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Federal Highway Administration Research and Technology
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Publication Number: FHWA-RD-03-052
Date: May 2005
Field Observations and Evaluations of Streambed Scour At Bridges
CHAPTER 1. INTRODUCTION
Bridge piers and highway embankments leading to a bridge often obstruct the flow of floodwaters, causing an increase in velocity and the development of vortices. The increased velocity and vortices often cause scour near the bridge foundations. The damage to and failure of bridges caused by scour are problems of national concern, as illustrated by the number of bridges damaged or destroyed by floods in the United States during 1985-95 (Table 1).
The U.S. Federal Highway Administration (FHWA) has issued several guidance documents addressing scour. Hydraulic Engineering Circular (HEC)-18 presents methods for predicting local and contraction scour at planned and existing bridges.(6) Although the methods presented in HEC-18 represent the state-of-the-knowledge at the time of publication, some of the potential limitations of these methods were identified.
The current equations and methods for estimating scour at bridges are based primarily on laboratory research. Very little field data have been collected to verify the applicability and accuracy of the various design procedures for the range of soil conditions, streamflow conditions, and bridge designs encountered throughout the United States. (p. 3)(6)
The lack of and need for reliable and complete field data on scour at bridges has been a recurring conclusion of many researchers (See references 8, 9, 10, 11, and 12.) Other researchers have compiled field measurements on local pier scour.(3,13,14) These historical data sets contain valuable information, but most do not contain information on all major factors known to affect scour. Froehlich was unable to include the effect of sediment gradation in his analysis because many data sets did not include this information.(13) Johnson, in a comparison of seven published pier scour equations with field data, assumed uniform sediment size because sediment gradation information was not available for most of the data.(15)
Field measurements of contraction and abutment scour are significantly fewer than of local pier scour. In a recent review of published field data on contraction and abutment scour, 29 references were found. Of these only Norman(16) presented detailed data collected during floods and only two other references included data on abutment scour.(17,18) Seven of the 29 papers presented data on contraction scour, and another 14 discussed sites that could yield contraction scour data, but only if additional data were available.
Despite the recognized need for the collection of field data, few data were collected until the late 1980s, a deficiency that is primarily a reflection of the difficulty in collecting the necessary data. Accurate and complete field measurements of scour are difficult to obtain because of complex hydraulic conditions at bridges during floods, inability to get skilled personnel to bridge sites during floods, and problems associated with existing measuring equipment.(19,20) Cooperative research among FHWA, State highway departments, and the U.S. Geological Survey (USGS) has allowed the collection of scour data at bridges during floods. Landers and Mueller published 394 local pier scour measurements made by the USGS during the first national bridge scour study. (21) Most of these data collected by USGS contain bed material data and provide supporting channel cross sections and site-characterization data.
PURPOSE AND SCOPE
This report describes the results of the second USGS national field-data collection and analysis study on scour at bridges, funded by FHWA. The database originally developed during the first national study has been enhanced and many scour measurements added, including measurements of abutment and contraction scour.(22) Sufficient local pier scour data are now available to permit a detailed analysis of local pier scour. Scour depths computed from published pier scour equations are compared to the field measurements. Many commonly cited dimensionless variables believed to control the depth of scour are evaluated and compared with equations developed from laboratory data. The effect of the size and gradation of the bed material on the depth of scour is investigated, and a correction factor for the HEC-18 pier scour equation is proposed. Available data are insufficient to permit a detailed investigation of contraction and abutment scour; however, some basic comparisons and qualitative observations are presented on the basis of a review of the literature. The results of scour analyses for two contracted bridges are compared with real-time field data.