National Academies Press: OpenBook
« Previous: Executive Summary
Page 11
Suggested Citation:"Chapter 1: Introduction ." 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 11
Page 12
Suggested Citation:"Chapter 1: Introduction ." 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 12
Page 13
Suggested Citation:"Chapter 1: Introduction ." 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 13
Page 14
Suggested Citation:"Chapter 1: Introduction ." 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 14
Page 15
Suggested Citation:"Chapter 1: Introduction ." 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 15
Page 16
Suggested Citation:"Chapter 1: Introduction ." 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 16
Page 17
Suggested Citation:"Chapter 1: Introduction ." 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 17
Page 18
Suggested Citation:"Chapter 1: Introduction ." 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 18
Page 19
Suggested Citation:"Chapter 1: Introduction ." 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 19
Page 20
Suggested Citation:"Chapter 1: Introduction ." 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 20

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

9 CHAPTER 1 INTRODUCTION 1.1 Introduction Scour of foundation material at bridge piers is a long-standing design concern for bridge crossings of waterways. This report evaluates the current state of knowledge regarding bridge-pier scour, assesses the leading current methods to provide reliable design estimates of scour depth, and proceeds to recommend a structured approach to scour- depth estimation for design purposes. It focuses particularly on research information obtained since 1990, showing that this information provides considerable new insights that compel the need to change the approach currently recommended by the principal authoritative design guides (notably FHWA’s HEC-184, and AASHTO5 ) 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. A prominent theme running through the report is the importance of understanding the pier flow field and its erosion capacity for the range of pier sizes, and erodibility of various pier foundation materials. The flow field is highly three-dimensional and unsteady, and differs substantially in accordance with varying combinations of pier width and form, flow depth, and boundary-material diameter (or erosion resistance of channel boundaries). Its capacity to erode the foundation material at a pier often is difficult to predict accurately. Flow field complexities and diversity of foundation materials prevent design method elegance for scour-depth estimation. Instead, design is reliant on empirical and semi- empirical relationships, and on hydraulic models, to relate scour depths to the erosion capacity of pier flow fields, and erosion resistance of boundary materials. The large variation in factors potentially influencing pier flow field and boundary erosion resistance requires design methodology sufficiently comprehensive to account for the more important individual parameter pier scour influences to be considered. Yet the methodology must also treat pier scour from a holistic, or systems analysis, perspective when the number of parameters is too numerous, or the parameters are insufficiently independent, to be described practically in terms of a series of individual parameter influences. The report presents such a methodology. The evaluation and proposals presented here were prepared for eventual use in updating the two AASHTO manuals Policy for Design of Highway Drainage Facilities and Recommended Procedures for Design of Highway Drainage Facilities, so that these manuals present the best available guidelines for pier scour estimation and countermeasure design, and strong directions as to further research. The report’s proposals are intended for eventual use by AASHTO in developing policies and 4 Federal Highway Administration, Hydrologic Engineering Circular 18, ”Evaluating Scour at Bridges,” by Richardson et al. (2001) 5 AASHTO ~ Association of American State Highway and Transportation Officials

10 procedures in the bridge scour areas. The evaluation also identifies research and education needs regarding pier scour. 1.2 Motivation The need to evaluate present knowledge about pier scour, and determine the extents to which existing scour-estimation methods reflect this knowledge, has been expressed in several publications prepared since 1990 by national agencies and societies in the US: e.g., reports stemming from NCHRP Project 24-08, “Scour at Bridge Foundations: Research Needs” (Parola et al. 1996, Lagasse et al. 2004), also NCHRP Report 417 (Parola et al. 1998), Kattell and Eriksson (1998), U.S. Geological Survey (2002), and Mueller and Wagner (2005). It is generally considered that widely used scour estimation methods inadequately reflect present knowledge about scour processes, in particular how the primary length scales -- pier width, flow depth, and sediment size -- are of primary importance for defining the structure and geometric scale of the pier flow field, and thereby scour depth. The present evaluation shows that, while the individual scour influences of the many bridge waterway variables are now well understood for simple cylindrical, or common pier designs, and that recently developed scour estimation methods attempt to encompass these influences, the principal difficulty confronting reliable scour depth estimation at the moment is development of a method that comprehensively accounts for the diverse site factors complicating scour development and estimation (e.g., debris accumulation at piers, piers of large and unusual design). The evaluation suggests that accurate scour-depth estimation is not always possible, and consequently that greater use might be made of contemporary methods for real-time monitoring of pier foundation conditions (so-called smart monitoring). Of particular concern is the inadequate capacity of the established methods to estimate reliable scour depths at wide piers (i.e., piers of width greater than nominally ten feet). Information used for the evaluation was drawn from a broad range of sources, including agency reports, books, and technical papers. Close attention was given to recent or current NCHRP projects on pier scour: notably, scour at wide piers and long, skewed piers (NCHRP Project 24-32, documented in Sheppard et al. 2011); effects of debris on bridge-pier scour (NCHRP Project 24-26); pier scour in cohesive soils (NCHRP Project 24-15(1)); and, pier scour in rock (NCHRP Project 24-29). There is considerable overlap between NCHRP 24-32 and the present evaluation, insofar that both studies examine the variables influencing scour, and compare the veracity of methods for estimating scour depth. Also of major use is the book Bridge Scour by Melville and Coleman (2000). Dr Bruce Melville is a member of the research teams for NCHRP Projects 24-27(01) and 24- 32. Companion studies to the present evaluation are NCHRP 24-27(02) “Evaluation of Abutment Scour Research: Processes and Prediction”; and NCHRP 24-27(03) “Evaluation of Bridge-Scour Research: Geomorphic Processes and Predictions.” Bruce Melville and Robert Ettema serve on the research teams for NCHRP projects 24-27(01) and 24-27(02).

11 1.3 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 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; 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, lab studies, and numerical investigations) to fill gaps where research results are not yet conclusive enough for adoption by AASHTO and wide-scale use by practitioners. The present report is written in a form so that its information can be readily incorporated in the bridge-scour sections of the AASHTO manuals Policy for Design of Highway Drainage Facilities and Procedures for Design of Highway Drainage Facilities. 1.4 Key Considerations The set of objectives listed in Section 1.3 required that the evaluation assess several key considerations: 1. To understand pier-scour processes and develop reliable relationships for design estimation of scour depth, it is necessary to understand the main flow-field features driving scour for varying pier situations;

12 2. The flow field, and the potential maximum scour depth, at a pier scale change in accordance with three variables – 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 embodies pier form and orientation to approach flow. For non-cohesive foundation material (silts, sand, and gravel) erodibility is expressible in terms of a representative particle diameter. 3. Because considerable uncertainty attends flow and foundation material at bridge waterways, design prudence requires estimation of a potential maximum scour depth, rather than scour-depth prediction. Potential maximum scour depths are 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. Prediction (not design estimation) of scour for most pier sites involves a significant level of uncertainty. 4. It is important to consider how pier flow field is affected by conditions in an entire bridge waterway. Several waterway factors alter pier flow field and complicate design estimation of pier scour. They 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. The report identifies relatively simple pier forms and situations, as well as common pier forms, and shows how these may be complicated by fairly common processes at bridge waterways. Such processes include abutment proximity, debris accumulation, channel morphology issues, and variable strata of boundary material. It also shows how the current scour-estimation methods may have limited applicability in certain situations where piers are of unusually large size or uncommon form. The three variables mentioned in item 2, and depicted in Figure 1-1, control the scale of maximum scour-hole depth, doing so principally by controlling the flow field and its erosive strength at a pier. Arguably, effective pier width and flow depth set maximum scour depth at a pier. Approach flow velocity is an important variable, but secondarily so in terms of defining the maximum design scour depth at a pier site; in this context, approach velocity is important in defining the effective width and form of a pier. The key considerations point to the need for a structured methodology for design estimation of pier scour depth. Figures 1-2 and 1-3, which show that pier location often cannot be considered in isolation from other components of a bridge waterway, illustrate the need for the methodology. The former figure shows the pier supports for a long multi-span bridge over a wide channel, while the latter figure shows the pier supports for a shorter, three-span bridge over a comparatively small channel. The central pier in

13 Figure 1-2 could be considered in isolation from the complicating considerations of abutment proximity and channel alignment, but the local flow fields at other piers are to varying extents affected by flow around the abutments and over the channel’s floodplain. For the shorter bridge shown in the latter figure, the two piers cannot be considered in isolation, and are markedly affected by flow around the abutment and over the floodplain, and likely by variable erodibility of foundation material. 1.5. Complexities Using Figures 1-1 through 1-3, it is useful to point out the main sources of substantial complexity that complicate the development of reliable comprehensive (accounting for all important variable influences) design relationships for estimating scour depth at piers: 1. Complexity of flow field (evolving; large-scale turbulence; highly three- dimensional); overall difficulty in comprehending all the phenomena involved and their interactions; 2. The fundamental problem of simultaneously scaling three lengths (flow depth, bed material size, structure size); 3. Variations in channel boundary material; 4. Differences in pier structure; 5. The complicating interaction of pier scour and other processes at bridge waterways, such as accumulation of woody debris, ice, bridge over-topping, abutment proximity, channel morphology, and fluvial bedforms; 6. The potentially large number of parameters involved; and, 7. Addressing the foregoing issues so as to arrive at a practicable method for design estimation of scour depth. Though much is known about scour processes, heretofore no comprehensive design methodology exists linking all the main factors that influence scour at a pier site. The highly unsteady, three-dimensional flows around piers marked by macro-turbulence structures are hard to visualize, let alone measure, in their entirety. It presently is practically impossible, by means of existing laboratory instrumentation and facilities, to visualize the entire instantaneous flow field around a bridge pier. For a given geometry of the pier, three length scales are involved (Figure 1-1) – structure width (assuming structure length and height are proportional to width), flow depth, and bed sediment diameter. Whereas it usually is straightforward (for an undistorted model) to have model structure dimensions proportionate with flow depth, there is a lower limit in linking particle diameter relative to structure width and flow depth. The physical behaviour of particle beds changes when particle diameter decreases. As particle diameter drops below about 0.7mm, a bed may form ripples. As diameter drops below

14 about 0.1mm, inter-particle cohesion becomes increasingly pronounced. These changes affect scour at a pier. Ripple bedforms scale with particle diameter, whereas dunes scale with flow depth (e.g., ASCE, 1975). For cohesive soil and weak rock as foundation material, particle size no longer is a meaningful index for erodibility. For laboratory modeling purposes, however, the erodibility or shear strength of such material scales with the hydraulic model’s length scale (ASCE 2000). These difficulties can be especially challenging when investigating scour at piers of more complex form. 1.6. Report Organization The report is organized in accordance with the evaluation’s objectives: 1. Description of the essential processes associated with pier scour for comparatively simple cylindrical piers in situations uncomplicated by additional considerations such as debris accumulation; 2. Development of a framework of essential parameters needed when discussing the essential pier scour processes, and assessing whether leading design methods adequately reflect scour processes at simple or typical pier forms; 3. Extension of the parameter framework to include processes further complicating scour at piers (e.g. accumulation of woody debris); 4. Assessment of design methods for estimating scour-depth at simple, single- column piers or piers of common form. The assessment examines the extents to which the leading existing methods reflect the important parameter influences; 5. Assessment of design methods for scour-depth estimation when further processes complicate scour (e.g., debris accumulation); 6. Recommendation of a structured set of methods for estimating pier scour at bridges. The set of methods must be suitable for likely adaptation by AASHTO; 7. Use contemporary instrumentation methods for monitoring scour at piers (smart monitoring); and, 8. Identification of the necessary research and education needs to improve scour depth estimation. A hierarchy of design methods obliges designers to adequately understand scour processes, and the limits to which scour depth can be accurately estimated. The principal outcome of the tasks is a structured (or graduated) design methodology that takes into account, to the extent practicable, individual parameter influences on scour depth, yet also recognizes the need for a “systems” or holistic approach to the set of connected flow and erosion processes active during pier scour.

15 The nomenclature used in HEC-18 (Richardson and Davis 2001) is used herein, as a substantial audience for the report will be engineers already familiar with this nomenclature. 1.7. Relationship to Other NCHRP Projects This project is one of three projects conducted under NCHRP Project 24-27 Evaluation of Bridge Scour. The projects similarly aim at assessing existing knowledge and estimation methods regarding scour at bridge waterways: • NCHRP 24-27(01) Evaluation of Bridge Pier-Scour Research • NCHRP 24-27(02) Evaluation of Bridge Abutment-Scour Research • NCHRP 24-27(03) Evaluation of Bridge-Scour Research: Geomorphic Processes and Prediction There is a close relationship between the present and NCHRP Project 24-32 Scour at Wide Piers and Long Skewed Piers (Sheppard et al. 2011). The two principal investigators for the latter project, which had started about one year before the present project, were involved on the Research Team and Expert Team for the present project. The extensive evaluation of pier scour processes and predictive methods conducted for NCHRP 24-32 were of immediate use for the present project, which accordingly utilizes the results from NCHRP 24-32. The proposals given by the present project, however, differ in several aspects from those given by NCHRP 24-32. In particular, the proposed design methodology presented herein places a lower importance, for the purpose of design scour estimation, on the exact mapping of scour depth to many parameter influences. Instead, the methodology focuses on the leading parameters and taking into account the uncertainties associated with them. Consequently, the present methodology acknowledges the influences of time-rate of scour and stage of live-bed scour, but considers them of secondary importance for most pier design situations. The typically high levels of uncertainty associated with the time development of scour, and stage of live-bed scour (as well as periods of live-bed scour), potentially introduce considerable uncertainty in design estimation of scour. Additionally, the design methodology presented here is broader in options than the single method NCHRP 24-32 proposes. The present project also builds on the prior work sponsored by NCHRP and other groups. In this respect, it will link to reports prepared for NCHRP Projects 24-08 Scour at Bridge Foundations: Research Needs, and NCHRP Project 20-07(Task 178) - Evaluation and Update of NCHRP Project 24-08, Scour at Bridges Foundations: Research Needs (Parola et al. 1996). In particular, the project reflects back to NCHRP Project 24-08 (Parola et al. 1996) to assess progress in pier-scour knowledge and estimation methods since then. It also uses the findings available from the following NCHRP studies recently completed or currently underway:

16 1. NCHRP Project 24-14, Scour at Contracted Bridge Sites; 2. NCHRP Project 24-15, Pier and Abutment in Cohesive Soils; 3. NCHRP Project 24-20, Prediction of Scour at Bridge Abutments; 4. NCHRP Project 24-26, Effects of Debris on Bridge-Pier Scour; 5. NCHRP Project 24-29, Scour at Bridge Foundations on Rock; and, 6. NCHRP Project 24-34, Risk-Based Approach for Bridge Scour Prediction. Additionally, the project evaluates information syntheses available in significant reports completed for federal agencies: notably, HEC-18, Evaluating Scour at Bridges (Richardson and Davis 2001), and Channel Scour at Bridges in the United States, (Landers and Mueller, 1996). Leading books or compendia on scour also were examined: notably Hoffmans and Verheij 1997, Richardson and Lagasse 1999, and Melville and Coleman 2000. Figure 1-1 Three length scales (structure, flow depth, and sediment size (or shear strength when considering laboratory hydraulic models)) prescribe the flow field at a pier. The inherent difficulty of equally scaling the three lengths makes hydraulic modeling intrinsically approximate

17 (a) (b) Figure 1-2 Sketches showing a “long” multi-span bridge with multiple piers and abutments; (a) oblique perspective, and (b) cross-sectional view. In some cases pier-pier and/or pier-abutment interactions may be significant

18 Figure 1-3 A sketch showing the foundations of a “short,” three-span bridge. Depending on the water level and scour around the foundation, the flow fields in the vicinity of the pier may be significantly different

Next: Chapter 2: Scour as Design Concern »
Evaluation of Bridge Scour Research: Pier Scour Processes and Predictions Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

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.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!