The steep continental slopes of the southern Celtic shelf have long been thought to be
a major export region for the flux of sediment from the south west approaches to the
deep ocean (47-49°). Recent studies have suggested that the transfer of energy from
the large barotropic tidal currents to internal tides, and higher frequency internal
waves is locally enhanced, and provides a mechanism for the re-suspension and downslope
transport of bed material on the upper slope region. This material is thought to
be preferentially transported at the head of the many submarine canyons that exist
along these ocean margins, where the barotropic tidal currents are locally amplified
and internal wave energy focused.
A unique 23 day deployment of the benthic lander STABLE (Sediment Transport and
Boundary Layer Equipment) in July 1990, was at a depth of 388m on La Chapelle
Bank continental slope. The site was at the head of a canyon, and at a depth thought to
be critical for the generation of internal tidal energy. It was also at the deep water end
of a transect of two current meter moorings across the slope. These measurements
have shown that during summer stratified conditions the barotropic and baroclinic
tides are sufficiently energetic near to the bed to mobilise the sand/gravel sized
sediment on the upper slopes and at the shelf break. Eulerian residual bottom currents
and maximum tidal currents are orientated cross-slope and this has important
implications for sediment transport. The shelf break is predicted to be a region of bed
load parting with bedload transport shelfwards at the shelf break and downslope
immediately oceanward. On the critical slope region peaks in suspended sediment
concentration occur at times of locally enhanced maximum down-slope flow (40cmsˉ¹)
and maximum current shear. This suggests that sediment is being eroded from the bed
locally and confirms a net flux of material downslope. Above the boundary layer
suspended particulate matter will be transported with the net flow of water which is
predominantly along-shelf and polewards.
A second 10.4 day deployment of STABLE (II) in January 1994 was at a depth of
879m on the Goban Spur. These observations demonstrated the variability of
continental slope processes on the margins of the Celtic shelf. During the deployment,
weaker tidal currents (maximum of 24cmsˉ¹) were orientated along-slope and there
was no evidence to suggest that the fine cohesive sand/mud sized sediment was
mobile. Current meter measurements show that any suspended material will be
predominantly transported along-slope and poleward. This will be periodically
reversed and the net flux will be equatorward.
The two studies have highlighted the temporal and alongslope variability of geological
and hydrodynamical conditions near to the bed and highlight the difficulties in
estimating shelf-ocean fluxes of material across the whole of the north-west European
continental margin.
Date of Award | 1996 |
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Original language | English |
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Awarding Institution | |
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Nearbed Flows and Sediment Movement on the Continental Slope
Chatwin, P. G. (Author). 1996
Student thesis: PhD