The Inherent Asymmetry of River Meander Planform

Abstract

Inspection of the planform of meandering rivers indicates that they are generally not symmetric but display at least two forms of asymmetry indicative of flow direction. One is downchannel delay in the inflection point of meanders, producing traverses (the part of the channel trace that crosses from one side of the valley to the other) which are dominantly convex facing down valley. The other is upvalley skew in the axis of gooseneck loops. An index of inflection asymmetry is presented and applied to 15 rivers: the data confirm the statistical preference for delayed inflection. An explanation of the phenomenon is offered based on the hydraulics of flow through bends. Of major importance appears to be the persistence downchannel of patterns of helical circulation and cross-sectional distribution of longitudinal velocity well past the bend that forms them. The upvalley skew of goosenecks is, in many cases, nothing more than an extreme form of convex-downvalley looping, maintained as meanders have increased their tortuosity. Some goosenecks, however, show no delayed inflection and appear to develop from differences in rates of downvalley migration between adjacent meander traverses. It is concluded that the minimum variance theory and sine-generated model of meanders should now be discarded.