Baffles are roughness elements added to a culvert in order to reduce water velocity to a level acceptable for fish passage. Along with reducing velocity, baffles create turbulence that in excess can create a barrier to fish passage. Turbulence represents the energy dissipated by falling water necessary to reduce velocities in the culvert. Energy dissipation is described by the Energy Dissipation Factor (EDF) and current research shows that the EDF should be between 3.0 and 4.0 ft-lb/ft3/sec to prevent sediment or debris accumulation and provide adequate passage for adult salmon.

The work that was done by Shoemaker in 1956 relates baffle dimensions to roughness in terms of the darcy friction factor (see summary of work in Appendix D of the 2003 WDFW's "Design of Road Culvers for Fish Passage" manual. Also, folks at WDFW have collected data on new baffled culverts and calculated friction factors for them.