Chézy drag law
= Chézy drag law
{c}
{title2=$\tau=\rho c_du|u|$}
A quadratic boundary-drag model relates <shear stress> to mean flow <velocity>, with dimensionless resistance coefficient $c_d>0$. Uniform <steady flow> down slope $S$ balances $\rho gAS=\tau\ell$, giving $|u|=\sqrt{gRS/c_d}$. Its sign is chosen so drag opposes the motion.