RIVER WIDTH ADJUSTMENT - I - PROCESSES AND MECHANISMS

In 1993 a Task Committee (TC) of the ASCE was established to study the hydraulics, bank mechanics, and modeling of width adjustment in alluv ial channels. The work of the TC in reviewing width adjustment process es and mechanisms is reported in this paper. A companion paper present s the findings of the TC with regard to width adjustment modeling. Thi s paper first establishes the geomorphic context within which width ad justments occur, and it demonstrates that width adjustment may take pl ace over a wide range of scales in time and space. In the past enginee ring analyses of channel width have tended to concentrate on predictio n of the equilibrium width for stable channels. Most commonly the regi me, extremal hypothesis, and rational (mechanistic) approaches are use d, and these are reviewed herein. More recently, attention has switche d to channels that are adjusting their morphology either due to natura l instability or in response to changes in watershed land use, river r egulation, or channel engineering. Characterizing and explaining the t ime-dependent behavior of width in such channels requires an understan ding of the fluvial hydraulics of unstable channels, especially in the near-bank regions. Existing knowledge is reviewed, useful engineering tools are presented, and gaps requiring further field and laboratory research are identified. Finally, this paper considers the mechanics o f bank retreat due to flow erosion and mass failure under gravity, and bank advance due to sedimentation and berm building. It is demonstrat ed that, while rapid progress is being made, most existing analyses of bank mechanics are still at the stage of being research tools that ar e not yet suitable for design applications. This paper ends with a ser ies of conclusions and recommendations that synthesize the findings of the TC.