Perspective on the risk that sediment-laden ice poses to in-stream tidal turbines in Minas Passage, Bay of Fundy

• We weaken the Sanders (2011) claim that sediment-laden ice is dangerous to turbines.
• Density of samples from sediment-containing ice-cakes has a bimodal distribution.
• Low buoyancy was only observed if ice contained coherent pieces of bottom substrate.
• Large ice cakes are far less likely to be found at depth than small ice cakes.
• Dynamically, the size of ice cakes scales with tide and so should subsurface turbines.

Large sediment-laden ice cakes form in the Minas Basin and concern has been raised that they might pose a substantial danger to in-stream tidal turbines deployed in strong tides of Minas Passage, Bay of Fundy. Consideration of buoyancy and drag shows that large ice cakes must have density within a much more narrow range than small ice cakes if they are to be dragged below the surface by turbulent motion. Density measurements of ice samples cut from large ice cakes show a bimodal distribution, with most samples being clearly buoyant and a minority being clearly negatively buoyant. Ice cakes are composed from materials (sediment, ice, salt, air) that are all substantially different in density from seawater. Very particular combinations of materials would be required to produce a large ice cake that could be entrained deep into the water column. Mechanisms for the production of large ice cakes fundamentally depend upon buoyant force, a large tidal range, and hypsometry having deep channels cut through tidal flats. We document the unlikely set of events that would be required in order to produce large ice cakes that are sufficiently near neutral buoyancy and also drift into the Minas Passage so as to pose a danger to large in-stream tidal turbines.