Drift
What is Drift?
Drift (floating debris) accumulation at bridges is a widespread problem. Drift reduces the capacity of bridge openings, contributes to scour, and increases lateral forces on bridges.
It contributes to more than one-third of the bridge failures in the United States.
Drift damages bridges mostly through local and contraction scour.
Where does Drift come from?
Drift that accumulates at bridges comes primarily from trees growing on the banks and bank tops of rivers. Most of the trees that become drift are undermined by bank erosion. Rivers with unstable channels have the most bank erosion and drift, but most rivers transport some drift during floods.
Floating drift is concentrated along the thread of the stream and moves at about the average flow velocity. Logs longer than the width of the channel accumulate in jams, or are broken into shorter pieces. Sunken woody debris moves more slowly and tends to accumulate in and along the channel, rather than being transported downstream to bridges.
How does Drift affect bridges?
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 accretion can cause an accumulation to grow downward to the streambed.
Because drift accumulations on single piers are based on logs extending the full width of the accumulation, the maximum width of single-pier accumulations is about equal to the length of large logs delivered to bridges. Spans are generally blocked by logs extending from pier to pier, so the same design log length is equal to the maximum width of spans likely to be blocked by drift.
What does Debris Free do to mitigate drift?
Debris Free systems help redirect the flow of drift through the center span of a bridge, either through pier protection or drift training upstream.
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