A suggested approach to low head hydropower at run-of-river dams and tidal estuary sites is to include an intermediate air transmission stage where water pressure is converted into air pressure. Several studies have been carried out to investigate the hydraulic performance of such system called siphonic turbine. However, their overoptimistic assessment of the performance of the system is not applicable to actual low head sites. This paper investigates the performance of a siphonic turbine considered to be installed in Grevelingen Brouwersdam in the Netherlands. Based on available experimental results on air water ow in downward sloping pipes, a one-dimensional numerical hydrodynamic model is developed to nd optimised combinations of air and water ow rates. The generated power of an air turbine, connected to the siphon, is calculated and the efficiency is determined for several hydraulic and geometrical conditions. A maximum efficiency of 7.2% is observed, which is way below the estimated values in previous studies. The total generated power is about 4 MW for a total water discharge of 4500 m(3)/s and an available head of 1.25 m. This relatively low performance has to be kept in mind when identifying the best available technology for low head sites. (C) 2014 Elsevier Ltd. All rights reserved.