These assumptions are necessary since the transport parameters, shape of the 

 equilibrium beach profile, and berm height are considered constant for the 

 entire beach being simulated. 



190. Although beach fills are constructed with a certain cross- 

 sectional area, after a certain time period, typically on the order of a few 

 weeks to months, the fill will be redistributed by wave action to arrive at 

 the equilibrium shape of the beach. As a shoreline response model, GENESIS 

 interprets any added width of beach as conforming to the equilibrium shape. 

 For implementation of fill in GENESIS, the modeler must compute the total 

 added distance Y add that the shoreline will be advanced. This distance is 

 known since the total volume of the fill equals the product of the depth of 

 closure plus berm height, alongshore length of the fill, and Y add . The 

 modeler must estimate if it is appropriate to remove a percentage of the total 

 fill volume that may be lost in fines. Such material is believed to be 

 carried offshore and out of the littoral system. GENESIS places the amount of 

 Y add on the beach in equal increments Ay of shoreline advance along the 

 specified length of the project per time step over the user -specified con- 

 struction period of the fill. The amount Ay is added whether the waves are 

 calm or active. 



191. The input change in shoreline position can also be negative, 

 resulting in shoreline recession instead of advance. This option is useful 

 for describing sand mining. In this case, the shoreline cannot recede 

 landward of a seawall. 



Longshore Transport Rate: Practical Considerations 



192. The empirical formula used to calculate the longshore sand trans- 

 port rate in GENESIS is given by Equation 2. The transport rate is obtained 

 as a function of the waves and shoreline/contour orientation at each time step 

 and at each grid point, except at pinned-beach boundaries. In this section 

 three important considerations are discussed which involve quantities composed 

 of transport rates as calculated from Equation 2. The topics usually 

 encountered in practical applications are: 



a. Multiple transport rates as produced by multiple wave sources. 



91 



