the beach backed by a seawall cannot recede behind the wall. In this report, 

 the seawall constraint is referred to as a boundary condition although it is 

 not a boundary condition in a true sense. 



41. Three terms will be defined to distinguish important transport sit- 

 uations which can occur at a seawall. 



Minus area (Figure 5a) 



42. The expression "minus" area (minus calculation cell in the numeri- 

 cal model) is applied if, at a given time, sand is transported out of both 

 sides of the area. If a minus area occurs where the shoreline has eroded to a 

 seawall, then the sand transport rate must be corrected in such a manner as to 

 conserve sand volume and preserve direction of transport, in order to pass in- 

 formation about the lateral boundary conditions. In the method described in 

 this report, transport rate corrections along the beach are made in the direc- 

 tion of sediment transport, i.e., in the downdrift direction. Therefore, 

 minus cells are starting points for corrections. 



Plus area (Figure 5b) 



43. If sand is moving into an area from both sides at a given time, 

 this condition defines a "plus" area (plus calculation cell in the numerical 

 model). The terminology "plus cell" describes the reverse situation of a 

 minus cell; consequently, transport rate corrections end at plus cells (or at 

 lateral boundaries). 



Regular area (Figure 5c) 



44. The most common situation is for a certain quantity of sand to en- 

 ter one side of an area and for a slightly different quantity of sand to leave 

 the area on the opposite side. This is called a "regular" area (regular cell 

 in the numerical model). Sand volume and direction of transport must be pre- 

 served whether or not there is a local net gain or net loss of material. If 

 the shoreline in a regular area is in contact with a seawall, no more sand can 

 leave the cell than enters it. If the converse occurs, causing the nonphys- 

 ical movement of the shoreline to a position landward of the seawall, the 

 transport rates must be corrected in an appropriate manner to move the shore- 

 line position to the seawall. 



45. If a wide beach exists in front of a seawall, it is not necessary 

 to distinguish between minus areas, plus areas, and regular areas. These 

 three concepts become important only when the shoreline makes contact with a 

 seawall. 



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