Phase differences between bed configuration and flow in natural environments, and their geological relevance

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Abstract
In nature where unsteady flows prevail, the characters of bed configurations lag, or differ in phase from, the flow conditions. This is demonstrated from quantitative data for dune beds in the Fraser River (Canada) and the Gironde Estuary (France), where the flow conditions change ahead of the bed form by as much as 1/4 period. Qualitative evidence for lag abounds, for instance, substantially unmodified dunes exposed during low water.The existence of lag appears to confuse attempts to establish from field data the relationships between bed‐form properties and flow conditions and, at the same time, makes difficult the confident application to field situations of formulae based on steady‐state experiments or theories. Unfamiliar types of field study and experiments are needed to increase understanding of the degree and precise significance of lag.