Water Resources and Environmental Engineering

Hydraulic Radius

Flow area divided by wetted perimeter, the size term in open-channel equations.

Hydraulic radius R = A/P measures how much flow area there is per unit of boundary causing friction. Manning’s equation raises it to the two-thirds power, so it carries most of the effect of channel shape on velocity.

A circular pipe flowing full has R = D/4, and so does one flowing exactly half full — the area and the wetted perimeter both halve. That coincidence is a favourite of exam questions about half-full sewers.

How much of the FE Civil exam is Water Resources and Environmental Engineering?