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<br />It is difficult to anticipate when a change in plan form may <br />occur. It may be gradual with time or the result of a major flood <br />event. Also, the direction and magnitude of the movement of the <br />stream is not easily determined. ALTHOUGH DIFFICULT TO DO, IT IS <br />NECESSARY TO PROPERLY EVALUATE THE VULNERABILITY OF A BRIDGE TO <br />CHANGES IN PLAN FORM AND TO IMPLEMENT COUNTERMEASURES. <br /> <br />Countermeasures may be changes in the bridge design, construction <br />of river control works, protection of piers and/or abutments with <br />riprap or even just careful monitoring of the river in the bridge <br />inspection program. It is seldom desirable to place <br />footings/foundations at a higher elevation than that of the main <br />channel just because they are located on the flood plain out of <br />the main channel. <br /> <br />To control lateral shifting requires river training works, bank <br />stabilizing by riprap and/or guide banks. The design of these <br />works is beyond the scope of this circular. Design methods are <br />given in FHWA (18), U.S. corps of Engineers (19, 20) and AASHTO <br />(21) publications. Of particular importance are "Hydraulic <br />Analyses for the Location and Design of Bridges," Volume VII- <br />Highway Drainage Guidelines, 1982 (21); "Highways in the River <br />Environment" (7); "Spur and Guide Banks" (22) and "Strea~ <br />Stability" Hydraulic Engineering Circular 20 (6). <br /> <br />21 <br />