Channel stability in the tay estuary: controls by bedrock and unconsolidated post-glacial sediment

Abstract

Borehole and geophysical data have been collated for two sections across the Tay estuary on the lines of the road and rail bridges at Dundee. Geological reconstructions reveal a complex late-glacial and Holocene fill resting on a basement of Devonian sandstones and lavas. Comparative analyses of bathymetric charts (1816–1970) have enabled areas of present-day relatively stable and unstable bed to be identified. Stable areas are underlain by either gravels or partially compacted clays; unstable areas by loose, coarse to fine, sands. Non-migratory channels coincide with the stable areas; shifting sand banks and migratory channels occur elsewhere. Channel wandering can only take place in areas where current strengths are competent to erode easily and to transport bed materials. Where the substrate consists of less easily erodible bed materials, the channel migration is restricted. Artificial dredging of new channels into stable substrates may produce stable, self-maintaining waterways. Dredging in unstable areas is often a short-term, uneconomic, expedient. The use of physical models to determine modifications to channels frequently entails mobile bed techniques, commonly using uniform beds of effectively infinite depth. Unless the substrates within and beneath the mobile bed are reproduced, the true “natural” behaviour of channels and banks cannot be accurately assessed.