Streambed simulation culvert designs: Assumptions
Roughness if "large scale" can reduce velocities adequately even on relatively steep gradient culverts (Thorne and Zeberhagen, 1985)
If culverts are wide enough (compared to the stream width) and within a given gradient range material will set up in the culvert or remain in the culvert if placed (White, 1996).
Large scale roughness creates velocity shadows that allows fish to move through even steep culverts.