Installing add-ons in GRASS?

In a GRASS session, if a compiler is available on the system, run

g.extension r.inund.fluv

This GRASS module is one of the very few modules written in Fortran, so you need a Fortran compiler environment to be installed on the system.

In order to use g.extension, you'll need to install Subversion, a version control system. Apache provides binaries for most platforms.