123. Alternatively, the depth of closure may be estimated by reference 

 to a maximum seasonal or annual wave height. Hallermeier (1983) found that 

 the maximum seaward limit of the littoral zone could be expressed by 

 Equation 5 if the wave height and period are given by the averages of the 

 highest significant waves occurring for 12 hr during the year. 



124. Since the depth of closure is difficult to estimate at most sites, 

 the modeler must use some external means to determine a value for the par- 

 ticular project. It is recommended that both bathymetry (profile) surveys and 

 Equation 5 be used as a check of the consistency of values obtained. On an 

 open-ocean coast, the depth of closure is not expected to show significant 

 longshore variation, since the wave climate and sand characteristics would be 

 similar. However, in the lee of large structures such as long harbor jetties 

 and breakwaters, the wave climate is milder due to sheltering, and the depth 

 of closure should be smaller. This effect is not accounted for in GENESIS, 

 which uses an average closure depth for the entire modeled reach. 



Wave Calculation 



125. Offshore wave information can be obtained from either a 

 "numerical" gage, i.e., a hindcast calculation, or from an actual wave gage. 

 Wave data are input to the model at a fixed time interval , typically in the 

 range of 6 to 24 hr. The wave height and direction at the gage must be trans- 

 formed to breaking at intervals alongshore for input to GENESIS. Monochro- 

 matic wave models hold the wave period constant in this process. 



126. The modeling system GENESIS is composed of two major submodels. 

 One submodel calculates the longshore sand transport rate and shoreline 

 change. The other submodel is a wave model that calculates, under simplified 

 conditions, breaking wave height and angle alongshore as determined from wave 

 information given at a reference depth offshore. This submodel is called the 

 internal wave transformation model . as opposed to another, completely indepen- 

 dent, external wave transformation model which can be optionally used to 

 supply nearshore wave information to GENESIS. The availability and reliabil- 

 ity of wave data as well as the complexity of the nearshore bathymetry should 

 be used to evaluate which wave model to apply. 



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