Environmental Engineering Reference
In-Depth Information
Karsten et al. (2008) estimated that extracting the maximum of 7 gigawatts (GW)
of power from the Minas Passage (Bay of Fundy) with in-stream tidal turbines could
result in large changes in the tides of the Minas Basin (greater than 30%) and signifi-
cant far-field changes (greater than 15%). Extracting 4 GW of power was predicted
to cause less than a 10% change in tidal amplitudes, and 2.5 GW could be extracted
with less than a 5% change. The model of Blanchfield et al. (2007) predicted that
extracting the maximum value of 54 megawatts (MW) from the tidal current of
Masset Sound (British Columbia) would decrease the water surface elevation within
a bay and the maximum flow rate through the channel by approximately 40%. On the
other hand, the tidal regime could be kept within 90% of the undisturbed regime by
limiting extracted power to approximately 12 MW.
In the extreme far field (i.e., thousands of kilometers), there is an unknown poten-
tial for dozens or hundreds of tidal energy extraction devices to alter major ocean
current such as the Gulf Stream (Michel et al., 2007). The significance of these
potential impacts could be ascertained by predictive modeling and subsequent oper-
ational monitoring as projects are installed.
a lTeraTion oF s ubsTraTes and s ediMenT T ransporT and d eposiTion
Operation of hydrokinetic or ocean energy technologies will extract energy from the
water, which will reduce the height of waves or velocity of currents in the local area.
This loss of wave/current energy could, in turn, alter sediment transport and the wave
climate of nearby shorelines. Moreover, installation of many of the technologies will
entail attaching the devices to the bottom by means of pilings or anchors and cables.
Transmission of electricity to the shore will be through cables that are either buried
in or attached to the seabed. Thus, project installation will temporarily disturb sedi-
ments, the significance of which will be proportional to the amount and type of bot-
tom substrate disturbed. There have been few studies of the effects of burying cables
from ocean energy technologies, but experience with other buried cables and trawl
fishing indicate the possible severity of the impacts. For example, Kogan et al. (2006)
surveyed the condition of an armored, 6.6-cm-diameter coaxial cable that was laid on
the surface of the seafloor off Half Moon Bay, California. The cable was not anchored
to the seabed. Whereas the impacts of laying the cable on the surface of the seabed
were probably small, subsequent movements of the cable had continuing impacts on
the bottom substrates. For example, cable strumming by wave action in shallower,
nearshore areas created incisions in rocky siltstone outcrops ranging from superficial
scrapes to vertical grooves and had minor effects on the habitats of aquatic organisms.
At greater depths, there was little evidence of effects of the cable on the seafloor,
regardless of exposure. Limited self-burial of the unanchored cable occurred over an
8-year period, particularly in deeper waters of the continental shelf.
During operation, changes in current velocities or wave heights will alter sedi-
ment transport, erosion, and sedimentation. Due to the complexity of currents and
their interaction with structures, operation of the projects will likely increase scour
and deposition of fine sediments on both localized and far-field scales. For example,
turbulent vortices that are shed immediately downstream from a velocity-reducing
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