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Publication No. FHWA-RD-97-028

April, 1997

This technical summary announces the key findings of a Federal Highway Administration (FHWA) study that is fully documented in a report of the same title (FHWA-RD-97-028).

Purpose of Study

Drift (floating debris) accumulation at bridges is a widespread problem. Drift reduces the capacity of bridge openings, contributes to scour, and increases lateral forces on bridges. It contributes to more than one-third of the bridge failures in the United States. Drift damages bridges mostly through local and contraction scour.

The purposes of this study were (1) to determine characteristics of drift and drift accumulations at bridges and (2) to develop a method for rating potential for drift accumulation at bridges as high, medium, or low, based on bridge and site characteristics. The main purposes of the report were (1) to provide a general description of drift characteristics and drift-related problems and (2) to present an example of guidelines for assessing the potential for drift accumulation at specific bridges, whether existing or under design.

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. Use of the guidelines requires engineering judgement and some familiarity with regional drift characteristics.

Research Approach

This study was 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.

State Departments of Transportation in Indiana, Massachusetts, Maryland, South Carolina, and Tennessee cooperated with the USGS in potential-scour studies used as data sources in this study. State Departments of Transportation in Arkansas, Delaware, Florida, Georgia, Idaho, New Mexico, North Carolina, Oregon, Pennsylvania, Virginia, and Wisconsin responded to a request for information on drift accumulations.


Drift that accumulates at bridges comes primarily from trees growing on the banks and bank tops of rivers. Most of the trees that become drift are undermined by bank erosion. Rivers with unstable channels have the most bank erosion and drift, but most rivers transport some drift during floods.

Floating drift is concentrated along the thread of the stream and moves at about the average flow velocity. Logs longer than the width of the channel accumulate in jams, or are broken into shorter pieces. Sunken woody debris moves more slowly and tends to accumulate in and along the channel, rather than being transported downstream to bridges.

Drift accumulates against obstacles such as bridge piers that divide the flow at the water surface. Groups of obstacles separated by narrow gaps trap drift most effectively. Drift accumulation begins at the water surface, but accretion can cause an accumulation to grow downward to the stream bed.

Because drift accumulations on single piers are based on logs extending the full width of the accumulation, the maximum width of single-pier accumulations is about equal to the length of large logs delivered to bridges. Spans are generally blocked by logs extending from pier to pier, so the same design log length is equal to the maximum width of spans likely to be blocked by drift.

28K GIF version of this graph

Figure 22. Effective width of drift-blocked spans outside the Pacific Northwest.

Design log length is defined at a given site by the smallest of three values:

This design log length does not represent the absolute maximum length of drift pieces; longer pieces were observed at several sites. Logs longer than the design log length are insufficiently abundant, or insufficiently strong throughout their full length, to produce drift accumulations equal to their length.

Potential is low for blockage of spans with effective width greater than the design log length. Most span blockages have involved spans with an effective width less than the design log length. Exceptions have involved other factors such as sediment accumulation or drifting objects larger than logs such as cabled log rafts.

Assessment of the potential for drift problems should take place early in the project planning process. Designers can then select design features appropriate for drift-prone streams at the outset of a project. Such features include adequate freeboard, long spans, solid piers, round (rather than square) pier noses, and pier placement away from the path of drift.

Researcher -- This study was performed by Timothy H. Diehl of the U.S. Geological Survey, Water Resources Division, 810 Broadway, Suite 500, Nashville, Tennessee 37203

Distribution -- This technical summary is being distributed according to a standard distribution. Direct distribution is being made to the the Regions and Divisions.

Availability -- Copies of this report are available from the National Technical Information Service, 5285 Port Royal Road, Springfield, Virginia 22161. A limited number of copies will be available from the R&T Report Center, HRD-11, 6300 Georgetown Pike, McLean, Virginia, 22101-2296, phone no. (703) 285-2144. The report can be examined on the World Wide Web through the U.S. Geological Survey Tennessee District home page, at http://tn.water.usgs.gov/index.html

Keywords -- Bridge failure, Channel morphology, Debris, Drift, Geomorphology, Large woody debris, Open channel flow, Stream erosion, Scour, Woody debris

Notice -- This technical summary is disseminated under the sponsorship of the Department of Transportation in the interest of information exchange. The summary provides a synopsis of the study's final publication. This summary does not establish policies or regulations, nor does it imply FHWA endorsement of the study's conclusions or recommendations. The United States Government assumes no liability for its contents or use thereof.

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U.S. Department of the Interior, U.S. Geological Survey
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