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Potential Drift Accumulation at Bridges

SUGGESTED GUIDELINES FOR ASSESSMENT OF DRIFT POTENTIAL

These guidelines summarize current knowledge about drift, and are an example of one method of assessing the potential for drift accumulations on specific parts of a bridge. The factors used in this assessment may not be the only factors significantly influencing drift accumulation, and this is not the only way in which they can be combined. The use of these guidelines requires engineering judgment and should be tempered by regional experience with drift problems.

The assessment of potential for drift accumulation at a bridge can be divided into three major phases. First, estimate the potential for the stream to deliver abundant drift to the bridge site, along with the likely maximum dimensions of individual pieces of drift, and divide the site among location categories that define the local potential for drift delivery. These location categories reflect site characteristics and do not depend on the bridge design. Secondly, evaluate the bridge characteristics that influence potential for trapping drift at individual piers and spans, and integrate the potential for drift delivery with drift-trapping characteristics to obtain estimates of potential for drift accumulation at individual piers and spans. Finally, combine potential for accumulation on individual elements of the bridge to estimate potential accumulations for the entire bridge. These three phases can be further divided into eight tasks (table 5):

Table 5. Major phases and tasks in evaluating potential for drift accumulation at a bridge.

Major Phase

Tasks

1. Estimate potential for drift delivery.

a. Estimate potential for drift delivery to the site.

b. Estimate size of largest drift delivered.

c. Assign location categories to all
parts of the highway crossing.
2. Estimate drift potential on individual bridge elements.

a. Assign bridge characteristics to all
immersed parts of the bridge.

b. Determine accumulation potential
for each part of the bridge.
3. Calculate hypothetical accumulations for the entire bridge.

a. Calculate hypothetical accumulation
of medium potential.

b. Calculate hypothetical accumulation
of high potential.

c. Calculate hypothetical chronic accumulation.

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