Hydraulic radius
= Hydraulic radius
{title2=$R=A/\ell$}
The <hydraulic radius> is cross-sectional flow <area> divided by wetted perimeter. A perimeter-averaged boundary <shear stress> $\tau$ exerts drag per unit fluid volume $\tau\ell/A=\tau/R$. It differs from both actual depth and the radius of a noncircular channel.