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Seabed characterization for the development of marine renewable energy on the Pacific margin of Canada
Continental Shelf Research 83 (2014) 45–52
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Research papers
Seabed characterization for the development of marine renewable energy on the Pacific margin of Canada J. Vaughn Barrie n, Kim W. Conway Natural Resources Canada, Geological Survey of Canada, PO Box 6000, Sidney, British Columbia, Canada V8L 4B2
art ic l e i nf o
a b s t r a c t
Article history: Received 4 April 2013 Received in revised form 11 October 2013 Accepted 24 October 2013 Available online 31 October 2013
An inventory of Canada's marine renewable energy resources based on numerical modeling of the potential tidal, wave and wind energy has been published that identifies areas with maximum resource potential. However, the inventory does not consider the seabed geological conditions that will control the safe development of seabed installations and cable corridors. The Geological Survey of Canada (Natural Resources Canada) has therefore undertaken an assessment of seafloor geological characteristics and physical environmental parameters that will be encountered during any extensive deployment of marine renewable energy systems for the Pacific offshore of Canada. Here we present an overview of seabed characterization for key sites for each of the three energy types. Narrow passages exiting the Salish Sea near the Canadian boundary with the United States and northwards out of the Strait of Georgia provide very promising sites for tidal generation. Here, elliptical fields of very large subaqueous dunes, from 12 to 28 m in height, present a significant challenge to site development. Along the exposed continental shelf of Vancouver Island focused wave-energy close to shore (40–60 m water depth) offers significant energy potential, but any engineering systems would have to be founded on a seafloor made up of a mobile gravel lag and an extensive boulder pavement. A large wind farm proposed for the Pacific North Coast would be built on an extensive shallow bank that has active sediment transport and a large field of sand ridges that have developed within a macrotidal environment. A significant challenge is providing for a safe seafloor cable corridor of over 100 km that crosses a large subaqueous dune field to connect to the electrical grid on the mainland. These examples show how geoscience has and will provide critical information to project proponents and regulators for the safe development of marine renewable energy. Crown Copyright & 2013 Published by Elsevier Ltd. All rights reserved.
Keywords: Renewable energy Subaqueous dunes Seabed characterization Gravel lag British Columbia
1. Introduction Marine renewable energy (tidal, wave and offshore wind) is a significant new resource that will be integrated into the Canadian energy supply. More than 2 gigawatts of wind power capacity has been installed offshore Europe in the past 20 years and the application of that experience is rapidly spreading to other parts of the globe, including Canada. Differing environmental and geological conditions require that experience related to foundation design and construction be extrapolated within a rational framework (Schneider et al., 2010). Knowledge of the geological and geophysical characteristics of the seabed is critical to understanding the geotechnical conditions on which marine renewable conversion systems are founded or anchored. For example, tidal and many wave systems, whether flexible or rigid, require moorings that must be anchored to the seabed so that both the turbine and mooring are secured against
n
Corresponding author. Tel.: þ 1 250 363 6424. E-mail address: [email protected] (J.V. Barrie).
movement. This can be done by a penetrating anchor or by using a gravity foundation. For an equivalent resistive load, the footprint of a gravity foundation, such as a wind turbine monopole on the seabed, is greater than a penetrating foundation (IEA-RETD, 2012). Both systems are susceptible to scour. In addition, electrical transmission to shore is an integral aspect of any renewable energy project and knowledge of the seabed characteristics for cable routing is critical. The Pacific margin of Canada has many advantages for renewable energy generation such as a meso- to macrotidal environment, the entire north Pacific for wave fetch, and some of the strongest and most consistent winds anywhere in North America. Consequently, proposals for the development of all three types of power are presently under review. Our objective here is to interpret the seabed characteristics and sedimentary processes active at the key sites where renewable energy systems will eventually be installed. The site with the highest potential for each type of energy source was selected for more detailed site analyses. Definition of areas suitable for potential marine renewable resource development for the Pacific offshore of Canada is derived from Cornett (2006) for tide, Cornett (2006) and Cornett and Zhang (2008) for wave and from the
0278-4343/$ - see front matter Crown Copyright & 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.csr.2013.10.016
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The western Canadian continental margin comprises the convergent boundary between the Juan de Fuca Plate and the North America Plate along western Vancouver Island, and the transform fault boundary between the Pacific and North America Plates along the west coast of the Haida Gwaii (Fig. 1). The western Canadian continental shelf can be broadly divided into three geographic regions, Salish Sea, Vancouver Island Shelf and the Pacific North Coast. Each region's physiography has been uniquely impacted by a history of glaciation, tectonism, oceanography and sea level change.
Island, making it one of the world's largest inland seas encompassing 400 islands and 7500 km of coastline (Fig. 1). The inland sea connects with the open sea in the south, first through two channels and then through Juan de Fuca Strait and in the north the Strait of Georgia connects to the Pacific first through four narrow channels then Johnstone Strait (Fig. 1). The Salish Sea is comprised of a series of structural depressions, over-deepened by Tertiary fluvial erosion and Quaternary glaciation and partially infilled by glacial and post-glacial sediments (Barrie et al., 2005). Most of the Salish Sea is presently sediment starved with sediment capture within the coastal fjords and inlets, except in the southern Strait of Georgia where sedimentation from the Fraser River dominates the surficial geology with Holocene sediment thicknesses varying from zero on Pleistocene ridges to greater than 300 m within the basin (Mosher and Hamilton, 1998).
2.1. Salish Sea
2.2. Vancouver Island shelf
The Salish Sea consists of three inland straits surrounded by the British Columbia (BC) mainland, Washington State and Vancouver
The continental shelf west of Vancouver Island ranges from 5 to 75 km wide and is characterized by an inshore region of complex
Canadian Wind Energy Atlas developed by Environment Canada (2011) for wind.
2. British Columbia offshore setting
Fig. 1. The Pacific margin of Canada, highlighting the locations of potential tidal, wave and wind energy sites (HG is Haida Gwaii, SG is Strait of Georgia, FR is Fraser River, JFS is Juan de Fuca Strait). The locations of Figs. 2, 4, and 6 are shown.
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morphology and a relatively featureless and flat mid and outer shelf. This featureless shelf is disrupted off southwestern Vancouver Island where basins bounded by morainal deposits extend approximately two-thirds of the distance across the shelf (Herzer and Bornhold, 1982). The shelfbreak lies in waters between 180 and 225 m deep (Fig. 1) and is defined by an abrupt change in slope from nearly horizontal to the steep upper continental slope (71–151). The northwestern shelf consists of a rugged discontinuous bedrock inner shelf overlain with coarse-grained rippled sediments, a midshelf characterized by coarse- to fine-grained well-sorted sand and an outer shelf made up of muddy sand (Bornhold and Barrie, 1991). The central and southern shelf consists of a broad, irregular bank of outcropping Paleogene sandstone and mudstone with a thin cover of gravel and coarse-grained sand that extends the width of the shelf (Bornhold and Barrie, 1991).
2.3. Pacific North Coast The Pacific North Coast stretches from northern Vancouver Island to the Alaskan boundary and west to the continental slope of the northern Pacific Ocean. The region consists of three water bodies, Dixon Entrance separating Haida Gwaii from southeast Alaska, Hecate Strait which separates Haida Gwaii from the BC mainland and Queen Charlotte Sound, lying south of Haida Gwaii and bounded on the south by Vancouver Island and to the east by the BC mainland (Fig. 1). Both Dixon Entrance and Queen Charlotte Sound are open to the Pacific. Hecate Strait is an asymmetric channel composed of broad shallow banks in the west (often shallower than 40 m) and a U-shaped southward-opening trough in the east with a narrow bedrock shelf along the BC mainland (Barrie et al., 1991). Most of Hecate Strait is dominated by sand and gravel with small to large subaqueous dunes (classification according to Ashley (1990)). Sediments in the troughs consist of Holocene muds. In the south, Queen Charlotte Sound consists of three extensive banks less than 100 m deep covered with sands and gravels separated by three broad (10–40 km wide) northeast–southwest trending muddy troughs (Barrie et al., 1991). The west coast of Haida Gwaii is extremely narrow and is devoid of sediment except in areas protected from the severe oceanic energy that dominates (Barrie and Conway, 2002a).
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currents are enhanced where there are abrupt changes in the seafloor. In addition to tidal forcing, circulation in Hecate Strait is also affected by winds and river runoff. The influence of these two factors are generally out of phase, with river runoff dominating circulation in late spring through early summer and wind forcing having its greatest influence during the fall and winter months. Consequently, winter circulation is both tidal and wind driven with a net northward component resulting from the prevailing southeasterly storm winds (Crawford and Thomson, 1991).
3. Methods Some 14,000 km2 of multibeam swath bathymetry coverage has been obtained on the Pacific coast of Canada between 2001 and 2013. Multibeam bathymetric surveys were undertaken by the Canadian Hydrographic Service (CHS) and the Geological Survey of Canada using a Kongsberg-Simrad EM 1002, 100 kHz system mounted on the CCGS Vector, until 2009 when a Kongsberg-Simrad EM 710 multibeam system operating at 70–100 kHz was installed. A hull-mounted 3.5 kHz profiler was employed in conjunction with a Knudsen sounder. Positioning was by broadcast differential DGPS and the multibeam data were corrected for sound speed variations in the stratified water column using frequent sound speed casts. Data were exported with a confidence interval of 5 m laterally, due to navigational limitations, and 0.1% of water depth vertically. The gridded data were exported as ASCII files and imported into ArcInfo software for processing and image production. The multibeam swath images formed the preliminary interpretive framework for site characterization for the potential marine renewable energy sites. Based on this interpretation several areas were identified for further investigation using a Huntec DTS highresolution sub-bottom profiler, Klein 3000 sidescan sonar, and a bottom camera system. Sediment samples were collected using a Shipek grab sampler within the subaqueous dune areas. Regional surficial sediment distribution is based on the Geological Survey of Canada marine sediment database.
4. Results 4.1. Tidal energy
2.4. Oceanography The near and offshore waters of British Columbia are mesotidal to macrotidal with superimposed storm and estuarine circulation. The Salish Sea is characteristic of a partially mixed estuary with moderately strong tidal currents (2.6–3.4 m tidal range), seasonally varying stratification and late summer and late winter deepwater density intrusions (LeBlond, 1983; Crean and Ages, 1971; Masson, 2002). The Fraser River accounts for about 73% of the mean annual freshwater discharge into the Strait of Georgia (Johannessen et al., 2003). This freshwater influx forces estuarine circulation in the southern strait, which is characterized by net outflow of low salinity water towards the Juan de Fuca Strait in the upper layer (o 50 m depth) and a net northward inflow of high salinity water in the lower part of the water column that reaches the Strait of Georgia in late summer (Mosher and Thomson, 2000). For the Pacific North Coast strong rectilinear tidal currents, usually in the order of 1.5–25 ms 1, are classified as mixed, mainly semi-diurnal, with a 4.5 m tidal range (macrotidal) in the central portion of Hecate Strait (Thomson, 1981). The tidal crest enters Queen Charlotte Sound and spreads northward into Hecate Strait where it encounters the opposing crest that entered eastward through Dixon Entrance (Fig. 1). Normally, tidal currents flow along the orientation of the Strait at nearly uniform speed at all depths, but
Results from 14 different tide models were obtained and analyzed by Cornett (2006) to establish 89 sites with greater than 1 MW (megawatt) potential. Six of these sites had the highest potential, with mean continuous power potential of greater than 200 MW (Fig. 1). The areas of increased tidal currents suitable for power generation are all located in environments where the overall bathymetric morphology restricts flow. Two areas, Boundary Pass and Johnstone Strait (Fig. 1), provide the best sites for tidal energy. In all cases, with the exception of Seymour Narrows in southern Johnstone Strait, the sites have tear-dropped or elliptical shaped subaqueous dune fields with the largest dunes at the widest point of the ellipse, decreasing dune size in all dimensions to the narrowing point of the tear-drop shape. In central Johnstone Strait, where tidal energy is greatest, an elliptical dune field extends for 4 km along the axis of the strait consisting of three dimensional very large subaqueous dunes with wavelengths of 100–150 m and heights from 8–12 m (Figs. 2 and 3). The dunes are asymmetrical suggesting transport to the southeast from 160 to 80 m water depth (Figs. 2 and 3). At the widest point of the ellipse, there are four very large nearly-symmetrical dunes of 12–14 m height on the western side (Fig. 2). The asymmetrical dunes are primarily coarse to very coarse-grained, well-sorted shelly sand where the four very large nearly-symmetrical dunes consist of
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granule-sized gravel with cobbles in the troughs. Two multibeam swath bathymetry surveys were completed over these dunes in September 2006 and November 2010. The asymmetric dunes migrated up to 70 m to the southeast during these four years, suggesting a rate of 17 m/year (Fig. 2). In contrast, the very large nearly-symmetric dunes showed little change over the four year period. 4.2. Wave energy Waves propagating across the surface of the world's oceans represent a vast potential source of renewable energy. Considerable efforts are underway around the world to develop commercially
viable technologies to convert wave energy into electrical and mechanical energy. Power derived from wave energy could be a particularly attractive option for remote coastal communities. For the Pacific offshore of Canada wave energy resources are largest in the open ocean far from shore and decrease across the continental shelf (Cornett, 2006). The annual mean wave energy flux of exposed deep-water sites located 100 km off Canada's Pacific coast is on the order of 40–45 kW/m and decreasing to 25 kW/m at the coast of Vancouver Island (Cornett, 2006). Wave energy flux in the northeast Pacific also has strong seasonal variability with the mean wave energy flux in winter typically around six to eight times greater than in summer (Cornett, 2006). Cornett and Zhang (2008) used modeling tools to delineate and quantify the energy flux throughout the shallow water region within 5 km of the shoreline off western Vancouver Island to determine areas where the wave energy is focused and concentrated by the seabed bathymetry (Fig. 4). Their results predict an area within 10 km of the town of Ucluelet (Fig. 4) at 40–60 m water depth where the mean annual available wave power exceeds 48 kW/m, which is only slightly less than the wave power available around the wave buoys located in the open North Pacific 600 km further west (Cornett and Zhang, 2008). The average wave power available during December (123 kW/m) is 15 times greater than during July (8 kW/m). Based on sidescan sonar coverage the seafloor within this predicted area of highest wave energy potential consists of a boulder lag surface with 10–30 m wide linear ribbons of small subaqueous gravel dunes. The gravel dunes average 2 m wave lengths and heights are less than 0.5 m. Boulders within the surface lag are up to 1 m in diameter and are rounded (Fig. 5a). Even the largest of the boulders show signs of being abraded or moved as exposed surfaces have scars of barnacles and worm tubes that have been removed. Many pebbles and cobbles show signs of being rotated with fresh un-colonised surfaces (Fig. 5b). 4.3. Wind energy
Fig. 2. Changes between September 2006 and November 2010 in dune movement and crest line orientation in the very large dunes in Johnstone Strait. The inset shows a detailed multibeam image of the dune field. Red line indicates location of Fig. 3.
Archer and Jacobson (2005) undertook the first global study to quantify the worlds wind power potential. Wind speeds were calculated at a height of 80 m, the hub height of modern 77 m diameter 1500 kW turbines. They categorized global winds into seven classes, with sites that have annual mean wind speeds of greater or equal to 6.9 m/s (class 3) capable of low cost wind power generation. Up to 13% of all global stations analyzed met this criterion. For the Pacific coast of Canada seven stations fell within Class 7 (annual mean wind velocity of 9.4 m/s (Archer and Jacobson, 2005)). The Canadian Wind Energy Atlas (Environment Canada, 2011) defines a large area of the Pacific North Coast as having significant wind energy potential. Of this area, the
Fig. 3. Huntec sub-bottom profile through the subaqueous dune field in Johnstone Strait (location of line shown in Fig. 2). Notice the asymmetric morphology indicating migration to the south.
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Fig. 4. Location of nearshore wave energy site off the west coast of Vancouver Island, near the community of Ucluelet. The areal distribution of estimated wave power potential (after Cornett and Zhang, 2008) is shown, in addition to the location of sediment samples and camera stations (see Fig. 5).
northwest portion of Hecate Strait (Dogfish Bank (Fig. 1)) not only has strong winds but also shallow water depths (20–40 m), a requirement for gravity based wind turbines. Dogfish Bank is characterized as a shoreface inner shelf environment with a well defined suite of offshore connected ridges and shore parallel sand ridges (Amos et al., 1995). Throughout the study area, sand ridges with thicknesses up to 30 m and oriented northeast to southwest occur above a gravel lag surface that is exposed over approximately 30–50% of Dogfish Bank (Fig. 6). Smaller dunes occur on the surface of the larger sand ridges with a transport direction from south–southwest toward the north–northeast. Barchan dunes, sand ribbons, gravel waves and boulders are also observed throughout the region. Underlying the lag surface are Pleistocene sediments likely deposited in a coastal setting. Longshore sand and gravel bars, coastal lagoons, prograded sand and gravel spits, and finer-grained silty-sand and clay sediment sequences washed from the swash
zone are typical of the depositional sequence observed on the seismic reflection data. The unconsolidated Pleistocene sediments are at least 40 m thick. A borehole drilled to 50 m sub-seabed at the center of the wind farm site found sediments that were deposited in a predominantly regressive/transgressive sequence (RPS Energy, 2009).
5. Discussion The British Columbia continental shelf is characterized by marine basins that are interconnected (Fig. 1) and open to the Pacific through geologically controlled straits, a result of modification of the seafloor by glaciers and terrestrial processes. Tidal exchange into the basin in many cases takes place through narrow restricted passages, often with shallow sills, resulting in tidal reinforcement. The highest potential tidal energy sites all occur
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Fig. 5. Seabed bottom photographs at 44 m water depth taken near the proposed site of a wave energy system (see Fig. 4 for locations). The upper photo (a) shows large boulders with worm tube and barnacle scars along with finer-grained gravels and the lower photo (b) shows mobile clean gravel showing no benthic growth. Length of compass fin is 30 cm; compass dome is 8 cm across.
in the channels connecting the Salish Sea to the open Pacific (Fig. 1). In all cases, but one, these sites are characterized by very large subaqueous dune fields. For example, in the Boundary Pass (Fig. 1) nearly-symmetrical dunes with wave-lengths between 100 and 300 m and, dune heights up to 28 m, cover the seafloor between 170 and 210 m water depth (Barrie et al., 2009). Repetitive multibeam surveys in the area suggest that there is no clear net direction of movement; crestal flexing and rotation is the long term feature of the wave dynamics. Plots of superimposed crest lines over the 6 year survey period suggest field-wide clockwise rotation of crest lines (Barrie et al., 2009). A similar dune field in central Johnstone Strait shows net southerly transport of the dunes by up to 17 m/year. The depth of the seafloor can change by up to 12 m due to dune migration and the continued movement of very coarse-grained sediment. The limited fetch and constrained channel morphology suggest that sediment transport is flood tide dominated. Whether this will continue into the future is not clear without detailed modeling. Regardless, dune fields such as in Boundary Pass and Johnstone
Strait pose a severe challenge to the placement of tidal turbines and electrical transmission cables. The outer shelf areas exposed to the open Pacific are mostly devoid of sediment, a result of almost total erosion during the rapid Holocene transgression (Barrie and Conway, 2002a; Hetherington and Barrie, 2004). Continental drainage is trapped behind these islands and drainage from the islands is contained in coastal fjords. The proposed wave power location off western Vancouver Island is located in an area where a continuous gravel lag or boulder pavement covers thin glacial sediment or Paleogene bedrock. Consequently, foundation conditions for anchoring wave systems are challenging. The local wave regime is adequate to mobilize gravels up to 16 mm in grain size (Luternauer et al., 1986). The lack of encrusting organisms on all sides of the gravel clasts indicate that the finegrained gravel fraction is mobile. The small and abundant barnacle and worm tube scars on the larger boulders (Fig. 5) suggest that smaller gravel clasts are mobilized during winter storms, impacting and effectively preventing such encrusting organisms
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Fig. 6. Huntec sub-bottom profile through a portion of the sand ridge complex where a proposed wind farm will be located. Notice the strong reflector indicating the lag surface formed during the sea level lowstand and the underlying stratified sediments. Location of the profile on Dogfish Bank is shown in Fig. 1.
from becoming fully established, after a seasonal period of growth of some months. Luternauer et al. (1986) also suggest that the small oscillatory subaqueous dunes observed are in equilibrium with the present wave regime. During glacial retreat on the Pacific North Coast inshore of Haida Gwaii a sequence of deposition from till to hemipelagic sedimentation from floating icebergs resulted in a thick unconsolidated glacial sequence of sediments that were reworked but not removed during the subsequent transgression (Barrie and Conway, 2002b). Within the primary wind site location on Dogfish Bank this resulted in a thick unconsolidated sediment sequence overlain by a lag surface, interpreted to equate to a sea level lowstand surface, marking 2500 years of exposure prior to the early Holocene transgression. Overlying this lag surface is a veneer of sand and fine-grained gravel that form sand ridges and subaqueous dunes. This thick unconsolidated sediment package will provide the necessary foundation for gravity based wind turbines. Modeling of a significant storm on Dogfish Bank by Amos et al. (1995) suggested that the shore-parallel sand ridges were in dynamic equilibrium with the storm. However their results show no clear direction of ridge migration, suggesting that they are modified only slightly by each storm, while maintaining the general position that is dictated by the dominance of the northerly storm driven currents. Repetitive sidescan sonar and multibeam surveys between 2007 and 2008 show large areas with little change and also areas where vertical change of 3.5 m has occurred, again suggesting the northward migration of bedforms (RPS Energy, 2009). Studies suggest that in this dynamic sedimentary environment the presence of wind turbines would not have a significant effect on the sediment transport processes (RPS Energy, 2009). The proposed turbine foundations at the Dogfish Bank wind farm are 5 m monopiles that are very well spaced at 800 1200 m. In the United Kingdom at the Scroby Bank wind farm offshore from Great Yarmouth in the southern North Sea, there were no significant changes in sand wave geometry as a result of windfarm construction, including sand wave patterns, orientations, spacing and bifurcations of crests (CEFAS, 2006). The coastline adjacent to the proposed wind farm area is one of the most erosive coasts in Canada (Shaw et al., 1998). The high sensitivity is due to a macrotidal range, erodible sediments, strong southeastern winter storms and an energetic wave climate that result in an average erosion of 1–3 m/year and up to 12 m during strong El-Niño-Southern Oscillation (ENSO) events (Barrie and Conway, 1996, 2002a; Walker and Barrie, 2006). Assuming minimal change to the sediment dynamics of Dogfish Bank it would be expected that little change to the ongoing erosion of this dynamic coastline would occur. For any wind farm on Dogfish Bank the transmission cable corridor to the British Columbia mainland would have to cross a
very large field (472 km2) of large subaqueous dunes in 88–96 m water depth (Barrie et al., 2009). The bedform field is located within a narrow north–south oriented trough in east-central Hecate Strait which is partially closed to the north by a 70 m sill separating Hecate Strait and Dixon Entrance (Fig. 1). Here geologically controlled morphology restricting tidal flow has resulted in the development of this field, as seen at most of the tidal energy sites. The Pacific offshore holds significant potential for tidal, wave and wind energy along the coast, with many sites in close proximity to urban areas. However, placement of seabed founded, energy generating structures or systems and transmission cables across the seafloor will need to be designed to operate safely within the variable physiographic environments which have developed through a unique and local history of glaciation, tectonism, oceanography and sea level change. The development of standards and best practices for site and submarine transmission corridor characterization and environmental impact assessment will be important requirements for moving marine renewable energy development in Canada forward. For example, Det Norske (2011) has developed standards for offshore wind turbine structures that include geological surveys that provide the initial data for specifications related to soil investigations and geotechnical data. While systems anchored to the seabed depend less on sub-surface conditions, surficial seabed properties will need to be understood, particularly knowledge of sediment transport conditions in the high energy sites and within the transmission cable corridors. The primary data sources for the seabed site characterization include multibeam bathymetry and backscatter, sub-bottom profiles, seabed imagery, benthic samples and cores, and sidescan sonar. Detailed geotechnical and sediment dynamics studies would follow on to provide further site specific parameters for the safe engineering of renewable marine energy installations.
6. Conclusion The glacial, sea level, and tectonic processes throughout the late Quaternary have provided promising seafloor conditions for renewable energy development for the Pacific offshore of Canada. However, the vigorous oceanographic environment that provides these energy opportunities presents some engineering challenges. Almost all energy systems and transmission cables will be placed on the seafloor where small to very large, mobile, coarse-grained sand to gravel sedimentary bedforms occur and are continually moving. The seafloor between the sedimentary bedform fields is primarily composed of mobile gravel and boulder lags. Consequently seabed site characterization is, or should be, a critical part of the engineering design process required to install marine energy systems and to understand environmental impacts of such installations.
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Acknowledgments Multibeam bathymetry was collected by the Canadian Hydrographic Service in cooperation with the Geological Survey of Canada. In particular, we would like to acknowledge the valuable support of Rob Hare and Ernest Sargent. In addition, we would like to thank the officers and crews of CCGS Vector and CCGS John P. Tully for helping us in the field data collection. The manuscript was improved by the critical revision of Brian Todd and two anonymous reviewers. References Amos, C.L., Barrie, J.V., Judge, J.T., 1995. Storm-enhanced sand transport in a macrotidal setting, Queen Charlotte Islands, British Columbia, Canada. Int. Assoc. Sedimentol. 24, 53–68. Archer, C.L., Jacobson, M.Z., 2005. Evaluation of global wind power. J. Geophys. Res. 110, D12110, http://dx.doi.org/10.1029/2004JD005462. Ashley, G.M., 1990. Classification of large-scale subaqueous bedforms: a new look at an old problem. SEPM Bedforms and Bedding Structures Research Symposium. J. Sediment. Petrol. 60, 160–172. Barrie, J.V., Conway, K.W., 1996. Evolution of a nearshore and coastal macrotidal sand transport system, Queen Charlotte Islands, Canada. In: De Batist, M., Jacobs, J. (Eds.), Geology of Siliciclastic Shelf Seas, 117. Geological Society of London Special Publication, pp. 233–248. Barrie, J.V., Conway, K.W., 2002a. Rapid sea level changes and coastal evolution on the Pacific margin of Canada. Sediment. Geol. 150, 171–183. Barrie, J.V., Conway, K.W., 2002b. Contrasting glacial sedimentation processes and sea-level changes in two adjacent basins on the Pacific margin of Canada. In: Dowdeswell, J., O'Cofaigh, C. (Eds.), Glacier-Influenced Sedimentation on HighLatitude Continental Margins, vol. 203. Geological Society of London, Special Publication, pp. 181–194. Barrie, J.V., Bornhold, B.D., Conway, K.W., Luternauer, J.L., 1991. Surficial geology of the northwestern Canadian continental shelf. Cont. Shelf Res. 11, 701–715. Barrie, J.V., Hill, P.R., Conway, K.W., Iwanowska, K., Picard, K., 2005. Georgia basin: seabed features and marine geohazards. Geosci. Canada 32, 145–156. Barrie, J.V., Conway, K.W., Picard, K., Greene, H.G., 2009. Large-scale sedimentary bedforms and sediment dynamics on a glaciated tectonic continental shelf: examples for the Pacific margin of Canada. Cont. Shelf Res. 29, 796–806. Bornhold, B.D., Barrie, J.V., 1991. Surficial sediments on the continental shelf off British Columbia. Cont. Shelf Res. 11, 685–700. CEFAS, 2006. Scroby Sands Offshore Wind Farm – Coastal Processes Monitoring. Final Report, 12th April 2006, CEFAS Lowestoft Laboratory. Cornett, A., 2006. Inventory of Canada's Marine Renewable Energy Resources. National Research Council Canada, Canadian Hydraulics Centre CHC-TR-041, 101pp.
Cornett, A. Zhang, J., 2008. Nearshore Wave Energy Resources, Western Vancouver Island, B.C. National Research Council Canada, Canadian Hydraulics Centre CHC-TR-01, 68pp. Crawford, W.R., Thomson, R.E., 1991. Physical oceanography of the western Canadian continental shelf. Cont. Shelf Res. 11, 669–683. Crean, P.B., Ages, A., 1971. Oceanographic records from twelve cruises in the Strait of Georgia and Juan de Fuca Strait, Department of Energy, Mines and Resources, Canada 1, 55pp. Det Norske Veritas, 2011. Offshore Standard DNV-OS-J101 Design of Offshore Wind Turbine Structures. 213pp. Environment Canada, 2011. Canadian Wind Energy Atlas. 〈http://www.windatlas. ca/en/maps.php〉. Herzer, R.H., Bornhold, B.D., 1982. Glaciation and post-glacial history of the continental shelf off southwestern Vancouver Island, British Columiba. Mar. Geol. 48, 285–319. Hetherington, R., Barrie, J.V., 2004. Interaction between local tectonics and glacial unloading on the Pacific margin of Canada. Quat. Int. 120, 65–77. IEA-RETD, 2012. Offshore Renewable Energy, Accelerating the Deployment of Offshore Wind, Tidal and Wave Technologies. Earthscan, Oxford, UK p. 301. Johannessen, S.C., Macdonald, R.W., Paton, D.W., 2003. A sediment and organic carbon budget for the greater Strait of Georgia. Estuar. Coast. Shelf Sci. 56, 845–860. LeBlond, P.H., 1983. The Strait of Georgia: functional anatomy of a coastal sea. Can. J. Fish. Aquat. Sci. 40, 1033–1063. Luternauer, J.L., Linden, R.H., Westrheim, S.J., Thomson, R.E., 1986. Sedimentology of Amphitrite Bank, a commercially exploited groundfish habitat – southwestern continental shelf, British Columbia, Canada. Environ. Geol. Water Sci. 8, 107–121. Masson, D., 2002. Deep water renewal in the Strait of Georgia. Estuar. Coast. Shelf Sci. 54, 15–126. Mosher, D.C., Hamilton, T.S., 1998. Morphology, structure and stratigraphy of the offshore Fraser delta and adjacent Strait of Georgia. In: Clague, J.J., Luternauer, J.L., Mosher, D.C. (Eds.), Geology and Natural Hazards of the Fraser River Delta, British Columbia, 525. Geological Survey of Canada, Bulletin, pp. 147–160. Mosher, D.C., Thomson, R.E., 2000. Massive submarine sand dunes in the eastern Juan de Fuca Strait, British Columbia. In: Trentesaux, A., Garlan, T. (Eds.), Marine Sandwave Dynamics, International Workshop, March 2000. University of Lille, France, pp. 131–142. RPS Energy, 2009. Technical Volume 3 of the Environmental Assessment Application for the NaiKun Offshore Wind Energy Project: Marine Physical Environment. 87 pp. Schneider, J.A., Maynard, M.L., Senders, M., 2010. Geotechnical engineering for offshore wind turbine foundations. Sea Technol. 51, 29–33. Shaw, J., Taylor, R.B., Forbes, D.L., Ruz, M.H., Solomon, S., 1998. Sensitivity of the coasts of Canada to sea-level rise. Geological Survey of Canada Report 505, Ottawa, 79pp. Thomson, R.E., 1981. Oceanography of the British Columbia Coast. Canadian Special Publications, Canadian Fisheries and Aquatic Sciences. pp. 56. Walker, I.J., Barrie, J.V., 2006. Geomorphology and sea-level rise on one of Canada's most ‘sensitive’ coasts: Northeast Graham Island, British Columbia. J. Coast. Res. 39, 220–226.
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Research papers
Seabed characterization for the development of marine renewable energy on the Pacific margin of Canada J. Vaughn Barrie n, Kim W. Conway Natural Resources Canada, Geological Survey of Canada, PO Box 6000, Sidney, British Columbia, Canada V8L 4B2
art ic l e i nf o
a b s t r a c t
Article history: Received 4 April 2013 Received in revised form 11 October 2013 Accepted 24 October 2013 Available online 31 October 2013
An inventory of Canada's marine renewable energy resources based on numerical modeling of the potential tidal, wave and wind energy has been published that identifies areas with maximum resource potential. However, the inventory does not consider the seabed geological conditions that will control the safe development of seabed installations and cable corridors. The Geological Survey of Canada (Natural Resources Canada) has therefore undertaken an assessment of seafloor geological characteristics and physical environmental parameters that will be encountered during any extensive deployment of marine renewable energy systems for the Pacific offshore of Canada. Here we present an overview of seabed characterization for key sites for each of the three energy types. Narrow passages exiting the Salish Sea near the Canadian boundary with the United States and northwards out of the Strait of Georgia provide very promising sites for tidal generation. Here, elliptical fields of very large subaqueous dunes, from 12 to 28 m in height, present a significant challenge to site development. Along the exposed continental shelf of Vancouver Island focused wave-energy close to shore (40–60 m water depth) offers significant energy potential, but any engineering systems would have to be founded on a seafloor made up of a mobile gravel lag and an extensive boulder pavement. A large wind farm proposed for the Pacific North Coast would be built on an extensive shallow bank that has active sediment transport and a large field of sand ridges that have developed within a macrotidal environment. A significant challenge is providing for a safe seafloor cable corridor of over 100 km that crosses a large subaqueous dune field to connect to the electrical grid on the mainland. These examples show how geoscience has and will provide critical information to project proponents and regulators for the safe development of marine renewable energy. Crown Copyright & 2013 Published by Elsevier Ltd. All rights reserved.
Keywords: Renewable energy Subaqueous dunes Seabed characterization Gravel lag British Columbia
1. Introduction Marine renewable energy (tidal, wave and offshore wind) is a significant new resource that will be integrated into the Canadian energy supply. More than 2 gigawatts of wind power capacity has been installed offshore Europe in the past 20 years and the application of that experience is rapidly spreading to other parts of the globe, including Canada. Differing environmental and geological conditions require that experience related to foundation design and construction be extrapolated within a rational framework (Schneider et al., 2010). Knowledge of the geological and geophysical characteristics of the seabed is critical to understanding the geotechnical conditions on which marine renewable conversion systems are founded or anchored. For example, tidal and many wave systems, whether flexible or rigid, require moorings that must be anchored to the seabed so that both the turbine and mooring are secured against
n
Corresponding author. Tel.: þ 1 250 363 6424. E-mail address: [email protected] (J.V. Barrie).
movement. This can be done by a penetrating anchor or by using a gravity foundation. For an equivalent resistive load, the footprint of a gravity foundation, such as a wind turbine monopole on the seabed, is greater than a penetrating foundation (IEA-RETD, 2012). Both systems are susceptible to scour. In addition, electrical transmission to shore is an integral aspect of any renewable energy project and knowledge of the seabed characteristics for cable routing is critical. The Pacific margin of Canada has many advantages for renewable energy generation such as a meso- to macrotidal environment, the entire north Pacific for wave fetch, and some of the strongest and most consistent winds anywhere in North America. Consequently, proposals for the development of all three types of power are presently under review. Our objective here is to interpret the seabed characteristics and sedimentary processes active at the key sites where renewable energy systems will eventually be installed. The site with the highest potential for each type of energy source was selected for more detailed site analyses. Definition of areas suitable for potential marine renewable resource development for the Pacific offshore of Canada is derived from Cornett (2006) for tide, Cornett (2006) and Cornett and Zhang (2008) for wave and from the
0278-4343/$ - see front matter Crown Copyright & 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.csr.2013.10.016
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The western Canadian continental margin comprises the convergent boundary between the Juan de Fuca Plate and the North America Plate along western Vancouver Island, and the transform fault boundary between the Pacific and North America Plates along the west coast of the Haida Gwaii (Fig. 1). The western Canadian continental shelf can be broadly divided into three geographic regions, Salish Sea, Vancouver Island Shelf and the Pacific North Coast. Each region's physiography has been uniquely impacted by a history of glaciation, tectonism, oceanography and sea level change.
Island, making it one of the world's largest inland seas encompassing 400 islands and 7500 km of coastline (Fig. 1). The inland sea connects with the open sea in the south, first through two channels and then through Juan de Fuca Strait and in the north the Strait of Georgia connects to the Pacific first through four narrow channels then Johnstone Strait (Fig. 1). The Salish Sea is comprised of a series of structural depressions, over-deepened by Tertiary fluvial erosion and Quaternary glaciation and partially infilled by glacial and post-glacial sediments (Barrie et al., 2005). Most of the Salish Sea is presently sediment starved with sediment capture within the coastal fjords and inlets, except in the southern Strait of Georgia where sedimentation from the Fraser River dominates the surficial geology with Holocene sediment thicknesses varying from zero on Pleistocene ridges to greater than 300 m within the basin (Mosher and Hamilton, 1998).
2.1. Salish Sea
2.2. Vancouver Island shelf
The Salish Sea consists of three inland straits surrounded by the British Columbia (BC) mainland, Washington State and Vancouver
The continental shelf west of Vancouver Island ranges from 5 to 75 km wide and is characterized by an inshore region of complex
Canadian Wind Energy Atlas developed by Environment Canada (2011) for wind.
2. British Columbia offshore setting
Fig. 1. The Pacific margin of Canada, highlighting the locations of potential tidal, wave and wind energy sites (HG is Haida Gwaii, SG is Strait of Georgia, FR is Fraser River, JFS is Juan de Fuca Strait). The locations of Figs. 2, 4, and 6 are shown.
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morphology and a relatively featureless and flat mid and outer shelf. This featureless shelf is disrupted off southwestern Vancouver Island where basins bounded by morainal deposits extend approximately two-thirds of the distance across the shelf (Herzer and Bornhold, 1982). The shelfbreak lies in waters between 180 and 225 m deep (Fig. 1) and is defined by an abrupt change in slope from nearly horizontal to the steep upper continental slope (71–151). The northwestern shelf consists of a rugged discontinuous bedrock inner shelf overlain with coarse-grained rippled sediments, a midshelf characterized by coarse- to fine-grained well-sorted sand and an outer shelf made up of muddy sand (Bornhold and Barrie, 1991). The central and southern shelf consists of a broad, irregular bank of outcropping Paleogene sandstone and mudstone with a thin cover of gravel and coarse-grained sand that extends the width of the shelf (Bornhold and Barrie, 1991).
2.3. Pacific North Coast The Pacific North Coast stretches from northern Vancouver Island to the Alaskan boundary and west to the continental slope of the northern Pacific Ocean. The region consists of three water bodies, Dixon Entrance separating Haida Gwaii from southeast Alaska, Hecate Strait which separates Haida Gwaii from the BC mainland and Queen Charlotte Sound, lying south of Haida Gwaii and bounded on the south by Vancouver Island and to the east by the BC mainland (Fig. 1). Both Dixon Entrance and Queen Charlotte Sound are open to the Pacific. Hecate Strait is an asymmetric channel composed of broad shallow banks in the west (often shallower than 40 m) and a U-shaped southward-opening trough in the east with a narrow bedrock shelf along the BC mainland (Barrie et al., 1991). Most of Hecate Strait is dominated by sand and gravel with small to large subaqueous dunes (classification according to Ashley (1990)). Sediments in the troughs consist of Holocene muds. In the south, Queen Charlotte Sound consists of three extensive banks less than 100 m deep covered with sands and gravels separated by three broad (10–40 km wide) northeast–southwest trending muddy troughs (Barrie et al., 1991). The west coast of Haida Gwaii is extremely narrow and is devoid of sediment except in areas protected from the severe oceanic energy that dominates (Barrie and Conway, 2002a).
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currents are enhanced where there are abrupt changes in the seafloor. In addition to tidal forcing, circulation in Hecate Strait is also affected by winds and river runoff. The influence of these two factors are generally out of phase, with river runoff dominating circulation in late spring through early summer and wind forcing having its greatest influence during the fall and winter months. Consequently, winter circulation is both tidal and wind driven with a net northward component resulting from the prevailing southeasterly storm winds (Crawford and Thomson, 1991).
3. Methods Some 14,000 km2 of multibeam swath bathymetry coverage has been obtained on the Pacific coast of Canada between 2001 and 2013. Multibeam bathymetric surveys were undertaken by the Canadian Hydrographic Service (CHS) and the Geological Survey of Canada using a Kongsberg-Simrad EM 1002, 100 kHz system mounted on the CCGS Vector, until 2009 when a Kongsberg-Simrad EM 710 multibeam system operating at 70–100 kHz was installed. A hull-mounted 3.5 kHz profiler was employed in conjunction with a Knudsen sounder. Positioning was by broadcast differential DGPS and the multibeam data were corrected for sound speed variations in the stratified water column using frequent sound speed casts. Data were exported with a confidence interval of 5 m laterally, due to navigational limitations, and 0.1% of water depth vertically. The gridded data were exported as ASCII files and imported into ArcInfo software for processing and image production. The multibeam swath images formed the preliminary interpretive framework for site characterization for the potential marine renewable energy sites. Based on this interpretation several areas were identified for further investigation using a Huntec DTS highresolution sub-bottom profiler, Klein 3000 sidescan sonar, and a bottom camera system. Sediment samples were collected using a Shipek grab sampler within the subaqueous dune areas. Regional surficial sediment distribution is based on the Geological Survey of Canada marine sediment database.
4. Results 4.1. Tidal energy
2.4. Oceanography The near and offshore waters of British Columbia are mesotidal to macrotidal with superimposed storm and estuarine circulation. The Salish Sea is characteristic of a partially mixed estuary with moderately strong tidal currents (2.6–3.4 m tidal range), seasonally varying stratification and late summer and late winter deepwater density intrusions (LeBlond, 1983; Crean and Ages, 1971; Masson, 2002). The Fraser River accounts for about 73% of the mean annual freshwater discharge into the Strait of Georgia (Johannessen et al., 2003). This freshwater influx forces estuarine circulation in the southern strait, which is characterized by net outflow of low salinity water towards the Juan de Fuca Strait in the upper layer (o 50 m depth) and a net northward inflow of high salinity water in the lower part of the water column that reaches the Strait of Georgia in late summer (Mosher and Thomson, 2000). For the Pacific North Coast strong rectilinear tidal currents, usually in the order of 1.5–25 ms 1, are classified as mixed, mainly semi-diurnal, with a 4.5 m tidal range (macrotidal) in the central portion of Hecate Strait (Thomson, 1981). The tidal crest enters Queen Charlotte Sound and spreads northward into Hecate Strait where it encounters the opposing crest that entered eastward through Dixon Entrance (Fig. 1). Normally, tidal currents flow along the orientation of the Strait at nearly uniform speed at all depths, but
Results from 14 different tide models were obtained and analyzed by Cornett (2006) to establish 89 sites with greater than 1 MW (megawatt) potential. Six of these sites had the highest potential, with mean continuous power potential of greater than 200 MW (Fig. 1). The areas of increased tidal currents suitable for power generation are all located in environments where the overall bathymetric morphology restricts flow. Two areas, Boundary Pass and Johnstone Strait (Fig. 1), provide the best sites for tidal energy. In all cases, with the exception of Seymour Narrows in southern Johnstone Strait, the sites have tear-dropped or elliptical shaped subaqueous dune fields with the largest dunes at the widest point of the ellipse, decreasing dune size in all dimensions to the narrowing point of the tear-drop shape. In central Johnstone Strait, where tidal energy is greatest, an elliptical dune field extends for 4 km along the axis of the strait consisting of three dimensional very large subaqueous dunes with wavelengths of 100–150 m and heights from 8–12 m (Figs. 2 and 3). The dunes are asymmetrical suggesting transport to the southeast from 160 to 80 m water depth (Figs. 2 and 3). At the widest point of the ellipse, there are four very large nearly-symmetrical dunes of 12–14 m height on the western side (Fig. 2). The asymmetrical dunes are primarily coarse to very coarse-grained, well-sorted shelly sand where the four very large nearly-symmetrical dunes consist of
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granule-sized gravel with cobbles in the troughs. Two multibeam swath bathymetry surveys were completed over these dunes in September 2006 and November 2010. The asymmetric dunes migrated up to 70 m to the southeast during these four years, suggesting a rate of 17 m/year (Fig. 2). In contrast, the very large nearly-symmetric dunes showed little change over the four year period. 4.2. Wave energy Waves propagating across the surface of the world's oceans represent a vast potential source of renewable energy. Considerable efforts are underway around the world to develop commercially
viable technologies to convert wave energy into electrical and mechanical energy. Power derived from wave energy could be a particularly attractive option for remote coastal communities. For the Pacific offshore of Canada wave energy resources are largest in the open ocean far from shore and decrease across the continental shelf (Cornett, 2006). The annual mean wave energy flux of exposed deep-water sites located 100 km off Canada's Pacific coast is on the order of 40–45 kW/m and decreasing to 25 kW/m at the coast of Vancouver Island (Cornett, 2006). Wave energy flux in the northeast Pacific also has strong seasonal variability with the mean wave energy flux in winter typically around six to eight times greater than in summer (Cornett, 2006). Cornett and Zhang (2008) used modeling tools to delineate and quantify the energy flux throughout the shallow water region within 5 km of the shoreline off western Vancouver Island to determine areas where the wave energy is focused and concentrated by the seabed bathymetry (Fig. 4). Their results predict an area within 10 km of the town of Ucluelet (Fig. 4) at 40–60 m water depth where the mean annual available wave power exceeds 48 kW/m, which is only slightly less than the wave power available around the wave buoys located in the open North Pacific 600 km further west (Cornett and Zhang, 2008). The average wave power available during December (123 kW/m) is 15 times greater than during July (8 kW/m). Based on sidescan sonar coverage the seafloor within this predicted area of highest wave energy potential consists of a boulder lag surface with 10–30 m wide linear ribbons of small subaqueous gravel dunes. The gravel dunes average 2 m wave lengths and heights are less than 0.5 m. Boulders within the surface lag are up to 1 m in diameter and are rounded (Fig. 5a). Even the largest of the boulders show signs of being abraded or moved as exposed surfaces have scars of barnacles and worm tubes that have been removed. Many pebbles and cobbles show signs of being rotated with fresh un-colonised surfaces (Fig. 5b). 4.3. Wind energy
Fig. 2. Changes between September 2006 and November 2010 in dune movement and crest line orientation in the very large dunes in Johnstone Strait. The inset shows a detailed multibeam image of the dune field. Red line indicates location of Fig. 3.
Archer and Jacobson (2005) undertook the first global study to quantify the worlds wind power potential. Wind speeds were calculated at a height of 80 m, the hub height of modern 77 m diameter 1500 kW turbines. They categorized global winds into seven classes, with sites that have annual mean wind speeds of greater or equal to 6.9 m/s (class 3) capable of low cost wind power generation. Up to 13% of all global stations analyzed met this criterion. For the Pacific coast of Canada seven stations fell within Class 7 (annual mean wind velocity of 9.4 m/s (Archer and Jacobson, 2005)). The Canadian Wind Energy Atlas (Environment Canada, 2011) defines a large area of the Pacific North Coast as having significant wind energy potential. Of this area, the
Fig. 3. Huntec sub-bottom profile through the subaqueous dune field in Johnstone Strait (location of line shown in Fig. 2). Notice the asymmetric morphology indicating migration to the south.
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Fig. 4. Location of nearshore wave energy site off the west coast of Vancouver Island, near the community of Ucluelet. The areal distribution of estimated wave power potential (after Cornett and Zhang, 2008) is shown, in addition to the location of sediment samples and camera stations (see Fig. 5).
northwest portion of Hecate Strait (Dogfish Bank (Fig. 1)) not only has strong winds but also shallow water depths (20–40 m), a requirement for gravity based wind turbines. Dogfish Bank is characterized as a shoreface inner shelf environment with a well defined suite of offshore connected ridges and shore parallel sand ridges (Amos et al., 1995). Throughout the study area, sand ridges with thicknesses up to 30 m and oriented northeast to southwest occur above a gravel lag surface that is exposed over approximately 30–50% of Dogfish Bank (Fig. 6). Smaller dunes occur on the surface of the larger sand ridges with a transport direction from south–southwest toward the north–northeast. Barchan dunes, sand ribbons, gravel waves and boulders are also observed throughout the region. Underlying the lag surface are Pleistocene sediments likely deposited in a coastal setting. Longshore sand and gravel bars, coastal lagoons, prograded sand and gravel spits, and finer-grained silty-sand and clay sediment sequences washed from the swash
zone are typical of the depositional sequence observed on the seismic reflection data. The unconsolidated Pleistocene sediments are at least 40 m thick. A borehole drilled to 50 m sub-seabed at the center of the wind farm site found sediments that were deposited in a predominantly regressive/transgressive sequence (RPS Energy, 2009).
5. Discussion The British Columbia continental shelf is characterized by marine basins that are interconnected (Fig. 1) and open to the Pacific through geologically controlled straits, a result of modification of the seafloor by glaciers and terrestrial processes. Tidal exchange into the basin in many cases takes place through narrow restricted passages, often with shallow sills, resulting in tidal reinforcement. The highest potential tidal energy sites all occur
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Fig. 5. Seabed bottom photographs at 44 m water depth taken near the proposed site of a wave energy system (see Fig. 4 for locations). The upper photo (a) shows large boulders with worm tube and barnacle scars along with finer-grained gravels and the lower photo (b) shows mobile clean gravel showing no benthic growth. Length of compass fin is 30 cm; compass dome is 8 cm across.
in the channels connecting the Salish Sea to the open Pacific (Fig. 1). In all cases, but one, these sites are characterized by very large subaqueous dune fields. For example, in the Boundary Pass (Fig. 1) nearly-symmetrical dunes with wave-lengths between 100 and 300 m and, dune heights up to 28 m, cover the seafloor between 170 and 210 m water depth (Barrie et al., 2009). Repetitive multibeam surveys in the area suggest that there is no clear net direction of movement; crestal flexing and rotation is the long term feature of the wave dynamics. Plots of superimposed crest lines over the 6 year survey period suggest field-wide clockwise rotation of crest lines (Barrie et al., 2009). A similar dune field in central Johnstone Strait shows net southerly transport of the dunes by up to 17 m/year. The depth of the seafloor can change by up to 12 m due to dune migration and the continued movement of very coarse-grained sediment. The limited fetch and constrained channel morphology suggest that sediment transport is flood tide dominated. Whether this will continue into the future is not clear without detailed modeling. Regardless, dune fields such as in Boundary Pass and Johnstone
Strait pose a severe challenge to the placement of tidal turbines and electrical transmission cables. The outer shelf areas exposed to the open Pacific are mostly devoid of sediment, a result of almost total erosion during the rapid Holocene transgression (Barrie and Conway, 2002a; Hetherington and Barrie, 2004). Continental drainage is trapped behind these islands and drainage from the islands is contained in coastal fjords. The proposed wave power location off western Vancouver Island is located in an area where a continuous gravel lag or boulder pavement covers thin glacial sediment or Paleogene bedrock. Consequently, foundation conditions for anchoring wave systems are challenging. The local wave regime is adequate to mobilize gravels up to 16 mm in grain size (Luternauer et al., 1986). The lack of encrusting organisms on all sides of the gravel clasts indicate that the finegrained gravel fraction is mobile. The small and abundant barnacle and worm tube scars on the larger boulders (Fig. 5) suggest that smaller gravel clasts are mobilized during winter storms, impacting and effectively preventing such encrusting organisms
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Fig. 6. Huntec sub-bottom profile through a portion of the sand ridge complex where a proposed wind farm will be located. Notice the strong reflector indicating the lag surface formed during the sea level lowstand and the underlying stratified sediments. Location of the profile on Dogfish Bank is shown in Fig. 1.
from becoming fully established, after a seasonal period of growth of some months. Luternauer et al. (1986) also suggest that the small oscillatory subaqueous dunes observed are in equilibrium with the present wave regime. During glacial retreat on the Pacific North Coast inshore of Haida Gwaii a sequence of deposition from till to hemipelagic sedimentation from floating icebergs resulted in a thick unconsolidated glacial sequence of sediments that were reworked but not removed during the subsequent transgression (Barrie and Conway, 2002b). Within the primary wind site location on Dogfish Bank this resulted in a thick unconsolidated sediment sequence overlain by a lag surface, interpreted to equate to a sea level lowstand surface, marking 2500 years of exposure prior to the early Holocene transgression. Overlying this lag surface is a veneer of sand and fine-grained gravel that form sand ridges and subaqueous dunes. This thick unconsolidated sediment package will provide the necessary foundation for gravity based wind turbines. Modeling of a significant storm on Dogfish Bank by Amos et al. (1995) suggested that the shore-parallel sand ridges were in dynamic equilibrium with the storm. However their results show no clear direction of ridge migration, suggesting that they are modified only slightly by each storm, while maintaining the general position that is dictated by the dominance of the northerly storm driven currents. Repetitive sidescan sonar and multibeam surveys between 2007 and 2008 show large areas with little change and also areas where vertical change of 3.5 m has occurred, again suggesting the northward migration of bedforms (RPS Energy, 2009). Studies suggest that in this dynamic sedimentary environment the presence of wind turbines would not have a significant effect on the sediment transport processes (RPS Energy, 2009). The proposed turbine foundations at the Dogfish Bank wind farm are 5 m monopiles that are very well spaced at 800 1200 m. In the United Kingdom at the Scroby Bank wind farm offshore from Great Yarmouth in the southern North Sea, there were no significant changes in sand wave geometry as a result of windfarm construction, including sand wave patterns, orientations, spacing and bifurcations of crests (CEFAS, 2006). The coastline adjacent to the proposed wind farm area is one of the most erosive coasts in Canada (Shaw et al., 1998). The high sensitivity is due to a macrotidal range, erodible sediments, strong southeastern winter storms and an energetic wave climate that result in an average erosion of 1–3 m/year and up to 12 m during strong El-Niño-Southern Oscillation (ENSO) events (Barrie and Conway, 1996, 2002a; Walker and Barrie, 2006). Assuming minimal change to the sediment dynamics of Dogfish Bank it would be expected that little change to the ongoing erosion of this dynamic coastline would occur. For any wind farm on Dogfish Bank the transmission cable corridor to the British Columbia mainland would have to cross a
very large field (472 km2) of large subaqueous dunes in 88–96 m water depth (Barrie et al., 2009). The bedform field is located within a narrow north–south oriented trough in east-central Hecate Strait which is partially closed to the north by a 70 m sill separating Hecate Strait and Dixon Entrance (Fig. 1). Here geologically controlled morphology restricting tidal flow has resulted in the development of this field, as seen at most of the tidal energy sites. The Pacific offshore holds significant potential for tidal, wave and wind energy along the coast, with many sites in close proximity to urban areas. However, placement of seabed founded, energy generating structures or systems and transmission cables across the seafloor will need to be designed to operate safely within the variable physiographic environments which have developed through a unique and local history of glaciation, tectonism, oceanography and sea level change. The development of standards and best practices for site and submarine transmission corridor characterization and environmental impact assessment will be important requirements for moving marine renewable energy development in Canada forward. For example, Det Norske (2011) has developed standards for offshore wind turbine structures that include geological surveys that provide the initial data for specifications related to soil investigations and geotechnical data. While systems anchored to the seabed depend less on sub-surface conditions, surficial seabed properties will need to be understood, particularly knowledge of sediment transport conditions in the high energy sites and within the transmission cable corridors. The primary data sources for the seabed site characterization include multibeam bathymetry and backscatter, sub-bottom profiles, seabed imagery, benthic samples and cores, and sidescan sonar. Detailed geotechnical and sediment dynamics studies would follow on to provide further site specific parameters for the safe engineering of renewable marine energy installations.
6. Conclusion The glacial, sea level, and tectonic processes throughout the late Quaternary have provided promising seafloor conditions for renewable energy development for the Pacific offshore of Canada. However, the vigorous oceanographic environment that provides these energy opportunities presents some engineering challenges. Almost all energy systems and transmission cables will be placed on the seafloor where small to very large, mobile, coarse-grained sand to gravel sedimentary bedforms occur and are continually moving. The seafloor between the sedimentary bedform fields is primarily composed of mobile gravel and boulder lags. Consequently seabed site characterization is, or should be, a critical part of the engineering design process required to install marine energy systems and to understand environmental impacts of such installations.
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