Relaxation time of dunes in decelerating aqueous flows

The relaxation time of a bed form is the time required by the feature to equilibrate to a new flow condition after a change of flow. In the case of dunes formed by flows that periodically decelerate into the ripple existence-field, or into the field of no bed-material movement, the relaxation time (minimum value) may be calculated in terms of dune geometry and the total sediment transport permitted by the governing flow regime. Application of the analysis to intertidal dunes on a sand bank at an East Anglian locality afforded a relaxation time very much greater than the effective period of the ebbing tide and significantly larger than the effective period of the spring-neap tidal cycle. This finding is consistent with the observation that a dune pattern is a permanent feature of the bank.