How do bedform patterns arise? New views on the role of bedform interactions within a set of boundary conditions

摘要

One explanation for bedform patterns is self-organization in which the pattern emerges because of interactions among the bedforms themselves. Models, remote images, fi eld studies and lab experiments have identifi ed bedform interactions that involve whole bedforms, only bedform defects, or that are remote interactions between bedforms. It is proposed that bedform interactions form a spectrum from constructive to regenerative in pattern development. Constructive interactions, including merging, lateral linking, cannibalization, and remote transfer of sediment, push the system toward fewer, larger, more widely spaced bedforms. Regenerative interactions, including bedform splitting, defect creation and calving, push the system back toward a more initial state. Other interactions, including off-center collision, defect migration, and bedform and defect repulsion, cause pattern change, but may not be strongly constructive or regenerative. Although bedform interactions are ubiquitous to any fi eld of bedforms, their dynamics, fl ow-fi eld modifi cation, and impact upon measurable pattern parameters are yet poorly understood. Most bedform interactions span bedform types and fl uids, supporting the hypothesis that pattern emerges from dynamics at the bedform level in a hierarchy that includes lower levels of bedform-fl ow and grain–fl uid interactions. Bedform interactions alone, however, cannot account for the rich diversity of bedform patterns in nature. It is proposed that fi eld diversity arises because of boundary conditions, which are the environmental variables within which a fi eld evolves. Conceptually, boundary conditions modify the shape of the attractor toward which a fi eld evolves, possibly by altering the type and frequency of bedform interactions. Boundary conditions are broadly similar within system types, but are unique for each bedform fi eld so that no two are ever exactly alike. Although aeolian and fl uvial systems share some types of boundary conditions, fl ow depth is a unique boundary condition in shallow fl uvial systems. Copyright 2009 John Wiley & Sons, Ltd.