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Potential Drift Accumulation at Bridges

Depth, Shape, and Structure of Drift Accumulations

Most drift accumulations form at the water surface as a raft. Logs and smaller pieces of drift accrete to the upstream edge of the raft. The accumulation can grow toward the river bed through accretion of logs on the underside of the raft as they are washed under it by the plunging flow at the upstream edge. Alternatively, the raft can become thicker by collapsing in compression along the direction of flow as the lateral hydraulic forces on the raft exceed its compressive strength (Kennedy, 1962). Drift accumulations are typically deepest at the piers that support them, and widest at the surface. The potential to achieve a roughly rectangular cross section (transverse to approaching flow) from the bed to the water surface may be related to abundant drift, prolonged periods of high water, or multiple floods without intervening drift removal.

The depth of a blockage is limited by the depth of flow. Several observed drift accumulations extended upward nearly to the maximum flood stage even after the flood receded. Allowing for compression of the jam as the water level dropped, these accumulations likely occupied the entire depth of flow. The maximum vertical extent of drift observed in this study was about 12 m (40 ft), but a larger vertical extent of drift seems possible. Australian and New Zealand's design practices incorporate limits to the vertical extent of drift accumulations, but the basis for selecting such limits is unclear (National Association of Australian State Road Authorities, 1976; Apelt, 1986a; Wellwood and Fenwick, 1990; Dr. Arthur Parola, University of Louisville, written commun., 1992).

Accumulations may be irregular, but most large accumulations are similar in shape. Logs are initially trapped perpendicular to approaching flow in most accumulations, but as accretion continues, logs are added parallel to the upstream edge of the raft. Accretion is fastest where the path of most drift in the river intersects the accumulation. The net result is often an accumulation with a curved upstream edge, and with the upstream nose of the raft near the thalweg. Drifting logs encountering the nose, where the edge of the raft is perpendicular to the flow, are more likely to be trapped at the upstream edge of the raft, or swept under it, than logs that encounter the edges diagonal to the approaching flow.

The sides of an accumulation may be trimmed by the breakage of protruding logs. This effect is most noticeable where water velocities are high, as at bridges where most of the channel is blocked by drift.

Drift accumulations originate at the water surface, but ultimately become part of the streambed. As the water level rises during a flood, drift already on the bridge generally remains in place as new drift continues to be added at the water surface. When the water level falls, accumulated drift typically slides downward on the pier or piers in contact with it until it rests on the bed. Most accumulations do not float to the surface during subsequent floods, but form a solid mass with irregular protrusions around the base of the pier or piers against which they rest.

Drift accumulations typically begin as loose accumulations of multiple logs; eventually, the gaps between logs fill with branches, twigs, and leaves. Sediment fills some of the remaining voids in the accumulation. This mass of wood and sediment may persist indefinitely, potentially forming a base for further accumulations.


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