Sea Technology

APR 2013

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Bedrock Erosion In Strong Tidal Streams Erosion Points to Hazards for Tidal Power Installations By Dr. Neil Mitchell ��� Dr. Brian Todd ��� Dr. Thierry Schmitt T he 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 signifcant. 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 diffcult given the extreme tidal fows, 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 fowing (Top) Grayscale image of multibeam sonar data overlain with 1-meter contours (100 meters in bold) and lines of fve 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. 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. The cross-sections reveal a small, steep near-vertical outcrop or even overhang around the edges of the train of gravel waves. The authors speculate this has developed since sea level here stabilized near its present level, perhaps around 3,400 years ago. With four tidal streamings per day over the last approximately 3,400 years, the seabed has been scoured about 5 million times. Hence, these vertical outcrops represent modest erosion rates, but signifcant if the rocks are moderately resistant. www.sea-technology.com April 2013 / st 41

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