Interactions of flow, tidal stream turbines and local sediment bed under combined waves and tidal conditions (INSTRON)

The increased political recognition in recent years of the economic and societal benefits of renewable energy systems has led to an increased pace of technological advances in such systems and an increased urgency to deploy renewable energy devices on land and in coastal waters. At the same time, there has been a realisation that, in order to exploit fully the opportunities for renewable power generation, it is necessary to be aware of (and to adjust to) a wide range of environmental disturbances in the vicinity of the power generation installations themselves. The present project examines this potential conflict for the case of a tidal stream turbine (TST) - a device installed in tidal waters to generate power by the rotation of a set of rotor blades driven by the tidal current. It is known that flow passing TST support structure combined with the rotation of the turbine rotors produces a turbulent downstream wake that can be sufficiently energetic to disturb the stability of the sediments on the sea bed on which the turbine is constructed and affect the sediment suspension. This may have significant impact on the sea floor topography and adverse consequences for the indigenous marine flora and fauna displaced by the geomorphologiocal changes and changes of suspended sediment in water. As a result, large-scale implementation of TST devices is often viewed with serious concern by environmental and ecological agencies, the fisheries authorities and local communities dependent economically on the affected zones.

Thus far, little is known about the nature of the pressure distributions within the turbulent wakes of the rotors and the mechanisms by which these wake flows perturb the sediments in water and on the seabed. For cases in which several tidal stream turbines are constructed in an array, the configuration of the sea bed sediments is subjected to complex pressure distributions arising from from each of the constituent installations and the prediction of sea floor sediment response is even more uncertain. Because of these knowledge gaps in predicting the sea bed response, developers of TST projects have presently to carry out costly environmental monitoring programs in order to obtain environmental permission for installation of devices.

The marine environment, especially in those areas that are subject to strong tidal currents, is usually subject to complex sediment transport dynamics. The main aim of the proposed research is, therefore, to develop advanced computational tools to overcome the above knowledge gaps, in order to predict the consequences of complex interactions between tidal flow, tidal stream turbines and the sea floor sediment bed under combined waves and tidal conditions. The research will build upon results from previous research programmes such as Supergen I and II.

To tackle the above uncertainties, computational and laboratory modelling studies will be carried out (i) to investigate the fundamental processes controlling the complex flow-TST-sediment interactions and (ii) to improve practical prediction methods that can be used not only by engineers in full-scale TST planning and design but also by regulatory authorities monitoring environmental and ecological consequences of installing TST arrays. The research will take a systematic, multidisciplinary, evidence-based approach involving analysis, physical model experiments and numerical modelling components to address and delineate the key processes affecting the sea bed response to TST placements in coastal waters

The non-academic beneficiaries of the research will be drawn from (i) the private commercial sector (principally civil engineering consulting companies), (ii) national (and eventually international) policy makers concerned with low carbon (i.e renewable) energy production, (iii) environmental monitoring agencies, (iv) nature protection agencies and (iv) local communities, conservation groups/societies and local businesses (fishing, tourism etc) affected by new coastal developments of tidal stream turbines.

The prediction of the consequences of installing arrays of tidal stream turbines is of relevance to all of the above groups, not least because one of the most threatening consequences of such developments, namely the disturbance produced to the sea bed on which the turbines are constructed, has wide commercial, environmental and societal repercussions. The private sector has a direct commercial interest in being supplied with predictions on the stability and erodibility of the sea bed sediments around the construction. This affects primarily civil engineering consultancies concerned with (i) the design, stability and integrity of the structures and (ii) the prediction of the resulting sediment erosion and deposition patterns that can affect navigability, trapping of contaminants from anthropogenic discharges into coastal waters and sea bed infrastructure (pipes, cables) close to shore. The other commercial sector affected by uncertainties in knowledge of the effects of TST installations and the risk to the stability of such structures on an erodible sea bed (e.g scour and downstream erosion) is the insurance industry charged with substantial investment in such structures and a need for awareness of the risks of structural and environmental costs of inadequate predictions of TST installation consequences.

The environmental consequences of the TST installations generating significant sediment transport and geomorphological changes to the original site are potentially very serious; the disturbances to the local ecology and the potentially-disruptive redistribution of indigenous benthic species can affect in a complex way the survival of communities of sea bed plants and animals (and the birds and higher forms of marine life that predate these species). It is with the research- and evidence-based studies typified by this proposed project that the environmental constituency derives benefits, initially via the environmental regulatory and monitoring agencies charged with understanding and incorporating into regulation new research on this relatively-unknown set of problems and then via local and national opinion formers and vested interests (politicians, marine protection groups and special interest lobbies such as those depending upon shellfish and fin fish harvesting). The impact of the research is far reaching, even on a local scale; the wider consequences of deleterious effects of TST construction are of international significance with so many tidal stream devices being developed and deployed overseas. The benefits of evidence-based, additional predictive tools to UK civil engineering consultancies (and insurance companies) who operate globally in this specialised area are clear.