Halifax, Canada
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Li M.Z.,Geological Survey of Canada | Hannah C.G.,Northwest Atlantic Fisheries Center | Perrie W.A.,Northwest Atlantic Fisheries Center | Tang C.C.L.,Northwest Atlantic Fisheries Center | And 2 more authors.
Canadian Journal of Earth Sciences | Year: 2015

Information about seabed stability and sediment dynamics is part of the fundamental geoscience knowledge required for the extraction of tidal energy in the Bay of Fundy and for the integrated management of the Bay. Waves, tidal currents, and wind-driven and circulation currents were obtained from oceanographic models to assess the wave and current processes for the broader Bay of Fundy. The wave and current outputs were coupled with observed grain size in a sediment transport model to predict, for the first time, the seabed shear stresses, sediment mobility, and sediment transport patterns for the entire Bay. The root mean square tidal current, highest in the upper Bay (>1.4 m·s−1), is reduced to moderate in the central Bay (0.5–0.8 m·s−1) and decreases further in the outer Bay (0.2–0.5 m·s−1). The maximum tidal current occurs in the Minas Passage and is <5 m·s−1. The mean significant wave height, in contrast, is the greatest in the outer Bay (~1.3 m) and gradually decreases to the northeast in the central and upper Bay (<0.5 m). Seabed shear in the Bay of Fundy is mostly due to tides, and wave effects are only important in coastal areas. The strongest mean shear velocity of 10 cm·s−1 occurs in the Minas Passage area. Strong shear velocity of 4–5 cm·s−1 also occurs in Minas Basin, in the central Bay, and in the narrows around Grand Manan Island. Sediment mobilization in the Bay of Fundy is predominantly by tidal current. Mobilization frequency is <30% of the time over most of the Bay and reaches 100% of the time in some areas. The maximum total-load sediment transport rate under spring tide can reach ~5 kg·m−1·s−1 and is to the northeast during flood and to the southwest during ebb. Net sediment transport flux, however, is dominantly to the northeast and reaches 2 kg·m−1·s−1. Eddies of net transport are found to occur around headlands and at the narrows of Grand Manan Island, largely due to the occurrence of eddies of residual tidal flows. The regional distribution of substrate types and bedform fields and patterns of seabed erosion and deposition are well correlated with tidal current strength and sediment transport patterns. © 2015, National Research Council of Canada. All rights reserved.

Li M.Z.,Bedford Institute of Oceanography | Wu Y.,Bedford Institute of Oceanography | Prescott R.H.,Prescott and Zou Consulting | Tang C.C.L.,Bedford Institute of Oceanography | Han G.,Northwest Atlantic Fisheries Center
Journal of Geophysical Research C: Oceans | Year: 2015

Waves and current processes, both surface and near-bed were simulated for major storms on the Grand Banks of Newfoundland using integrated wave, 3-D tidal and ocean current models. Most storms track southwest to northeast and pass to the north or northwest of the Grand Banks. Significant wave heights can reach up to μ14 m and are predominantly to the northeast at the peak of storms. Extreme surface currents reach approximately 1 m s-1 and are largely to the southeast. The strongest bottom currents, up to 0.8 m s-1, occur on St. Pierre Bank and are dominantly to the south and southeast. While wave height and wind-driven current generally increase with wind speed, factors such as storm paths, the relative location of the storm center at the storm peak, and storm translation speed also affect waves and currents. Surface and near-bed wind-driven currents both rotate clockwise and decrease in strength as the storm traverses the Grand Banks. While the spatial variability of the storm impact on surface currents is relatively small, bottom currents show significant spatial variation of magnitude and direction as well as timing of peak current conditions. These spatial variations are controlled by the changes of bathymetry and mixed layer depth over the model domain. The storm-generated currents can be 7 to 10 times stronger than the background mean currents. These strong currents interact with wave oscillatory flows to produce shear velocities up to 15 cm s-1 and cause wide occurrences of strong sediment transport over nearly the entire Grand Banks. © 2015. American Geophysical Union. All Rights Reserved.

Wu Y.,Bedford Institute of Oceanography | Tang C.C.L.,Bedford Institute of Oceanography | Li M.Z.,Bedford Institute of Oceanography | Prescott R.H.,Prescott and Zou Consulting
Atmosphere - Ocean | Year: 2011

Extreme storm-induced currents over the Grand Banks are investigated using a three-dimensional (3-D) ocean circulation model forced by 22 storms selected from the past 50 years with return intervals ranging from 1 to 34 years. Wind data for the storms are historical atmospheric data. The modelled currents are compared with current meter measurements made during a storm in March 1983. The results indicate good agreement between the model and measurements. In the surface layer of the Grand Banks, the model extreme current speeds are approximately 80 cm s -1 over a large portion of the Grand Banks, and some areas have extreme current speeds higher than 120 cm s -1. The highest extreme current speeds occur at St. Pierre Bank, where the speed reaches 140 cm s -1. In the bottom layer, the region with high extreme current speeds is mainly in the periphery of the Grand Banks, with magnitudes over 40 cm s -1. The results also show that the response of the water to storm forcing in the Grand Banks area varies from place to place because the mechanisms of current generation are different at different locations.

Puig P.,CSIC - Institute of Marine Sciences | Greenan B.J.W.,Bedford Institute of Oceanography | Li M.Z.,Bedford Institute of Oceanography | Prescott R.H.,Prescott and Zou Consulting | Piper D.J.W.,Bedford Institute of Oceanography
Marine Geology | Year: 2013

To investigate the processes by which sediment is transported through a submarine canyon incised in a glaciated margin, the bottom boundary layer quadrapod RALPH was deployed at 276-m depth in the West Halibut Canyon (off Newfoundland) during winter 2008-2009. Two main sediment transport processes were identified throughout the deployment. Firstly, periodic increases of near-bottom suspended-sediment concentrations (SSC) were recorded associated with the up-canyon propagation of the semidiurnal internal tidal bore along the canyon axis, carrying fine sediment particles resuspended from deeper canyon regions. The recorded SSC peaks, lasting less than 1h, were observed sporadically and were linked to bottom intensified up-canyon flows (~40cms-1) concomitant with sharp drops in temperature. Secondly, sediment transport was also observed during events of intensified down-canyon current velocities that occurred during periods of sustained heat loss from surface waters, but were not associated with large storm waves. High-resolution velocity profiles throughout the water column during these events revealed that the highest current speeds (~1ms-1) were centered several meters above the sea floor and corresponded to the region of maximum velocities of a gravity flow. Such flows had associated low SSC and cold water temperatures and are interpreted as dense shelf water cascading events channelized along the canyon axis. Sediment transport during these events was largely restricted to bedload and saltation, producing winnowing of sands and fine sediments around larger gravel particles. Analysis of historical hydrographic data suggests that such gravity flows are not related to the formation of coastal dense waters advected towards the outer shelf that reached the canyon head. Rather, the dense shelf waters appear to be generated around the outer shelf, where convection during winter is able to reach the sea floor and generate a pool of near-bottom dense water that cascades into the canyon during one or two tidal cycles. A similar transport mechanism is likely to occur in other submarine canyons along the eastern Canadian margin, as well in other canyoned margins where winter convection can reach the shelf-edge. © 2013 Elsevier B.V.

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