Sediment entrainment threshold
= Sediment entrainment threshold
{title2=$\tau_{\mathrm{th}}$}
= Threshold shear stress
{synonym}
The <sediment entrainment threshold> is the bed <shear stress> at which resting grains begin moving. An elementary <force balance> using spherical grains, tangential <quadratic drag>, and <static friction> gives $\Theta_{\mathrm{th},0}=4\mu_s/(3C_D)$ when the <drag coefficient> is defined relative to the bed <shear velocity>.