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.
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 stream bed. 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. 28K GIF version of this graph Figure 22 .