|Abstract or Summary
- The ocean deployment of arrays of Wave Energy Converters (WEC arrays) appears likely in the near future, and deployment of offshore wind turbines has already started. These technologies tap into a potential renewable energy resource but also involve complex systems with uncertain environmental consequences that will likely scale with the size of their ocean footprint. This synthesis talk will concentrate on the potential physical effects of these array technologies.
Both WEC arrays and offshore wind farms consist of sizable structures placed in the water column; hence, their mere presence is a potential environmental stressor. Possible effects on the physical environment include wave scattering and wave shadowing; added drag on the coastal current field; modifications to sediment transport (by way of the aforementioned changes to the wave and current forcing); and changes to local sediment characteristics (due to anchors and pilings). In many ways, these effects are similar to those caused by other ocean structures that have been studied for some time (e.g., offshore platforms). However, there are additional potential effects of WECs and wind turbines that require further attention. For example, extraction of wave energy by WECs could have additional environmental consequences. Similarly, offshore wind farms can alter the local wind field, in turn altering locally-generated waves. We will address effects due to wave or wind installations on the wave field, on local ocean circulation, and on sediment transport characteristics.
Because WECs partially extract and scatter incident wave energy, they cause significant modifications in the near-field. In fact, if device performance can be optimized at field scales, then by definition the near-field effects will be maximized, i.e., if energy extraction is maximized the potential physical effects of WECs are also maximized. Over the past decade a sizable number of studies have applied theoretical principles using varying assumptions and simplifications to the problem of WEC-wave interactions. Some of these assumptions (e.g., “optimal” motions, monochromatic wave conditions, etc.) have now been shown to be unrealistic, and there has been a convergence toward classes of models that appear to produce reasonable estimates. While recent model studies have managed to bound the problem, significant uncertainties remain. The primary cause for the remaining uncertainties is the lack of observational studies, particularly data sets that provide spatial information about the wave field in the vicinity of in situ devices. Nonetheless, a few studies have undertaken scaled laboratory testing, and these data sets are beginning to lend confidence to the available numerical model results and shed light on the dominant processes.
Once near-field effects are understood, far-field effects can be assessed. Far-field effects influence the wave field near beaches, which, in turn, influences the sand transport processes that govern the morphodynamics of the beach face. Fortunately, hydrodynamic modelling of large-scale wave propagation processes in the absence of structures is highly advanced, i.e., if given accurate incident wave conditions in the lee of an installation and bathymetry for the model domain, models can well-simulate local wave conditions, wave-driven currents and sediment transport patterns. Therefore, once near-field WEC/wave dynamics are understood, expanding our understanding to the far-field will be relatively straightforward. Nonetheless, observational studies of far-field beach modifications shoreward of an installation will help to further solidify our understanding of beach behaviour.
Offshore wind farms can also potentially influence the local wind field around them. Previous studies of such modifications at land-based wind farm installations serve as a reasonable basis for predictions offshore . Any changes to offshore winds will also influence the local wave field, especially where local winds are the dominant source of waves. Such effects will be minimal near coasts where the local wave climate is dominated by incident swells generated at large distances (e.g., the U.S. West Coast). In contrast, locally generated waves are a more important component of the wave climate on the East Coast of the U.S
Modification to ocean currents by an array of structures can be assessed by considering the additional frictional effects (“form” drag) caused of the array. If the drag caused by a dense of array of structures is large, circulation will be altered, which might result in reduced current velocities or the diversion of currents toward an area of less drag. Note that ocean currents already experience drag due to bottom friction; hence, the question hinges on the relative magnitude of the drag induced by structures versus the pre-existing frictional drag.
Finally, any near-field modifications to the wave and circulation field (due to either WEC arrays or wind farms) will necessarily result in changes in sediment transport. Any local reduction in flow velocities can result in a reduction of the sediment carrying capacity of circulation leading to sediment accumulation at the site. Small-scale modification to a current will also likely cause bumps and holes around the pilings or anchors. These effects are similar to those observed around existing offshore structures and pilings, and can be accounted for in the design of the structures.
Far-field modification of waves and associated changes in wave-induced currents can also result in changes in sediment transport patterns near beaches. Although some recent studies exist, questions regarding far-field effects on beaches are still relatively poorly addressed.
- Haller, M.C. & T. Özkan-Haller. 2012. Physical-Environmental Effects of Wave and Offshore Wind Energy Extraction: A Synthesis of Recent Oceanographic Research. In: Boehlert, G., C. Braby, A. S. Bull, M. E. Helix, S. Henkel, P. Klarin, and D. Schroeder, eds. 2013. Oregon Marine Renewable Energy Environmental Science Conference Proceedings. U.S. Department of the Interior, Bureau of Ocean Energy Management, Cooperative Agreement with Oregon State University M12AC00012. OCS Report BOEM 2013-0113. 149 pp.