55. The numerical model permits various types of boundary conditions 

 among which are the following: 



a. "No flow" (wall) . This type of boundary condition is used at 

 closed boundaries such as the still-water line on beaches and 

 at impermeable structures. The normal velocity is set to zero 

 in this case. 



b. Uniform flux. In this type of open boundary condition, the 

 flux at a boundary cell is made equal to that at the next 

 interior cell. Thus the condition assumes 9(Ud)/9x = or 

 9(Vd)/9y = at the boundary. This type of condition is used 

 for the lateral boundaries since it is a passive condition. 



c. Radiation. This open boundary condition requires that any 

 transients developed initially inside the numerical grid should 

 propagate out of the grid as gravity waves. It is of the form 

 9ri/9t + c(9ti/9.x) = where c is the phase speed of a surface 

 disturbance Ti(x,t) . It is often used by the wave-induced 

 current model at the offshore boundary and is found preferable 

 to a wall or constant elevation condition there. Both of the 

 latter conditions are highly reflective, and, as a result, the 

 transients tend to bounce back and forth between the offshore 

 and nearshore boundaries and take a long time to damp out. On 

 the other hand, the radiation condition seems to work quite 

 well, allowing the transients to propagate out of the grid and 

 permitting the setdown at the offshore boundary to assume an 

 appropriate value. 



56. The boundary conditions frequently used in the wave-induced current 

 model are illustrated in Figure 9. 



57. At present, the model allows for subgrid (thin-wall) barriers such 

 as jetties, provided they are impermeable and nonovertopping. The program 

 essentially sets to zero the velocity component normal to the appropriate cell 

 face. 



The Sediment Transport Model 



58. The sediment transport model predicts the transport, deposition, 

 and erosion of noncohesive sediments such as sands in open coast areas as well 

 as in the vicinity of tidal inlets. It accounts for both tides and wave ac- 

 tion by using for input the results of WIFM, RCPWAVE, and CURRENT in terms of 

 tidal elevations and currents, wave climate information, wave-induced cur- 

 rents, and setups at the centers of grid cells. The model computes transport 

 separately for straight open coast areas and areas in the vicinity of tidal 

 inlets. In the case of the former, transports inside and outside the surf 

 zone are treated separately. 



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