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Potential Drift Accumulation at Bridges
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 the most drift, but some drift is present in most rivers 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 streambed. An 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. Drift damages bridges mostly through local and contraction scour.
Further research on drift is needed in five main areas:
A large amount of under-used information on drift accumulations remains in bridge files, damage reports, and the memories of active and retired maintenance engineers. Further studies could take advantage of these resources to identify many more drift-laden rivers and drift-prone bridges.
High-resolution scanning sonar has the potential to yield new and valuable information on the size and shape of drift accumulations and associated scour; improvements are needed in determining the precise location of the sonar transducer relative to the bridge and in discriminating the drift accumulation from backscatter due to turbulence, bubbles, and suspended sediment. Drift accumulations are irregular in shape because of the random accumulation of individual logs, some of which are as long as the dimensions of the entire accumulation. Past investigations have not examined accretion to the underside of a raft and crushing of the raft, which account for nearly all the vertical thickness of the accumulation. The recommended method for calculating the depth of drift-related local scour at pier noses depends on the estimate of an equivalent pier width. Estimation of equivalent pier widths would benefit from field data on scour depths associated with drift accumulations.
Further field investigations could lead to the definition of channel types in which drift delivery is negligible. In channels where the branches of fallen trees drag on the bed, the minimum water velocity necessary for drift transport has not yet been established. Some channels may be so shallow that practically no drift is transported. In other channels, flow deep enough to transport drift also inundates wooded flood plains, and most flood flow moves through the woods rather than along the channel. Such channels may be incapable of transporting much drift further than the woods on the neck of the next meander bend downstream.
The maximum sturdy-log length controls the potential for span blockage in wide channels. The relation between this length and regional riparian-forest characteristics has not been firmly established. Field studies of drift length and forest characteristics could refine regional estimates of the maximum sturdy-log length.
Some spans shorter than the design log length may have low potential for drift accumulation. Further research may identify bridge design features that promote transport of all floating drift through the bridge, allowing the use of shorter span lengths without incurring high potential for drift accumulation.
Many rivers transport abundant drift in every period of high water, which can create chronic drift-accumulation problems. Maintenance engineers typically know which rivers present drift problems. Assessment of the potential for drift problems should take place early in the project planning process, and communication between maintenance engineers and design engineers should continue throughout this process. Designers can then select design features appropriate for drift-prone streams at the outset of a project. Such features include adequate freeboard, long spans, solid piers, round (rather than square) pier noses, and pier placement away from the path of drift.
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