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Publication Number: FHWA-RD-03-052
Date: May 2005

Field Observations and Evaluations of Streambed Scour At Bridges


The analysis and prediction of scour at bridges is complex. Scale models and design methodology must account for the variability of site conditions and the potential interaction of the various components of scour. Scour at bridges has traditionally been classified into the categories of degradation, contraction scour, and local scour (abutment and pier). These categories do not explicitly account for the natural scour and deposition that occurs in a river during a flood or series of floods. Data collected at bridges during floods must be carefully analyzed to determine the magnitude of local and contraction scour. If appropriate reference surfaces are not selected, the scour reported may reflect scour caused by other processes.

Researchers and design engineers have agreed that field data on bridge scour are needed to validate laboratory experiments and to ensure the reliability of design methodology. The USGS, in cooperation with FHWA and many State highway agencies, has collected and compiled data on scour at bridges. The national database has been expanded and now contains 493 local pier scour measurements, 18 contraction scour measurements, and 12 abutment scour measurements from 79 sites located in 17 States.

Various researchers have proposed many pier scour equations, but none have accurately and conservatively predicted the scour observed in the field. Most equations are based on scaled laboratory experiments that did not account for the complexity of the field conditions: Relations among dimensionless variables developed from these laboratory experiments did not compare well with the field data. The Froehlich Design, HEC-18, HEC-18-K4, HEC-18-K4Mu, HEC-18-K4Mo (>2 mm), and Mississippi equations proved to be better than other equations for predicting pier scour for design purposes. The comparison of the scour depths predicted from these equations with scour depths measured in the field clearly showed that processes are reflected in the field data that are not correctly accounted for in these equations. Analysis of the pier scour data indicated the importance of bed material characteristics as an explanatory variable in the predictive equations. A new K4 term for the HEC-18 equation was developed based on the relative bed material size (b/D50).

Although caused by the same geometric contraction of the floodplain, contraction and abutment scour have traditionally been studied and treated separately. Contraction scour equations are based on theoretically developed equations that do not account for the complexity of the flow conditions in the field. Likewise, most abutment scour equations are developed from laboratory experiments that oversimplify or ignore many complexities common at highway bridges. Field data are needed to evaluate published approaches to computing contraction and abutment scour. A review of the literature found 29 references with mention of contraction and (or) abutment scour data, but only one presented detailed data collected during floods. Only two references included data on abutment scour. Published comparisons of field data with computed scour showed mixed results. Four papers showed the contraction scour equation typically overpredicted the observed scour and in a few instances severely overpredicted the scour; however, one paper showed the Laursen contraction scour equation underpredicted a number of measurements, and no severe overprediction was observed. The accuracy of the contraction and abutment scour equations recommended in HEC-18 may depend greatly upon the degree of contraction, the flow distribution, the configuration of the approach, and how well the hydraulic model represents the true flow distribution.

Comparison of computed abutment and contraction scour depths with depths measured in the field for U.S. Route 12 and Swift County Route 22 over the Pomme de Terre River in Minnesota provides insight to the capabilities and limitations of using one-dimensional models and the available abutment and contraction scour equations to predict scour at contracted bridge openings. The application of the methods outlined in HEC-18 to these sites showed a variability of results similar to the comparisons published in the literature. HEC-RAS and the equations recommended in HEC-18 provided reasonable predictions for maximum total scour at the two bridges; however, the magnitudes of the individual components (abutment and contraction) of scour did not compare well with the field data. Although field data in the approach sections were inadequate to provide a comprehensive evaluation of the ability of a one-dimensional model to represent a complex two-dimensional flow field, the comparisons that could be made showed the one-dimensional model computed flow distributions comparable to the field data for the fully developed scour hole conditions, but were less accurate for initial conditions and in areas of highly curvilinear flow.

Overall, the methodology for computing scour at bridges published in HEC-18 provides estimates that are generally conservative, in that the depth of scour is usually overpredicted. The complexity and variability of conditions at bridges make the development of predictive methodology difficult. The equations oversimplify most conditions, but modification of the methodology to account for site complexity and variability is not a simple task. New methodologies must balance the desire to fully explain complex processes with the need to provide procedures that are time and cost effective to apply. Additional field data and model studies are needed to continue to improve scour prediction methodology.

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