier alter for common pier forms. ii. Pier flow field interactions with bridge features. The pier flow field can be altered substantially by flow around other features of a bridge waterway. Flow around other features, notably an abutment, may substantially alter the structure of the pier flow field, or may alter approach-flow orientation and magnitude. Two interactions are common for piers, and require additional research:

132 a. Abutment proximity. A pier within the influence region of an abutment flow field is affected by the abutment flow field, to the extent that scour at the pier can be primarily attributable to the abutment flow field. Though some work on abutment proximity has been done, further research is needed to determine how the distance of abutment influence extends for varying conditions of abutment length and channel geometry, and then to use this information for use in design estimation of scour depth at piers near abutments; and, b. Bridge-deck submergence. The useful work done on the influence of deck submergence on pier scour under clear-water scour conditions should be extended for live-bed conditions. iii. Influence of debris rafts, and ice accumulations, on pier flow field, and thereby on scour depth. The final report for NCHRP Project 24-26 (Lagasse et al. 2010) states the following topics require further research: a. Adaptation of the design method for use with the design methodology recommended in the present project; b. Undertaking laboratory tests extending findings from debris accumulation at simple cylindrical piers to common pier forms comprising multiple components, and with varying approach-flow angle; and, c. Undertaking further tests with a broader variation of debris raft geometries, including rafts that extend upstream of the pier (as with an ice cover), and debris rafts extending across more than one pier. This factor is directly similar to scour at a submerged bridge deck. 2. Erosion of foundation material. The essential concern is the pier-site presence of an armouring layer of material overlaying strata of more erodible material, which if exposed to the pier flow field would produce a deeper scour than if the pier foundation were in a single layer of material. The following factors complicate scour-depth estimation, and require further investigation: i. Scour development in layered boundary material. A fairly frequent pier site complication concerns the presence of a gravel stratum overlaying strata of sands or finer gravels. Though this situation has been researched, further research is needed especially regarding the following aspects: a. Investigation and documentation of field situations; and, b. Development of design scenarios indicating a possible potential maximum scour depth associated with scour in these conditions. ii. Scour in layered weak rock. This site complication relates to item 2 in Section 7.2. a. Investigation and documentation of field situations; and,

133 b. Confirming whether or not pier scour in various rock types attains greater potential maximum depths than scour in cohesionless sediments. iii. Scour at piers on a vegetated or grassy floodplain. A vegetated or grassy surface may extend the clear-water condition scour beyond the critical entrainment condition for the foundation soil or sediment beneath. However, once flow strips the vegetation from the foundation surface, scour occurs more widely around the pier as well as at the pier. Several issues require resolution: a. Investigation and documentation of field situations; and, b. Confirming whether or not pier scour in grassy floodplains attains greater potential maximum depths than scour in cohesionless sediments.

134 Table 8-1 Priority range for research needs (Adapted from NCHRP Project 24-8 (Parola et al. 1996)) Critical Priority • Research that is necessary for improved solutions to widespread and costly problems • Research that is necessary to ensure public safety • Research that is essential to maintain an effective research agenda • Development of guidelines for the application of methodologies High Priority • Research that is judged to have a high benefit-to-cost ratio • Research that is applicable to a large number of bridges • Research that will lead to substantial long-term improvement in scour prediction methodology Medium Priority • Research on problems that relate to large numbers of bridges, but where benefits of a solution cannot be estimated • Research that will increase the accuracy of predictive procedures in the long- term, but without immediate impact Low Priority • Research on problems specific to a small number of bridges • Research on problems that infrequently cause bridge damage

135 Table 8-2 Research topics and priorities for single-column piers Topics Priority DESIGN ISSUES 1. Estimation of potential maximum scour depth for piers in the changeover range from transition- to wide-pier categories, especially for live-bed conditions Critical 2. Combination of pier scour and contraction scour, especially for cohesive soils High 3. Procedure for ascertaining probability that potential maximum scour will occur High PHYSICAL PROCESSES 1. Flow field i. Systematic changes in flow field for piers in the transition- to wide-pier categories, determining potential maximum scour depth High ii. Influence of bedforms on flow field during live- bed scour Medium 2. Erosion of Foundation Material i. Changes in erosion processes and scour form with changes from transition to wide-pier categories. Especially important are alignment and shape factors for use in scour-depth estimation High ii. Erosion of non-cohesive material by turbulence structures Medium iii. Erosion of cohesive material by turbulence structures Medium iv. Temporal development of scour for the transition- pier category of scour, and for non-cohesive and cohesive foundation materials Medium

136 Table 8-3 Research topics and priorities for considerations complicating scour-depth estimation Topics Priority DESIGN ISSUES – COMMON PIER FORMS 1. Reliable shape and alignment factors for common pier forms. Adapt the Sheppard-Melville method, notably as expressed by Eq. (7.2), for use with common pier forms Critical 2. A body of scour data for common piers. The data can be used with the full Sheppard-Melville method Medium 3. A procedure for ascertaining probability that potential maximum scour will occur Medium DESIGN ISSUES – COMPLEXITIES 1. Accounting for bridge-deck submergence when estimating pier scour depth High 2. Pier scour in layered sediment, particularly when the top layer acts as an armoring layer; and, in vegetated floodplains High 3. Scour-depth estimation for selected common pier forms should account for debris accumulation Medium DESIGN ISSUES – COMPLEX PIER FORMS 1. Scour depth estimation for piers of unusual or intricate geometry (development of hybrid approach) Medium PHYSICAL PROCESSES 1. Flow field i. Main features of flow field at common pier shapes with multiple components (notably – column, pile- cap, piles) High ii. Pier flow field interaction with bridge components, especially a bridge deck High iii. Influence of debris rafts or ice accumulations Medium

137 iv. Flow field interaction with channel features Medium 2. Erosion of Foundation Material The erosion processes associated with the flow-field complexities mentioned above Medium

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TRB’s National Cooperative Highway Research Program (NCHRP) Web-Only Document 175: Evaluation of Bridge Scour Research: Pier Scour Processes and Predictions explores the current state of knowledge regarding bridge-pier scour, assesses several methods for design estimates of scour depth, examines a structured methodology for scour-depth estimation for design purposes, and highlights aspects of pier-scour in need of potential further research.

In September 2012 TRB released NCHRP Research Results Digest 378: Evaluation of Bridge Scour Research, which summarizes key finding of NCHRP Web-Only Document 175 along with two other NCHRP projects that explored processes and predictions related to pier scour, abutment and contraction scour, and geomorphic scour.

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