Fig. 4: Long- and short-term processes that contribute to sudden scour hole formation.

a, Geological timescale, with typical uplift and subsidence rates of horsts and grabens up to centimetres per century; rivers incise into horsts. b, Engineering timescale in which natural rivers (upper panel) are engineered (lower panel) and sediment transport is modified, leading to incision rates up to centimetres per year. c, Event timescale during which flow conditions change rapidly and erosion and deposition processes develop fast; labels d, e and f along the plot line refer to moments of the processes in panels d, e and f, respectively. d, Armoured bed with increasing suspended load from upstream and developed small sand dunes. Underneath the armour, a thin mixture of gravel and sand overlays a thick layer of fine sand. e, Increased flow disrupting the armour and forming gravel dunes. f, Rapid development of scour hole in fine sands^58,59.