Culvert Studio computes various parameters for a wide variety of channel shapes. In all cases, steady flow is assumed and thus normal depth is assumed. Manning’s equation is used exclusively. When a known Q is specified it solves for the depth using an iterative procedure.

**Manning’s Equation**

Where:

Q = flow rate in cfs

n = roughness coefficient

A = cross-sectional area in sqft

R = hydraulic radius = A / WP

S = channel slope in ft/ft

**Composite Manning’s n**

With user-defined sections that have varying n values, Culvert Studio uses the following Horton-Einstein equation to first compute a composite roughness coefficient. Then it employs Manning’s equation as described above.

Where:

nc = composite n-value

Pi = wetted perimeter of subdivision i

ni = n-value for subdivision i

p = total wetted perimeter of channel

**Critical Depth**

Yc, or critical depth is computed using the following equation along with an iterative procedure:

Where:

Q = flow rate

A = cross-sectional area

T = top width