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Potential Drift Accumulation at Bridges
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.
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