Mass-movement processes acting at up to three scales influence the intertidal forms of drainage in the Severn Estuary and its bordering estuarine alluvium. The open mudflats are marked by delicate drainage rills into which some freshly deposited silt is transferred at low tide by downslope creep, as shown by widespread systems of small extensional fractures. The pills traverse the areas of reclaimed alluvium and the saltings. They retain a relatively symmetrical cross-sectional profile, even where tortuous, and are remarkably stable in both general position and detailed form. The pills are maintained by large rotational slips accompanied in many cases by channelled mudflows formed from much smaller failures. A profile of long-term sediment accumulation that favours the loading of the upper sides of the pills, irrespective of local channel curvature, appears to be a significant factor in the maintenance of these drainage lines.

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