Tunnel inner diameter

For shapes such as squares or rectangular ducts and tunnels where the height and width are comparable, the characteristic dimension for internal-flow situations is taken to be the hydraulic diameter, defined as $D=4*A/P$, where A is the cross-sectional area, and P is the wetted perimeter. The wetted perimeter for a tunnel is the total perimeter of all inner tunnel walls that are in contact with the flow.

For circular tunnels, the diameter is an input value.

$\frac{4 A_{t}}{2 h_{t} + 2 w_{t}}$