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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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