LARGE-SCALE TEXTURAL PATTERNS AND SEDIMENTARY PROCESSES ON TIDAL SALT MARSHES IN THE SEVERN ESTUARY, SOUTHWEST BRITAIN

Approximately 840 km2 of largely reclaimed estuarine alluvium border the Severn Estuary and inner Bristol Channel today. Using a direct measure of grain size, and the proxy evidence of sediment-controlled, early land-use (medieval-early modern), it is shown that the grain size of the tidal mud deposited on the marshes prior to reclamation declined outward from the main and larger tidal channels toward the bedrock limit of the alluvium. Variations in the altitude of the marsh surfaces, as they survive after reclamation, suggest that the deposition rate of this sediment also decreased outward from these sources. At Elmore, a representative narrow (ca. 1 km) marsh, grain-size change is steep and well defined. In contrast, at Hill, in a representative sector from a large and broad (ca. 5 km) marsh, the measured change is gradual and disorderly, and disturbed by an extensive, arcuate belt of relatively coarse sediments. Based on a review of processes and sedimentation, and appealing to a broad geomorphological and hydraulic analogy between tidal wetlands and river floodplains. it is suggested that marshes of the Elmore type may be supplied with sediment largely by transverse diffusion from the tidal channel, whereas Hill-type marshes may receive considerable mud by convection as well as diffusion.