Channel erosion due to subsurface flow.

We study the physical processes that create eroded channels and drainage networks by doing laboratory experiments. The apparatus consists of a sloping granular bed inside a 90 cm 119 cm 30 cm acrylic box. The granular material consists of 0.5 mm diameter glass beads. Water is fed from a reservoir in the back and the pressure is maintained at a constant level. The water seeps through the porous medium and creates channels on the slope above a critical pressure which depends on the slope of the bed. We obtain the depth of channels as a function of time using the intersection of a laser generated light sheet with the pile surface which is imaged with a digital camera. An example of a spontaneous set of eroded channels measured using the laser aided topography technique and then rendered using a false color scale is shown in Fig. 1. In order to simplify the phenomenon, we follow the evolution of an initial channel created with a triangular cross section. Contour maps of channels obtained using this laseraided topography technique after time t=30, 60, 90, and 120 min are shown in Fig. 2. The channel head first develops a smooth amphitheater shape before the shape becomes unstable. The onset of the instability can be delayed by increasing the slope of the bed. This work was funded by Department of Energy Grant Nos. DE-FG0202ER15367 Clark and DEFG0299ER15004 MIT .