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Potential Drift Accumulation at Bridges
Bridge Characteristics
Certain characteristics of a bridge have a strong influence on the potential for drift accumulation. If an existing bridge is the object of study, select values for these characteristics based on the current location and design of bridge components. If these guidelines are part of a design process, use alternative locations and designs to determine how they affect the relative potential for drift accumulations at the new bridge. For each design under evaluation:
1. Assign each of the following to one of the location categories described above:
2. Determine whether the effective width of each gap exceeds the design log length for the site.
3. Determine whether each pier or superstructure section immersed in the design flood includes apertures that carry flow.
Where the bridge is skewed to approaching flow, the effective width of horizontal gaps is reduced. In order to estimate the effective width, one must first estimate the direction of flow approaching the gap during the design discharge. Project the positions of the upstream noses of fixed elements parallel to this flow direction onto an imaginary plane perpendicular to the flow direction. The effective width of a horizontal gap is the distance between the projected positions of the elements defining it (figure 21).
A horizontal gap should be assigned to the most drift-prone location category occupied by the fixed elements that define the gap. A horizontal gap from a pier to a bank or abutment has the same location as the pier. (For example, if the pier is on the bank slope, the gap is "on the bank." If the pier is in the drift path, so is the gap.) A horizontal gap between a pier on the bank and a pier in the channel should be classified as "in the channel." Where one of the fixed elements is sheltered and the other is not, the gap should be regarded as unsheltered.
Vertical gaps with low steel in the water
The fixed elements defining this vertical gap are low steel and the streambed beneath it. Measure the width of the gap vertically. The height of this gap will vary along the bridge as the elevations of low steel and the streambed change.
The location of a vertical gap can also vary from point to point along the bridge, even within an individual span. A vertical gap from the bank, bank top, or flood plain to low steel is in the flood-plain/bank location category. A vertical gap over the channel belongs to that location category, and a vertical gap at the drift path should be assigned to the "in the path" category.
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