Fig. 1

Phenomenology and setup. a Bed sediment of the River Wharfe, U.K., that shows a pronounced surface armour. Photo courtesy D. Powell⁵⁸. b Sketch of the experiment, showing position of the camera and laser plane used for imaging inside the granular bed. c–e Snapshots during armour development for \(\tau _s^* = 3.8\tau _{cs}^*\). Also shown is the fluid boundary stress, which is computed as τ = ηU _f /h _f ¹¹ U _f and h _f are the top-plate speed and flow depth, respectively. The red curve shows the long-term-averaged streamwise particle velocity u _x (z), where I and II correspond to the bed load and creep zones, respectively. The directions x and z are indicated