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208 From the literature review and laboratory studies, several key points emerge. The following sections summarize the conclusions for countermeasures involving wing-wall abut- ments, spill-through abutments, and flow guidance. 11.1 Wing-Wall Abutments Experiments were performed for a wing-wall abutment located close to the main channel edge. Countermeasures tested were riprap, cable-tied blocks, geobags, parallel walls, spur dikes, and collars. In addition, two-dimensional model- ing of the flow field was compared with laboratory results. Test were also performed in a large-scale flume, and the results were compared with experimental results. 11.1.1 Riprap, Cable-Tied Blocks, and Geobags Local scour in the general vicinity of an abutment cannot be eliminated completely by an apron of riprap or geobags, because an apron only shifts the scour region away from an abutment. The experiments show that an apron can prevent scour from developing at the abutment itself, but significant scour can occur readily near the downstream edge of the apron. A possible concern in using an apron is to ensure that shifting of scour does not imperil a nearby pier or portion of riverbank. Moreover, if the scour is likely to extend to an adjacent pier, then the abutment and pier countermeasure apron should be placed so as to protect both elements of a bridge. The experiments show that it is necessary to protect the following regions of the river bed and banks near an abutment: ⢠The river bed at the abutment pile cap, ⢠The riverbank immediately upstream of the abutment and a short distance downstream of the abutment, ⢠The side slopes of embankment immediately behind the abutment (i.e., the standard stub for a wing-wall abutment or spill-through abutment), and ⢠The area beneath and immediately behind the pile cap. For use of riprap or cable-tied blocks alone, the following conclusions emerged from this study: ⢠For the range of experimental investigation in this study, the scour at wing-wall abutments in live-bed conditions directly relates to the level of the deepest bed-form trough that propagates past the abutment (which can be predicted using existing expressions) and to any localized scour that may occur. ⢠Stones on the outer edge of riprap aprons tend to settle and move away from the abutment, thereby pushing the troughs of the bed forms farther away from the abutment. Conversely, cable-tied block mats remain intact during set- tlement. The outer edge of the apron settles vertically, allowing the troughs of the bed forms to pass closer to the abutment face than for an equivalent riprap apron. ⢠Equations 7-10 and 7-11 allow prediction of the minimum apron width remaining horizontal after erosion. Equation 7-12 allows prediction of the horizontal distance between the abutment face and the point of deepest scour. These pre- dictions, along with prediction of apron settlement, facilitate assessment of the stability of an abutment structure. With regard to the specific use of geobags for wing-wall abutments, the following conclusions can be drawn: ⢠Geobags are a promising alternative to riprap for use as a bridge abutment scour countermeasure. ⢠It is necessary to connect the geobags placed as an apron around an abutment. The initiation of the failure of geobag apron, shown in Figure 7-58, was due to the failure of an individual geobag placed in front of the abutment. C H A P T E R 1 1 Conclusions
⢠The apron should have a perimeter toe whose lower level approximately coincides with the average elevation of dunes moving through the channel in the vicinity of the bridge. ⢠The geobags should be placed in a shingled manner, whereby adjoining geobags overlie joints between underly- ing geobags. ⢠It is necessary to place geobags or riprap immediately under the pile cap in order to prevent the winnowing of embankment sediment from beneath the pile cap. ⢠Geobags may serve as a useful alternative to a geotextile fil- ter cloth placed beneath a riprap apron because geobags are more readily placed than an underlay cloth for blocking the winnowing of sediment from between bed-armor ele- ments like riprap stone. However, the geobags should be placed somewhat below bed level so as not to increase riprap exposure to flow. 11.2 Spill-Through Abutments The flowfield and the behavior of riprap and cable-tied blocks were studied. Pertinent conclusions are as follows. 11.2.1 Abutment Flow Field The following conclusions can be drawn from the abut- ment flow field study: ⢠Velocity, vorticity strength, and normalized bed shear stress in the flow field at the end of the abutment all increase with increasing abutment length and floodplain width. ⢠The normalized bed shear stress on the main channel bank upstream of the abutment increases significantly as the abut- ment setback distance from the main channel bank decreases. The transverse component of the flow that is diverted around the abutment is strongest at the upstream end of the abut- ment. The smaller the setback distance is, the stronger the transverse velocity component over the top of the main chan- nel bank is. The transverse velocity component destabilizes the bed material on the main channel bank, making the bed material susceptible to significant erosion in that region. ⢠Regions of high vorticity in the wake of the abutment cor- respond to the zone where scour is initiated. There is a strong correlation between the scour hole position and the line of strongest vorticity. ⢠The effect of placing a protection apron around an abut- ment is to inhibit the initiation of scour at the point where the vorticity is strongest. Consequently, the scour hole devel- ops farther away from the end of the abutment, where the bed is unprotected. The vorticity strength is weaker farther away from the abutment, thereby decreasing the size of the scour hole. ⢠Zones of excess shear stresses are responsible for the ero- sion that occurs near the main channel, where the vorticity is weaker. 11.2.2 Riprap and Cable-Tied Blocks in Clear-Water Conditions The conclusions from the spill-through abutment experi- mental study are as follows: ⢠Apron countermeasure protection at spill-through abut- ments does not significantly reduce the depth of local scour. Rather, the apron deflects the scour development away from the end of the abutment, preventing the devel- oping scour hole from undermining the abutment toe. ⢠By increasing the apron extent, the scour hole is deflected farther away from the end of the abutment. When the scour hole forms on the floodplain, the depth of scour typically reduces as the scour hole is deflected farther away. How- ever, for abutment and compound channel configurations where the scour hole forms close to the main channel bank, the depth of scour relative to the bed of the floodplain increases as the scour hole is deflected away from the abut- ment and into the main channel. ⢠The scour depth for spill-through abutments, situated on the floodplain of a compound channel, is given by Equa- tion 8-13. Alternatively, the scour depth can be deter- mined by Equation 8-19, using the abutment flow field parameters. ⢠Cable-tied block aprons allow scour holes to form closer to the abutment than equivalent riprap aprons. Therefore, wider cable-tied block aprons are needed to provide the same level of protection to an abutment as equivalent riprap aprons. Consequently, riprap is likely to be eco- nomically preferable to cable-tied blocks as a form of apron protection at spill-through abutments. ⢠The minimum apron width required to prevent under- mining of the toe of the spill-through abutment is related to the depth of scour and is given by Equation 8-16. ⢠The extent of apron protection required to ensure that spill-through abutment fill material is stable can be deter- mined using the design procedure given in Equation 10-15. 11.2.3 Two-Dimensional Modeling Conclusions regarding the two-dimensional modeling using FESWMS are as follows: ⢠The local peak velocity on the floodplain and in the main channel was well predicted by the two-dimensional model, although the location of the local maximum was not. 209
⢠The average velocity in the channel was well predicted by the two-dimensional model. One unusual velocity obser- vation in the particle-tracking velocimetry (PTV) meas- urements causes the PTV point data to deviate from the two-dimensional model results. The average velocity for the PTV floodplain data is significantly higher than that of the two-dimensional model for transects 8.18 and 8.67. ⢠The PTV-measured velocity in the floodplain (from 0 m to 1.6 m along the transect) is higher than that computed from the two-dimensional modeling. 11.2.4 Large-Scale Tests of Riprap Apron Performance Relative to abutment position, the locations of deepest scour coincide reasonably well. Scour depths, though, were proportionately less for the large-scale tests. Given the differences in geometric scale and layout of abut- ment for the large-scale test, this agreement is a substantial validation of the design relation developed from the smaller- scale tests presented in Section 8.1. 11.3 Flow Guidance Three flow modification countermeasures were investi- gated in this project for scour reduction at wing-wall abut- ments located close to the main channel bank: parallel walls, spur dikes, and abutment collars. 11.3.1 Scour with No Countermeasures In order to understand abutment scour and its mecha- nisms, scour without any countermeasures was explored first. Scour was studied for both clear-water and live-bed scour conditions. 11.3.2 Clear-Water Conditions Conclusions regarding the clear-water conditions are as follows: ⢠As shown in Figure 9-5, five locations of scour were found in the whole scour region, provided that the roughness on the floodplain was the same as it was in the main channel and the velocity ratio between the floodplain flow and the main channel flow was relatively high. The floodplain flow tended to shoot into the main channel at a distance upstream from the upstream corner of the abutment. Under this condition, the maximum scour in the whole region was normally found in Zone B of Figure 9-5. ⢠The more the floodplain flow was constricted at the abut- ment, the deeper the scour hole would be at the upstream corner of the abutment. When the roughness on the flood- plain was increased and the velocity ratio between the floodplain and the main channel decreased, the floodplain flow moved to the corner of the abutment. As a conse- quence, the scour zones A, B, and E from Figure 9-5 con- verged into a single scour hole, and the maximum scour depth was found at the upstream corner of the abutment. ⢠The principal vortex systems and secondary vortex sys- tems at the upstream corner of the abutment were stronger than the wake vortex systems at the downstream corner of the abutment. Consequently, the scour hole induced by the principal vortex systems and by the sec- ondary vortex systems reaches equilibrium more quickly than the scour hole induced by the downstream wake vortex systems. ⢠Clear-water scour with no countermeasures resulted in a scour depth of 77.7 mm. 11.3.3 Live-Bed Conditions Conclusions regarding the live-bed conditions are as follows: ⢠Maximum scour took place at the upstream corner of the abutment. Time-averaged scour depth at the upstream cor- ner was less than the scour depth under critical clear-water conditions, while instantaneous scour depths were between values near zero to values nearly twice the maximum scour under clear-water flows because of the superposition of the trough of the bed forms (see Figure 9-10). ⢠Live-bed scour reaches equilibrium more quickly than clear-water scour. 11.3.4 Parallel-Wall Countermeasures Conclusions regarding the parallel-wall countermeasures are as follows: ⢠A parallel solid wall attached at the upstream corner of the abutment parallel with the flow can be used as a counter- measure against abutment scour. The length of the solid wall should be 1.6L to obtain an acceptable scour reduction rate at the abutment for the conditions tried in this study. ⢠A parallel solid wall attached at the upstream corner of the abutment parallel with the flow may or may not be able to reduce the amplitude of the bed forms that pass through the bridge opening, depending on the changes of the flow parameters from the approaching channel after entering the bridge crossing. ⢠There may be significant scour at the upstream solid wall end, so no other structures should be located in this region. ⢠Parallel rock walls attached at the upstream of the abut- ment can also be used as countermeasures against scour at 210
the abutment. The foot of the wall should not protrude into the main channel beyond the abutment, and a top wall length of 0.5L will provide sufficient protection. The side slope of the rock wall should be on the order of 30 degrees, but in no case should it be steeper than about 70 percent of the rocksâ angle of repose. ⢠Rock walls have more advantages than solid walls in terms of efficiency, stability, and cost. 11.3.5 Spur Dikes Under Clear-Water and Live-Bed Scour Conclusions regarding the spur dikes under clear-water and live-bed conditions are as follows: ⢠A single spur dike made of a solid plate having the same protrusion length as, or less protrusion length than, the abutment and placed upstream of the abutment was not able to protect the abutment. The downflow and the prin- cipal vortex are very strong at the stream end of the struc- ture. As a consequence, a huge scour hole was always found at the end of the structure, which threatened both the structure and the channel bank. ⢠Rock spur dikes show several advantages over rigid spur dikes and are preferred. ⢠Three rock spur dikes-as configured in Tests Sp-9 (Table 9-9) and Sp-13 (Table 9-10)-were considered the best configura- tion for protecting the abutment. This configuration can provide 100-percent protection to the abutment under the velocity ratios of 0.9, 1.5, and 2.3. Two spur dikes at the upstream and downstream corners of the abutment were also successful at preventing scour in both clear-water and live- bed experiments. 11.3.6 Abutment Collars in Clear-Water Scour Conditions Conclusions regarding abutment collars in clear-water conditions are as follows: ⢠Collars were found to be effective at preventing local scour at vertical-wall bridge abutments. The collars isolated the turbulent flow and vortex systems from the bed material and thereby prevented the bed underneath the collar from scouring. ⢠The farther the collar extended downstream of the abut- ment, the farther downstream the scour hole was located. As the transverse width of the collars increased, the depth of the scour hole at the edge of the collar decreased. The scour became insignificant as the main channel edge of the collar was extended beyond the local scour hole area meas- ured in the baseline case without countermeasures. The trailing edge of the collar should extend to a location downstream of the abutment. ⢠Based on these experiments, the collar elevation should be 0.08ym below the original bed level, and the collar width should be at least 0.23L, where L is the abutment length perpendicular to the flow direction. 211
