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Potential Drift Accumulation at Bridges
Drift Transport and Storage
Previous studies have shown that drift can be abundant in large floods or in prolonged periods of high water, that most drift is transported as individual logs, and that these logs tend to move along the thalweg of the stream (Chang and Shen, 1979; Lagasse and others, 1991). Effects of meanders on the orientation and location of drift have been noted (Klingeman, 1971; Pangallo and others, 1992).
Most drift floats at the water surface in a zone of surface convergence, generally where flow is deepest and fastest. As a result, floating drift is transported at about the average water velocity. Submerged drift is carried to the banks and point bars by the slower, diverging flow near the bed.
Drift accumulates most frequently and in the greatest amounts where the path of floating drift encounters fixed objects that divide the flow. Accumulated drift remains intact for years unless removed, and sometimes is remobilized and transported downstream.
Logs are typically observed floating individually, with only temporary contact between them (figure 9) (Lagasse and others, 1991). Typical logs observed in this study lay approximately horizontal in the water, were exposed over their full length or nearly so, and did not rotate about a horizontal axis. Logs on the surface are not consistently aligned with the flow or across it, but rotate under the influence of large moving eddies.
Figure 9. Logs floating along the center of the Harpeth River at Wray Bridge, Williamson County, Tennessee.
Drift commonly aggregates into short-lived clumps. Most transported aggregations observed in this study were broken apart by turbulence, or when they struck a stationary object such as a pier or an accumulation of drift. Large drift aggregations occasionally are transported downstream (Helmericks, 1968; Rowe, 1974). In the failure of the temporary Harrison Road bridge over the Great Miami River at Miamitown, Ohio, a huge pile of drift including parts of a boat and dock struck the drift accumulation on the bridge just before failure occurred (National Transportation Safety Board, 1990).
Floating drift was typically observed in a surface-convergence zone, usually in the thread of the stream. In moderate bends, drift was observed more often along the thread between the center of the channel and the outside bank than in contact with bank vegetation. Whether floating drift followed the thread or the bank, it was typically concentrated in a path occupying only a small fraction of the channel width (figure 10).
Figure 10. Generalized plan view of the path of floating drift in a meandering river.
This drift path is created by surface convergence of flow. Such convergence tends to occur at the thread of the stream. Net downward flow and divergence at the bed under the zone of surface convergence typically coincide with the thalweg. In straight rectangular or trapezoidal channels, secondary flow currents typically form a double longitudinal vortex (figure 11) (Toebes and Sooky, 1967; Chiu and Hsiung, 1981; Tominaga and others, 1989).
Figure 11. Patterns of secondary flow in straight and curving channels.
Bends cause the formation of a single large vortex in which the surface flow is directed toward the outside bank, and flow along the bed is directed inward and upward onto the point bar (Klingeman, 1971; Bathurst and others, 1977; Nouh and Townsend, 1979; Thorne and Hey, 1979; LaPointe and Carson, 1986; Bathurst, 1988; Johannesson and Parker, 1989). A small vortex can develop along the outside of a bend, with surface flow directing floating material away from the bank (figure 11) (Bathurst and others, 1977; Bathurst and others, 1979; Thorne and Hey, 1979; Thorne and others, 1985; Damaskinidou-Georgiadou and Smith, 1986; Bathurst, 1988).
Sunken drift is transported downstream near the bed, presumably dragging, bouncing, or tumbling along the bed. Secondary flow currents carry sunken drift to the banks in straight reaches and onto point bars in bends. As a result, sunken drift moves more slowly than floating drift and typically comes to rest away from bridges. Drift transported along the bed in deep flow is hard to observe; its abundance and importance relative to floating drift can only be inferred.
When floating drift strikes fixed objects such as piers, abutments, island heads, the streambed, or trees on the outside banks of bends, it generally continues to move downstream. Contact with fixed objects breaks the branches of floating trees, and converts them into the bare trunks with root masses common in drift accumulations. Most isolated logs on sediment bars and in pools come to rest with the root mass upstream, and the trunk and any remaining branches pointing downstream. The prevalence of this position suggests that the root mass is more likely to drag than the trunk and branches.
When a piece of drift slows relative to the flow, a visible surface wake forms around it. Such wakes were observed where floating wood struck driftwood rafts, where driftwood contacted the outside bank of bends and bank vegetation, and where driftwood removed by maintenance crews from a bridge accumulated on the river bed. Drift dragging on the bed was not observed during flood flow.
Drift can also be transported tumbling in the flow, rotating around a horizontal or inclined axis. A few tumbling pieces of drift were observed during floods.
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