Effects of Debris on Bridge Pier Scour (2010) / Chapter Skim
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92.1 Review of Current Practice 2.1.1 Introduction Debris (or drift) , for the purposes of this project, is defined as floating woody debris that is delivered to and transported along a stream or river.
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et al.
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this model with tree-growth and "fall-down" (risk-rating) models may be useful for identifying trees that will have a high probability of providing coarse woody debris to a stream in later years.
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12 Location Channel Network Position Results References Western Oregon 0.10 to 0.30 channel gradients LWD in small streams randomly distributed LWD easily transported in larger rivers leading to size sorting and accumulations in distinct jams Swanson et al. 1976 Western Oregon 0.02 to 0.50 channel gradients Debris loading highest in small, steep streams, decreasing downstream In 1st and 2nd order streams, LWD randomly located because streams too small to redistribute In 3rd to 5th order streams, flows large enough to redistribute debris from distinct accumulations that directly affect channel width In large rivers, LWD thrown on islands or on banks, having little influence on channel, except during high flows Swanson and Lienkaemper 1978 Indiana, North Carolina, Oregon 0.001 to 0.40 channel gradients Loading (kg/m2)
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width and remains in place during most flow events whereas wood in larger, higher order streams is generally shorter than the channel width and is, therefore, more easily transported, leading to a reduction in LWD frequency due to flushing of smaller pieces and clumping of the remaining LWD pieces. Evidence suggests that pieces shorter than bankfull width and with a diameter less than bankfull depth are more likely to be transported downstream (Bilby 1984, Bilby and Ward 1989)
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the debris. In steep, low order streams, woody debris mobilization and transport is accomplished primarily by debris torrents triggered by heavy rainfall and flood flows (Swanson et al.
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at about the average water velocity. In moderate radius bends, drift is observed more often along the thread of a stream (i.e., thalweg)
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Based on a study conducted in the Queets River basin in Washington, Abbe and Montgomery (2003) determined that individual pieces of woody debris in an accumulation or jam can be separated into key, racked, and loose members.
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bers and fluvially transported racked and loose members. Within these categories, Abbe and Montgomery (2003)
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plunging flow at the upstream edge .
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In addition, accumulations between the bank or abutment and an adjacent pier are similar in structure to accumulations between piers (Diehl 1997)
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Examining the drag forces identified by Parola et al.
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Melville and Dongol combine the debris mass with the pile to arrive at an effective diameter. For general scour, HEC-18 (Richardson and Davis 2001)
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Piers The placement, type, and skew of bridge piers have a significant influence on the extent of debris accumulation at a pier. Piers placed in the path of drift movement, such as in the thalweg, will have a higher trapping potential.
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is dependent on the amount and frequency of debris delivered to the bridge site as well as local site conditions and structure configuration (e.g., Lyn et al.
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Debris racks or catchers are placed across the channel and are used to collect debris before it reaches a bridge (Perham 1987, 1988)
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25 Geographic Region State Stream Source Number of Photos East North Deep River Debris Free, Inc. (Mike Collier)
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26 Geographic Region State Stream Source Number of Photos Pacific Bear River Ayres Associates Inc. 2 Coast Harris Creek Ayres Associates Inc.
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27 Geographic Region State Stream Source Number of Photos West Arizona New River Ayres Associates Inc. 2 Colorado Bijou Creek Ayres Associates Inc.
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Fenneman (1917) proposed the concept of physiographic subdivisions of the United States based on regional geomorphologic characteristics in the early 20th century.
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Source: after Barton et al.
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Source: after Bailey (1997) 100 POLAR DOMAIN 120 Tundra Division 124 Arctic Tundra Province 125 Bering Tundra (Northern )
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31 Geographic Region Source State Stream No. of Photos Physiographic Region Ecoregion California (Central Arroyo Grande 11 24 260 and South Coast)
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32 Geographic Region Source State Stream No. of Photos Physiographic Region Ecoregion Midwest Brad Rognlie, P.E.
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result of the more arid climate, which produces less widely distributed debris. The paucity of debris sites in the East may be tied to the greater overall stability of the streams and rivers in the region.
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framework, a sublist of survey responses can be developed. The database allows the user to specify multiple query criteria, so refinements to the sublist can also be readily performed.
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ephemeral or intermittent, reported that submerged material was equally as common as floating material. This phenomenon may be the result of accumulations that slide to the streambed during the falling limb of a flood event.
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damage caused by debris at bridges. Only about half of the respondents answered this question.
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Respondents reported that key logs less than 25 ft (7.6 m) in length are most often the initiators of drift accumulation.
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budget in years with little flooding and perhaps 2% to 5% in years that experience one or more significant events. • Relatively high standard deviation from the mean numerical responses within each region indicate that it is possible to experience the full range of drift-related problems in any given region; however, ranking the responses does provide an indication that some problems or issues are more common than others, as noted in the preceding subsections.
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sonar having to be mounted to point straight down, the sidelooking or backward-looking heads allow the transducer to be mounted to look horizontally under the debris pile. The head was attached to the articulating arm and positioned around the pile in the same manner as the downward-pointing sonar.
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