The velocity within a full flowing culvert is calculated as:

Where:            

V = Average velocity in the culvert barrel [ft/s or m/s]

Q = Flow rate through culvert [ft³/s or m³/2]

 A = Full cross sectional area of the flow [ft² or m²]

When flowing full, the friction loss within the culvert barrel, H_f, over a given length of culvert, L, is calculated based on the friction slope, S_f:

To calculate the friction slope, Manning’s equation is rearranged into the following form:

Where:

n = Manning’s roughness coefficient

j = Constant of 1.00 for metric and 1.49 for English units

R = Hydraulic radius [ft or m]

When full flow conditions exist, FishXing uses the friction slope and culvert slope, S_o, to calculate the height of the hydraulic grade line (HGL) and energy grade line (EGL) above the culvert bottom.

If the hydraulic grade line intersects the top of the culvert, FishXing switches between full flow and open channel flow calculations.