Radiation stress is the flux of momentum carried by ocean waves.
The fact that waves can exert a net force on the water body directed along the wave propagation direction was first recognised by Longuet-Higgins and Stewart (1962, 1964). The radiation stress is the momentum transferred through the water body per unit time (the flux of momentum) by wave orbital motion. It is called a stress because for obliquely incident waves, cross-shore momentum can be transferred by both cross-shore wave orbital motion and longshore wave orbital motion and longshore momentum can be transferred by both longshore wave orbital motion and cross-shore wave orbital motion. For obliquely incident waves, a cross-shore gradient in the wave orbital motion, for example due to wave breaking, will exert a stress on the water mass in cross-shore direction as well as in longshore direction. The stress in longshore direction generates a longshore current. The stress in cross-shore direction generates a water level set-up at the coast. Forcing by radiation stress gradients related to wave breaking is commonly an order of magnitude greater than forcing due to wind stress or other wave nonlinearities.
For a more detailed explanation, see Shallow-water wave theory#Radiation Stress (Momentum Flux) and Shallow-water wave theory#Radiation Stress Components for Oblique Waves.
- Longuet-Higgins, M.S. and Stewart, R.W. 1962. Radiation stress and mass transport in gravity waves, with application to 'surf beats'. Journal of Fluid Mechanics 13: 481–504
- Longuet-Higgins, M.S. and Stewart, R.W. 1964. Radiation stresses in water waves; a physical discussion, with applications. Deep Sea Research 11: 529–562