The following equations are used in FishXing to calculate the wetted perimeter, top width, and cross-sectional area of the flow through a circular culvert that is not-embedded.

Definition sketch for Geometry of a circular culvert:

Equations Used in the Calculation of Circular Culvert Geometry:

            Where:

d = Diameter of culvert

r  = Radius of culvert  

y = Depth of water in culvert  

P = Wetted Perimeter of water on the bottom

T = Top width of water surface

A = Cross sectional area of flow

Rh = Hydraulic radius

Another common method for determining culvert geometries involves the use of a table of geometric ratios developed for a partially full pipe that is not embedded. If the depth of flow is known, a ratio of depth to diameter can be used to calculate the area of flow, wetted perimeter and hydraulic radius.