185. To represent sand bypassing, a bypassing factor BYP is intro 

 duced and defined as 



BYP = 1 - -rr— , (D G < D LT ) (34) 



U LT 



implying a uniform cross-shore distribution of the longshore sand transport 

 rate. If D G > D LT , BYP - . Values of BYP thus lie in the range 

 < BYP < 1 , with BYP - signifying no bypassing, and BYP - 1 signifying 

 that all sand can potentially pass the position of the structure. The value 

 of BYP depends on the wave conditions at the given time step, since D LT is 

 a function of the wave height and period (Equation 4). 



186. Sand transmission . A permeability factor PERM is analogously 

 introduced to describe sand transmission over, through, and landward of a 

 shore -connected structure such as a groin. A high (in relation to the mean 

 water level), structurally tight groin that extends far landward so as to 

 prevent landward sand bypassing is assigned PERM = , whereas a completely 

 "transparent" structure is assigned the value PERM - 1 . Values of PERM 

 thus lie in the range of < PERM < 1 and must be specified through judgment 

 of the modeler based upon, for example, the structural characteristics of the 

 groin (jetty, breakwater), its elevation, and the tidal range at the site. 

 Aerial photographs are often helpful in estimating a structure's amount of 

 void space (hence PERM) in relation to other structures on the model grid. 

 The optimal value of PERM for each structure must then be determined in the 

 process of model calibration. 



187. With the values of BYP and PERM determined, GENESIS calculates 

 the total fraction F of sand passing over, around, or through a shore- 

 connected structure as (Hanson 1987) 



F - PERM(1 - BYP) + BYP (35) 



This fraction is calculated for each shore -connected (groin-type) structure 

 defined on or at the boundaries of the grid. 



89 



