Debris accumulation at bridge piers has been a significant problem at a number of bridge sites in Indiana, increasing risks of upstream flooding, scour, and stream instability. The current study aimed at identifying factors contributing to debris accumulation in order to formulate guidelines for the design of new bridges that would minimize their occurrence or impact. Three related approaches were taken: i) a systematic study of the available underwater bridge inspections reports, ii) a program of periodic visits and visual examination of bridge sites that were thought to be prone to debris accumulation, and iii) continued video monitoring of three bridge sites, and analysis of video images recorded during debris-transporting events. The available underwater bridge inspection reports spanned a period of 10 years and covered 370 structures. Heavy debris accumulation was observed at ¡Ö20% of these sites, with the heaviest accumulations being concentrated in southwestern Indiana. Almost all of the sites in south-central and southern Indiana experienced at least moderate debris accumulation at one time. Sites with estimated volume of debris accumulation greater than 1000 cubic yards (¡®mega¡¯ sites), sites where heavy debris accumulation was observed during more than one inspection (¡®chronic¡¯ sites), and sites with at most minor debris accumulation during more than one inspection (¡®lite¡¯ sites) were examined in greater detail with regards to their crosssectional geometry and the placement of the piers, and the specific location of debris accumulation. A program of periodic site inspections was undertaken that eventually covered 22 sites, including 4 ¡®mega¡¯ sites and 6 ¡®chronic¡¯ sites. The sites were visited at intervals of 3 ¨C 4 months over a 16-month period, and visually examined with regards to the occurrence and location of any debris accumulation, and other features, such as the presence of upstream bridges, that might be related to debris accumulation. These observations were combined with cross-section information from underwater bridge inspection reports to corroborate or refine the conclusions drawn from the broader study of underwater bridge inspection reports. Issues that could not be resolved from the bridge inspection reports, such as the behavior over time of debris accumulation, and the relationship to hydrologic events, were of particular interest in the periodic-site inspections. Although some sites suffered what might be classed as ¡®heavy¡¯ accumulation during the study period, no ¡®mega¡¯ accumulation occurred at any site, suggesting that average recurrence intervals for such events are more than 16 months, at least for larger rivers. Finally, video monitoring of debris-transporting events was conducted at three sites including one ¡®mega¡¯ site. This yielded the most detailed information regarding aspects that were visible above the water surface. This included including the variation of debris ¡®discharge¡¯ and amount of debris accumulated with time, typical debris trajectories, and even direct evidence of disaggregation of already accumulated debris. At two of the sites, extensive debris accumulations, possibly approaching ¡®mega¡¯ class at one site, were recorded. In contrast, little debris accumulation was noted at the third site in spite of quite heavy debris transport during at least one large flow event. For larger, longer-duration flow events, debris transport seemed to be concentrated towards the earlier rising part, such as the first 12 ¨C 18 hours, of the event. Little correlation between the lateral location within the stream where debris transport is high and the thalweg in the immediate vicinity of the bridge crossing was found. On the basis of the observations made during the study, recommended practices are suggested for designing bridges with the aim of minimizing debris accumulation.

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bridges, hydraulics, piers, debris, scour, SPR-2859

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Performing Organization

Joint Transportation Research Program

Publisher Place

West Lafayette, IN

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