Evaluation of Bridge-Scour Research: Abutment and Contraction Scour Processes and Prediction (2011)

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 This report reviews the present state of knowledge regarding bridge-abutment scour and evaluates the leading methods currently used for estimating design scour depth. It focuses on research information obtained since 1990, which is to be considered in updating the scour estimation methods that are recommended by AASHTO2 , and used generally by engineering practitioners. Though considerable further progress has been made since 1990, the findings indicate that several important aspects of abutment scour processes remain inadequately understood. Moreover, the current methods for scour depth prediction do not adequately take into account the physical scour processes. The state-of-art for abutment scour prediction is considerably less advanced than that for pier scour prediction. The review and its recommendations 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 abutment scour estimation and countermeasure design, and provide clear direction as to further research. The recommendations are particularly intended to be used by AASHTO in developing policies and procedures for use in addressing bridge abutment scour. The review draws upon a broad range of sources of information regarding abutment scour, including agency reports, books, and technical papers. Close attention was given to recent NCHRP project reports on abutment scour; e.g., “Estimation of Abutment Scour” (NCHRP Project 24-20), “Abutment Scour in Cohesive Materials” (NCHRP 24-15(2); and, “Abutment Scour Countermeasures” (NCHRP Project 24-18). Additionally, the review builds on that by Parola et al. (1996), who provide a useful earlier wide-ranging assessment of research needs regarding bridge waterway scour. 1.1 DEFINITIONS At the outset of this report it is necessary to define the terms abutment, abutment scour, and abutment failure. These terms are not clearly defined in scour literature or in the common vernacular about scour. Abutments comprise several structural parts, notably an abutment column supporting one end of a bridge deck, and the column which is set amidst, or backed by, a compacted earthfill approach embankment. This review may use the term “embankment/abutment” to describe the full structure – approach embankment and abutment column structure, but where necessary the separate terms will be applied for clarity. The “embankment” is considered to be the earthfill that extends from the abutment column into the floodplain away from the stream, while the term “abutment” refers to the column and the support structure facing the stream. Chapter 2 describes the main features of abutment structure and form. 2 AASHTO ~ Association of American State Highway and Transportation Officials

10 Abutment scour herein is taken to be scour at the bridge-opening end of an abutment, and directly attributable to the flow field developed by flow passing around an abutment. It includes the effects of flow acceleration due to channel flow constriction as well as local large- scale turbulence effects due to flow separation which are present in varying relative proportions depending on the upstream approach flow distribution and flow distribution at the bridge section; abutment column type, foundation type and location; flow curvature; and near-field river morphology. Other flow and channel-erosion processes cause scour at abutments. One such process often leading to abutment failure is lateral erosion and shifting of the approach channel immediately upstream of an abutment as part of a long-term geomorphic process; the approach flow then impinges against the abutment flank. Many field observations of abutment scour mix abutment scour (as defined above) and scour caused by channel shifting. Chapter 4 explains the current understanding of abutment scour. Abutment scour may cause embankment failure, abutment column failure, or both. Observations of abutment scour indicate that scour frequently may initiate a geotechnical- type failure of the earthfill embankment. Failure of the abutment column itself is less commonly observed. Although failure of the embankment may occur with the abutment column (and bridge structure) remaining intact, it is a most undesirable condition that renders the bridge approach dangerous for road vehicles. Chapter 3 elaborates abutment scour conditions and the various modes of bridge abutment failure. 1.2 MOTIVATION FOR REVIEW The need to evaluate present knowledge about abutment scour processes and failure conditions, and determine the extent to which existing scour-estimation methods reflect this knowledge, is expressed in several publications prepared by national agencies and societies in the US: e.g., NCHRP Reports 24-08 (“Scour at Bridge Foundations: Research Needs”) and 20- 07(178) (Parola et al. 1996, Lagasse and Zevenbergen 2004), as well as NCHRP Report 417 (Parola et al. 1998), USGS (2003), and Kattell and Eriksson (1998). However, few situations of water flow and boundary erosion are more complex and challenging to understand than those associated with scour of bridge abutments. The sketches in Figures 1-1 and 1-2 convey a sense of the complexities faced during estimation of scour depths at bridge waterways. Abutment sites may vary widely in their specific details. Figure 1-1 illustrates a wide, multi-span bridge whose abutments are considerably set back from the bank on broad flood plains. As depicted in Figure 1-2, the abutments for shorter bridges are in close proximity to each other; in such cases the abutments often may be set close to the bank of a channel whose morphology is quite irregular and varies markedly with flow stage. Both Figures 1-1 and 1-2 indicate how flow approaching a bridge waterway converges then diverges once through it. As it does so, it passes around bluff bodies, generating, transporting, and eventually dissipating large-scale turbulence structures (large eddies shed in a recognizable pattern due to flow separation albeit intermittently with time). The flow is bounded by erodible boundaries of complex and changing form that have widely varying compositions and characteristics. Even the classification of abutment scour as an independent bridge scour component is problematic, because contraction scour and abutment scour are linked processes that usually occur together during flood events.

11 Figure 1-1. Schematic of long, multi-span bridge over a compound channel. Figure 1-2. Schematic of relatively short bridge over a narrow main channel. Furthermore, the sketch in Figure 1-3 shows the effect of hydraulic erosion of bed and banks on the integrity of certain boundary components (banks and embankments) after a geotechnical slope-stability failure. Such failures add additional complexity to waterway flow and scour, and thereby to scour-depth estimation. It can be readily appreciated from Figures 1-1 through 1-3 that scour indeed is a long-standing and vexing problem in hydraulic-engineering research, not to mention bridge foundation design.

12 Figure 1-3. Abutment scour resulting in embankment failure by collapse due to geotechnical instability. The development of practical design methods for predicting scour depths at bridges has been hampered by inadequate knowledge about, and formulation of, important component processes and their interaction during scour. The scour-estimation methods presently available do not adequately take all these considerations into account. As would be expected, early work on abutment scour focused on the simpler and idealized situations of scour; notably, abutment scour simulated as scour at a rigid structure extending at depth into a bed of uniform sand. Commensurately, the existing relationships and guidelines apply to simplified abutment situations, such as an abutment placed in a straight rectangular channel, and are roughly based on empirical or regression equations fitted to a collection of data from laboratory tests with model abutments (whose construction does not always resemble that of actual abutments). Such design relationships can only be extrapolated with considerable uncertainty to actual field conditions. That extrapolation often results in overly conservative estimates of scour depth. Conservatism is understandable and indeed useful, but can be expensive for large abutment structures. Moreover, when existing design methods inadequately embody certain scour processes, there is a risk that the manner or location of actual scour failure will differ from that assumed for the estimation relationship or guideline. Additionally, an overlooked process may trigger or exacerbate scour at a site where a scour problem had not been anticipated. There are several prominent knowledge gaps about processes whereby scour could occur in ways and places not accounted for by existing prediction methods or programs of bridge monitoring (e.g., geomorphic change in channel alignment, inadequate estimation of peaks and periods of design flows, proximity of old or new bridges, the role of large-scale turbulence, inadequate control of storm-water drainage at the bridge site). These gaps are not only limited to flow and geomorphic processes but also relate to sediment type in terms of fine-grained sediments, which experience interparticle physico-chemical forces, and coarse grained sediments whose movement is resisted by gravity forces alone.

13 The threat posed by scour was realized early in the struggle to construct and maintain bridges. Over the ages it has been dealt with in several ways, but the threat has not yet been adequately addressed. In antiquity, for instance, Roman engineers recognized the threat. Whenever they built a new bridge they usually would place on the bridge an appeasing inscription to Janus, the Latin god of bridges (and portals generally), or to the local deity of the river or stream being crossed. Engineers in Japan and Korea reduce the threat for bridge abutments at major, levee- flanked rivers flowing through heavily urbanized regions. They do so by not locating bridge abutments on the floodplain, but instead locating them outside the levee; in this manner, flow contraction through a bridge waterway is minimized or practically eliminated, and the abutments are not exposed to scour. On the whole, though, bridge scour continues to be a threat. The case depicted in Figure 1-4 is an example of scour failure that occurred fairly recently (1993 flood in Midwestern U.S.) for an unusually large flow that exceeded the design flow for the bridge waterway. The maximum scour depth measured two months after the flood was 17 m in the floodplain on the upstream side of the bridge (Parola et al. 1998). Figure 1-4. Scour at I-70 bridge over Missouri River from 1993 flood. Flow was from left to right. (Photo from Parola et al. 1998).

14 1.3 OBJECTIVES The principal objectives of this review are as follow: 1. Complete a critical evaluation of knowledge about abutment scour processes, using especially research conducted since 1990; 2. Compare current scour-prediction practice with the present understanding of scour processes; and, 3. Develop recommendations for adoption of specific research results by AASHTO and use by the engineering community in general. In pursuing these objectives, the review constructs a well-illuminated explanation of the physical processes attendant to scour at abutments, and delineate the validity limits of existing scour- prediction methods. From this basis, the review indicates the prospects for substantial improvements in estimating scour depths, and thereby better abutment design. Two points require emphasis at the outset of this report: 1. The state of knowledge regarding abutment scour considerably lags that for pier scour; and, 2. A major education effort is needed to better inform engineers about the processes associated with abutment scour. In particular, the geotechnical aspects of abutment scour and failure have received inadequate attention heretofore. 1.4 Approach Three important considerations guide the approach taken for the present review: 1. Abutment scour must be viewed from the perspective of flow and scour through the entire bridge waterway, because abutment scour normally cannot be dissociated from flow and bathymetric conditions across the bridge waterway; 2. Abutment construction influences scour, as the type of abutment affects maximum scour depth and location; and, 3. Abutment scour comprises processes of hydraulic erosion, which may cause geotechnical instability of the embankment earthfill and possibly the foundation upon which the abutment is based. These considerations lead to the necessary insights regarding abutment scour, and provide the requisite framework of inquiry for understanding abutment scour.