Revised Clear-Water and Live-Bed Contraction Scour Analysis (2021)

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1-1   Introduction and Research Approach 1.1 Scope and Research Objectives 1.1.1 Background Bridge waterways commonly narrow or constrict natural channels, forcing water to flow through a contracted area, thereby increasing the magnitudes of velocity and turbulent kinetic energy of flow passing through the waterway. If these increases cause erosion of the waterway boundaries, the contracted section may scour. Substantial contraction of flow may occur when bridge abutments encroach into a channel, or when bridge approach embankments extend at length across the adjoining floodplains of a channel. The presence of bridge piers may aggravate the contraction effect through a bridge waterway. Existing procedures for estimating contraction scour assume a relatively simplified situation where the contracted channel is straight in alignment, rectangular in cross section, with banks that are resistant to lateral erosion. It is also generally assumed that the bed is formed of uniform non-cohesive sediment. The resulting vertical erosion or scour in a constricted channel is commonly termed “contraction scour” (Arneson et al. 2012). The assumption of a straight, rectangular, alluvial channel is reasonable for relatively wide channels of slight local curvature at a bridge site with erosion-resistant banks. Live-bed and clear-water scour can occur along channel contractions. The former scour condition commonly occurs in the main channel of an alluvial river, while the latter condition is more typical for a floodplain contraction or at a relief bridge located on the floodplain. For clear-water scour, the governing principle is that the depth of scour in the contracted section corresponds to the occurrence of critical bed shear stress or mean flow velocity when scour approaches its equilibrium state. For live-bed contraction scour, the limiting condition is continuity of sediment transport between the upstream approach flow section and the contracted section. An additional consideration in the live-bed condition (generally over- looked in prior studies) is the role of differing bedform morphology in the approach and contracted channels. HEC-18 provides equations for estimating contraction scour depth in straight rectangular channels subject only to vertical erosion. Existing equations are based on sediment transport theory using approaches developed over 50 years ago by Laursen (1960; live-bed contraction scour) and Laursen (1963; clear-water contraction scour). Both equations assume that the scour is due solely to the contraction effect and that local effects are negligible (i.e., that the contraction is hydraulically “long”), and both solve for the depth of flow (y2) in the contracted section after scour has occurred. The depth of scour of the bed material, (ys), is then calculated as the difference between two flow depths: ys = y2 – y0, where y0 is the depth of flow in the contracted section before scour occurs. C H A P T E R 1

1-2 Revised Clear-Water and Live-Bed Contraction Scour Analysis Analysis of existing laboratory datasets conducted under NCHRP Project 24-34 revealed that the clear-water contraction scour equation does not envelop the observed data as a design equation (Lagasse et al. 2013). Rather, it is a predictive equation that is seen to underpredict observed scour relatively frequently compared with scour estimates from pier and abutment scour equations. Although there are studies on live-bed scour at abutments, no laboratory datasets of live-bed contraction scour were identified during the NCHRP Project 24-34 study. Therefore, the live-bed contraction scour equation could not be assessed against observed data. However, there are contraction scour data embedded in live-bed abutment scour data (e.g., Ettema et al. 2010) that can be evaluated in the light of an improved contraction scour equation. The need for improved contraction scour estimates was also emphasized in NCHRP Web-Only Document 83: Scour at Contracted Bridges (Wagner et al. 2006). Practical, risk-based design methods for predicting total scour depths at bridge waterways necessarily involve accurate estimation of scour depth due to flow contraction through the bridge waterway. Although most bridge waterways impose relatively short flow contractions, bridge abutments and piers can produce conditions similar to long-contraction scour within the waterway, in addition to local scour at abutments and piers caused by flow obstruction and the resulting large-scale turbulence structures. Early work on contraction scour used rather simplified models of flow contraction. These models did not take into account additional factors that can influence scour depth that need to be considered for accurate estimation of the actual depth of contraction scour. The factors include the following: (a) There may be non-uniform slope of the water surface approaching and through the zone of flow contraction. (b) Contraction scour may evolve under unsteady flow conditions that occur when flow con- traction (especially at the higher discharges) causes choking of the approach flow. Choking may relax over time as scour develops. (c) For non-cohesive bed material, the presence of bedforms (usually dunes) in the scour zone may introduce uncertainty as to the average or maximum scour depth during live-bed conditions. (d) There may be variation in bed material within a waterway. Sands and gravels may form the bed of the main channel; silts and clay may predominate in riverbanks and underlying floodplains; and rock may be (or become) exposed at some locations. (e) For actual bridge waterways, vertical erosion may trigger geotechnical instabilities of waterway banks, and thereby also cause lateral erosion of a waterway. (f) Lateral erosion may become pronounced at bridge sites where the channel bed (notably rock) is more resistant to erosion than channel banks and earthfill abutments. (g) For some channel situations, the shape (a compound channel) and curved alignment (a channel bend) of a bridge waterway may introduce geometric complexity. (h) The presence of piers and abutments, as well as the interface between the main channel and floodplains, can introduce large-scale turbulence structures that may substantially affect the entrainment and transport of bed material in the contraction zone. Factors (a) through (h) raise interesting questions, including how to define “contraction ratio” for some bridge waterways, and how to treat contractions where large-scale turbulence structures enter the contraction scour zone. Factors (a) through (d) must be considered in the context of current recommended design practice. Factors (c) through (f) raise questions on how to estimate maximum scour in a scour zone of variable boundary material. Factors (g) and (h) introduce additional complexities for both the laboratory and computational analyses. The constraints of time and budget available for this project limited the investigation to addressing, primarily, factors (a) through (c).

Introduction and Research Approach 1-3   1.1.2 Objectives The objectives of this research were to develop live-bed and clear-water contraction scour equations suitable for use in risk-based bridge design encompassing a wide range of hydraulic conditions, including varying contraction ratios and various bed materials (cohesive, non- cohesive, and rock). NCHRP Research Report 971 addresses the following factors associated with the objectives: • Definition of contraction ratio • Summary of limitations/applicability of existing datasets and approaches • Multiple modeling techniques, such as physical and computational fluid dynamics (CFD) • Validation of models • Hydraulic force decay on bed • Applicability to new versus existing bridges • Relationship to other scour components • Applicability to 1D and 2D modeling given 2D data The objectives and the factors listed led to the following considerations aligned with the project’s scope: (a) Investigate vertical scour in a straight, rectangular channel formed with a bed of uniform non-cohesive sediment and erosion-resistant banks. One sediment size was considered: a fine sand. One contraction angle (45°) and three contraction ratios (Severe, Moderate, and Mild) were considered. (b) Investigate clear-water scour, whereby bed sediment is immobile in the approach reach upstream of the contraction. (c) Investigate live-bed scour, whereby bedload transport of sediment occurs in the approach reach. (d) Investigate scour depth associated with a long contraction. (e) Focus the analysis on the scour depth associated with a short contraction or the initial reach at the start of a long contraction (i.e., the so-called vena-contracta region of the contraction). These considerations are sufficient for determining the essential processes producing con- traction scour in a bridge waterway. Other factors, as mentioned in Section 1.1.1, may introduce complications but do not alter the essential processes. 1.2 Research Approach 1.2.1 Overview The research approach for this project consisted of a set of 10 tasks to produce improved contraction scour equations, taking into account the range of configurations of channel geo- morphology, bridge approaches, and flow contraction situations to be expected at most bridge waterways. The revised equations and the analyses supporting them address the eight bulleted factors listed in Section 1.1.2, and indicate how other factors may contribute uncertainty to contraction scour estimation for bridge foundations. These factors include currently overlooked interactions of flow and geotechnical aspects of bridge-waterway erosion, current attributes and limitations of both numerical and laboratory models, deficiencies in existing datasets on scour, and the concerted use of laboratory and numerical methods to address complex scour situations. The evaluations and recommendations previously developed for companion NCHRP projects concerning scour at bridge waterways were also considered.

1-4 Revised Clear-Water and Live-Bed Contraction Scour Analysis 1.2.2 Research Tasks A set of 10 tasks were conducted in two phases to implement the research plan. Phase I involved five information-gathering and planning tasks. Phase II comprised a further five tasks, including the laboratory and numerical work necessary to produce a final report documenting the project and presenting the improved contraction scour equations. Phase II also involved preparing educational materials and a Technical Memorandum for Implementation of Research Results (published as NCHRP Web-Only Document 294: Revised Clear-Water and Live-Bed Con- traction Scour Analysis Training Manual and herein referred to as the Training Manual) to facilitate application of the improved equations. Phase I Task 1: Review Technical Literature Task 2: Evaluate Existing Clear-Water and Live-Bed Contraction Scour Laboratory Data Task 3: Identify and Evaluate Field Data on Clear-Water and Live-Bed Contraction Scour Task 4: Develop a Laboratory Test Plan and Computational Applications Task 5: Produce an Interim Report Phase II Task 6: Conduct Laboratory Studies Task 7: Apply CFD and Other Computational Modeling Techniques Task 8: Develop Enhanced Clear-Water and Live-Bed Contraction Scour Equations Task 9: Prepare Educational Materials and a Technical Memorandum for Implementation of Research Findings Task 10: Submit a Final Report 1.2.3 Relationship to Other NCHRP Projects The Research Team considered recent prior work conducted as NCHRP projects, especially the following projects: • NCHRP Project 24-20, “Prediction of Scour at Bridge Abutments” [Draft Final Report: Estimation of Scour Depth at Bridge Abutments (Ettema et al. 2010)] • NCHRP Project 24-27(02), “Evaluation of Abutment-Scour Equations from NCHRP Projects 24-15(2) and 24-20 Using Field Data” [NCHRP Web-Only Document 181: Evalu- ation of Bridge-Scour Research: Abutment and Contraction Scour Processes and Prediction (Sturm et al. 2011)] • NCHRP Project 24-34, “Risk-Based Approach for Bridge Scour Prediction” [NCHRP Report 761: Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction (Lagasse et al. 2013)] • NCHRP Project 24-14, “Scour at Contracted Bridge Sites” [NCHRP Web-Only Document 83: Scour at Contracted Bridges (Wagner et al. 2006)] • NCHRP Project 24-18, “Countermeasures to Protect Bridge Abutments from Scour” [NCHRP Report 587: Countermeasures to Protect Bridge Abutments from Scour (Barkdoll et al. 2007)] These projects provided insights into contraction scour that were useful in re-analyzing existing methods for estimating scour depth and developing improved methods. Members of the Research Team participated in each of these projects.

Introduction and Research Approach 1-5   1.2.4 Summary The findings of Phase I of the project supported development of the test plan for laboratory and numerical modeling that was conducted in Phase II. The most important outcomes from Phase I included the following: • A detailed review of the approximately 10 existing methods for predicting contraction scour depth • An evaluation of existing laboratory data on clear-water and live-bed contraction scour • An evaluation of the fairly sparse (and quantitatively unreliable) sources of field data • A plan of study (in consultation with the NCHRP Project 24-47 panel) involving laboratory experiments and numerical simulation • Discussion of the procedures required to develop improved, risk-based clear-water and live-bed contraction scour prediction Chapter 2 of this report summarizes conclusions drawn from the literature review and evaluation of existing data. Chapter 3 describes the extensive series of flume experiments that provided observations and measurements of scour development in non-cohesive bed sediment. The flume experiments were augmented with insights obtained using 1D and 2D numerical models of flow through contractions and, where applicable, the use of three-dimensional (3D) numerical modeling to contribute to research objectives. Both the flume and numerical experiments examined contraction scour in the vicinity of the contracted reach (including short contractions typical of most bridge reaches) and along the so-called long contraction. Chapters 4, 5, and 6 present the results of clear-water laboratory testing, live-bed laboratory testing, and rigid-bed laboratory testing, respectively. Chapter 7 provides the results of 1D, 2D, and 3D modeling to supplement the results of laboratory testing. Chapter 8 presents the suggested revised clear- water and live-bed contraction scour equations and provides an appraisal of results, including application examples. Chapter 8 also includes a plan for implementation of research results and reference to the stand-alone Training Manual (NCHRP Web-Only Document 294) that supports the implementation plan. Finally, Chapter 9 provides conclusions from this research effort and identifies research opportunities that would extend the results of this research project.