81. The ratio between the amplitudes of the longshore sand transport 



2 

 rate in the two solution areas will be denoted as 6 (= Q ,/Q „). Mathemati- 



ol oz 



cally, the situation is formulated as 



1 a 2 

 9x 



3t 



x < 



(84) 



til 



1 a 2 

 9x 



3y, 



3t~ 



x > 



(85) 



yi (x,0) = y 2 (x,0) = 



(86) 



3x ol r2 o2 .2 9x 



x = 



(87) 



y l = y 2 



x = 



(88) 



yi = o 



y 2 = 



(89) 



It is assumed that the border between the two solution areas at x = is 

 stationary in time, although it moves somewhat in the x-direction as time 

 evolves. The solution is (for details, see Appendix D) 



y^x.t) = 



1 

 ol r 2 o2 



1 + 



2 / 



: . t -x /4e . t 



+ x erfc 



2/^7 



(90) 



for t > and x ^ 



62 



