Landscapes that are rhythmically dissected by natural drainage channels exist in various geologic and climatic settings. Such landscapes are characterized by a length-scale for the lateral spacing between channels. We observe a small-scale version of this process in the form of beach rills and reproduce channelization in a table-top seepage experiment. On the beach as well as in the experiments, channels are spontaneously incised by surface flow, but once initiated, they grow due to water emerging from underground. Field observation and experiment suggest the process can be described in terms of flow through a homogeneous porous medium with a freely shaped water table. According to this theory, small deformations of the underground water table amplify the flux into the channel and lead to further growth, a phenomenon we call "Wentworth instability". Piracy of groundwater can occur over distances much larger than the channel width. Channel spacing coarsens with time, until channels reach their maximum length.

[1] We study channelization and slope destabilization driven by subsurface (groundwater) flow in a laboratory experiment. The pressure of the water entering the sand pile from below as well as the slope of the sand pile are varied. We present quantitative understanding of the three modes of sediment mobilization in this experiment: surface erosion, fluidization, and slumping. The onset of erosion is controlled not only by shear stresses caused by surfical flows but also by hydrodynamic stresses deriving from subsurface flows. These additional forces require modification of the critical Shields criterion. Whereas surface flows alone can mobilize surface grains only when the water flux exceeds a threshold, subsurface flows cause this threshold to vanish at slopes steeper than a critical angle substantially smaller than the maximum angle of stability. Slopes above this critical angle are unstable to channelization by any amount of fluid reaching the surface.