specified for the beach in the vicinity of groins, and this slope will usually 

 be different (milder) than the equilibrium beach slope calculated with the 

 representative grain size.) 



266. Since a methodology does not presently exist to allow GENESIS or 

 the modeler to calculate groin permeability by a standard or objective 

 procedure, this quantity is best determined as part of model calibration. If 

 a shoreline reach has numerous groins of various construction types and states 

 of functioning, it is recommended that estimates of relative permeability be 

 given initially and then refined in the course of the model calibration by 

 observing the trend of shoreline change near the groins. As a rule of thumb, 

 an apparently fully functioning groin with a crest above MSL for most tides is 

 assigned an initial permeability value in the range of 0.0 to 0.1, whereas a 

 groin that has gaps or is overtopped during parts of the tidal cycle may have 

 a permeability in the range of 0.1 to 0.5. An effective method of estimating 

 relative groin permeability is to compare the condition (number and width of 

 gaps, thickness and height of groin) of groins on aerial photographs of the 

 model reach. 



267. Lines F.4 and F.5: YG1 . YGN . If a groin or jetty is located 

 on a boundary (grid cell number 1 or N+l) , the distance from the shoreline 

 outside the grid to the seaward end of the structure YG1 and/or YGN must 

 be specified (called "yGi" an< ^ "yGN" i n tne maln text). Since this loca- 

 tion is "off the grid," it must be given externally (by the modeler) and 

 cannot be calculated. This distance is used in the sand bypassing calculation 

 for the structure in situations where sand may be transported onto the grid. 



G. Detached breakwaters 



268. GENESIS treats a detached breakwater as a structure with two dif- 

 fracting ends. The tips of detached breakwaters can be placed at different 

 distances from the x-axis, and gap widths and breakwater lengths can also be 

 arbitrary if a line of segmented detached breakwaters is to be represented. 

 Generally speaking, detached breakwaters should be placed a distance offshore 

 that is at least as far as the location of the average wave breaker line, to 

 simulate the full diffracting effect of the detached breakwaters. If at any 

 time step the waves break seaward of a detached breakwater, the wave height at 



114 



