Flow and Benthic Ecology 4D (FLOWBEC)

The strength of currents near the seabed has a major effect on the organisms and communities, that is the benthos, that occur there. The currents may, for example, affect the amount of food supplied to the organisms or they may control the ability of spores and larvae to settle and grow on the seabed. By their very nature, tidal energy devices will disturb the current flow near the seabed with the degree of disurbance depending on the design of the device. In particular a wake will be produced downstream of the body of the device as well as a defined zone of increased turbulence downstream of the blades or rotor. These disturbances to the ambient flow may be expected to have an influence on the benthic communities present on the seabed. Observations of the benthos at three fixed sites downstream of the SeaGen tidal turbine over four years have shown no influence of the device on the benthos; however these observations are very limited spatially. Ultimately there is a need to be able to predict the possible influence of a tidal device or devices on the benthos rather than relying on costly post-installation surveys. One of the main difficulties in being able to make predictions has been the inabiltiy to predict changes in the ambient current flow resulting from tidal energy devices. However recent developments in numerical modelling have the potential to describe at a scale of 1m or better the flowfield downstream from tidal energy devices with early results indicating that there may be localised semi-permanent zones of increased or decreased flow associated with a typical device. Arising from these developments, the proposal intends to carry out detailed benthic surveys downstream of the SeaGen tidal energy device in Strangford Narrows and compare the benthic community distributions with the current field predicted from the high resolution numerical models. The comparison will make a major step in understanding the consequences of flow changes resulting from tidal energy devices on the benthos and also allow predictions to be made for future device deployments.
Queen's University Belfast will oversee the required benthic sampling programme, which will be carried out by personnel from NUI Galway, and will liaise closely with the University of Edinburgh (UoE) who are developing the necessary numerical model within a separate component bid of FLOWBEC. The model will allow current predictions at a spatial scale of 1m or better. Benthic distributions will be characterised by a major series of drop-down video images taken up to 200m downstream of SeaGen during the spring in two consecutive years. The position of each image will be recorded to better than 1m using a precision underwater DGPS system. The video images will be analysed by the University of Galway to quantify the benthos and the nature of the seabed at each sampling point. Statistical analyses will be carried out in close liaison with the UoE team to establish the relationship between the mean velocity and the turbulent velocity component and the distribution of the benthos. Close attention will be paid to assessing an appropriate parameter to describe the turbulent component of the velocity that is appropriate to the biology. The analysis will also consider the interaction between the benthos and the physical nature of the seabed.
The study, which will develop ongoing work establishing the relationship between currents and benthic communities on a coarser spatial scale, is supported by MCT, the developers of SeaGen. The results will be of major value to the marine energy industry as a whole by allowing the prediction of relationships between benthic communities and changes in the ambient current field resulting from the installation of tidal energy devices.

Given the focus of the study on SeaGen, the tidal turbine deployed in Strangford Narrows in April 2008 By Marine Current Turbines (MCT) and operated on a commercial basis since then by MCT, the immediate beneficiary of the proposed work will clearly be MCT. MCT, which is a fully commercial organisation, will benefit by being able to accurately predict what, if any, influence the flow perturbation associated with one SeaGen device may have on ambient benthic communities. The benthic surveys associated with the SeaGen environmental monitoring programme have been unable to show this. Depending on the development of suitable models of flow perturbation associated with array deployments of SeaGen or equivalent devices, the data to be obtained will also be able to be applied to multiple device deployments. The outcomes of the project will also be of major relevance to national and international marine environmental regulatory bodies by allowing them a firmer base on which to reach decisions on device deployment. Related to this, the presence of firm scientific data confirming what, if any, benthic effects may result from the deployment of a horizontal blade-axis tidal turbine will also provide reassurance for interested public environmental groupings.
Reassurance on possible environmental effects of tidal turbines is an important selling factor for any such device. MCT has a national and international presence and is a clear leader in their field and the ability to reassure their potential customers and associated regulatory bodies of the effects, if any, of the device on the benthos will be a valuable selling point for the SeaGen device. Because of the close commercial involvement of MCT with the proposed project, access to data obtained from the project would be restricted for 15 months after its collection. Following this period, it is expected that the data would be submitted for publication in peer-reviewed journals and would be openly available.