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28 CHAPTER 4. SCOUR CONDITIONS One method for classifying abutment scour depends on abutment location in a channel, the relative erodibilities of sediments forming the main-channel bed and soils forming the floodplain (see Figure 2-10), as well as to the shear strength of the compacted earthfill forming the approach embankment. In addition, other conditions such as stream morphologic changes and lack of control of highway runoff can lead to abutment scour under unexpected and less well- defined circumstances. 4.1 THREE COMMON CONDITIONS OF ABUTMENT SCOUR Figure 4-1a-c illustrates the three scour conditions for spill-through abutments: 1. Scour Condition A. Scour of the main-channel bed, when the channel bed is far more erodible than the floodplain. Figure 4-1a illustrates how scour of the main-channel bed causes the main-channel bank to become geotechnically unstable and collapse. The collapsing bank undercuts the abutment and embankment, which in turn collapses locally. Soil, and possibly riprap, from the collapsed bank and embankment slide into the scour hole; 2. Scour Condition B. Scour of the floodplain around the abutment. This condition also is equivalent to scour at an abutment placed in a rectangular channel, if the abutment is set back from the main channel. As the amount of bed-sediment transport on a floodplain usually is quite low, this scour condition usually occurs as clear-water scour. Figure 4-1b shows that the floodplain scours around the abutment, and especially slightly downstream of it. The scour hole locally destabilizes the embankment side slope, causing embankment soil, and possibly riprap, to slide into the scour hole; and, 3. Scour Condition C. Scour Conditions A and B may eventually cause the approach embankment to breach near the abutment, thereby fully exposing the abutment column. For this condition, scour at the exposed stub column essentially progresses as if the abutment column were a pier, as illustrated in Figure 4-1c. For the same reasons as given for Condition B, this scour condition usually occurs as clear-water scour.
29 Figure 4-1. Abutment-scour conditions: Scour Condition A - hydraulic scour of the main channel bed causes bank failure, which causes a failure of the face of the abutment embankment (a); Scour Condition B - hydraulic scour of the floodplain causes failure of the face of the abutment embankment (b); and, Scour Condition C - breaching of the approach embankment exposes the abutment column so that scour progresses as if the abutment were a form of pier (c) (Ettema et al. 2010).
30 The three scour conditions may occur also for wing-wall abutments. However, a couple of additional erosion processes can result in failure of the main-channel bank and the approach embankment: 1. The local flow field generated at the corners of the abutment can cause local scour at those locations; and, 2. Exposure of the piles beneath the abutment pile cap can cause river-bank and embankment soil to be eroded out from beneath the pile cap. Provided no substantial geotechnical failure of the abutment occurs for scour Conditions A and B, scour deepens to an equilibrium level commensurate with the abutment flow fieldâs capacity to attain a balance with the rate of sediment inflow to the scour region (live-bed scour) or the channel boundaryâs resistance to erosion (clear-water scour). A scour event (or series of events) at an abutment, may involve a sequence of all three scour conditions, resulting in several local maxima for scour depth for a wing-wall abutment. When an abutment is close to the main channel, Condition A may develop relatively quickly, with Condition B occurring at a slower rate. Either, or together, Scour Conditions A and B may eventually cause the approach embankment to undergo a slope-stability failure. If the embankment extensively washes out, so as to expose the abutment structure, scour may then develop at the abutment structure as if the abutment were a form of pier (Condition C). Accordingly, an important design consideration is that the stub or wing-wall abutment should not fail when exposed; i.e. foundations of wing-walls should be deep enough that the wing-walls do not fail when exposed to a pier-like scour condition. For design estimation of scour depth, it is useful to consider the likely rates or sequences in which the three scour conditions developed, and to ask -- What is the greatest scour depth that reasonably could occur near the abutment? Will that scour depth pose a slope-stability problem for the earthfill embankment adjoining an abutment foundation or for the floodplain bank of the main channel? What is the deepest scour that could occur at the abutment column foundation itself, and does that scour occur when the embankment is breached so as to fully expose the abutment column? The set of photographs in Figures 4-2 through 4-4 depict situations where Scour Conditions A, B, and C occurred at bridge abutments.
31 Figure 4-2. Field example of Scour Condition A. Figure 4-3. Field example of Scour Condition B.
32 Figure 4-4. Field example of Scour Condition C for a wing-wall abutment. 4.2 INFLUENCE OF PIER PROXIMITY The influence of pier proximity on the three scour conditions is slight, at least for the pier form and construction depicted previously in Section 2.5. Flume experiments (NCHRP 24-20) show that abutment scour is dominated by the flow field established by an abutment. Once scour initiates, and deepens below the pierâs pile cap, pier presence does not substantially increase flow contraction or the strength of large-scale turbulence structures. For Scour Condition A at spill-through abutments, pier presence may increase maximum scour depth by approximately 10% when Lp/W < 2; where, W = embankment top width and Lp = distance from abutment to pier. The increase results because pier presence close to an abutment slightly increases flow contraction, as flow is deflected around the pier (as if the abutment were lengthened). For Scour Condition B, pier presence acts to increase flow contraction but it also acts to partially block the dispersal of riprap stone. The net influence for Scour Condition B is a lessening of scour depth. 4.3. OTHER SCOUR PROCESSES Abutment scour may develop consequent to several processes of flow and bed-sediment movement: 1. Localized scour attributable to change in main channel alignment and morphology, which adversely affects abutment location and orientation relative to flow in the main channel. Lateral shift of a channel may direct flow adversely towards abutments not designed for a lateral shift in the channel thalweg. The deeper scour commonly resulting from this possibility must be considered in the scour design of abutments; 2. Scour of the approach embankment flank on the floodplain. This condition may occur when the floodplain flow converging towards the bridge waterway undercuts the flank of
33 the approach embankment. This scour mechanism differs from those discussed in Chapter 4, and is less common; 3. Erosion along the flanks of an abutment, which may develop because of inadequate control of road drainage along an abutment. Such erosion exposes the earthfill at the end of the abutment, making the abutment more prone to erosion by flow in the main channel; and, 4. Degradation of the main channel bed. This process occurs in response to an overall propensity of the main-channel flow to degrade associated with the reduction in the bed- sediment load along the channel. It also could result from the upstream advance of head- cutting of the channel bed, because the channel has steepened hydraulically. Bank erosion with channel widening may accompany degradation and lead to erosion attack of the embankment.
