The excess depth-integrated, time-averaged momentum flux of a surface gravity wave is its radiation stress. In deep water , so . 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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Radiation stress, in the context of oceanography and coastal engineering, refers to the force per unit area exerted by waves on the water column and seabed due to the momentum flux associated with wave energy. It is a critical concept for understanding how waves interact with coastal structures and the seabed, and for modeling sediment transport and beach erosion processes. In more detail, when waves propagate, they carry energy and momentum.