Objectives:To determine through experimental investigations if waves and currents experience the same physical bottom roughness in a combined wave-current boundary layer flow over a tippled movable bed. To update and improve the G-M model for turbulent combined wave-current boundary layers. Methodology:Laboratory experiments will be performed with waves, currents, and combined wave-current flows over a movable sediment bed consisting of 0.2 mm diameter quartz sand. Measurements of wave attenuation over the 17 meter long test section of the flume and velocity measurements with a SonTek ADV will be used in conjunction with an updated version of the G-M model (to be developed as part of this project) to extract values for the bottom roughness experienced by the waves and currents. The roughness will be interpreted in terms of the ripple geometry, which is to be measured by a bottom Profiling system designed, constructed and used in RC-33. Rationale:A previous Sea Grant Project, RC-28, demonstrated that waves and currents experience the same bottom roughness when flowing over a fixed rippled bed. To investigate the extent to which this result holds when the bed is movable and able to respond to the different flow conditions, is a natural follow up study. The G-M model for wave-current boundary layer flows is widely used, but has been extensively modified over the years in a number of separate papers. Given its wide use as a subroutine in numerical models for coastal circulation and transport of suspended matter, it is high time to present the complete and updated version of the model in a single coherent publication.