Bed forms and unsteady processes: Some concepts of classification and response illustrated by common one‐way types

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Abstract
In natural environments, where unsteady flows prevail, the common one‐way bed forms (ripples, dunes, antidunes, parting lineations on plane beds) show delayed responses to changes of flow. A dual classification may then be necessary for the features:1. Geometric, with no implication of flow conditions.2. Geometric‐hydraulic, in which the shape of the observed forms is combined with the character of the concurrent flow.The delayed responses arise because, under the simplest steady‐state equilibrium conditions, the bed forms behave deterministically as populations, which take average dimensions controlled by the bed‐material and/or flow conditions. The response of the bed to a change of flow is a rearrangement of the particles forming the bed in an attempt to meet the new conditions. The rate of rearrangement, however, is controlled by the sediment transport rate, in turn determined by the flow conditions, and so is finite.Two specific mechanisms of change of opposing tendency are quantifiable:1. Creation‐destruction of forms, where the new forms are better adjusted than predecessors.2. The imperfect modification of existing forms during their life‐spans.Both rates may be controlled by a combination of flow variables, the attributes of the bed forms, and ‘constants’ specific to each kind of form. A better knowledge of the unsteady responses of these bed forms should increase our understanding of and control over river and tidal systems and may provide models for a better appreciation of still larger features in landscapes and waterscapes.