Critical behavior and conservation in directed sandpiles
  We perform large-scale simulations of directed sandpile models with both deterministic and stochastic toppling rules. Our results show the existence of two distinct universality classes. We also provide numerical simulations of directed models in the presence of bulk dissipation. The numerical results indicate that the way in which dissipation is implemented is irrelevant for the determination of the critical behavior. The analysis of the self-affine properties of avalanches shows the existence of a subset of super-universal exponents, whose value is independent of the universality class. This feature is accounted for by means of a phenomenological description of the energy balance condition in these models. 
Romualdo Pastor-Satorras, Alessandro Vespignani