AbstractGrass-lined spillways are flow conveyance structures with environmental benefits. The stability of such spillways has been typically assessed considering soil erosion, but the design of grass-lined spillways based on hydraulic considerations has rarely been conducted. While subcritical flows in channels with grass have been studied extensively, studies of velocities and flow resistance in supercritical flows in spillways are limited. Herein, this experimental study investigated the application of a multilayer velocity model for supercritical self-aerated flows on a spillway with submerged artificial grass. Velocities were measured with a pitot tube and dual-tip air–water flow conductivity probe, providing the most systematic assessment of velocities and flow resistance in supercritical vegetated flows to date. The velocity distributions were well described with a multilayer velocity model previously developed for subcritical flow conditions, and a constant interfacial velocity supplemented the observed supercritical aerated free-surface layer. Based on this velocity model, explicit expressions for the mean flow velocity and the friction factor were developed for grass-lined spillways, which were also applicable for subcritical flow conditions with comparable vegetation cover. The flow resistance model provides a theoretically developed design option for grass-lined spillways that is solely based on vegetation properties and hydraulic boundary conditions.