Scour at Contracted Bridges (2006) / Chapter Skim
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From page 18... ...
7 CHAPTER 2: FINDINGS EXECUTIVE SUMMARY OF LITERATURE REVIEW AND FIELD DATA Scour at contracted bridge openings results from the complex interaction of hydraulics, bridge geometry, soils characteristics, and other site-specific conditions. The current understanding of bridge-scour processes, largely have been derived from laboratory investigations consisting of physical models in straight, rectangular flumes with uniform noncohesive bed material.
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8 methods for estimating contraction scour including (1) regime equations, (2)
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9 The literature describes a number of semi-empirical contraction-scour equations that were developed by use of conservation of flow and sediment in a control volume, in conjunction with laboratory derived concepts of sediment transport. These equations can be readily applied to a given site, which may account for their common use.
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10 from these and other assumptions, it is likely that the contraction-scour equations may not provide reasonable scour depths under field conditions. Local scour is the removal of bed material from around flow obstructions such as piers, abutments, spurs, and embankments caused by the local flow field induced by a pier or abutment.
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11 Coleman, 2000)
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12 SCOUR AT BRIDGE CONTRACTIONS Field observations of scour at many bridges indicate that conceptual separation of contraction and abutment scour as described in Hydraulic Engineering Circular-18 (HEC-18) (Richardson and Davis, 2001)
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13 Consideration of contraction and local-abutment scour as independent and separate processes in the abutment region is a particular concern. The assessment of contraction scour is often based on the simplifying assumption of uniform-flow distributions within a long contraction.
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From page 25... ...
14 necessary to understand how the depth of abutment scour or contraction scour was measured in the laboratory. In the laboratory, observed contraction scour that has occurred beyond the abutment region is subtracted from the total observed scour depth at a simulated abutment.
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15 Figure 1. Example of single scour hole at shorter bridges, as shown at U.S.
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16 Figure 2. Example of separate left and right abutment-scour holes at longer bridges, as shown at Road S-87 bridge, crossing the Coosawhatchie River in Jasper County, South Carolina, November 12, 1997.
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17 showed some bridges with abutment scour and no contraction scour (Figure 3) and other bridges (Figures 4)
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18 Figure 3. Plot of abutment scour measurements at the C.R.
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19 B Figure 4. Plot of contraction scour measurements at (A)
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20 CONTRACTION SCOUR Contraction scour traditionally has been classified as live-bed or clear-water. The livebed condition is characterized by bed material being transported into the contracted opening from upstream of the bridge.
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21 consideration of additional energy losses not accounted for in a roughness coefficient based on the channel composition and configuration (Matthai 1968; Schneider and others, 1977; Shearman and others, 1986)
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22 TABLE 1. Summary of live-bed contraction scour equation exponents.
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23 Richardson and Richardson (1994) modified Laursen's live-bed equation by removing the ratio of Manning's n in equation 1.
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24 shear stress of noncohesive sediments with a specific grain size is commonly estimated from the Shield's diagram. The critical velocity for incipient motion can be computed from the Shield's parameter by substituting the Manning equation for the slope term of the shear-stress equation and then using Strickler (1923)
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From page 36... ...
25 conditions; however, flow variables can change substantially after a scour develops. As a result large changes in flow area and backwater effects can affect the accuracy of scour-prediction equations.
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26 is based on hydraulic parameters from a one-dimensional model calibrated to the discharge and water-surface elevations observed in the field. The equations for live-bed contraction scour consistently overpredicted the observed scour (Table 2)
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27 Channel alignment upstream and through the bridge opening is another factor that can greatly affect the occurrence of contraction scour. The increase in flow conveyance caused by scour at an abutment can be sufficient to reduce overall velocities and shear stresses through the bridge opening, limiting scour to the area near the abutment.
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TABLE 2. Comparison of observed and theoretical contraction-scour depth (clear-water and live-bed)
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29 Figure 5. Relation of observed and theoretical clear-water contraction-scour depth for the 100year flow, in the Piedmont of South Carolina.
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30 at sites with long bridges that have extremely high rotational velocities at the abutments as a result of upstream meanders. Appreciable embankment skew and (or)
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From page 42... ...
31 combination of upstream approach alignment and riprap around the left abutment and piers. Abutment-scour holes were observed at both abutments and contraction scour was observed across the entire bridge section, but the deepest scour was observed downstream between the left abutment and the leftmost pier.
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Figure 6. Example of scour-hole low point located upstream of S.R.
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Figure 7. Example of scour-hole low point located downstream of S.R.
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Figure 8. Site configuration, flow patterns and approach section location for S.R.
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35 downstream of the bridge opening. Data from an ADCP section that cut-off the left floodplain flow accounted for all but 14 m3/s of the difference in discharge between the typical approach section and the section further upstream.
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37 Figure 10. Example of scour-hole low point located upstream of Road S-299, crossing Cannons Creek in Newberry County, South Carolina, November 24, 1997.
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38 abutments likely causes complex, unsteady flow patterns for shorter bridge lengths and creates the scatter within the longitudinal pattern. As bridge length increases beyond approximately 91.5 m, the interaction of flow from left and right abutments is diminished and steadier flow patterns are established around each separate abutment.
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39 flow reach. Models based on cross-section conveyance distributions alone can be highly inaccurate where non-uniform approach-flow conditions exist at bridges.
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40 ABUTMENT SCOUR The current knowledge on prediction of scour at abutments is derived from regime theory equations, equations used to estimate the depth of scour for spur dikes, and equations developed from small-scale physical-model studies conducted in laboratory flumes. Unfortunately, none of these approaches have resulted in a satisfactory prediction equation.
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From page 52... ...
41 The natural flow distribution in a river and its floodplain also can have an appreciable effect on the depth of scour at an abutment. The distribution of the approach flow blocked by the embankment is dependent upon the roughness and topography of the floodplain and alignment of the main channel.
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42 (1994) have used models that incorporate the floodplain, some channel-geometry effects, and non-uniform flow distributions.
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43 emulates the conditions in the field so that the equations developed are more representative of field conditions. Laboratory research on abutment scour has focused on equilibrium scour in non-cohesive materials.
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44 Research on scour in cohesive materials shows that, in general, cohesion increases the resistance of soils. In addition, the time required for maximum scour-depth conditions in cohesive soils is substantially longer than would occur with non-cohesive soils.
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45 4. floodplain roughness and topographic variation; 5.
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47 channel slope channel geometry (2) Variables describing the abutment embankment length skew abutment shape (3)
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48 dynamic viscosity (6) Temperature (7)
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Figure 14. Comparison of the embankment-length envelope for field observations of abutment-scour depth in South Carolina with the observed abutment scour for selected sites from the National Bridge Scour Database (BSDMS)
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50 application of one-dimensional hydraulic models and the selection of scour-prediction variables from these models. Errors in the hydraulic models or the selection of scour parameters from hydraulic models can lead to appreciable error in the scour predictions.
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51 Figure 15. Comparison of field observations of abutment-scour depth with the theoretical abutment-scour depth computed with the original Froehlich (1989)
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Figure 16. Comparison of field observations of abutment-scour depth with theoretical abutment-scour depth computed with the HEC18 (2001)
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TABLE 3. Comparison of observed abutment scour with scour calculated with the HEC-18 (2001)
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54 equations when calculating scour by methods documented in HEC-18. Comparing the observed to computed depths of scour using field measured hydraulic parameters in the selected predictive equations allows evaluation of the accuracy of the predictive equations.
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TABLE 4. Comparison of observed abutment scour with scour calculated by use of the HEC-18 (2001)
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56 abutment is always low compared to the rest of the flow field around the abutment and does not represent the acceleration of the high curvature flow especially near the point of flow separation (Table 5)
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TABLE 5. Comparison of measured and modeled abutment-tip velocities for use in the HIRE abutment scour prediction equation (m/s, meters per second)
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TABLE 6. Comparison of observed abutment scour with scour calculated by use of the HEC-18 (2001)
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TABLE 7. Comparison of observed abutment scour with scour calculated by use of the HEC-18 (2001)
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Figure 17. Aerial photograph of State Route 25 over the Minnesota River near Belle Plaine, Minnesota Flow 60
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61 The effect of changes in channel geometry on the computed depth of scour is evaluated by comparing depth of scour computed from modeled hydraulic parameters for pre-flood and flood-channel geometry. Because this comparison does not require detailed measured flow data in the approach section and floodplain, additional sites can be used in this evaluation.
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TABLE 8. Comparison of observed abutment scour with scour calculated by use of HEC-18 (2001)
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TABLE 9. Comparison of observed abutment scour with scour calculated by use of the HEC-18 (2001)
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64 SCOUR WITH DEBRIS In general, very little research has been done regarding bridge scour that is directly associated with woody-debris accumulation despite the fact that debris accumulations have contributed to one-third of all bridge failures in the United States (Chang, 1973)
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65 Current design guidelines treat debris rafts as a detriment to bridges, but do not provide methods for estimating the likelihood and size of debris accumulation. The majority of published information regarding debris scour is subjective and qualitative; although this information is useful, it is difficult to apply in bridge design.
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66 The local scour associated with the debris accumulation for each of the five measured floods was calculated using Melville and Dongol (1992) wherein the effect of a debris accumulation is converted to an effective pier diameter based on the thickness and diameter of the accumulation.
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TABLE 10. Comparison of Melville's debris-scour-estimating procedure with HEC-18 procedures and observed debris scour at S.R.
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TABLE 11. Comparison of debris width-design-criteria relationship (Diehl and Bryan, 1997)
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69 flood flow contracting through bridge openings is an inherently two-dimensional and many times three-dimensional hydrodynamic situation. One of the most important factors in using numerical models at contracted bridges is the ability for the model to accurately represent the velocity distribution laterally across the stream and floodplain.
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70 HEC-RAS did not duplicate this skewed flow pattern but rather computed a uniform flow distribution across the cross-section caused by the model assigning flow tubes of equal conveyance through the geometrically uniform bridge section. HEC-RAS was more accurate in reproducing the observed velocity distribution for the scoured channel geometry (Figure 18B)
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73 flow field, such as at the beginning of a flood and during the scouring process, the onedimensional model is severely limited in its ability to accurately distribute the flow. An analysis of scour computations in HEC-RAS revealed that the approach channel alignment is not accounted for in calculations of abutment scour.
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74 Figure 20. Plan view of topography and channel alignment for County Route 22 over the Pomme De Terre River near Fairfield, Minnesota (elevation referenced in feet above seal level; 1 ft = 3.2808 m)
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75 formation of a large scour hole at the left abutment during the time between the measurements. A large standing wave and area of reverse flow was witnessed during the 4/5/97 measurement (Figure 21)
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76 field, the lack of data between 4/5/97 and 4/9/97 prevented a more accurate representation of the hydraulics. The bed elevations of the sediment-transport model relative to scour measurements made on 4/9/97 are illustrated in the difference map found in Figure 25.
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Figure 22. Modeled flow field for County Route 22 over the Pomme de Terre River for conditions on April 5, 1997.
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78 Figure 23. Computational mesh for the two-dimensional model of County Route 22 over the Pomme de Terre River.
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80 Discussion of One-Dimensional and Two-Dimensional Model Comparisons Comparison of the output from the one- and two-dimensional models yielded surprising results. Despite the calibration complexities induced by geometry uncertainty and threedimensional flow, the two-dimensional model was able to reproduce the hydraulics in the bridge opening for the conditions measured on 4/5/97 more accurately than the one-dimensional model (Figure 26)
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Figure 25. Difference in bed elevation (in meters)
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Figure 27. Comparison of the original one-dimensional and two-dimensional model approach section location with an approach section in a location more representative of the actual blocked and main channel hydraulics at County Route 22 over the Pomme de Terre River.
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TABLE 12. Comparison of HEC-18 scour estimates (Froehlich, HIRE, and Live-bed equations)
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85 processes that are present in the field. Field hydraulic measurements and two-dimensional models are more representative of the processes that induce scour than one-dimensional models; however, the scour depths at the selected field sites were more accurately estimated when using HEC-18 equations and the output from the one-dimensional models, which simulate hydraulic conditions similar to those of laboratory experiments.
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From page 97... ...
86 cohesionless soils has a profound affect on the initiation of scour, the development of scour holes, and the final geometry of scour holes near bridges. Generally, at the sites examined in this study, the characteristics of sediments composing the streambed were different from those composing the upper layers of the floodplain and streambanks.
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From page 98... ...
87 Scour propagates away from the initial erosion point by (1) Undermining of vegetated soils and fine-grained soils strengthened by cohesion effects; (2)
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From page 99... ...
88 Examination of soils on floodplain surfaces, in scour holes, and in streambanks showed the affect of several factors that contributed to resistance to scour. Fine-grained soils such as silts and clays were observed near the surface of floodplain alluvium at all sites.
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89 The upstream edge of a scour hole formed during Hurricane Floyd at the U.S. 70 Bridge over Bear Creek in North Carolina (Appendix A, Case Study No.
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Figure 28. Looking upstream from the east bound bridge deck of higway 70 over Bear Creek near Lagrange, North Caroina during low-flow.
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From page 102... ...
91 the location and depth of the maximum point of scour and, more importantly, the scour around pier and abutment foundations. The upstream and lateral extension of the scour holes is one mechanism by which the non-uniform soil characteristics of floodplain sediments affect scourhole formation processes; other effects of the variation of soil characteristics also probably exist.
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