in these units. (The only exception is median grain size diameter on Line 

 C.l, which must be given in millimeters.) 



227. Line A. 3: NN . DX . The total number of calculation cells NN 

 (called "N" in the text of this report) and the cell length DX (called 

 "Ax") are entered here. The product NN'DX gives the total length of the 

 modeled reach. 



228. Line A. 4: ISSTART . N This request allows the user to perform 

 simulations over a portion of the grid through specification of starting and 

 ending grid cells (boundaries) other than 1 and N+l , respectively. This 

 option is useful if a long grid has originally been prepared but, in a 

 particular application, details of shoreline change along a subsection are to 

 be studied. It is cautioned that the numbers of the starting cell ISSTART 

 and ending cell N of the subsection grid must be located in physically 

 reasonable areas for meaningful results to be obtained. In almost all circum- 

 stances, lateral boundaries should be placed either at a long groin or jetty 

 or at a historically stable section of coast. It is recommended that this 

 option not be exercised until experience is gained running GENESIS. If 

 simulation of shoreline change in a subsection is not performed, the values of 

 ISSTART and N should be 1 and NN (as specified on Line A. 3), respec- 

 tively. By setting N equal to -1 , GENESIS will set N equal to NN , and 

 the value of N does not have to be changed for each new application. 



229. Line A. 5: DT For a specific simulation interval, smaller 

 values of the duration of the time step DT (called "At" in the main text 

 of this report) increase the computational run time, whereas larger values of 

 DT result in a less accurately predicted shoreline position. A time step of 

 6 hr is recommended for design, but longer time steps may be used, for 

 example, 24 hr, depending on the variability of the input waves. Scoping 

 applications will typically use a long time step (on the order of 24 hr). The 

 wave data input file (WAVES) must be designed to provide wave data at the 

 specified time step. To satisfy this requirement, DT must be a proper 

 fraction (e.g., 1/2, 1/4) of the time step DTW defining entries in the wave 

 file (Line B.6) . 



230. Line A. 6: SIMDATS The date when the calculation starts 

 SIMDATS is needed to key GENESIS for selecting the correct season of waves, 



105 



