defined alternately. Q-points define calculation cell faces and y-points lie 

 at the centers of cells. Subscripts denote locations of points along the 

 beach. Both Q-grid points and y-grid points are separated by a constant dis- 

 tance Ax alongshore; the distance between a Q-point and an adjacent y-grid 

 point is Ax/2 . Lateral boundary conditions must be specified at the ends 

 of the grid, e.g., at Q^ and Q N+1 . Alternatively, it is possible to spec- 

 ify boundary conditions at y« and yjt , or impose a condition on y at one 

 end of the grid and a condition on Q at the other end. 



y „ 



Y///?///////7% 



SHORELINE 



-SEAWALL 



YSBEG- 



AX 



YSEND 



Figure 6. Definition sketch for finite difference discretization 



47. For simplicity, only one seawall will be considered. Its beginning 

 and ending coordinates on the x-axis are denoted by YSBEG and YSEND, re- 

 spectively, as shown in Figure 4. A general y-position at the seawall is de- 

 noted by ys i . 



48. In a standard explicit scheme, Equation 1 is discretized as 



y! = 2B (Q. - Q. .) + y. 



J l l i+1 J l 



(5) 



where 



B = At/(2DAx) , s/m 

 At = time step, s 

 Ax = space interval, m 



20 



