Simulations of the Influence of MHK-Turbine Operation on Hydrodynamics and Sediment Transport for Scotlandville Bend, Mississippi River

Water-current MHK turbines are receiving growing interest in many parts of the world with hydrokinetic resources. However, little is known about the potential effects of MHK device operation in coastal waters, estuaries, or rivers, or of the cumulative impacts of these devices on aquatic ecosystems over years or decades of operation. This lack of knowledge affects the actions of regulatory agencies, the opinions of stakeholder groups, and the commitment of energy project developers and investors. There is an urgent need for practical, accessible tools and peer-reviewed publications to help industry and regulators evaluate environmental impacts and mitigation measures and to establish best siting and design practices. This study presents a methodology to assess the hydrokinetic potential and its environmental effects in a reach of the Mississippi river. The potential changes to the physical environment imposed by operation of MHK turbine arrays were evaluated using the modeling platform SNL-EFDC. Energy extraction is simulated using momentum sinks recently coded into SNL-EFDC, which is an augmented version of US EPA's Environmental Fluid Dynamics Code (EFDC). Three different scenarios (4-, 23- and 112-piling arrays) were development for SNL-EFDC simulations. The four-piling array included 12 turbines; the 23-piling, 132 turbines; and the 112-piling, 638 turbines. As expected, average velocities decrease downstream of each MHK device due to energy removal and blunt-body form drag from the MHK support structures. Furthermore, the velocity profiles exhibit a wake velocity deficit downstream of the last MHK row, which disappears within about 15 array widths downstream. This mirrors the approximate recovery of the wake for a single MHK turbine in a straight channel, which recovers to a ~10% deficit around 15 device diameters downstream. Changes in the flow field also alter sediment transport dynamics around and downstream of an MHK array. Model results with and without an MHK array were compared to facilitate an understanding of how MHK-turbine arrays might alter the river environment. These simulations and scenario analyses can assist cost-effective planning before proceeding to detailed siting, engineering designs, and deployment of devices.