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Potential Drift Accumulation at Bridges
CHARACTERISTICS OF DRIFT
The predictability of many drift characteristics and processes creates
opportunities for bridge engineers to improve their methods of avoiding
drift problems. The processes of drift generation, transport, and
accumulation interact with bridge characteristics to control the
likelihood that a large accumulation will occur. The characteristics of
typical pieces of drift determine the maximum size of accumulations.
Drift production and transport are natural characteristics of most
rivers. The rate of drift production depends on the degree of channel
instability. Channels wider than the length of logs, and deep enough
that logs do not drag on the bed, transport drift efficiently. Floating
trunks with attached root masses make up most of the drift volume
transported to bridge sites.
The greatest amounts of drift accumulate where flow separates to pass around obstacles. Logs are the structural members of accumulations, so log characteristics determine accumulation characteristics. The length of the largest sturdy logs defines the maximum width of accumulations on single piers and the maximum length of bridge span that can be completely blocked.
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