= Wave radiation stress
{title2=$S_{xx}=\mathcal E(2c_g/c_p-1/2)$}
{wiki=Radiation_stress}
The excess depth-integrated, time-averaged <momentum flux> of a <surface gravity wave> is its radiation stress. In deep water $c_g/c_p=1/2$, so $S_{xx}=\mathcal E/2=\rho_wga^2/4$. The force per transverse width on a reflecting object is the incident plus reflected minus transmitted radiation stress. Reflection increases force because the reflected wave carries momentum in the opposite direction.
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