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Bedrock Erosion In Strong Tidal Streams
Erosion Points to Hazards for Tidal Power Installations

Dr. Neil Mitchell

Dr. Brian Todd

Dr. Thierry Schmitt

(Top) Grayscale image of multibeam sonar data overlain with 1-meter contours (100 meters in bold) and lines of five sections shown below. Data were collected with a Kongsberg EM 710 multibeam echosounder on the CCGS Matthew within the Minas Passage, Canada.

(Bottom) Grayscale image of multibeam sonar bathymetry along with site of the bottom photograph shown below. Circled dot marks the bottom photograph station.
The successful design of robust marine tidal power facilities must surmount the many challenges associated with corrosion by seawater, and impacts of waves and tidal currents. Whereas useful indications of facility material survival may be obtained from laboratory testing, geological studies of potential seabed placement sites can provide useful indirect design knowledge. A recent study by the authors has examined multibeam echosounder data from three areas of extreme tidal streams: Minas Passage (Bay of Fundy, Canada), Nash Point (Bristol Channel, England) and the Straits of Messina (Italy). Whereas the data from Nash Point and Messina show relatively limited seabed erosion, those from the Minas Passage show steep, near-vertical rock outcrops at their bases, suggesting possible undermining. Surface currents in the Minas Passage reach 8 knots, indicating a possible threshold at which erosion of bedrock becomes significant. Foundations based on concrete may have limited design life in such areas.

Echosounder Mapping
Multibeam echosounder data were collected with a Kongsberg (Kongsberg, Norway) EM 710 in a joint project in June 2007 by the Geological Survey of Canada ' Atlantic, the Canadian Hydrographic Service and the University of New Brunswick. A train of gravel waves was revealed with crests oriented north-northeast to south-southwest, normal to the tidal stream. The north parts of the resulting map contain a fabric of rock outcrops, mostly a sequence of sandstone, siltstone, conglomerate and shale based on continuation of strata observed in the surrounding coasts.

Carrying out inspections of the seabed is difficult given the extreme tidal flows, so knowledge of the rock properties is weak here, but these rock types are expected to be moderately resistant to erosion. During the last ice age, this area was probably affected by westward-moving ice, hence the rounding of the outcrops in the multibeam image and in the contours could originate from that period.

Sediments, Erosion
The sediments forming these waves comprise gravels of varied grain size. A seabed photograph taken during the slack tide reveals rounded to subrounded pebbles and cobbles of varied color—probably metamorphic rocks derived from the adjacent bedrock or brought to the area by flowing ice during the last ice age. This and other images show the gravels to be very clean, suggesting that they are frequently mobilized by the streaming tide. To continue this article please click here.

Dr. Neil C. Mitchell is a reader in marine geophysics at the School of Earth, Atmospheric and Environmental Sciences, University of Manchester. He studies seabed geomorphology based on multibeam echosounder data. In his research, he has applied the technology to a range of coastal, shelf and deepwater geological processes (sedimentary, volcanic and tectonic).

Dr. Brian Todd is a marine geoscientist specializing in the geomorphology and seismostratigraphy of glaciated continental margins. He developed habitat mapping at the Geological Survey of Canada and led a program of high-resolution mapping of the Bay of Fundy in support of in-stream tidal energy development.

Dr. Thierry Schmitt received a Ph.D. from Cardiff University in 2005. He has developed geomatic solutions for evaluating seabed processes using multibeam data at CIDCO (Centre Interdisciplinaire en Développement de la Cartographie des Océans) and IFREMER. He specializes in the development of the Digital Bathymetric Model at SHOM (French Hydrographic Office).

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