Normal depth is the depth of flow in a channel or culvert when the slope of the water surface and channel bottom is the same and the water depth remains constant. Normal depth occurs when gravitational force of the water is equal to the friction drag along the culvert and there is no acceleration of flow. In culverts, water flows at normal depth when outside the influence of the inlet and outlet tailwater.   Normal depth is undefined for culverts placed at horizontal or adverse slopes.

FishXing uses Manning's Equation to calculate normal depth at each flow based on the channel roughness, wetted area and hydraulic radius.  Knowing normal depth aids in classifying the hydraulic slope of the culvert.

Note:  Flow at normal depth in culverts often presents the highest average velocities and shallowest depths at that flow.  Using normal depth in designing for fish passage is a conservative approach, and ignores potential backwater effects that can increase the range of passable flows.

See also: Roughness Coefficients,Mannings Equation, Geometry of Circular Culverts, Flow Profiles