Interaction between flow, transport and vegetation spatial structure

Gives the basic idea. (Not on the model, but how to parameterize seagrass effect.)

Starting from your mesh you can create a dfsu file in which you can specify the desired value of  roughness (z0) for each node, obviously different for seagrass and rock. In this way you can load a space-varying roughness as input. 

Thank you for your idea.

I know this is a very late answer, but you can also input a bed roughness map in dfs2 formula, which may be faster to do than creating a new dfsu file from the mesh.