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Potential Drift Accumulation at Bridges
Forces exerted on bridge decks and superstructures by drift accumulations have caused displacement of bridge spans and damage to piers. Simply supported trusses are most vulnerable to this kind of damage (O'Donnell, 1973). The sieve-like structure of the truss acts as a trash rack, and spans are not designed to resist the resulting lateral forces. Forces on piers due to drift have also contributed to failure by placing additional forces on piers subjected to scour (O'Donnell, 1973; National Transportation Safety Board, 1990; I. Nagai, California Highway Department, written commun., 1992; James Schall, written commun., 1993).
Published work on drag coefficients of drift accumulations at bridges is limited to studies of idealized models of drift accumulations (Apelt, 1986a). The drag coefficient varied significantly with variations in the porosity and internal structure of the drift. Apelt (1986a) recommended studies of drag forces on real drift accumulations. Such a study is now underway (Thomas Fenske and Arthur Parola, University of Kentucky, written commun., 1995). Other studies have used flume experiments and field measurements to estimate drag coefficients for drift accumulations away from bridges (Shields and Smith, 1991; Gippel and others, 1992; Shields and Gippel, 1995).
Impact of individual pieces of drift on bridges has been cited as a cause of damage, especially to the upstream pile in a bent (Chang, 1973). Drift impact alone, however, has caused few bridge failures (Bowser and Tsai, 1973; National Transportation Safety Board, 1990). In the case of the failure of the temporary Harrison Road bridge over the Great Miami River at Miamitown, Ohio, an eyewitness reported that a large mass of drift including a boat and part of a dock collided with the pier (which already had a drift accumulation on its nose) and that the pier then failed (National Transportation Safety Board, 1990). This failure may have been due primarily to impact forces. In other cases, the cause of failure is not well documented (Bowser and Tsai, 1973). A study of actual forces due to drift impact is now underway (Thomas Fenske and Arthur Parola, University of Kentucky, written commun., 1995).
Loss of flow capacity in the bridge opening may greatly increase flood depths upstream (Williams, 1990). Resulting overflow of approaches may protect the bridge itself from structural damage, possibly at the cost of embankment erosion (O'Donnell, 1973). Increased backwater is combined with increased hydrostatic forces on the drift accumulations, increased flow velocities and contraction scour, and an increased chance that the superstructure will become immersed.
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