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Potential Drift Accumulation at Bridges

Technical Report Documentation Page

1. Report No. FHWA-RD-97-028 2. Government Accession No. 3. Recipient's Catalog No.

4. Title and Subtitle

POTENTIAL DRIFT ACCUMULATION AT BRIDGES
5. Report Date
6. Performing Organization Code

7. Author(s)

Timothy H. Diehl
8. Performing Organization Report No.

9. Performing Organization Name and Address

U.S. Geological Survey, Water Resources Division

640 Grassmere Park, Suite 100, Nashville, TN 37211
10. Work Unit No. (TRAIS)

11. Contract or Grant No.

DTFH61-92-Y-30058

12. Sponsoring Agency Name and Address

Office of Research and Development

Federal Highway Administration

U.S. Department of Transportation, Washington D.C. 20590

13. Type of Report and Period Covered

Final Report

April 1992 to September 1995
14. Sponsoring Agency Code

15. Supplementary Notes

Contracting Officer's Technical Representative (COTR) - Steven Chase [routing code]

16. Abstract

Drift (floating debris) increases lateral forces on bridges and promotes scour. This report presents the results of a study of drift accumulation at bridges performed by the U.S. Geological Survey from 1992 through 1995, in cooperation with the Federal Highway Administration. The study included a review of published literature on drift, analysis of data from 2,577 reported drift accumulations, and field investigations of 144 drift accumulations.

The potential for drift accumulation depends on basin, channel, and bridge characteristics. Drift that accumulates at bridges comes primarily from trees undermined by bank erosion. Rivers with unstable channels have the most bank erosion and the most drift. Most drift floats along the thread of the stream. Logs longer than the width of the channel accumulate in jams, or are broken into shorter pieces.

Drift accumulates against obstacles such as bridge piers that divide the flow at the water surface. Groups of obstacles separated by narrow gaps trap drift most effectively. Drift accumulation begins at the water surface, but an accumulation may grow downward to the stream bed through accretion. A drift accumulation on a single pier grows no wider than the length of the longest logs it contains. The gap between two piers is not blocked by drift unless individual logs can reach from pier to pier. Design features to reduce the potential for drift accumulation include adequate freeboard, long spans, solid piers, round (rather than square) pier noses, and pier placement away from the path of drift.

17. Key Words

Bridge Failure, Channel morphology, Debris, Drift, Geomorphology,

Open channel flow, Stream erosion, Scour

18. Distribution Statement

No restrictions. This document is

available to the public through the

National Technical Information

Service, Springfield, Virginia 22161

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. of Pages

114
22. Price

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