Temporal and spatial depositional patterns in the Severn Estuary, southwestern Britain: intertidal studies at spring-reap and seasonal scales, 1991-1993

The estuary is a high-energy and very variable system, with a hypertidal, semidiurnal tidal regime. The amount and coarseness of the sediment trapped on filter papers, and the suspended silt concentration in the flood tidal front, were measured daily from mudflat and marsh stations at each of six sites during eight spring-neap cycles spread over a two-year period. Four of the cycles were associated with vernal equinoxes and four with autumnal equinoxes. River discharge, wind speed, tidal height and the suspended sediment concentration all affect the amount and texture of the trapped mud. Complex tidal patterns are unlikely to be registered other than very imperfectly in these tidal deposits. At any site, the silts deposited on the mudflats are slightly coarser than those accreted on the marshes. Both marsh and mudflat deposits tend to become less sandy but more clayey toward the mouth of the estuary. The sediments trapped on the mudflats and marshes during vernal equinoxes tend at a site to be sandier and less clayey than their autumnal counterparts. As the vernal and autumnal, equinoctial tidal regimes are virtually identical, and river discharge has a negligible influence, winter storminess and the substantial annual change in water temperature, affecting viscosity, may largely be the explanation, with biological factors possibly augmenting the effect. (C) 1998 Elsevier Science B.V. All rights reserved.