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Potential Drift Accumulation at Bridges

Size of Potential Accumulations

Drift accumulations can grow to maximum sizes that depend mostly on log dimensions, flow depth, and the number and proximity of gaps and piers affected. Accumulations in the channel can reach their maximum size during a single flood where delivery is high, but accumulations grow more slowly where the drift supply is low. Large accumulations are less likely to occur in a single flood outside of the channel and at sites where delivery of drift is low.

The values of drift-accumulation potential estimated in the preceding section are relative, and do not address the likely size of an accumulation. For example, a "high potential" for drift accumulation at a single pier indicates high potential for a drift accumulation relative to the potential for accumulation at other piers. Accumulations will not necessarily form on all piers with high potential. If an accumulation does form, its width may be as great as the design log length based on channel width, and it may extend vertically to the depth of flow, or it may be much smaller.

The accumulation of drift on a single pier begins with a single piece of drift at the water surface, and the width of the accumulation may reach the length of the design log. If initially narrow, the accumulation can grow to the maximum width through the accretion of a single log. Where drift accumulates across a horizontal gap, the initial accumulation extends the full width of the gap at the water surface, and may ultimately extend beyond the vertical elements defining the gap.

Additional accretion causes the drift accumulation to grow downward toward the streambed. The accumulation can continue to grow downward until it extends over the full depth of flow, with about the same width over its full height.

To be conservative, assume that a drift accumulation extends from the water surface to the streambed. Also assume that an accumulation on a single pier will have a width equal to the design log length over its full depth, and that an accumulation across two or more piers will extend laterally half the design log length beyond them. These assumptions are consistent with the largest observed accumulations, and will probably maximize effective pier width and predicted forces and scour depths. In calculating scour, however, it may be more conservative to assume that an accumulation extends only part of the distance downward from the surface to the streambed.

The maximum size of drift accumulations on superstructures cannot be estimated based on the few existing descriptions of such accumulations. Abundant drift can close all apertures in trusses and parapets as they are submerged. To be conservative, assume that these apertures will be closed. Where drift accumulates across the vertical gap from low steel to the streambed, the entire gap could ultimately be closed. Accumulations on the upstream side of a bridge may extend vertically beyond low steel and the top of the superstructure. The design length used for this vertical extension may have a large effect on estimates of the forces on the bridge and the upstream water level. Data on the distance of vertical extensions are scarce. The Australian design practice of allowing for 1.2 m (4 ft) of total vertical extension above the top of the parapet and below low steel does not seem excessively conservative (Wellwood and Fenwick, 1990).

Effects of Accumulations that Modify Analysis

Drift accumulations change bridge hydraulics and trapping characteristics, and may increase the potential for additional trapping. When all the possible drift accumulations at a given bridge have been assigned a potential for occurrence, alter the assumed bridge characteristics and water levels as needed, then run the analysis again to determine whether some individual accumulations increase in potential. For example, a high-potential blockage across the channel may cause skewed flow through bents in the flood plain, raising the trapping potential at these bents from medium to high. If so, accumulation at these bents should be regarded as having high potential.

Deflection of flow from drift accumulations changes the angle of approaching surface flow at nearby piers. If adjacent piers include multiple columns, assume they are no longer aligned. Also, decrease the effective width of adjacent horizontal gaps as needed to reflect increased skew.

Drift constricts the bridge opening and increases the backwater effect of the bridge. Use a one-dimensional step-backwater flow model (for example, WSPRO) to determine a new upstream water-surface elevation (Shearman and others, 1986; Shearman, 1990). If backwater from drift causes immersion of the superstructure, evaluate the potential for accumulation on the superstructure and across vertical gaps.

Unremoved accumulations change the pier shape to "with apertures" (figure 32).

Mid-channel piers in the drift path may retain drift for a long time after it accumulates. This problem is greatest where potential for accumulation is chronic and where the pier is difficult or expensive to clear. Factors impeding removal include a location in mid-channel beyond the reach of cranes on either bank, a pier nose overhung by the deck impeding removal from above, hammerheads with drift wedged under the top of the pier, and multiple columns with entangled drift.

Overall Potential for the Entire Bridge

The potential for drift accumulation at a bridge is the maximum of the potentials estimated for each pier, superstructure section, or gap between fixed elements. For example, if any part of the bridge is assigned a high potential for accumulation, assign the bridge a high potential for accumulation.

The drift accumulation that has a high potential is the sum of all individual high-potential accumulations on single elements or across gaps, assuming each one grows to its maximum size. Similarly, the drift accumulation over the entire length of the bridge that has a medium potential is the sum of all individual medium-potential and high-potential accumulations on single elements or across gaps, assuming each one grows to its maximum size. If the stream frequently delivers abundant drift to the bridge site, combine the individual accumulations assigned high, chronic potential to estimate the location and maximum extent of drift requiring regular removal.

The overall potential for problems related to drift accumulations at a bridge depends on the probability or frequency of events that have the potential to produce accumulations as well as the potential for accumulations to occur at the bridge and the potential size they could reach. An assessment of high potential for drift accumulation could result from assumptions of radical changes in land use in the basin, followed by enlargement of the channel and a large flood. On the other hand, an assessment of high potential could result from the assumed occurrence of a 2-year flood with existing channel conditions. These two assessments would have different implications for bridge design and bridge maintenance.


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