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Potential Drift Accumulation at Bridges
Pier and Superstructure in Flow: Solid, or with Apertures
The hydraulic characteristics of parts of the bridge exposed to floating drift determine whether drift is deflected or trapped. In this regard, the most important hydraulic characteristic is flow through narrow apertures at the water surface. Piers and superstructures with narrow apertures that carry flow are significantly more likely to trap drift.
One well-recognized example of flow through apertures occurs with substructures made up of two or more parallel rows of pilings exposed at the water surface. Many such piers are designed as such so that concrete for the footing can be poured above water. Others result from degradation of the river bed below pier footings, or exposure through bank erosion of piles originally buried below the flood plain. In any case, flow through the narrow apertures between piles pins drift against them. When such clusters of piles are skewed to the approaching flow, the increased width of surface flow passing through the apertures brings with it an increased likelihood of drift accumulation.
A pile bent, or a pier made of a single row of cast columns, may be aligned to the approaching flow so that each vertical pile or column is directly downstream from the one furthest upstream and no significant flow passes through the relatively narrow apertures between them. Alternatively, the approaching flow may be skewed to the line of the columns or piles, so that flow passes through each aperture. The width of surface flow passing through the apertures depends on the length of the line of columns or piles and the angle between the line and the approaching flow. Trapping associated with skewed flow through a line of columns is indicated by accumulated drift lying along one side of the line of columns.
Observing the site during high water may be the only way to determine whether approaching flow is skewed or aligned to the pier. The flow direction may change, particularly when stage increases beyond bank-full. Surface flow may cease to be roughly parallel to the banks and become roughly parallel to the valley or meander belt. Flow direction can be altered by bridges that impede flow along part of the valley.
Other changes can cause skewed flow at piers that were originally aligned to flow. If enough drift accumulates on one part of a bridge, flow directions at nearby piers will be altered. Channel evolution can produce dramatic changes in flow direction at a pier.
A drift accumulation has poor ability to deflect additional incoming drift because many logs and branches protrude from the main body of the accumulation. These protrusions are apparently more common at lower velocities, while at higher velocities the protrusions are broken off by collision with other pieces of drift, particularly logs. Where drift delivery is frequent and removal is difficult (for example, where blockages form underneath the bridge deck in mid-channel), assume that drift will be in place at the beginning of a flood, and that this drift accumulation will trap drift regardless of the pier's design.
Superstructures that include apertures at or below the water surface are particularly vulnerable to drift accumulation. Open trusses retain most drift over a wide stage range from low steel up to truss top, and this drift typically becomes entangled in the truss. Simply supported trusses are subject to lateral displacement when the water level is above low steel and drift delivery is high (Chang and Shen, 1979). This is one situation in which lateral forces due to drift may be the primary cause of damage to a bridge.
Open parapets with pillars and rails also incorporate narrow apertures. The vertical extent of these apertures is much less than in a truss, extending from the stage at which flow starts to pass through apertures in the parapet up to shallow submergence of the top of the parapet. Drift can entangle in a parapet but apertures are typically small, and entangling is probably less severe than on a truss. Some arrangements of pier caps, beams, and deck create apertures that carry flow and trap drift. Flow may pass over beams and under the deck. Some arrangements of diagonal bracing create triangular apertures through which flow passes. Some arch bridges include openings between the deck and arches that are too small for logs to pass through.
Many bridges lack small apertures conveying flow. Single-column piers, whether walls, cylinders, or hammerheads, deflect all flow through wide adjacent gaps. A deck resting directly on solid beams, with a solid parapet, is another example. The apertures in a single row of columns carry no flow and do not contribute to drift trapping if the row is aligned to approaching flow.
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