Distribution and Behavior of Fine Sediment in the Severn Estuary and Inner Bristol Channel, U.K.

Abstract

During the 10 yr, 1970–80 a variety of field measurements using some new techniques have allowed the distribution of muddy sediment in the Severn Estuary and Inner Bristol Channel to be defined. Based on these measurements, a behavioral model has been adopted comprising mobile and stationary suspensions, and settled mud. The regional distribution of mobile suspensions shows a suspended solids front located along the main channel axis with the more turbid water on the southern, English, side. At maximum current velocities on spring tides, vertical suspended sediment profiles are homogeneous. As velocity decreases, a stepped structure develops due to settling, which is then remixed on the next accelerating semidiurnal tidal phase. Over the lunar timescale these steps become more stable, eventually settling to the bed. The steps are unrelated to salinity, temperature, grain-size, or mineralogy. On neap tides, layered, acoustically detectable stationary suspensions, which occasionally consolidate to form settled mud, develop from the mobile suspensions. An idealized "three-zone" model has been devised characterizing the structures developed in the mobile suspensions.Key words: Severn Estuary, vertical profiling, siltmeter, densimeter, suspended solids, fronts, stratification