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92 CHAPTER 3: INTERPRETATIONS, APPRAISAL AND APPLICATION RECOMMENDED MODIFICATIONS TO SCOUR PREDICTION METHODOLOGY The analysis of field data collected at the 15 sites during the NCHRP 24-14 project, and 146 sites in South Carolina (Benedict, 2003) has provided further recognition of the complex nature of scour at contracted bridges and a basis to recommend modifications to the HEC-18 (Richardson and Davis, 2001) scour-prediction methodology. Use of BSDMS in Bridge Evaluation With the contributions of this study there are now 93 measures of scour at bridges in the BSDMS database. This database can be used by state highway agencies (and their consultants) to make comparisons between bridges being evaluated and those in the database where scour has been measured. The database should provide a basis for evaluating complex sites and how individual factors may contribute to limiting or causing scour. The case studies in Appendix A provide detailed information on methods of evaluation and observed scour. Appendix A and the BSDMS can both be used as a training tool.
93 Adding Abutment and Contraction Scour 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) is problematic because the hydrodynamic mechanisms that induce the individual scour components work together. It is clear from the field observations of this study that the scour that occurs near the ends of the abutment is the result of a complex combination of flow contraction and flow curvature. Scour prediction methods published in HEC-18 indicate that contraction and abutment scour are separate and additive for all contracted bridge openings. HEC-18 follows a conservative approach of adding the scour components to create a scour prism for design and assessment purposes, because of an insufficient amount of field data to develop an understanding of the interaction of scour components. Therefore, to compute the total scour at an abutment, the individual components of long-term streambed change, contraction scour, and abutment scour within the abutment region must be estimated and then summed. Isolating the effect of an individual scour component is difficult because the various components interact in the development of the total depth of scour. Laboratory investigations typically have focused on understanding each scour component in isolation, necessitating the approach for estimating total scour outlined in HEC-18. Analyses of field observations, in conjunction with the theory of flow patterns in short contractions, indicate that this view of scour in the abutment region may be inappropriate.
94 Although the overall effects of flow contraction and the local flow curvature that occurs around abutments can be conveniently separated conceptually, the resulting scour pattern cannot be separated into contraction- and abutment-scour components. The cause of the specific scour patterns is believed to be highly sensitive to local field conditions. The field observations collected during this study are not adequate to develop a definitive classification system based on site characteristics that could indicate the expected scour pattern. Effects of Channel Bends Channel alignment and in particular, channel bends upstream of bridges can have an appreciable effect on the depth and distribution of scour, including the location of maximum scour. Hydraulic parameters should be adjusted to account for bend effects on flow distributions. Although the scour-prediction methodology provided in HEC-18 typically over-predicts scour depths, scour depths greater than those predicted can result where upstream channel bends centrifuge flow into floodplains and toward piers or abutments. Channel bends can present a unique, site-specific problem at bridges because approaching flow distributions at flood stage can be appreciably altered when flows leave a channel and enter a floodplain at a channel bend. When channel bends occur just upstream of a bridge, concentrated channel flows can be directed to a section of a bridge opening that would not typically experience this magnitude of flow if the channel were straight. Under these conditions, large scour holes can develop even when embankment lengths and geometric-contraction ratios are small (Benedict, 2003).
95 Two-dimensional models such as FESWMS-2DH can provide information on the bend effects on flow distribution; however, judgment in the adjustment of flow parameters may be required where two-dimensional models may not be cost effective. Considerations such as the relative amount of flow in the channel and floodplain, the bend radius of curvature, the position of the bend with respect to the bridge opening, and the flow velocity in the channel and floodplain are a few factors that influence the effect of bends on the flow distribution near a bridge. Other factors such as the distribution of floodplain roughness, topographic variation, and features such as drainage ditches and flow obstructions can appreciably affect the distribution of flow approaching bridges and the depth and distribution of scour. Although one-dimensional backwater models can represent some of these effects, they are incapable of propagating these effects in the downstream direction. In general, a detailed topographic map and aerial photography can be used to qualitatively assess the severity of flow contraction at bridge crossings as well as the location of contracted and uncontracted cross-sections. The use of these maps can provide valuable insight on site characteristics such as flow structures, geomorphic setting, floodplain topography and land cover, and upstream channel configuration; all of which greatly affect the potential for scour at a bridge site. Where the floodplain is narrow and embankments are short, potential for abutment scour is low. In contrast, when floodplains are wide and embankments are long, the potential for abutment scour is high. Although there are exceptions to these generalizations, qualitative assessments need to be taken into account during scour computations.
96 Location of Scour Holes Analysis of the field data also has revealed that the location of scour in a contracted bridge opening is highly variable and does not follow the patterns typically reported from laboratory experiments. The longitudinal location of contraction and abutment scour holes can be dependent upon site specific factors such as the configuration of scour protection, guide banks, bridge length, channel alignment, and bed material. The location of scour holes observed at the 15 sites in this study and the 146 sites in the South Carolina study (Benedict, 2003) were highly variable, especially for shorter bridges (less than 91 m long). Field observations show contraction and abutment scour holes commonly are formed upstream and (or) downstream of the bridge. Although this study, Benedict (2003) and others have discussed factors that contribute to the position of scour holes, no method for predicting the location has been developed. Consequently, the present scour-prediction methods found in HEC-18 recommend that the scour hole low point be located at the bridge. Additional research and data collection is needed to determine the factors that control scour and to develop a method for predicting the location of scour holes. Application of Contraction Scour Equations All contraction-scour equations were shown to consistently over-predict observed scour depths; however, the clear-water scour equations grossly over-predicted field observations by 2 to 40 times the measured scour depth (Table 2). The collection and analysis of additional field data at clear-water scour sites, similar to what was collected as part of the South Carolina bridge
97 scour study (Benedict, 2003), may provide the information necessary to develop prediction equations that will more accurately represent field conditions and reduce the costs of over- designing bridge foundations. Clear-water scour equations should be used with the knowledge that the selection of the critical-shear stress for the bed material will substantially affect the computed depth of scour. Vegetation and soil cohesion, both of which are difficult to quantify, greatly affect the soilâs ability to resist scour. Predicting Abutment Scour Analysis of the recommended methods for predicting abutment scour indicates that current methods are not reliable. The methods include procedures for assessing flow hydraulics using one-dimensional backwater models and abutment-scour prediction procedures recommended in HEC-18. Comparison of measured flow velocities and those computed using one-dimensional backwater programs showed that in most cases average and local velocities required for use in abutment scour equations were appreciably in error. Computed velocities near the abutment were always appreciably lower than the peak measured velocity near the upstream tip of the abutments. Despite the consistently low prediction of velocities near abutments, computed scour depths under most conditions were still high compared to measured scour depths. Under a few conditions where velocities were not measured, the scour depths computed using flow velocity from one-dimensional models were slightly lower than measured scour depths.
98 Comparison of abutment-scour predictions with observed scour depths showed that typically the abutment-scour equations over-predict the depth of scour, often substantially. Analysis of the cause of the inaccuracies of the predictions showed that the primary problem lies in the abutment-scour equations rather than in the model used to estimate the hydraulic parameters. Scour at contracted bridges is complex and is highly dependent upon site conditions and channel geometry (curvature and alignment). Simple equations based on simple experiments are not able to account for the complexities of typical field conditions. The current approach to predicting scour at abutments is unreliable. Envelope curves from field observations of abutment scour can be useful tools for assessing abutment-scour depths; however, analysis of the envelope curves developed for South Carolina shows that these types of curves may be regionally specific and cannot be applied without sufficient consideration of the site conditions on which they are based. Scour with Debris The New Zealand debris scour prediction methodology (Dongol, 1989 and Melville and Dongol, 1992) worked well at the Chariton River site near Prairie Hill, Missouri (see Appendix A, Case Study No. 10) and may be appropriate for application at other sites. The methodology suggested by Diehl and Bryan (1997) for determining the design width of a debris accumulation based on channel characteristics also proved accurate predictions for the Chariton River site. Unfortunately, a single favorable comparison is not sufficient to prove general accuracy and
99 applicability of these methods. These methods should be applied with caution and substantiated with additional field observations. GUIDELINES FOR NUMERICAL MODELING When using HEC-RAS to predict scour at contracted bridge openings, engineers should closely inspect the model output and input parameters used for the internal scour computations especially in simulations with complex upstream channel configurations. The approach channel alignment is not accounted for in HEC-RAS calculations of abutment scour; default HEC-RAS hydraulic parameters used for abutment scour calculations can provide erroneous predictions based on incorrect projection of bridge opening to approach section. All default scour parameters in HEC-RAS should be closely inspected to assure they represent site configuration and any available field data. It also is important to note that because of the limitations of the model, it is extremely common for appreciable differences to exist between the contraction and abutment scour variables used in HEC-RAS and those measured in the field. Multi-dimensional numerical models have the capability to provide a better representation of the complex flow conditions that exist at a contracted bridge site. Using such models requires more topographic information than one-dimensional models and hence more time to develop. The comparisons with field data showed that if the application of the abutment- and contraction-scour equations is the primary goal of the effort, a multi-dimensional model may not be worth the additional cost; however, where flow conditions are particularly complex a multi-dimensional model will likely provide insights into the flow patterns that cannot be identified with a one-dimensional model. The coupling of sediment transport with a multi-
100 dimensional model appears to be a better alternative to simply using the multi-dimensional model to determine the hydraulic parameters for the scour equations; therefore, the need for a multi-dimensional model is highly site specific but where applicable can provide valuable information on the expected flow and scour patterns. ERODIBILITY AND GEOTECHNICAL PROPERTIES OF MATERIALS The drastic overprediction of scour using HEC-18 methods on floodplains where no scour was observed at a large percentage of sites in the study of Benedict (2003) and of this study can be, at least in part, attributed to the treatment of soil-erosion resistance without the effects of fine-grained soil behavior and vegetation. On floodplains in low-gradient environments (valley slopes less that 0.5 percent) consideration should be given to increased erosion resistance afforded by vegetation and fine-grained soil properties. Unfortunately, the uncertainty in predicting erosion resistance caused by the variability in fine-grained characteristics of floodplain soils (Benedict 2003) may preclude complete reliance on apparent or true fine-grained soil cohesion; however, the combined effect of root reinforcement and fine grained soil behavior appears to be reliable, at least in humid environments. The effectiveness and reliability of vegetation in preventing scour should be developed on a regional basis such that regional climate, soils, bridge-design methods, and vegetation can be considered. Typically vegetation is not a significant factor in the river main channel, in heavily shaded areas beneath the superstructure, or in arid climates where plant growth is unreliable. NCHRP Project 24-15 has developed techniques for predicting scour depths in cohesive soils that include temporal effects and variation in soil properties.
