tests. Additional laboratory experiments are needed to verify new two- 

 dimensional numerical models applied to situations with irregular bathy- 

 metry, coastal structures (breakwaters, groins, etc.), and irregular wave 

 fields. 



A large number of wave, wind, geometry, fluid, and sediment factors 

 interact to create the currents observed. Considerable simplification 

 and idealization is necessary to formulate the theory and equations needed 

 to obtain a solution. 



2. Theory and Experimental Verification . 



The time-averaged equations of horizontal motion and continuity, 

 including radiation stress terms, have become the accepted basis for a 

 theory of coastal hydrodynamics in the 1970 's. Short-period wind waves 

 when time-averaged create additional momentum flux terms, i.e. the radia- 

 tion stress gradients, which become the primary driving forces in the 

 theory. Solution of these three equations determines the three dependent 

 variables of interest: the MWL change n, the longshore current v, and the 

 cross-shore current u. Wave heights must be specified a priori to affect 

 a solution because the local wave energy appears in the radiation stress 

 terms. This requires use of wave shoaling, refraction, diffraction, 

 reflection etc. theory outside the breakers, a wave breaking criterion, 

 and some model (general, empirical) of surf zone energy dissipation. 

 Additional stress gradient terms appear in the motion equations to account 

 for surface wind and bottom friction shears and lateral, internal, turbu- 

 lent mixing stresses. All require semiempirical turbulence stress models 

 with appropriate closure coefficients determined experimentally. 



The wave breaking and surf zone empirism along with general knowledge 

 of the required closure coefficients remain the weak links in the theory. 

 The original theory makes extensive use of linear (Airy) wave theory. 

 It forms the basis for calculation of the radiation stress components both 

 outside and within the highly nonlinear surf zone and is used in wave 

 shoaling, refraction, diffraction, etc. calculations. Applications to 

 simplified geometries with appropriate boundary conditions hat^e tradition- 

 ally been in the following categories: 



(1) Mean water level changes (setdown and setup), 



(2) uniform longshore current profiles, and 



(3) nearshore circulation systems with rip currents. 



The findings in these three areas are summarized below. Major modi- 

 fications to the original theory have taken place in 



(1) Bed shear-stress formulations, 



(2) extensions to irregular waves. 



221 



