  The role of turbulent large-scale streaks in forming subaqueous sediment
ridges on an initially flat sediment bed is investigated with the aid of
particle-resolved direct numerical simulations of open channel flow at bulk
Reynolds numbers up to 9500. The regular arrangement of quasi-streamwise ridges
and troughs at a characteristic spanwise spacing between 1 and 1.5 times the
mean fluid height is found to be a consequence of the preferential spanwise
organization of turbulence in large-scale streamwise velocity streaks. Sediment
ridges predominantly appear in regions of weaker erosion below large-scale
low-speed streaks and vice versa for troughs. The interaction between the
dynamics of the large-scale streaks in the bulk flow and the evolution of
sediment ridges on the sediment bed is best described as a `top-down' process,
as the arrangement of the sediment bedforms is seen to adapt to changes in the
outer flow with a time delay of several bulk time units. The observed
`top-down' interaction between the outer flow and the bed agrees fairly well
with the conceptual model on causality in canonical channel flows proposed by
Jimenez (J. Fluid Mech., vol. 842, 2018, P1, section 5.6). Mean secondary
currents of Prandtl's second kind of comparable intensity and lateral spacing
are found over developed sediment ridges and in single-phase smooth-wall
channels alike in averages over O(10) bulk time units. This indicates that the
secondary flow commonly observed together with sediment ridges is the
statistical footprint of the regularly organized large-scale streaks.
