## Defining Normal Depth |

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