Streamwise Erosional Structures in Muddy Sediments, Severn Estuary, Southwestern UK

ABSTRACT

Muddy sediments ranging from stiff clays and silts of early post-glacial age to fresh deposits of a buttery consistency are widely encountered in the intertidal zone of the estuary, a large macrotidal system swept by vigorous currents. Erosion of these muddy sediments is frequent, widespread and, in places, long-lasting. It is commonly expressed by the development of streamwise furrows and ridges on three distinct transverse wavelength scales (order 0.1 m, 1 m, 10 m). These occur on scoured surfaces ranging in extent from a few tens to many thousands of square metres. Coarse debris—the chief if not sole agent of erosion—is normally present in the bottoms of the furrows. The structures are discounted as obstacle marks because, with rare exceptions, they are in spacing and location independent of the distribution of bed irregularities. They appear instead to be related to secondary currents that arise due to the instability of the tidal shear flows, for which there is circumstantial field evidence. The furrows and riges of intermediate and large size probably depend on an instability mechanism constrained by flow depth. A second mechanism, taking a much smaller length scale, appears necessary to explain the small-scale structures, which in places appear to be active at the same time as those of intermediate dimensions.