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. Key Words Bridge Failure, Channel morphology, Debris, Drift, Geomorphology, Open channel flow, Stream erosion, Scour 18.