The potential risk to marine mammals colliding with turbines is one of the primary environmental concerns slowing tidal energy development in the US and Europe. Few field observations of interactions between marine animals and tidal turbines have been reported , necessitating supplemental risk analyses and modeling to help fill the gap. This study provides a surrogate measure of the potential consequences of collision between harbor seals and a two-bladed unducted turbine. By combining estimates of the consequences of collision with the probability of collision, the risks to harbor seals around tidal turbines can be estimated, supporting regulatory decisions, and providing feedback on turbine design to minimize potential harm. In conjunction with future assessments of interactions of marine mammals with other tidal turbine designs, a more generalizable estimate of the risk of tidal turbines to marine mammals could be developed.