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Calibration of the Maryland Abutment-Scour Equation Using a Modified Critical Velocity

Project Number: 2519-CWJ01
Project Chief: Stephen Benedict
Cooperator: Maryland State Highway Administration
Period of Project: October 1, 2006 to September 30, 2008

INTRODUCTION

I-90 bridge collapse on Schoharie Creek, April 5, 1987. Sid Brown, Schenectady Gazette

I-90 bridge collapse on Schoharie Creek, April 5, 1987. Sid Brown, Schenectady Gazette.

In the late 1980's, bridge scour caused the catastrophic failure of the I-90 bridge crossing the Schoharie Creek in New York State resulting in the loss of life. This tragic event brought the issue of bridge scour to the forefront of the highway engineering community highlighting the need for research to better understand and predict this phenomenon. Since the I-90 failure, many laboratory investigations have led to the development of various equations for predicting bridge scour, including the components of pier, abutment, and contraction scour. Of particular interest to the current investigation is an abutment-scour prediction equation developed by the Maryland State Highway Administration (MSHA).

PROBLEM

Failure of Route SC 418 bridge crossing Enoree River

Failure of Route SC 418 bridge crossing Enoree River (photograph by Michael Hall, August, 1995).

In the mid 1990's, MSHA initiated an investigation to develop an improved equation for predicting abutment scour at bridges. The equation, called the Maryland abutment-scour equation, was derived from analytical methods and laboratory data and was initially presented by Chang and Davis (1999) with slight modifications later presented in the MSHA manual for hydrologic and hydraulic design (Maryland State Highway Administration, 2005). The performance of the Maryland abutment-scour equation was evaluated with field measurements in an investigation conducted by the U.S. Geological Survey (USGS), in cooperation with the Federal Highway Administration (FHWA) (Benedict and others, 2006). The investigation indicated that the equation did not consistently perform well when compared with field data and identified the selection of the sediment critical velocity within the equation as a potential source of error. These findings indicated that performance of the equation could improve if a better method for estimating critical velocity was used. To improve prediction with the Maryland abutment-scour equation, the USGS in cooperation with the MSHA, initiated the current investigation to determine the best method for estimating sediment critical velocity.

APPROACH

Chart showing comparison of selected methods for estimating critical velocity using field data collected in South Carolina.

Comparison of selected methods for estimating critical velocity using field data collected in South Carolina.

A limited literature review identified 10 different methods for estimating sediment critical velocity (Benedict and others, 2006). While some methods are similar, a comparison showed that large discrepancies can often exist between some methods (figure 3), yielding significant differences in predicted abutment scour when applied to the Maryland abutment-scour equation. These 10 methods for estimating sediment critical velocity were reviewed and 4 representative methods were selected to test in the Maryland abutment-scour equation. These methods included those presented in Richardson and Davis (2001), Neill (1973), Vanoni (1977), and Fortier and Scobey (1926). The application of these critical velocity methods for to the Maryland abutment-scour equation indicated that each had strengths and weakness and none worked well in all circumstances. Based on the results of this analysis, a refined method for estimating critical velocity is under development that will generally improve the performance of the Maryland equation when applied to the South Carolina field data. Upon completion of this analysis the results will be documented in a paper that provides guidance for applying the calibrated Maryland abutment-scour equation along with its limitations.

BENEFITS

The results of this investigation should improve the ability of the Maryland abutment-scour equation to estimate abutment scour in the field setting. While this improvement will benefit the MSHA, it also will benefit the engineering community, in general, by providing an improved method for estimating abutment scour in other parts of the United States.

REFERENCES

Benedict, S.T., Deshpande, N., Aziz, N.M., and Conrads, P.A., 2006, Trends of abutment-scour prediction equations applied to 144 field sites in South Carolina: U.S. Geological Survey, Open-File Report 03-295, 52 p. Report and electronic data available online at http://pubs.water.usgs.gov/ofr2003-295/. Accessed November 15, 2006.

Chang, F. and Davis, S.R.. 1999, Maryland SHA procedure for estimating scour at bridge abutments, Part II - Clear Water Scour. ASCE Compendium, Stream Stability and Scour at Highway Bridges, Richardson and Lagasse (eds.), Reston, VA., 1999, pp. 412-416

Fortier, Samuel. and Scobey, F.C., 1926, Permissible canal velocities: Transactions of the American Society of Civil Engineers, v. 89, Paper no. 1588, p. 940-984.

Maryland State Highway Administration, 2005, Evaluating scour at bridges: Manual for hydrologic and hydraulic design, v. 2, chapter 11

Neill, C.R., 1973, Guide to Bridge Hydraulics: Roads and Transportation Association of Canada; University of Toronto Press, 191 p.

Richardson, E.V., and Davis, S.R., 2001, Evaluating scour at bridges: Federal Highway Administration Hydraulic Engineering Circular No. 18, Publication FHWA-NHI-01-001, 378 p.

U.S. Geological Survey, Ten years since major bridge collapse: http://ny.water.usgs.gov/projects/scour/text.html , accessed January 17, 2008.

Vanoni, V.A., 1977, Sedimentation Engineering, American Society of Civil Engineers Task Committee for the Preparation of the Manual on Sedimentation of the Sedimentation Committee of the Hydraulics Division.

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