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Potential Drift Accumulation at Bridges
Accumulation at Bridges
Drift accumulates at bridges when it encounters structural components that trap it. Most observed drift accumulations fall into two classes: single-pier accumulations and span blockages. Consistent features of observed accumulations allow prediction of probable locations and maximum size of potential accumulations.
A probable maximum width of drift accumulations and blocked spans can be estimated on the basis of known drift characteristics or, lacking detailed information on drift, on the basis of channel width upstream from the site. Wider span blockages and single-pier accumulations are rare, and seem to involve the formation of mid-channel bars through massive sedimentation or the accumulation of exceptionally large drift.
No limit to the vertical extent of accumulations has been established other than the depth of flow. In the process of formation, single-pier accumulations often take on a form roughly resembling the inverted half-cone shape implied by New Zealand's design criteria (Dongol, 1989; Dr. Arthur Parola, University of Louisville, written commun., 1992). Under some circumstances, drift accumulations can reach from the water surface to the river bed. The maximum vertical extent of drift accumulation observed in this study was more than 12 m (40 ft).
Unlike drift delivery, which is commonly beyond the control of bridge engineers, drift trapping can be reduced by appropriate design features such as adequate freeboard, long spans, solid piers, and careful pier placement. Measures designed to guide drift through existing structures have had mixed results.
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