Radiation stress

Definition of Radiation stress: Radiation stress is the flux of momentum carried by ocean waves. This is the common definition for Radiation stress, other definitions can be discussed in the article

Notes

The fact that spatial gradients in wave energy can induce a net force on the water body 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 and by the dynamic pressure (the deviation from hydrostatic pressure). Formulas are given in Shallow-water wave theory#Radiation Stress (Momentum Flux).

For obliquely incident waves, cross-shore momentum is transferred by both cross-shore wave orbital motion and longshore wave orbital motion and longshore momentum is 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. A gradient in the stress in cross-shore direction generates a water level set-up at the coast and a gradient in the stress in longshore direction generates a longshore current. 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.

Analytical expressions for the radiation stresses based on linear wave theory are given in Shallow-water wave theory#Radiation Stress (Momentum Flux) and Shallow-water wave theory#Radiation Stress Components for Oblique Waves. These expressions underestimate the radiation stresses when waves become skewed and asymmetric while propagating into shallow water - see Shallow-water wave theory#Finite amplitude waves. For example, calculations with 5th order nonlinear Stokes theory give a 17% higher value of [math]S_{XX}[/math] in the case of very steep waves.

For irrotational periodic (regular) gravity waves an exact expression of [math]S_{XX}[/math] is given by

[math]S_{XX} = 4 E_k - 3 E_p + \rho h \bigl\langle u_b^2 \bigr\rangle , \quad E_k = ½ \rho \Bigl\langle \int_{-h}^{\eta} (u^2+w^2)dz \Bigr\rangle , \quad E_p = ½ \rho g \bigl\langle \eta^2 \bigr\rangle , [/math]

where [math]u=[/math] cross-shore wave orbital velocity, where [math]u_b=[/math] cross-shore wave orbital velocity at the bottom, [math]w=[/math] vertical wave orbital velocity, [math]\eta=[/math] wave surface elevation, [math]h=[/math] water depth, [math]\rho=[/math] water density, [math]g=[/math] gravitational acceleration, and where [math] \bigl\langle … \bigr\rangle [/math] represents the average over the wave period.