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64 Rivers prone to channel migration may be spanned by struc- tures and paralleled by fixed highway alignments and other appurtenances. Channel migration (alluvial river meander planform deformation) is a major consideration in designing bridge crossings and other transportation facilities in affected areas; it causes the channel alignment and approach conditions present during construction to deteriorate as the upstream channel location changes. Channel migration can result in the following: (1) excessive bridge pier and abutment scour, (2) flanking of bridge approaches and other highway infra- structure, (3) excessive scour because of debris problems, and (4) obstructed conveyance through bridge openings. Channel migration includes lateral channel shift (expressed in terms of distance moved perpendicular to the channel cen- terline per year) and down-valley migration (expressed in dis- tance moved along the valley per year). Engineers are con- cerned about predicting channel migration as it moves through the bridge elements (piers and abutments) or endangers other highway infrastructure during the design life of the structure. The Handbook will assist practicing engineers in locating and designing new bridges, highway facilities, or other struc- tures to accommodate anticipated channel migration. The techniques developed in the Handbook could also be used to evaluate the risk to existing facilities and, if necessary, deter- mine the need for, and design of, measures to counteract the effects of channel migration. A reliable prediction of chan- nel migration could also alert bridge inspection personnel to the potential for channel change that could affect the safety of a bridge. In addition, the techniques developed in the Handbook can be applied to channel restoration and stabi- lization efforts. The map and aerial photo comparison and frequency analy- sis techniques provide a practical quantitative methodology for predicting channel migration in proximity to transportation facilities. The Handbook will be useful in reconnaissance, design, rehabilitation, maintenance, and inspection of highway facilities. The result will be a more efficient use of highway resources and a reduction in costs associated with the impacts of channel migration on highway facilities. The prediction techniques can also be used by other practitioners responsible for river channel maintenance, river restoration/rehabilitation, and floodplain planning and management. CHAPTER 9 SUMMARY
