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Potential Drift Accumulation at Bridges
Chronic Accumulations and Countermeasures
Drift accumulation is chronic where abundant drift is frequently delivered to piers or spans that have a high trapping potential. Chronic accumulation repeatedly subjects the bridge to the ill effects associated with drift. Drift-removal cost may become a significant component of total bridge cost during the life of the structure (Pangallo and others, 1992). Although prompt and complete drift removal is the most commonly used countermeasure, structural means of guiding drift through bridge openings have been applied at some sites. Scour countermeasures such as river-training structures and bed and bank armor may also be effective drift countermeasures.
The potential for accumulations to grow over the course of two or more floods depends on the difficulty of drift removal. At bridges where removal is difficult or expensive, accumulations are likely to remain in place longer. For example, at State Route 59 on the Eel River near Clay City, Indiana, at State Route 59 on the Vermilion River near Cayuga, Indiana, and at Sneed Road on the Harpeth River of Tennessee, accumulated drift accessible from the banks had been removed at the time of the visit, while drift in the center of the channel remained in place. The presence of drift accumulations from previous floods can have several consequences. A pier with accumulated drift on it may shed additional drift less effectively than if it were bare. An accumulation developing on top of a previous accumulation grows more rapidly through interaction with the drift below it. Drift remaining on the bridge has the potential to promote the growth of an island or bar.
The use of long spans allows the bridge designer to place fewer piers in the water, and makes it easier to avoid placing piers in the path of drift or near the center of the channel, where access is most difficult. Long spans, especially those longer than the design log length, are less likely to be blocked by drift. Because of these advantages, the use of long spans should decrease the frequency and difficulty of drift removal.
Wall piers that extend upstream to the edge of the parapet are easier to clear than piers of other types. Drift not only accumulates more readily on multiple-column piers, but also may become entangled with the columns along the full width of the underside of the bridge, possibly creating access problems for drift-removal crews. Drift trapped on trusses and piers with multiple columns can be entangled among multiple structural elements. Entanglement makes removal more difficult and adds to the possibility of damage to the bridge during clearing operations. Hammerhead piers are an alternative to multiple columns, but have the disadvantage of placing the pier nose well under the bridge and making it difficult to lift drift off the top of the accumulation (figure 27). At worst, drift may fill the space between the overhang of the pier and the bed, causing even more difficulty.
Figure 27. Drift under the upstream side of a bridge deck.
Superstructures that allow access to the pier nose from directly above ease drift removal. At best, a crane or log-picker operated from the bridge deck during the flood can remove drift before it forms an entangled accumulation (Rowe, 1974; Turner, 1992). A wide deck with a simple parapet and adequate load-bearing capacity for heavy equipment at the upstream edge affords the best opportunity for drift removal from the deck.
Access to the substructure of the bridge is important in allowing prompt and complete drift removal. Tracked vehicles may be able to remove all drift from a small channel during low water. On large rivers, access for barge launching may be needed.
Drift accumulation can be prevented where drift is accumulated upstream from the bridge, deflected away from piers, or guided through wide openings. Various measures to collect or guide drift have been suggested by several sources (Brice and others, 1978a; Lagasse and others, 1991; Richardson and others, 1991). Effective drift deflectors at two sites are mentioned by Lagasse and others, but no locations or designs are given. In their compilation and analysis of scour problems and countermeasures, Brice and others (1978a) provide the location of one deflector that failed, and have this to say about countermeasures in general:
"Except for well-known design features relating to bridge clearance, pier spacing, and in particular to webbing or enclosure of multiple column piers or pile bents, no successful devices for the prevention of debris accumulation were reported in the interviews."
A laboratory study of floating booms includes recommendations for glance booms as deflectors for pulpwood in currents of 1.4 meters per second (4.6 feet per second) or less (Kennedy and Lazier, 1965). The design depends on a smooth, vertical face at a small angle to the approaching flow, with a horizontal lip projecting upstream to prevent pulpwood from being carried under the boom by the strong induced current plunging under it. Pulpwood logs 1.2 to 2.4 m (4 to 8 ft) long (the range considered by Kennedy and Lazier) would float above this lip. The larger size and greater irregularity of logs that include branches and root masses might reduce the effectiveness of this design. No recommendations are offered for mooring such a boom in water subject to large changes in stage. No field experience with such designs is mentioned.
Perham mentions several deflection booms used in reservoirs and one in the Clark River in Idaho (Perham, 1988). These booms have smooth, vertical, upstream faces, but no horizontal lip is mentioned. Deflection booms work best when the angle to the flow is 20 degrees or less. Water velocity and depth are important considerations in boom design, but values of depth and velocity at effective booms are not given. Other effective deflectors include steel barges moored or sunk at a small angle to the approaching flow.
McFadden and Stallion (1976) recommended the installation of a system of pilings intended to align logs to pass through a set of gates 7.6 m (25 ft) wide at a dam on the Chena River in Alaska. These pilings were installed, and a model test of their ability to align drift was performed. Results were inconclusive:
"In many cases [the pilings] aligned the model logs so that they passed through the structure without incident. However, in some instances the model logs would form jams around the pilings. Clearly more work is needed to determine if these pilings are of sufficient value to warrant their installation." (McFadden and Stallion, 1976)
River training may prevent skew at multiple-column piers, thus reducing their tendency to trap drift. By stabilizing the channel location at the bridge, river training may keep piers in appropriate locations to avoid contact with drift in the zone of surface convergence. Any measures that stabilize the upstream channel will also reduce the supply of drift to the channel.
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