The condition n > is known as setup, and ri < is called setdown. 



50. Bottom friction. At present, the numerical model uses a linear 

 formulation for friction (Longuet-Higgins 1970) . Thus 



where c is a drag coefficient (of the order of 0.01) and <|u , |> is the 

 time average, over one wave period, of the absolute value of the wave orbita] 

 velocity at the bottom. From linear wave theory 



2H 

 <'"orb'^= Tsinhkh ^^^^ 



Equations 47 and 48 are based on the assumption that the velocity components 



U and V of the current are small compared with the wave orbital velocity, 



< lu , I > . 

 ' orb ' 



51. Radiation stresses. The radiation stresses are of major importance 

 since they furnish the main forces for creating wave-induced currents. Refer- 

 ring to Longuet-Higgins (1970), for monochromatic waves, they are defined in 

 terms of the local wave climate as follows: 



S = E 



XX 



( 2n - 2 ) cos ' 



S = E n cos e sin e 

 xy 



S = E 



yy 



(2n - 2 1 sm 



(n - I) sin^ e (50) 



f n - 2 ) cos 6 



(51) 



(52) 



where 



n =|( 



' + iiirW) f"> 



33 



