“…Among the possible benefits, the present effort may pave the way to a sound mechanical foundation for certain phenomenological dynamical-systems (DS) models (Aranson and Tsimring, 1998;Rothman, 1998;Venkataramani and Ott, 1998;Jeong and Moon, 1999;Jeong et al, 2000;Blair et al, 2000), models that have been remarkably successful in capturing several qualitative aspects of granular-layer dynamics. With a similar motivation, Eggers and Riecke (1999) offer a model with a more evident mechanical content, but it still relies on a phenomenological ''granular diffusion", whose mechanical origins are not obvious to us.…”