I lost touch with the CGI industry, but sidefx latest release of houdini includes a dry and wet sand (snow, ...) simulator.
https://www.sidefx.com/index.php?option=com_content&task=vie...
They used to be on the fore front of this kind of R&D, so I guess it's pretty state of the art (as a CGI package of course).
Sand behaves much differently than liquid. The friction between sand grains is not like viscosity and it lacks surface tension too. I don't know any continuous model that describes the movement of sand similar to Navier-Stokes for liquid.