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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.

Wide Gaps Between Fixed Elements of the Bridge Opening

A bridge includes one or more gaps through which any drift carried by the stream is intended to pass. Where the width of a gap is less than the length of the longest pieces of drift delivered to it, the potential for drift accumulation can be high. Estimate the effective width of gaps between fixed elements of the bridge opening including piers, banks, and abutments. In addition to the gap between each pair of adjacent piers, assign a width to the gap between each bank and the nearest pier in the channel and the gap between each abutment and the nearest pier. If low steel is submerged in the maximum design discharge, estimate the width of vertical gaps between sections of the superstructure and the streambed or flood plain below them. (Gap "width" as used in this analysis may be the horizontal distance across the direction of approaching flow between projected positions of vertical elements or the vertical distance across the gap between low steel and the streambed.)

Horizontal gaps

Horizontal gaps are common locations for large accumulations of drift. Piers, nearby banks, and abutment bases are fixed elements of the bridge opening that can interact to trap drift. Pieces of drift in the longest size fraction delivered to the site will typically come into contact with one such element, then rotate downstream until they lodge against another element. Once one log is lodged across a gap at the surface, other pieces of drift can lodge against it and against the bridge, speeding the accumulation process. Alternatively, the horizontal gap may extend from a drift accumulation on one element to another element previously free of drift.

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

When the water level is at or above the bottom of the superstructure ("low steel"), drift may become trapped vertically between the superstructure and the streambed below it. Most drift is transported at the surface. When floating drift hits the superstructure, most pieces rotate to one side, remaining at the water surface. The drift then accumulates against the superstructure at the surface or is swept under the superstructure. However, some pieces of floating drift hit roughly endwise, and the upstream end rotates downward until it encounters the streambed. Some such pieces remain lodged against the superstructure and the streambed.

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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