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Potential Drift Accumulation at Bridges
Drift(1) accumulation at bridges is a
widespread problem. Drift reduces the capacity of bridge openings,
contributes to scour, and increases lateral forces on bridges. Drift
contributes to more than one-third of the bridge failures in the United
States and has been a primary cause of a number of failures (Chang,
1973). Current design guidelines treat drift as a threat to bridges,
but do not include methods for estimating the size and likelihood of
drift accumulations. Most published information regarding drift is
anecdotal and qualitative. Such information is valuable, but difficult
to apply in bridge design.
This report presents the results of a study of drift accumulation at
bridges conducted by the U.S. Geological Survey (USGS), in cooperation
with the Federal Highway Administration (FHWA). The study was conducted
from 1992 through 1995, and included a review of published literature
on drift, analysis of data from 2,577 reported drift accumulations, and
field investigations of 144 drift accumulations.
The guidelines for the assessment of drift potential presented in this report summarize the main conclusions of this study in the form of a detailed drift-assessment method. The guidelines include methods for estimating the likelihood that drift will accumulate at a bridge and the maximum size of drift accumulations. These guidelines assign a relative potential for drift accumulation and do not estimate the probability of an accumulation occurring in a given year. Their use requires engineering judgement and some familiarity with regional drift characteristics.
1. Drift is defined as "any type of debris that is floating on or through a river" (Pangallo and others, 1992). "Floating debris" is a synonymous term. In this report, the term "debris" is sometimes used in discussing previous studies that use this term for drift and refers to floating debris (Lagasse and others, 1991). However, "debris" is often used to refer to rocks transported by flowing water or dense, non-Newtonian mixtures of sediment and water, and the less ambiguous term "drift" is preferable (Perham, 1987).
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