River Width Adjustment. I: Processes and Mechanisms

Abstract

In 1993 a Task Committee (TC) of the ASCE was established to study the hydraulics, bank mechanics, and modeling of width adjustment in alluvial channels. The work of the TC in reviewing width adjustment processes and mechanisms is reported in this paper. A companion paper presents the findings of the TC with regard to width adjustment modeling. This paper first establishes the geomorphic context within which width adjustments occur, and it demonstrates that width adjustment may take place over a wide range of scales in time and space. In the past engineering analyses of channel width have tended to concentrate on prediction of the equilibrium width for stable channels. Most commonly the regime, extremal hypothesis, and rational (mechanistic) approaches are used, and these are reviewed herein. More recently, attention has switched to channels that are adjusting their morphology either due to natural instability or in response to changes in watershed land use, river regulation, or channel engineering. Characterizing and explaining the time-dependent behavior of width in such channels requires an understanding 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 of bank retreat due to flow erosion and mass failure under gravity, and bank advance due to sedimentation and berm building. It is demonstrated that, while rapid progress is being made, most existing analyses of bank mechanics are still at the stage of being research tools that are not yet suitable for design applications. This paper ends with a series of conclusions and recommendations that synthesize the findings of the TC.