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1Problem Statement Most of the techniques and guidelines that are available for protecting bridge abutments against scour have been developed from small-scale, hydraulic modeling conducted in laboratories, and a limited amount of empirical data along with anecdotal observations has been acquired from field sites. Though quite useful advances have been made with scour-protection countermeasures, there is a widely recognized need for a more extensive study, one that links modeling efforts in the laboratory to priorities of countermeasure needs and to observed field performance of counter- measures. In addition, there is a perceived need to explore innovative concepts for scour counter- measures. None of the existing approaches has been totally successful, as bridge abutments and their approach embankments are the most commonly damaged bridge components during floods. It has been recognized, therefore, that along with new countermeasure concepts, better design and construction guidelines need to be developed to protect bridge abutments and approach embankments from scour damage and to reduce the depths to which expensive deep foundations may have to be placed. In addition, there are substantial needs for guidelines and selection criteria that address limitations imposed by environmental regulation, relative cost, availability, serviceability, constructability, and design constraints. Such guidelines will assist prac- titioners in preventing, reducing, or mitigating the damage incurred with abutment failure owing to scour. Scour and Abutment Forms Scour is caused by a complicated flow pattern through the bridge opening consisting of downward flow and vortices around the abutment leading edge and rear. In addition, the river- bank can be eroded and thereby remove soil from behind the abutment. Other processes can also threaten abutments, such as riverbed degradation, headcuts, river meander migration, and embankment-eroding drainage from the roadway. Two primary types of abutments are commonly used: wing-wall abutments, which have a ver- tical wall and are typically close to the main channel banks, and spill-through abutments, which are typically located back from the main channel banks on the floodplain. Wing-wall abutments are typically found on smaller streams, while spill-through abutments are on wider rivers. In addition, the orientation of the bridge may vary from being lateral, angled upstream, or down- stream in relation to the flow. Each orientation changes the scour behavior in complicated ways. Countermeasure can be located on existing bridges or on new ones, with construction possibly more difficult on existing bridges with limited access to the bridge abutment. The proximity of the first pier to the abutment can be a problem since the scour holes from each can merge and further complicate countermeasure placement. S U M M A R Y
Countermeasure Concepts and Criteria Abutment scour can be mitigated by several approaches, including upstream or downstream channel control, armoring, flow modification, bridge modification, and drainage control. Upstream channel control can be accomplished by spur dikes, hard points, or vanes that prevent a channel from migrating laterally and thereby bypassing the bridge opening. Downstream con- trol includes a weir or checkdam to prevent headcuts from migrating upstream and threatening the bridge. Armoring consists of riprap or cable-tied blocks that protect the soil from scour. Bridge modification means adding an additional span to allow increased flow area, and flow modification entails guiding the flow smoothly through the bridge opening, typically with a wall of some kind. Drainage control ensures no adverse impact from drainage water around the bridge. The criteria for selecting a countermeasure usually encompass the following set of con- siderations: technical effectiveness (including no substantial adverse effects), constructability, durability and maintainability, aesthetics and environmental issues, and cost. Survey Findings A survey was conducted of the state offices of the U.S. Departments of Transportation. The results revealed that the most common form of scour countermeasure was riprap. Other methods were employed on a more limited basis, including sheet-piling and grout bags. Monitoring, most often by visual inspection, was employed by all the respondents with varying frequency. Wing-Wall Experimental Results Local scour in the general vicinity of a wing-wall abutment next to the main channel cannot be eliminated completely by an apron of riprap or geobags. An apron shifts the scour region away from an abutment. The experiments show that an apron can prevent scour from developing at the abutment itself, but that 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 ele- ments of a bridge. It is necessary to protect all areas around an abutment. For the use of geobags (sacks of geotextile material filled with gravel), it is necessary to tie them together and extend the mat thus formed at the lowest dune level under the pile cap. The results obtained show that decreasing wall angle (from 90 degrees) to flow reduces the scour depth under either live-bed or clear-water conditions of scour. Decreasing the wall angle at an abutment was observed to weaken downflow and the horseshoe vortex. Accordingly, an approach-flow guide wall likely can be effective in reducing scour depth at a vertical-wall or wing- wall abutment. The brief ancillary experiments on scour at various alignments of abutment show that scour depth is at maximum when an abutment is perpendicular to the channel crossed. Spill-Through Abutment Results The results of the characterization of the flow field through a bridge opening with a spill- through abutment show that the velocity increases at the abutment as the flow accelerates toward and past the end of the abutment, causing a local increase in bed shear stress on the floodplain. There is a small plan-view counterclockwise rotation in the flow field at the upstream corner of the abutment and a larger plan-view counterclockwise rotation in the flow field downstream of the abutment, which extends out past the end of the abutment and increases with abutment length. There is also a smaller clockwise rotation in the flow field at the downstream corner of the abutment next to the larger region of counterclockwise rotation. The apron protection around the abutment inhibited the development of scour at the abutment toe. Scour was initi- ated at the edge of the apron, increasing in depth with the passage of time. As the scour hole deep- ened, bed material on the sides of the scour hole would fall into the scour hole, progressively undermining the protection apron. The response of the apron to the undermining process 2
depended on the protection type. Two-dimensional numerical modelling was also performed on this flow field and was found to be within about 15 percent of the experimental velocity results. As the riprap aprons were undermined, the stones at the outer edge would roll into the scour hole, protecting the bed of the hole from further scour. This would deflect the erosion zone far- ther away from the abutment. As the cable-tied block aprons were undermined, the outer edge of the apron would fold down onto the side of the scour hole, because the cables would prevent the blocks from sliding into the scour hole. As the apron folded down onto the side slopes of the scour hole, the horizontal distance between the toe of the abutment and the edge of the apron decreased, allowing the erosion zone to move closer toward the abutment. The scouring process would continue until the equilibrium scour depth was reached. The velocity flow fields mentioned above, measured at the abutments, showed that the velocity at the contracted bridge section increased with increasing abutment length and floodplain width. Both parameters have the effect of reducing the flow area at the contracted bridge section, thereby increasing the velocity and flow strength. Consequently, the vorticity and bed shear stress also increase with increasing abutment length and floodplain width. Concurrently, similar effects were observed regarding the influence of the abutment length and floodplain width on the equilibrium scour hole depths, showing that the scour hole size is related to the flow field around the abutment. Flow Modification Experimental Results Experiments were performed investigating the use of a wall composed of piled rocks. The wall extended parallel to the flow and was higher than the flood level, thereby smoothing flow through the bridge opening and also preventing return floodplain flow from scouring the abutment foun- dation. These parallel walls were found to work well and reduce scour at the abutment substan- tially. Solid walls did not work as well and could have foundation problems in a prototype bridge. Spur dikes located locally to the abutment were also modeled and found to work well. Three dikes were warranted, one upstream of the abutment, one at the upstream abutment corner, and one at the downstream corner. If the abutment were sufficiently long, then other dikes could be located in between the two corner dikes. Abutment collars were also investigated and found to work well, but could cause problems with debris and could be difficult to construct. Design Guidelines and Suggestions A complete set of design guidelines and suggestions have been developed, consisting also of criteria for selection of the most appropriate countermeasure. For bed degradation, some kind of grade-control structure can be used. For meander migration, upstream control needs to be implemented. For local protection of the abutment, either the flow can be modified or the mate- rial armored. For flow control, one can align the approach-channel banks, shift the abutment back and add a bridge span, add a relief bridge, or place flow-deflection spur dikes or guidebanks upstream of the bridge. For armoring, one can use riprap, cable-tied blocks at the abutment and/or at the drainage outlets, parallel rock walls, or spur dikes locally at the abutment. Selection should be guided by a life-cycle cost assessment, including environmental impacts. Study Participants Work for this project was performed at various locations. The riprap and cable-tied block experiments were carried out at the University of Auckland, New Zealand. Experiments on geobags and a large-scale experiment on riprap on spill-through abutments were carried out at the University of Iowa. Experiments on parallel walls and spur dikes were performed at the U.S. Department of Agriculture Agricultural Research Service (USDA-ARS) National Sedimentation Laboratory. The researchers involved visited the other laboratories involved, and the results are synthesized in this final report. 3
