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Potential Drift Accumulation at Bridges
Effects of Pier Type and Placement
Pier placement can result in a high potential for single-pier accumulation even if the span length is above the threshold value for low span-blockage potential. A pier in the path of drift will likely trap it. Along the Harpeth River, Tennessee, even bridges with long spans and round pier noses were subject to single-pier accumulations. In addition, if the effective span width from a poorly placed pier to the bank is smaller than the threshold value for span length, potential for accumulations across the gap between the pier and the bank will also be high. In a narrow channel, a single pier near the center of the channel may have high potential to initiate a channel-wide drift accumulation.
Piers on the banks are less likely to trap drift than piers in the channel. Among 3,581 selected bridges in Tennessee, those with one pier in the channel were several times more likely to have single-pier drift accumulations than bridges with two piers on the banks and none in the channel.
Like the potential for single-pier accumulations, the potential for span blockages depends on pier placement, channel curvature, and other channel shape features. In relatively narrow channels with one or two piers in the channel, placement of piers at or near the bank bases seems to create less potential for span blockage than placement in the main channel. Where the reach upstream from the bridge is a long curve, drift is likely to move along the outer (concave) bank, and a span from the outer bank across much of the channel probably has the lowest trapping potential for a given span length. Where the reach upstream is a long, straight channel segment and the channel banks are wooded down to the bank bases, a span over the center of the channel likely has lowest trapping potential. In any situation where most drift follows a relatively narrow path along the surface of the stream, and piers are located outside of this path, span blockage is somewhat less likely. Thus, pier placement is an important factor regardless of whether span length is above or below the span-length threshold for high trapping potential.
Several pier types aggravate the potential for trapping drift. Multiple columns can act as a sieve unless exactly aligned with flow. The gaps between columns are narrow relative to drift length, and logs spanning the gap between two or more columns can be firmly held. Logs can become entangled in a group of columns in ways not possible at a single-column pier. Of the bridges included in the Tennessee scour-potential study, those with skewed pile bents in the water were about twice as likely to have drift on them as those with unskewed bents in the water.
Where clusters or multiple rows of piles are exposed to drift, accumulation is likely (figure 26). This situation can result from the intentional placement of a footing above the water surface in the channel, or from the exposure of piles through erosion.
Figure 26. Downstream side of a pile cluster with accumulated drift.
The flat noses of rectangular piers and pier footings also can provide stable resting places for drift. A square nose supports trapped drift at both vertical corners, which are sometimes widely separated. The accumulation must rotate around one of these points to become dislodged from the pier.
Some existing design recommendations favor rounded pier noses over sharp noses. Drift may slide more easily along a rounded pier nose, increasing the ability of the round-nosed pier to shed drift. However, selected Tennessee bridges with round-nosed piers were not significantly less likely to accumulate drift than bridges with sharp-nosed piers.
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