LES of bed form initiation in an experimental flume (Venditti et al. 2005, JGR) after 70 sec. from an initial flat bed. The domain size is 5m x 1m x 0.15m in streamwise, spanwise, and vertical directions, respectively. Coupled stratified flow and bed morphodynamics (with suspended sediment and bed load modules) are solved on 43 million grid nodes.