Channel stability in the tay estuary: controls by bedrock and unconsolidated post-glacial sediment
✍ Scribed by Antony T. Buller; John McManus
- Book ID
- 103070113
- Publisher
- Elsevier Science
- Year
- 1971
- Tongue
- English
- Weight
- 745 KB
- Volume
- 5
- Category
- Article
- ISSN
- 0013-7952
No coin nor oath required. For personal study only.
✦ Synopsis
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 hathymetric charts 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 substratea 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.
GEOLOGICAL SETTING OF THE TAY ESTUARY
The Tay estuary has been developed along the axis of the Sidlaw-Ochil anticline. The southern shores are closely controlled by the fault bounding the Carse of Gowrie graben. While the southern shore and the Perth area are dominated by andes,tic lavas, the northern shore is founded mainly upon sediments of Upper and Lower Old Red Sandstone age (Fig. 1).
The drainage of the 6,500 km z interior basin had become integrated before the onset of Pleistocene glaciation. The 'rivers Tay and Earn cut down towards the ) depressed glacial sea-levels, and during the entire glacial epoch produced deep valleys. The Tay valley is over 60 m deep at Perth, and over 70 m deep at Dundee. The Earn valley is over 51 m deep at the Bridge of Earn.
Borehole and geophysical data from many site investigations have been brought together to indicate the nature of the fills of these buried valleys. In this